Journal of Colloid and Interface Science 436 (2014) 99–110
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Structure of the electrical double layer at aqueous gold and silver interfaces for saline solutions Zak E. Hughes ⇑, Tiffany R. Walsh Institute for Frontier Materials, Deakin University, Geelong 3216, VIC, Australia
a r t i c l e
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Article history: Received 9 June 2014 Accepted 22 August 2014 Available online 4 September 2014 Keywords: Molecular simulation Salt solutions Noble metals Interfaces
a b s t r a c t We report the structure of the electrical double layer, determined from molecular dynamics simulations, for a range of saline solutions (NaCl, KCl, MgCl2 and CaCl2) at both 0.16 and 0.60 mol kg1 on different facets of the gold and silver aqueous interfaces. We consider the Au/Ag(1 1 1), native Au/Ag(1 0 0) and reconstructed Au(1 0 0)(5 1) facets. For a given combination of metallic surface and facet, some variations in density profile are apparent across the different cations in solution, with the corresponding chloride counterion profiles remaining broadly invariant. All density profiles at the higher concentration are predicted to be very similar to their low-concentration counterparts. We find that each electrolyte responds differently to the different metallic surface and facets, particularly those of the divalent metal ions. Our findings reveal marked differences in density profiles between facets for a given metallic interface for both Mg2+ and Ca2+, with Na+ and K+ showing much less distinction. Mg2+ was the only ion for which we find evidence of materials-dependent differences in interfacial solution structuring between the Ag and Au. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Understanding the structuring of water and salt ions at metal interfaces is of fundamental importance in surface science as well as electrochemistry and materials science [1–3]. Also, in recent years there has been considerable interest in identifying peptide sequences that show adsorption selectivity to a particular material, or even a particular facet [4–12]. There is increasing evidence that the structure of water molecules at the metal interface plays a major role in the mechanism of peptide adsorption [13–16,12]. Despite this there are still many unanswered questions regarding the behaviour of metal–aqueous interfaces. If we are to gain greater understanding of what drives the selectivity of some biomolecules to particular surfaces we need to gain a more detailed understanding of the behaviour of metal–aqueous interfaces and how the presence of ionic species may modulate these binding preferences, as has been explored experimentally, e.g. for SiO2 interfaces [17]. In addition, there is significant interest in the phenomenon of the electrical double layer (EDL) that is formed when a surface is in the presence on an electrolyte [18–28]. Here too, a more detailed, atomic-level understanding of the behaviour of water and ions at the interface is needed.
⇑ Corresponding author. E-mail address: [email protected] (Z.E. Hughes). http://dx.doi.org/10.1016/j.jcis.2014.08.045 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
From an experimental perspective it remains very challenging to gain information about the structure of aqueous solutions at metal surfaces. As such a number of theoretical and simulation studies have reported investigations of metal–aqueous interfaces [18,19,21,29,22,30,23,24,31,32,25,33,34,26,27,35,14–16,28]. The starting point for many of the theoretical descriptions of the EDL is the Gouy–Chapman–Stern (GCS) theory, which is described by the Poisson–Boltzmann (PB) equation [36,24,27]. This theory predicts that the EDL comprises a strongly adsorbed layer of ions at the surface, known as the Stern layer, and a second ‘diffuse layer’ of ions where the electric potential decays to the bulk value [24,27]. This theory can accurately predict the long-ranged behaviour of the charge distribution at the interface; it has the advantage of simplicity and is a reasonable approximation for systems of low ion concentration, low valence and low surface charge [24,27]. However, this theory cannot capture the short-ranged structuring of the ions at the interface, due to the assumption that particle interactions are negligible. Other theoretical treatments have been developed to address these limitations [18,19,22,23,26,28]; these are capable of predicting more persistent non-monotonic ion profiles [18,19,22,23]. In addition, the differing behaviour of cations and anions can be observed, even for charge-neutral surfaces [18,19,25,28]. Molecular dynamics (MD) and Monte Carlo (MC) simulations of aqueous interfaces provide an alternative way of investigating the molecular structure of such systems, and allow
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Table 1 Radius of first solvation shell, r (Å), and the coordination number of water molecules in the first solvation shell, n. The data are shown for the 0.16 mol kg1 with the data for the 0.60 mol kg1 solution given in parentheses. Parameter
System
Na+
K+
Mg2+
Ca2+
Cl
r/Å n
Solution Solution Au(1 1 1) Ag(1 1 1) Au(1 0 0)(1 1) Ag(1 0 0)(1 1) Au(1 0 0)(5 1)
2.30 5.8 (5.6) 5.7 (5.6) 5.7 (5.6) 5.5 (5.4) 5.5 (5.4) 5.7 (5.6)
2.70 6.8 (6.6) 7.1 (7.0) 7.2 (7.1) 7.3 (7.2) 7.2 (7.2) 7.2 (7.1)
1.96 6.0 (5.9) 6.0 (6.0) 6.0 (6.0) 6.0 (5.6) 6.0 (6.0) 6.0 (6.0)
2.26 7.0 (6.2) 6.6 (6.6) 6.6 (6.7) 7.3 (7.3) 6.7 (7.1) 7.0 (6.8)
3.15 7.7 (7.7) 7.8 (7.8) 7.9 (7.9) 7.8 (7.8) 7.9 (7.9) 7.9 (7.8)
evaluation theoretical models, contributing to the development and refinement of such models [22,30,23,24,26,25]. While past simulations of metal–electrolyte interfaces have provided useful details, some of these studies have sought to reduce the complexity of the problem by incorporating assumptions such as an implicit solvent [19,24,28], by modelling anions and cations as having opposite charge but being otherwise identical [22,23], or by considering the surface at the aqueous materials interface to be an unstructured wall [27]. Assumptions such as the latter may neglect important chemical and physical details of these interfaces that can lead to material and facet dependent differences in solvent structuring. For example, first-principles Car–Parrinello [33] and Born–Oppenheimer [34] MD simulations of the aqueous Au(1 1 1) interface have shown the water molecules in the first adsorption layer are not distributed isotropically parallel in the
plane of the surface. Rather, oxygen atoms were predicted to show a preference for a position atop the metal atoms. While first-principles simulations of metal–aqueous interfaces [37,33,34] are able to describe the chemical and physical properties of the systems in great detail, the computational expense of such simulations typically limits these to tens of picoseconds of trajectory. Use of molecular mechanics force-fields (FFs) allows longer time- and length-scales to be reached while at the same time retaining more of the physical/chemical detail of the metal surface and the interfacial structure than many theoretical treatments. However, the parametrisation of metal–aqueous FFs is not trivial. Despite this, a number of accurate, yet efficient FFs, capable of simulating gold/silver–aqueous interfaces have been developed, in particular the CHARMM-METAL [38], GolP [39], and GolPCHARMM/AgP-CHARMM [14–16] FFs, with the latter two explicitly
Fig. 1. The structure of water at the gold interfaces. Normalised density of water oxygens and hydrogens at the (a) Au(1 1 1), (b) Au(1 0 0)(1 1) and (c) Au(1 0 0)(5 1) interfaces and (d) the charge density at the three Au interfaces.
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incorporating metal polarization contributions. The relative merits of each class of FF have been discussed in detail elsewhere [40,14,16,12]. Here, we have used the GolP-CHARMM and AgP-CHARMM FFs [39,14–16], which capture the polarisability of the Au and Ag surfaces via the use of a rigid-rod dipole [41]. In these FFs, the metal atoms are fixed in space but the dipoles attached to each atomic site are free to rotate. In addition, these FFs include virtual sites in the uppermost surface layer of metal atoms, to direct noncovalent adsorption of water and other molecules to surface atop sites, as has been indicated as preferable in recent simulations of the interface between Au/Ag and liquid water [37,33,34,32]. The description of the aqueous Au(1 1 1) interface by the GolPCHARMM FF agrees well with those of first principles simulations [33,34], not only in terms of the density profile of water atoms perpendicular to the surface, but also in terms of the distribution of the first adsorbed layer of water molecules parallel to the interface. In addition, both the first-principles simulations [33,34] and GolPCHARMM simulations [14–16,12] indicate that the interfacial water molecules in the first adsorbed layer are most likely to have the oxygen atom directed towards the surface, meaning that these
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water molecules are more likely to be hydrogen-bond donors compared with those in the bulk. By investigating the structuring of water and ions at metal interfaces via atomistic MD simulations, we provide data that can be usefully compared both with new analytical models as well as with future experimental findings. While the (1 1 1) facet is the lowest in energy for Ag and Au, metal surfaces/nanoparticles do, in general, feature defects as well as a range other facets at the metal–aqueous interface, such as the (1 0 0) facet. Thus, to predict and elucidate the behaviour of aqueous saline solutions at nanoparticle interfaces, we chose to model both of these major facets. Both Au facets are more complex to describe than may be appreciated at first sight, as both undergo reconstruction in an aqueous environment atp300 ffiffiffi K and pH 7. In the case of the reconstructed Au(1 1 1) 22 3 surface, the high degree of structural similarity between it and the native Au(1 1 1) surface means that the unreconstructed surface provides a suitable model for molecular simulation. In contrast, there are significant structural differences between the native Au(1 0 0)(1 1) and the reconstructed Au(1 0 0)-hex surfaces [15]. Unlike the native surface, the Au(1 0 0)-hex surface is not atomically flat, featuring atomic-scale undulations. The Au(1 0 0)(5 1) reconstruction makes an excellent
direction at the Au(1 1 1) surface, (b) in the [1 1 2] direction at the Fig. 2. 2-dimensional density profiles of water oxygen atoms at different gold interfaces: (a) in the [110] 1] direction at the Au(1 1 1) surface, (c) in the [0 1 1] direction at the Au(1 0 0)(1 1) surface, (d) in the [0 1 1] direction at the Au(1 0 0)(5 1) surface and (e) in the [0 1 Au(1 0 0)(5 1) surface. Crosses indicate the position of the surface metal atoms.
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approximation to the Au(1 0 0)-hex surface, admitting a unit cell size that is practical for molecular simulations [15]. When considering typical Au nanoparticles in aqueous solution, it is not known if the (1 0 0) surface is reconstructed or not. On this basis, we chose to model the native Au(1 1 1), native Ag(1 1 1), native Au(1 0 0)(1 1), native Ag(1 0 0)(1 1), and the reconstructed Au(1 0 0)(5 1) aqueous interfaces. Atomic-scale structures of the (1 1 1), (1 0 0)(1 1) and (1 0 0)(5 1) surfaces are provided in Fig. S1 of the Supplementary Material. In this study we have investigated the interfacial solvent structure of four different saline solutions; NaCl, KCl, MgCl2 and CaCl2, at the Au(1 1 1), Au(1 0 0)(1 1), Au(1 0 0)(5 1), Ag(1 1 1), and Ag(1 0 0)(1 1) interfaces. Simulations have been performed for solutions at physiologically-relevant concentrations ( 0.16 mo lkg1) as well concentrations close to seawater ( 0.6 mol kg1). Overall, we found that the facet of the aqueous metallic interface, of overall neutral charge, exerted a strong influence over how the salt ions ordered at the interface. In contrast, we saw little change in the interfacial solution structuring for the range of solution concentrations considered here. While the different cations shared some similarities in their interfacial solution structure, there were also significant differences in the spatial distribution of these ions at the interface, particularly for the divalent cations.
2. Computational methods Molecular dynamics simulations were carried out for each of the four saline solutions in contact with each of the five metallic interfaces. These simulations were performed using the GROMACS software package, version 4.5.5. [42]. The interactions of the water and ions with the gold and silver interfaces were modelled using the GolP-CHARMM [14,15] and AgP-CHARMM [16] FFs, respectively. The modified version of the TIP3P [43,44] water model was used for the water molecules, and the CHARMM22⁄ FF parameters was used for the salt ions [45,46]. Further details about the FF and atomistic parameters are provided in Section S1 and Tables S1 and S2 in the Supplementary Material. For all simulations a timestep of 1 fs was used, with the Lennard-Jones (LJ) non-bonded interactions switched off between 10.0 and 11.0 Å and a cut-off of 13.0 Å used for the Particle Mesh Ewald summation [47]. For the Ag and Au(1 1 1) and (1 0 0)(1 1) surfaces, we used slabs comprising p(14 16) and p(14 14) supercells respectively, of five atomic layers thick. For the Au(1 0 0)(5 1) surface a p(2 2) supercell, 9 atomic layers thick was modelled. Lattice parameters of 4.145 and 4.165 Å were used for Au and Ag, respectively. All of the surfaces modelled here were charge-neutral. The systems simulated contained 2045 or 3198 water molecules for the (1 1 1)/(1 0 0)(1 1) and (1 0 0)(5 1) surfaces, respectively. The cell dimension perpendicular to the surface plane, along the z-axis, was adjusted such that the bulk density of the water midway between the slab and its periodic image in the z-direction was equal to that obtained from a MD simulation of salt solution modelled at the same temperature, pressure and concentration. The inter-slab spacing in the z-direction was typically 38 Å. This gave rise to a bulk density region 18–20 Å thick in the centre of the inter-slab space. Once the correct solvent density was obtained, the production run was performed in the Canonical (NVT) ensemble with the Nosé–Hoover thermostat [48,49], used to maintain the temperature at 300 K. The production runs were of 50 ns and 20 ns duration for the 0.16 and 0.60 mol kg1 systems, respectively. For all MD simulations reported in this section, the actual gold and silver atoms in the metal slab were held fixed in space, while the dipole particles were free to rotate (according to their restraint potential and the temperature of the thermostat). Recently reported tests indicate that there is very little difference
between binding free energies obtained using a rigid substrate compared with those calculated using a slab where all atoms can move [50]. In addition to the simulations of the different interfaces, simulations of each salt solution (without the presence of the metallic surface) at both 0.16 and 0.60 mol kg1 were performed to obtain the coordination number of the first solvation shell of each ion, as given in Table 1. 3. Results and discussion 3.1. Liquid water The structuring of water at the different interfaces is thought to have a strong effect on how other species interact with the different metal/facets. [14–16] These previous studies have suggested that both the structuring of water molecules is greater on the (1 0 0) facet than the (1 1 1) facet (with the corresponding structuring on the (1 0 0)(5 1) facet predicted to be intermediate between these two native facets), and that this interfacial solvent structuring is greater on Ag interfaces than on Au interfaces, for a given facet. At all interfaces, a strong peak in the vertical density profile (along the direction perpendicular to the plane of the surface) of the water oxygen is found at 2.5 Å from the surface, with a second smaller peak at 6.0 Å, see Fig. 1. The water molecules in the first solvation layer tended to direct the hydrogen atoms away from the surface [14–16]. As such, the water molecules in the first
Fig. 3. Normalised density of (a) Na+ and (b) Cl ions at concentrations of 0.16 and 0.60 M at the Au(1 1 1) interface. The normalised density of water is also shown.
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solvation layer are more likely to be hydrogen bond donors when compared against water molecules in bulk solution. The water molecules in this first layer are also structured in the lateral (x–y) plane [14–16], with an increased oxygen density apparent atop the surface metal atom sites, in agreement with Cicero et al. [33]. The total charge density of system at the gold interfaces is shown in Fig. 1(d); the behaviour at Ag(1 1 1) and Ag(1 0 0)(1 1) is very similar and thus not shown. The presence of the water oxygen atoms atop the metal surface atoms causes a degree of alignment in the dipoles of the metal atoms; this is responsible for the positive peak in the charge density profile just above the interface. All of the different facets show this type of behaviour, but to different degrees. The positions of the oxygen and hydrogen atoms of the water molecules give rise to the series of peaks and troughs above the gold/silver interfaces in Fig. 1(d). As seen for the density profiles, the Au(1 0 0)(5 1) interface more closely resembles the Au(1 1 1) than the Au(1 0 0)(1) interface. Fig. 2 shows the 2-D density profiles of the water oxygens at the different Au facets. The strong 3-D ordering of the water molecules can be seen, with water molecules situated above the metal atoms (see also Fig. S2 in the Supplementary Material). The corresponding density profiles for the Ag(1 1 1) and Ag(1 0 0)(1 1) interfaces (not shown) yielded very similar structuring to the Au(1 1 1) and Au(1 0 0)(1 1). This strong adsorption of water atop the metal atoms has also been seen for other metal surfaces [51,52]. In particular, Chandler and co-workers have used a FF containing both two- and three-body terms to investigate the structure of water at the Pt(1 1 1) and Pt(1 0 0) surfaces [30,32,35]. As discussed below, the structuring of the water molecules is an important factor in
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determining the structuring of the salt ions in the saline solutions at the aqueous metallic interface. 3.2. NaCl Solution The normalised vertical density profiles of 0.16 and 0.60 mol kg1 NaCl solutions at the Au(1 1 1) interface are shown in Fig. 3. Increasing the salt concentration does not change the shape of the density profile significantly; only for Cl was there a noticeable difference in peak heights. Neither of the two ions, Na+ or Cl, approached the Au(1 1 1) interface as closely as the water molecules. The Na+ profile showed strong structuring with multiple peaks, at 4.6, 7.2 and 10.0 Å from the interface. The position of these peaks corresponded with troughs in the water density profile. In addition, both the first and second peaks featured shoulders located at the positions of the peaks in the water density profiles. The Cl profile showed fewer features, with a strong first peak at 5.7 Å, aligned with the position of the second peak in the water density profile, with a shoulder. There may be a second peak at 8.0 Å but noise in the profile means that we cannot state this with certainty. The vertical density profiles for the 0.16 mol kg1 NaCl solution at the other interfaces are shown in Fig. 4. As for Au(1 1 1), the effect of the increased NaCl concentration did not alter the density profiles (not shown) of the ions at the Ag(1 1 1), Au(1 0 0)(1 1), Au(1 0 0)(5 1) or Ag(1 0 0) interfaces significantly. Comparing the Au(1 1 1) and Ag(1 1 1) interfaces, the positions of the peaks in the Na+ profile appeared the same, but the height of the first peak was slightly greater in the case of Ag(1 1 1). This may be
Fig. 4. Normalised density of water, Na+ and Cl at a concentration of 0.16 M at the (a) Ag(1 1 1), (b) Ag(1 0 0)(1 1), (c) Au(1 0 0)(5 1) and (d) Au(1 0 0)(1 1) interfaces.
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due to the more structured first layer of water on the Ag(1 1 1) surface compared with the Au(1 1 1) surface. Alternatively, the greater polarisability of Ag might be responsible for these differences. In the case of the Cl profiles the differences between the metals is very small. The second peak in the chloride density profile is more clearly defined for the Ag(1 1 1) than the Au(1 1 1) interface. On the (1 0 0)(1 1) surfaces the shoulder there is a distinct peak at 4.0 Å observable (rather than a shoulder to the peak at 5.7 Å on the (1 1 1) surfaces). Like in the case of Na+ the Cl profile at the Au(1 0 0)(5 1) resembles the Cl profile at the (1 1 1) interface more than the (1 0 0)(1 1) interface. The different facets also gave rise to changes in the density profiles of the ions. For the native (1 0 0) facets (both Ag and Au) there was an extra peak at 3:5 Å in the Na+ profile. In contrast, the reconstructed Au(1 0 0)(5 1) surface yielded a sodium ion profile very similar to that of the Au(1 1 1) surface. In the case of chloride, different density profiles were observed for each facet. For both the Au and Ag(1 0 0)(1 1) interfaces, the shoulder of the first peak resolved into a separate peak, while in the case of the Au(1 0 0)(5 1) surface the same shoulder was diminished with respect to Au(1 1 1). In Fig. S3 in the Supplementary Material we provide the charge density profiles for the surfaces with the NaCl solutions. Even at a
NaCl concentration of 0.60 mol kg1 the total charge density of the system was still dominated by the water molecules and thus the profiles were practically unchanged from the pure water systems. As in the case of pure liquid water, the ions were not only arranged in one dimension, but two. The 2-D density maps of the ions are shown in Fig. 5. The distribution of ions further than 6 Å from the surface (i.e. corresponding to the second layer of interfacial water molecules and beyond) is largely isotropic along the [0 1 1] direction. However, at closer distances the distribution of ions is clearly anisotropic in the lateral plane. Moreover, these 2-D maps indicated that the lateral distribution of the ions was correlated with those of the first layer of water molecules (see Fig. 2). The Na+ ions closest to the interface were found above the hollows of the metal surface, i.e. offset from the water oxygens. In contrast, the Cl ions were found above the oxygens of the first layer of water molecules. The likely locations of the two ions can be understood if we consider the structuring of water molecules around the ions, when free in solution, compared with the water structuring at the interfaces. Table 1 gives the radius of the first solvation shell around each ion, while Fig. S4 in the Supplementary Material shows the radial distribution function of the ion with the water molecules for the five different ion species. The radius of the first hydration shell of Na+ is
0] direction of Au(1 1 1) (a and b), the [0 1 1] Fig. 5. 2-Dimensional density profiles of Na+ (a, c and e) and Cl (b, d and f) at 0.60 mol kg1 concentrations, along the [1 1 direction of Au(1 0 0)(1 1) (c and b) and the [0 1 1] direction of Au(1 0 0)(5 1) (e and f). Crosses indicate the position of the surface metal atoms.
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2:3 Å; this is smaller than the 2:9 Å distance between surface metal atoms for both Ag and Au. Thus, on the (1 1 1) facet a Na+ ion sitting in the hollow provided by three laterally-adjacent water molecules in the first solvation layer can coordinate with each of these water molecules, as well as two/three other water molecules in the second interfacial layer. A similar scenario is seen for the (1 0 0)(1 1) surface, where a sodium ion positioned above a hollow site on the metal surface can also coordinate with water molecules in the first interfacial layer. The larger first solvation shell of the chloride ion (in terms of both radius and coordination number) makes it more favourable for Cl to be located above a surface metal atom and coordinate to a number of water molecules in the first interfacial layer.
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The overall coordination number of the first layer of sodium and chloride ions (defined as ions within 5.5 Å of the interface) is given in Table 1. For the (1 1 1) and (1 0 0)(5 1) interfaces the coordination of the ions at the surface is very close to those found in bulk solution. In the case of the (1 0 0)(1 1) surfaces, there is a small reduction in the number of water molecules surrounding the sodium ion. In summary, both Na+ and Cl are more likely to be found at the metal surface than in the bulk. The effect of the different facets on the structuring of the ions at the metal interfaces is greater than the effect arising from the different metals. These results agree quite well with the results of simulations of 1 M LiCl solutions at Pt(1 0 0), which concluded ‘‘the absorption of cations at the electrode is
Fig. 6. Normalised density of water, K+ and Cl at a concentration of 0.16 M at the (a) Au(1 1 1), (b) Ag(1 1 1), (c) Au(1 0 0)(1 1), (d) Ag(1 0 0)(1 1) and (e) Au(1 0 0)(1 1) interfaces.
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strongly inhibited by the requirement for them to reorganize their hydration shells to approach the electrode surface’’ [30]. Moreover, their potential of mean force calculations showed an energy minimum at 4 Å in the adsorption profile of Li+. This simulation of the LiCl solution likewise showed a relatively weak interaction between the Cl anions and the metal surface again agreeing with the results of our simulations. In contrast, experimental and theoretical studies have shown quite strong interactions between Cl and transition metal surfaces, with the chloride ions forming adlayers [19–21,53,54]. The case of this discrepancy between these simulations and experiment is due to the form of the 12–6 potentials often used to model the LJ non-bonded interactions between atoms. The large r value given to Cl to reproduce the behaviour of Cl in bulk solution means that the LJ interactions, generated via Lorentz–Berthelot mixing rules, between the metal atoms and the anions is too repulsive at short range. Thus, the formation of an anionic adlayer is blocked. In future refinements of the GolP/ AgP-CHARMM FFs it would be possible to construct a bespoke Au/ Ag–Cl 12–6 LJ term that could more accurately model the interaction of Cl (and other halides) with the metal surfaces.
at the (1 0 0)(1 1) interfaces, where the height of the peak at 4 Å was increased. This change in the charge density at the interface could have an effect on the adsorption of biomolecules, especially those containing charged species. The density profiles of the MgCl2 and CaCl2 solutions are shown in Figs. 8 and 9, respectively. As for Na+, the peaks in the vertical density profile for both Mg2+ and Ca2+ coincided with the troughs in the water density profile. Unlike Na+, however, the ratio of the height of the first and second peaks varied strongly with the facet, with the (1 1 1)/(1 0 0)(5 1) interfaces yielding marked differences in behaviour compared with the (1 0 0)(1 1) interface. However, the corresponding profiles for the Cl counterion appeared very similar across all interfaces. In the case of Mg2+, at the Au(1 1 1) and Au(1 0 0)(5 1) interfaces, the height of the first peak was approximately equal to the height of the second peak. In contrast, Ag(1 1 1) featured a significantly greater degree of structuring, as evidenced in the height of the first peak in the vertical density profile compared with Au(1 1 1). The Mg2+ solution provided the only instance where a distinction in vertical density profiles was observed between Ag
3.3. KCl solution The vertical density profiles generated from the KCl 0.16 mol kg1 solution simulations are shown in Fig. 6, yielding Cl profiles that are essentially equivalent to those predicted for the NaCl solution. As in the case of Na+, the difference in solvent structuring between 0.16 and 0.60 mol kg1 is negligible. While the Cl density profiles appeared very similar to those predicted for NaCl, the density profiles of K+ were quite different to those of Na+. At all five interfaces the peaks in the density profile of K+ were shifted out to a greater distance from the surface compared with the data obtained for Na+. We suggest this is due to the larger radius of the first solvation shell of K+ compared with Na+. In addition, the ratio of the height of the first and second peaks in the vertical density profiles is closer to 1.0 compared with this ratio in the case of Na+, where the height of the second peak was roughly double that of the first. This could also be due to the fact that the K+ ions are further (on average) from the metal surfaces. Fig. 7 shows the 2-D density profiles of K+ at the three different facets. In the case of the (1 1 1) and (1 0 0)(5 1) interfaces there is distinctly less ordering of the cations parallel to the [0 1 1] direction, compared with the analogous data for Na+, with the (1 0 0)(1 1) interface supporting the greatest degree of lateral structuring. We propose that the larger atomic radius of K+, relative to Na+ (see Table 1), means that it is less favourable for the K+ ions to interdigitate between the water molecules in the first interfacial water layer; they are however located in the more loosely-structured second interfacial water layer. In contrast to our predictions for sodium, a change in the coordination number of potassium ions in the first layer was observed for all interfaces; there was a small but significant increase in the number of water molecules in the first solvation shell. This could be ascribed to the fact that position of the first layer of K+ ions is coincident with the position of the second interfacial layer of water molecules. Thus, the density of water surrounding the potassium ions is higher than that in the bulk, leading to a increase in the coordination number of the potassium ions. 3.4. MgCl2 and CaCl2 solutions Unlike the monovalent cation solutions, the divalent cations solutions do give rise to some, minor, changes in the charge density profiles of the interfaces at the higher concentration of 0.60 mol kg1 (as shown in Fig. S5 in the Supplementary Material). The strongest effect of the ionic species was seen for CaCl2 solution
Fig. 7. 2-Dimensional K+ density profiles of 0.60 mol kg1 KCl solutions along (a) 0] direction of Au(1 1 1), (b) the [0 1 1] direction of Au(1 0 0)(1 1) and (c) the [1 1 the [0 1 1] direction of Au(1 0 0)(5 1). Crosses indicate the position of the surface metal atoms.
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Fig. 8. Normalised density of water, Mg2+ and Cl at a concentration of 0.16 M at the (a) Au(1 1 1), (b) Ag(1 1 1), (c) Au(1 0 0)(1 1), (d) Ag(1 0 0)(1 1) and (e) Au(1 0 0)(1 1) interfaces.
and Au for a given facet. In contrast, for Ca2+ the height of the second peak was greater than that of the first peak at the both the (1 1 1)/(1 0 0)(5 1) interfaces, while the (1 0 0)(1 1) interfaces yielded an extremely sharp and pronounced first peak in the density profile. The 2-D density maps given in Fig. 10, highlight the ordering even more, with the interaction of Ca2+ on the (1 0 0)(1 1) being especially marked. The position of the first peak in the density profile of the divalent cations at the (1 1 1) surface is at a slightly shorter distance to the metal interface than in the case of Na+, (4.4 as opposed to 4.6 Å). Again, we propose that the driving force for the structuring of the ions at these interfaces is the nature of the first solvation shell around the different ions. In the case of Mg2+ the first solvation
shell is extremely tightly bound (as can be seen from Fig. S3 in the Supplementary Material), indicating that it is unfavourable for Mg2+ to lose a water molecule. This is suggested to promote a favourable interaction with the hex interfaces, where the ions can remain coordinated to six water molecules (three in the first water layer and three in the second water layer). However, our data indicate that it should not be as favourable for a Mg2+ adsorbed above a hollow site to coordinate to six water molecules on the squarepatterned (1 0 0)(1 1) surfaces, as suggested by the reduction in the coordination number of Mg2+ ions within 5.5 Å of the interface. In contrast, the first solvation shell of the Ca2+ ion is larger (both in radius and number of water molecules) and thus is spatially commensurate with the (1 0 0)(1 1) interfaces. In addition, we predict
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Fig. 9. Normalised density of water, Ca2+ and Cl at a concentration of 0.16 M at the (a) Au(1 1 1), (b) Ag(1 1 1), (c) Au(1 0 0)(1 1), (d) Ag(1 0 0)(1 1) and (e) Au(1 0 0)(1 1) interfaces.
that the water molecules in the first solvation shell calcium ions are not as tightly bound, compared to Mg2+. Indeed, in bulk solution there was a decrease in water coordination number of Ca2+ at 0.60 mol kg1, relative to 0.16 mol kg1 (see Table 1), due the fact that a water molecule was often displaced by a chloride ion. [55] In the 0.16 mol kg1 solution the calcium ions at the interface have a similar coordination number to the calcium ions in bulk solution (Table 1). At 0.60 mol kg1 the interfacial calcium ions have a larger water coordination number than those in the bulk, as the likelihood of the formation of Ca2+–Cl ions pairs was reduced. Obviously the ability of the FF to accurately describe the behaviour of the interaction of species is of paramount importance in molecular simulation. As such, it is important to recognise that
the fixed-charge, 12–6 LJ potential ion models used in CHARMM (and most biological FFs) have their limitations [56,57]. In the case of monovalent ionic species these models give reasonable agreement with experimental data [58,56,57] (if the simulation conditions of the system are not too different from those for which the FF was parametrised). However, in the case of divalent species the limitations become more apparent [55]. As such, while simulation can play an invaluable tool in helping to elucidate the structure of the EDL at metal interfaces, as with any method, its limitations need to be born in mind. By using molecular simulations in combination with theory and experiment, we can obtain a more accurate picture of the behaviour of water and ions at metal surfaces, which can in turn be used to help refine molecular FFs.
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0] direction of Au(1 1 1) (a and b), the [0 1 1] Fig. 10. 2-Dimensional density profiles of Mg2+ (a, c and e) and Ca2+ (b, d and f) at 0.60 mol kg1 concentrations, along the [1 1 direction of Au(1 0 0)(1 1) (c and b) and the [0 1 1] direction of Au(1 0 0)(5 1) (e and f). Crosses indicate the position of the surface metal atoms.
In summary, despite a long history of the investigation of the EDL, many unanswered questions remain. The GCS theory is known to have a number of limitations and the development/ refinement of new theoretical models will require data obtained from simulation and experimental studies. While the present study has only investigated charge-neutral interfaces, it has highlighted, in agreement with previous simulation studies [33,34,32], that the arrangement of the water molecules at the metal interface has a strong influence on the adsorption behaviour of species at the interface. These results concur with the development of newer theoretical models that explicitly consider the solvent molecules [19,23,26] and are to be favoured over the primitive model (PM) where the solvent is simply represented via a dielectric continuum. In addition, the fact that our results show that ions of the same valency behave differently suggests that modelling the water–ion interaction could be important. Unfortunately, at this time there is a general lack of direct structural experimental data on the metal–aqueous interface available for comparison, despite their importance in a wide range of areas. Recently-reported experiments have found that salt concentration can significantly influence the adsorption strength of molecules at the aqueous silica interface [17]. We anticipate that studies such as ours will
in future assist in evaluating such corresponding experimental data obtained for gold and silver surfaces.
4. Conclusions In conclusion, we have predicted the interfacial solution structuring at the Au/Ag(1 1 1), native Au/Ag(1 0 0) and reconstructed Au(1 0 0) (5 1) facets for four common saline solutions, at two solution concentrations. Unlike many previous molecular simulations of such systems, the model we have used employs both an explicit water model as well as atomistically-detailed polarisable metal surfaces, as opposed to a flat wall, thus keeping much of the chemistry of the system. For a given combination of metallic surface and facet, we found the largest variations in density profile for the cations in solution, with these density profiles showing little appreciable change across the concentration range considered here. The divalent metal ions were found to exhibit the most variable response to the different metallic surface and facets. Mg2+ was the only ion for which we predicted materials-dependent differences in interfacial solution structuring between the Ag and Au. Our findings were rationalized in terms of the structuring of liquid
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water at the aqueous metallic interfaces, considered in conjunction with our predictions for the coordination spheres of each type of cation in solution. Our results agree with previous studies in that the structuring of liquid water at the metal interface appears to be a major factor in the behaviour of the ionic species in solution at the interface. There is also good agreement with previous data on the structuring of the cationic species. However, the force-fields used appeared to underestimate the strength of the interaction of the chloride anion with the metal surfaces. To address this limitation, modification of these force-fields is identified as an area of future work. In addition, the adaptation of our polarisable forcefields to describe metal surfaces that carry an overall charge is another area for future investigation. Our findings also provide a basis for future studies to predict the impact of ion structuring at the aqueous metal interface on biomolecular adsorption at the interface, particularly in high-salinity and low-salinity cases. Acknowledgements The authors thank the Victorian Life Sciences Computational Initiative (VLSCI) and the National Computing Infrastructure (NCI) for provision of computational resources. ZEH and TRW thank veski for research funding, and TRW thanks veski for an Innovation Fellowship. This work was partially supported by the Air Force Office for Scientific Research (Grant #FA9550-12-1-0226). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.08.045. References [1] B.E. Conway, J. Electrochem. Soc. 138 (1991) 1539–1548. [2] C.K. Harnett, J. Templeton, K.A. Dunphy-Guzman, Y.M. Senousy, M.P. Kanouff, Lab Chip 8 (2008) 565–572. [3] J.K. Nørskov, T. Bligaard, J. Rossmeisl, C.H. Christensen, Nat. Chem. 1 (2009) 37– 46. [4] S.R. Whaley, D.S. English, E.L. Hu, P.F. Barbara, A.M. Belcher, Nature 405 (2000) 665–668. [5] R. Braun, M. Sarikaya, K. Schulten, J. Biomater. Sci. Polym. Ed. 13 (2002) 747– 757. [6] H. Heinz, B.L. Farmer, R.B. Pandey, J.M. Slocik, S.S. Patnaik, R. Pachter, R.R. Naik, J. Am. Chem. Soc. 131 (2009) 9704–9714. [7] Y. Fang, N. Poulsen, M.B. Dickerson, Y. Cai, S.E. Jones, R.R. Naik, N. Kroger, K.H. Sandhage, J. Mater. Chem. 18 (2008) 3871–3875. [8] J.W. Han, Top. Catal. 55 (2012) 243–259. [9] M. Hnilova, C.R. So, E.E. Oren, B.R. Wilson, T. Kacar, C. Tamerler, M. Sarikaya, Soft Matter 8 (2012) 4327–4334. [10] J. Schneider, L. Colombi Ciacchi, J. Am. Chem. Soc. 134 (2012) 2407–2413. [11] L. Ruan, H. Ramezani-Dakhel, C.-Y. Chiu, E. Zhu, Y. Li, H. Heinz, Y. Huang, Nano Lett. 13 (2013) 840–846. [12] Z. Tang, J.P. Palafox-Hernandez, W.-C. Law, Z.E. Hughes, M.T. Swihart, P.N. Prasad, M.R. Knecht, T.R. Walsh, ACS Nano 7 (2013) 9632–9646. [13] A. Calzolari, G. Cicero, C. Cavazzoni, R. Di Felice, A. Catellani, S. Corni, J. Am. Chem. Soc. 132 (2010) 4790–4795. [14] L.B. Wright, P.M. Rodger, S. Corni, T.R. Walsh, J. Chem. Theory Comput. 9 (2013) 1616–1630. [15] L.B. Wright, P.M. Rodger, T.R. Walsh, S. Corni, J. Phys. Chem. C 117 (2013) 24292–24306.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Structure of the electrical double layer at aqueous gold and silver interfaces for saline solutions Zak E. Hughes ⇑, Tiffany R. Walsh Institute for Frontier Materials, Deakin University, Geelong 3216, VIC, Australia
a r t i c l e
i n f o
Article history: Received 9 June 2014 Accepted 22 August 2014 Available online 4 September 2014 Keywords: Molecular simulation Salt solutions Noble metals Interfaces
a b s t r a c t We report the structure of the electrical double layer, determined from molecular dynamics simulations, for a range of saline solutions (NaCl, KCl, MgCl2 and CaCl2) at both 0.16 and 0.60 mol kg1 on different facets of the gold and silver aqueous interfaces. We consider the Au/Ag(1 1 1), native Au/Ag(1 0 0) and reconstructed Au(1 0 0)(5 1) facets. For a given combination of metallic surface and facet, some variations in density profile are apparent across the different cations in solution, with the corresponding chloride counterion profiles remaining broadly invariant. All density profiles at the higher concentration are predicted to be very similar to their low-concentration counterparts. We find that each electrolyte responds differently to the different metallic surface and facets, particularly those of the divalent metal ions. Our findings reveal marked differences in density profiles between facets for a given metallic interface for both Mg2+ and Ca2+, with Na+ and K+ showing much less distinction. Mg2+ was the only ion for which we find evidence of materials-dependent differences in interfacial solution structuring between the Ag and Au. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Understanding the structuring of water and salt ions at metal interfaces is of fundamental importance in surface science as well as electrochemistry and materials science [1–3]. Also, in recent years there has been considerable interest in identifying peptide sequences that show adsorption selectivity to a particular material, or even a particular facet [4–12]. There is increasing evidence that the structure of water molecules at the metal interface plays a major role in the mechanism of peptide adsorption [13–16,12]. Despite this there are still many unanswered questions regarding the behaviour of metal–aqueous interfaces. If we are to gain greater understanding of what drives the selectivity of some biomolecules to particular surfaces we need to gain a more detailed understanding of the behaviour of metal–aqueous interfaces and how the presence of ionic species may modulate these binding preferences, as has been explored experimentally, e.g. for SiO2 interfaces [17]. In addition, there is significant interest in the phenomenon of the electrical double layer (EDL) that is formed when a surface is in the presence on an electrolyte [18–28]. Here too, a more detailed, atomic-level understanding of the behaviour of water and ions at the interface is needed.
⇑ Corresponding author. E-mail address: [email protected] (Z.E. Hughes). http://dx.doi.org/10.1016/j.jcis.2014.08.045 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
From an experimental perspective it remains very challenging to gain information about the structure of aqueous solutions at metal surfaces. As such a number of theoretical and simulation studies have reported investigations of metal–aqueous interfaces [18,19,21,29,22,30,23,24,31,32,25,33,34,26,27,35,14–16,28]. The starting point for many of the theoretical descriptions of the EDL is the Gouy–Chapman–Stern (GCS) theory, which is described by the Poisson–Boltzmann (PB) equation [36,24,27]. This theory predicts that the EDL comprises a strongly adsorbed layer of ions at the surface, known as the Stern layer, and a second ‘diffuse layer’ of ions where the electric potential decays to the bulk value [24,27]. This theory can accurately predict the long-ranged behaviour of the charge distribution at the interface; it has the advantage of simplicity and is a reasonable approximation for systems of low ion concentration, low valence and low surface charge [24,27]. However, this theory cannot capture the short-ranged structuring of the ions at the interface, due to the assumption that particle interactions are negligible. Other theoretical treatments have been developed to address these limitations [18,19,22,23,26,28]; these are capable of predicting more persistent non-monotonic ion profiles [18,19,22,23]. In addition, the differing behaviour of cations and anions can be observed, even for charge-neutral surfaces [18,19,25,28]. Molecular dynamics (MD) and Monte Carlo (MC) simulations of aqueous interfaces provide an alternative way of investigating the molecular structure of such systems, and allow
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Table 1 Radius of first solvation shell, r (Å), and the coordination number of water molecules in the first solvation shell, n. The data are shown for the 0.16 mol kg1 with the data for the 0.60 mol kg1 solution given in parentheses. Parameter
System
Na+
K+
Mg2+
Ca2+
Cl
r/Å n
Solution Solution Au(1 1 1) Ag(1 1 1) Au(1 0 0)(1 1) Ag(1 0 0)(1 1) Au(1 0 0)(5 1)
2.30 5.8 (5.6) 5.7 (5.6) 5.7 (5.6) 5.5 (5.4) 5.5 (5.4) 5.7 (5.6)
2.70 6.8 (6.6) 7.1 (7.0) 7.2 (7.1) 7.3 (7.2) 7.2 (7.2) 7.2 (7.1)
1.96 6.0 (5.9) 6.0 (6.0) 6.0 (6.0) 6.0 (5.6) 6.0 (6.0) 6.0 (6.0)
2.26 7.0 (6.2) 6.6 (6.6) 6.6 (6.7) 7.3 (7.3) 6.7 (7.1) 7.0 (6.8)
3.15 7.7 (7.7) 7.8 (7.8) 7.9 (7.9) 7.8 (7.8) 7.9 (7.9) 7.9 (7.8)
evaluation theoretical models, contributing to the development and refinement of such models [22,30,23,24,26,25]. While past simulations of metal–electrolyte interfaces have provided useful details, some of these studies have sought to reduce the complexity of the problem by incorporating assumptions such as an implicit solvent [19,24,28], by modelling anions and cations as having opposite charge but being otherwise identical [22,23], or by considering the surface at the aqueous materials interface to be an unstructured wall [27]. Assumptions such as the latter may neglect important chemical and physical details of these interfaces that can lead to material and facet dependent differences in solvent structuring. For example, first-principles Car–Parrinello [33] and Born–Oppenheimer [34] MD simulations of the aqueous Au(1 1 1) interface have shown the water molecules in the first adsorption layer are not distributed isotropically parallel in the
plane of the surface. Rather, oxygen atoms were predicted to show a preference for a position atop the metal atoms. While first-principles simulations of metal–aqueous interfaces [37,33,34] are able to describe the chemical and physical properties of the systems in great detail, the computational expense of such simulations typically limits these to tens of picoseconds of trajectory. Use of molecular mechanics force-fields (FFs) allows longer time- and length-scales to be reached while at the same time retaining more of the physical/chemical detail of the metal surface and the interfacial structure than many theoretical treatments. However, the parametrisation of metal–aqueous FFs is not trivial. Despite this, a number of accurate, yet efficient FFs, capable of simulating gold/silver–aqueous interfaces have been developed, in particular the CHARMM-METAL [38], GolP [39], and GolPCHARMM/AgP-CHARMM [14–16] FFs, with the latter two explicitly
Fig. 1. The structure of water at the gold interfaces. Normalised density of water oxygens and hydrogens at the (a) Au(1 1 1), (b) Au(1 0 0)(1 1) and (c) Au(1 0 0)(5 1) interfaces and (d) the charge density at the three Au interfaces.
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incorporating metal polarization contributions. The relative merits of each class of FF have been discussed in detail elsewhere [40,14,16,12]. Here, we have used the GolP-CHARMM and AgP-CHARMM FFs [39,14–16], which capture the polarisability of the Au and Ag surfaces via the use of a rigid-rod dipole [41]. In these FFs, the metal atoms are fixed in space but the dipoles attached to each atomic site are free to rotate. In addition, these FFs include virtual sites in the uppermost surface layer of metal atoms, to direct noncovalent adsorption of water and other molecules to surface atop sites, as has been indicated as preferable in recent simulations of the interface between Au/Ag and liquid water [37,33,34,32]. The description of the aqueous Au(1 1 1) interface by the GolPCHARMM FF agrees well with those of first principles simulations [33,34], not only in terms of the density profile of water atoms perpendicular to the surface, but also in terms of the distribution of the first adsorbed layer of water molecules parallel to the interface. In addition, both the first-principles simulations [33,34] and GolPCHARMM simulations [14–16,12] indicate that the interfacial water molecules in the first adsorbed layer are most likely to have the oxygen atom directed towards the surface, meaning that these
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water molecules are more likely to be hydrogen-bond donors compared with those in the bulk. By investigating the structuring of water and ions at metal interfaces via atomistic MD simulations, we provide data that can be usefully compared both with new analytical models as well as with future experimental findings. While the (1 1 1) facet is the lowest in energy for Ag and Au, metal surfaces/nanoparticles do, in general, feature defects as well as a range other facets at the metal–aqueous interface, such as the (1 0 0) facet. Thus, to predict and elucidate the behaviour of aqueous saline solutions at nanoparticle interfaces, we chose to model both of these major facets. Both Au facets are more complex to describe than may be appreciated at first sight, as both undergo reconstruction in an aqueous environment atp300 ffiffiffi K and pH 7. In the case of the reconstructed Au(1 1 1) 22 3 surface, the high degree of structural similarity between it and the native Au(1 1 1) surface means that the unreconstructed surface provides a suitable model for molecular simulation. In contrast, there are significant structural differences between the native Au(1 0 0)(1 1) and the reconstructed Au(1 0 0)-hex surfaces [15]. Unlike the native surface, the Au(1 0 0)-hex surface is not atomically flat, featuring atomic-scale undulations. The Au(1 0 0)(5 1) reconstruction makes an excellent
direction at the Au(1 1 1) surface, (b) in the [1 1 2] direction at the Fig. 2. 2-dimensional density profiles of water oxygen atoms at different gold interfaces: (a) in the [110] 1] direction at the Au(1 1 1) surface, (c) in the [0 1 1] direction at the Au(1 0 0)(1 1) surface, (d) in the [0 1 1] direction at the Au(1 0 0)(5 1) surface and (e) in the [0 1 Au(1 0 0)(5 1) surface. Crosses indicate the position of the surface metal atoms.
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approximation to the Au(1 0 0)-hex surface, admitting a unit cell size that is practical for molecular simulations [15]. When considering typical Au nanoparticles in aqueous solution, it is not known if the (1 0 0) surface is reconstructed or not. On this basis, we chose to model the native Au(1 1 1), native Ag(1 1 1), native Au(1 0 0)(1 1), native Ag(1 0 0)(1 1), and the reconstructed Au(1 0 0)(5 1) aqueous interfaces. Atomic-scale structures of the (1 1 1), (1 0 0)(1 1) and (1 0 0)(5 1) surfaces are provided in Fig. S1 of the Supplementary Material. In this study we have investigated the interfacial solvent structure of four different saline solutions; NaCl, KCl, MgCl2 and CaCl2, at the Au(1 1 1), Au(1 0 0)(1 1), Au(1 0 0)(5 1), Ag(1 1 1), and Ag(1 0 0)(1 1) interfaces. Simulations have been performed for solutions at physiologically-relevant concentrations ( 0.16 mo lkg1) as well concentrations close to seawater ( 0.6 mol kg1). Overall, we found that the facet of the aqueous metallic interface, of overall neutral charge, exerted a strong influence over how the salt ions ordered at the interface. In contrast, we saw little change in the interfacial solution structuring for the range of solution concentrations considered here. While the different cations shared some similarities in their interfacial solution structure, there were also significant differences in the spatial distribution of these ions at the interface, particularly for the divalent cations.
2. Computational methods Molecular dynamics simulations were carried out for each of the four saline solutions in contact with each of the five metallic interfaces. These simulations were performed using the GROMACS software package, version 4.5.5. [42]. The interactions of the water and ions with the gold and silver interfaces were modelled using the GolP-CHARMM [14,15] and AgP-CHARMM [16] FFs, respectively. The modified version of the TIP3P [43,44] water model was used for the water molecules, and the CHARMM22⁄ FF parameters was used for the salt ions [45,46]. Further details about the FF and atomistic parameters are provided in Section S1 and Tables S1 and S2 in the Supplementary Material. For all simulations a timestep of 1 fs was used, with the Lennard-Jones (LJ) non-bonded interactions switched off between 10.0 and 11.0 Å and a cut-off of 13.0 Å used for the Particle Mesh Ewald summation [47]. For the Ag and Au(1 1 1) and (1 0 0)(1 1) surfaces, we used slabs comprising p(14 16) and p(14 14) supercells respectively, of five atomic layers thick. For the Au(1 0 0)(5 1) surface a p(2 2) supercell, 9 atomic layers thick was modelled. Lattice parameters of 4.145 and 4.165 Å were used for Au and Ag, respectively. All of the surfaces modelled here were charge-neutral. The systems simulated contained 2045 or 3198 water molecules for the (1 1 1)/(1 0 0)(1 1) and (1 0 0)(5 1) surfaces, respectively. The cell dimension perpendicular to the surface plane, along the z-axis, was adjusted such that the bulk density of the water midway between the slab and its periodic image in the z-direction was equal to that obtained from a MD simulation of salt solution modelled at the same temperature, pressure and concentration. The inter-slab spacing in the z-direction was typically 38 Å. This gave rise to a bulk density region 18–20 Å thick in the centre of the inter-slab space. Once the correct solvent density was obtained, the production run was performed in the Canonical (NVT) ensemble with the Nosé–Hoover thermostat [48,49], used to maintain the temperature at 300 K. The production runs were of 50 ns and 20 ns duration for the 0.16 and 0.60 mol kg1 systems, respectively. For all MD simulations reported in this section, the actual gold and silver atoms in the metal slab were held fixed in space, while the dipole particles were free to rotate (according to their restraint potential and the temperature of the thermostat). Recently reported tests indicate that there is very little difference
between binding free energies obtained using a rigid substrate compared with those calculated using a slab where all atoms can move [50]. In addition to the simulations of the different interfaces, simulations of each salt solution (without the presence of the metallic surface) at both 0.16 and 0.60 mol kg1 were performed to obtain the coordination number of the first solvation shell of each ion, as given in Table 1. 3. Results and discussion 3.1. Liquid water The structuring of water at the different interfaces is thought to have a strong effect on how other species interact with the different metal/facets. [14–16] These previous studies have suggested that both the structuring of water molecules is greater on the (1 0 0) facet than the (1 1 1) facet (with the corresponding structuring on the (1 0 0)(5 1) facet predicted to be intermediate between these two native facets), and that this interfacial solvent structuring is greater on Ag interfaces than on Au interfaces, for a given facet. At all interfaces, a strong peak in the vertical density profile (along the direction perpendicular to the plane of the surface) of the water oxygen is found at 2.5 Å from the surface, with a second smaller peak at 6.0 Å, see Fig. 1. The water molecules in the first solvation layer tended to direct the hydrogen atoms away from the surface [14–16]. As such, the water molecules in the first
Fig. 3. Normalised density of (a) Na+ and (b) Cl ions at concentrations of 0.16 and 0.60 M at the Au(1 1 1) interface. The normalised density of water is also shown.
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solvation layer are more likely to be hydrogen bond donors when compared against water molecules in bulk solution. The water molecules in this first layer are also structured in the lateral (x–y) plane [14–16], with an increased oxygen density apparent atop the surface metal atom sites, in agreement with Cicero et al. [33]. The total charge density of system at the gold interfaces is shown in Fig. 1(d); the behaviour at Ag(1 1 1) and Ag(1 0 0)(1 1) is very similar and thus not shown. The presence of the water oxygen atoms atop the metal surface atoms causes a degree of alignment in the dipoles of the metal atoms; this is responsible for the positive peak in the charge density profile just above the interface. All of the different facets show this type of behaviour, but to different degrees. The positions of the oxygen and hydrogen atoms of the water molecules give rise to the series of peaks and troughs above the gold/silver interfaces in Fig. 1(d). As seen for the density profiles, the Au(1 0 0)(5 1) interface more closely resembles the Au(1 1 1) than the Au(1 0 0)(1) interface. Fig. 2 shows the 2-D density profiles of the water oxygens at the different Au facets. The strong 3-D ordering of the water molecules can be seen, with water molecules situated above the metal atoms (see also Fig. S2 in the Supplementary Material). The corresponding density profiles for the Ag(1 1 1) and Ag(1 0 0)(1 1) interfaces (not shown) yielded very similar structuring to the Au(1 1 1) and Au(1 0 0)(1 1). This strong adsorption of water atop the metal atoms has also been seen for other metal surfaces [51,52]. In particular, Chandler and co-workers have used a FF containing both two- and three-body terms to investigate the structure of water at the Pt(1 1 1) and Pt(1 0 0) surfaces [30,32,35]. As discussed below, the structuring of the water molecules is an important factor in
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determining the structuring of the salt ions in the saline solutions at the aqueous metallic interface. 3.2. NaCl Solution The normalised vertical density profiles of 0.16 and 0.60 mol kg1 NaCl solutions at the Au(1 1 1) interface are shown in Fig. 3. Increasing the salt concentration does not change the shape of the density profile significantly; only for Cl was there a noticeable difference in peak heights. Neither of the two ions, Na+ or Cl, approached the Au(1 1 1) interface as closely as the water molecules. The Na+ profile showed strong structuring with multiple peaks, at 4.6, 7.2 and 10.0 Å from the interface. The position of these peaks corresponded with troughs in the water density profile. In addition, both the first and second peaks featured shoulders located at the positions of the peaks in the water density profiles. The Cl profile showed fewer features, with a strong first peak at 5.7 Å, aligned with the position of the second peak in the water density profile, with a shoulder. There may be a second peak at 8.0 Å but noise in the profile means that we cannot state this with certainty. The vertical density profiles for the 0.16 mol kg1 NaCl solution at the other interfaces are shown in Fig. 4. As for Au(1 1 1), the effect of the increased NaCl concentration did not alter the density profiles (not shown) of the ions at the Ag(1 1 1), Au(1 0 0)(1 1), Au(1 0 0)(5 1) or Ag(1 0 0) interfaces significantly. Comparing the Au(1 1 1) and Ag(1 1 1) interfaces, the positions of the peaks in the Na+ profile appeared the same, but the height of the first peak was slightly greater in the case of Ag(1 1 1). This may be
Fig. 4. Normalised density of water, Na+ and Cl at a concentration of 0.16 M at the (a) Ag(1 1 1), (b) Ag(1 0 0)(1 1), (c) Au(1 0 0)(5 1) and (d) Au(1 0 0)(1 1) interfaces.
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due to the more structured first layer of water on the Ag(1 1 1) surface compared with the Au(1 1 1) surface. Alternatively, the greater polarisability of Ag might be responsible for these differences. In the case of the Cl profiles the differences between the metals is very small. The second peak in the chloride density profile is more clearly defined for the Ag(1 1 1) than the Au(1 1 1) interface. On the (1 0 0)(1 1) surfaces the shoulder there is a distinct peak at 4.0 Å observable (rather than a shoulder to the peak at 5.7 Å on the (1 1 1) surfaces). Like in the case of Na+ the Cl profile at the Au(1 0 0)(5 1) resembles the Cl profile at the (1 1 1) interface more than the (1 0 0)(1 1) interface. The different facets also gave rise to changes in the density profiles of the ions. For the native (1 0 0) facets (both Ag and Au) there was an extra peak at 3:5 Å in the Na+ profile. In contrast, the reconstructed Au(1 0 0)(5 1) surface yielded a sodium ion profile very similar to that of the Au(1 1 1) surface. In the case of chloride, different density profiles were observed for each facet. For both the Au and Ag(1 0 0)(1 1) interfaces, the shoulder of the first peak resolved into a separate peak, while in the case of the Au(1 0 0)(5 1) surface the same shoulder was diminished with respect to Au(1 1 1). In Fig. S3 in the Supplementary Material we provide the charge density profiles for the surfaces with the NaCl solutions. Even at a
NaCl concentration of 0.60 mol kg1 the total charge density of the system was still dominated by the water molecules and thus the profiles were practically unchanged from the pure water systems. As in the case of pure liquid water, the ions were not only arranged in one dimension, but two. The 2-D density maps of the ions are shown in Fig. 5. The distribution of ions further than 6 Å from the surface (i.e. corresponding to the second layer of interfacial water molecules and beyond) is largely isotropic along the [0 1 1] direction. However, at closer distances the distribution of ions is clearly anisotropic in the lateral plane. Moreover, these 2-D maps indicated that the lateral distribution of the ions was correlated with those of the first layer of water molecules (see Fig. 2). The Na+ ions closest to the interface were found above the hollows of the metal surface, i.e. offset from the water oxygens. In contrast, the Cl ions were found above the oxygens of the first layer of water molecules. The likely locations of the two ions can be understood if we consider the structuring of water molecules around the ions, when free in solution, compared with the water structuring at the interfaces. Table 1 gives the radius of the first solvation shell around each ion, while Fig. S4 in the Supplementary Material shows the radial distribution function of the ion with the water molecules for the five different ion species. The radius of the first hydration shell of Na+ is
0] direction of Au(1 1 1) (a and b), the [0 1 1] Fig. 5. 2-Dimensional density profiles of Na+ (a, c and e) and Cl (b, d and f) at 0.60 mol kg1 concentrations, along the [1 1 direction of Au(1 0 0)(1 1) (c and b) and the [0 1 1] direction of Au(1 0 0)(5 1) (e and f). Crosses indicate the position of the surface metal atoms.
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2:3 Å; this is smaller than the 2:9 Å distance between surface metal atoms for both Ag and Au. Thus, on the (1 1 1) facet a Na+ ion sitting in the hollow provided by three laterally-adjacent water molecules in the first solvation layer can coordinate with each of these water molecules, as well as two/three other water molecules in the second interfacial layer. A similar scenario is seen for the (1 0 0)(1 1) surface, where a sodium ion positioned above a hollow site on the metal surface can also coordinate with water molecules in the first interfacial layer. The larger first solvation shell of the chloride ion (in terms of both radius and coordination number) makes it more favourable for Cl to be located above a surface metal atom and coordinate to a number of water molecules in the first interfacial layer.
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The overall coordination number of the first layer of sodium and chloride ions (defined as ions within 5.5 Å of the interface) is given in Table 1. For the (1 1 1) and (1 0 0)(5 1) interfaces the coordination of the ions at the surface is very close to those found in bulk solution. In the case of the (1 0 0)(1 1) surfaces, there is a small reduction in the number of water molecules surrounding the sodium ion. In summary, both Na+ and Cl are more likely to be found at the metal surface than in the bulk. The effect of the different facets on the structuring of the ions at the metal interfaces is greater than the effect arising from the different metals. These results agree quite well with the results of simulations of 1 M LiCl solutions at Pt(1 0 0), which concluded ‘‘the absorption of cations at the electrode is
Fig. 6. Normalised density of water, K+ and Cl at a concentration of 0.16 M at the (a) Au(1 1 1), (b) Ag(1 1 1), (c) Au(1 0 0)(1 1), (d) Ag(1 0 0)(1 1) and (e) Au(1 0 0)(1 1) interfaces.
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strongly inhibited by the requirement for them to reorganize their hydration shells to approach the electrode surface’’ [30]. Moreover, their potential of mean force calculations showed an energy minimum at 4 Å in the adsorption profile of Li+. This simulation of the LiCl solution likewise showed a relatively weak interaction between the Cl anions and the metal surface again agreeing with the results of our simulations. In contrast, experimental and theoretical studies have shown quite strong interactions between Cl and transition metal surfaces, with the chloride ions forming adlayers [19–21,53,54]. The case of this discrepancy between these simulations and experiment is due to the form of the 12–6 potentials often used to model the LJ non-bonded interactions between atoms. The large r value given to Cl to reproduce the behaviour of Cl in bulk solution means that the LJ interactions, generated via Lorentz–Berthelot mixing rules, between the metal atoms and the anions is too repulsive at short range. Thus, the formation of an anionic adlayer is blocked. In future refinements of the GolP/ AgP-CHARMM FFs it would be possible to construct a bespoke Au/ Ag–Cl 12–6 LJ term that could more accurately model the interaction of Cl (and other halides) with the metal surfaces.
at the (1 0 0)(1 1) interfaces, where the height of the peak at 4 Å was increased. This change in the charge density at the interface could have an effect on the adsorption of biomolecules, especially those containing charged species. The density profiles of the MgCl2 and CaCl2 solutions are shown in Figs. 8 and 9, respectively. As for Na+, the peaks in the vertical density profile for both Mg2+ and Ca2+ coincided with the troughs in the water density profile. Unlike Na+, however, the ratio of the height of the first and second peaks varied strongly with the facet, with the (1 1 1)/(1 0 0)(5 1) interfaces yielding marked differences in behaviour compared with the (1 0 0)(1 1) interface. However, the corresponding profiles for the Cl counterion appeared very similar across all interfaces. In the case of Mg2+, at the Au(1 1 1) and Au(1 0 0)(5 1) interfaces, the height of the first peak was approximately equal to the height of the second peak. In contrast, Ag(1 1 1) featured a significantly greater degree of structuring, as evidenced in the height of the first peak in the vertical density profile compared with Au(1 1 1). The Mg2+ solution provided the only instance where a distinction in vertical density profiles was observed between Ag
3.3. KCl solution The vertical density profiles generated from the KCl 0.16 mol kg1 solution simulations are shown in Fig. 6, yielding Cl profiles that are essentially equivalent to those predicted for the NaCl solution. As in the case of Na+, the difference in solvent structuring between 0.16 and 0.60 mol kg1 is negligible. While the Cl density profiles appeared very similar to those predicted for NaCl, the density profiles of K+ were quite different to those of Na+. At all five interfaces the peaks in the density profile of K+ were shifted out to a greater distance from the surface compared with the data obtained for Na+. We suggest this is due to the larger radius of the first solvation shell of K+ compared with Na+. In addition, the ratio of the height of the first and second peaks in the vertical density profiles is closer to 1.0 compared with this ratio in the case of Na+, where the height of the second peak was roughly double that of the first. This could also be due to the fact that the K+ ions are further (on average) from the metal surfaces. Fig. 7 shows the 2-D density profiles of K+ at the three different facets. In the case of the (1 1 1) and (1 0 0)(5 1) interfaces there is distinctly less ordering of the cations parallel to the [0 1 1] direction, compared with the analogous data for Na+, with the (1 0 0)(1 1) interface supporting the greatest degree of lateral structuring. We propose that the larger atomic radius of K+, relative to Na+ (see Table 1), means that it is less favourable for the K+ ions to interdigitate between the water molecules in the first interfacial water layer; they are however located in the more loosely-structured second interfacial water layer. In contrast to our predictions for sodium, a change in the coordination number of potassium ions in the first layer was observed for all interfaces; there was a small but significant increase in the number of water molecules in the first solvation shell. This could be ascribed to the fact that position of the first layer of K+ ions is coincident with the position of the second interfacial layer of water molecules. Thus, the density of water surrounding the potassium ions is higher than that in the bulk, leading to a increase in the coordination number of the potassium ions. 3.4. MgCl2 and CaCl2 solutions Unlike the monovalent cation solutions, the divalent cations solutions do give rise to some, minor, changes in the charge density profiles of the interfaces at the higher concentration of 0.60 mol kg1 (as shown in Fig. S5 in the Supplementary Material). The strongest effect of the ionic species was seen for CaCl2 solution
Fig. 7. 2-Dimensional K+ density profiles of 0.60 mol kg1 KCl solutions along (a) 0] direction of Au(1 1 1), (b) the [0 1 1] direction of Au(1 0 0)(1 1) and (c) the [1 1 the [0 1 1] direction of Au(1 0 0)(5 1). Crosses indicate the position of the surface metal atoms.
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Fig. 8. Normalised density of water, Mg2+ and Cl at a concentration of 0.16 M at the (a) Au(1 1 1), (b) Ag(1 1 1), (c) Au(1 0 0)(1 1), (d) Ag(1 0 0)(1 1) and (e) Au(1 0 0)(1 1) interfaces.
and Au for a given facet. In contrast, for Ca2+ the height of the second peak was greater than that of the first peak at the both the (1 1 1)/(1 0 0)(5 1) interfaces, while the (1 0 0)(1 1) interfaces yielded an extremely sharp and pronounced first peak in the density profile. The 2-D density maps given in Fig. 10, highlight the ordering even more, with the interaction of Ca2+ on the (1 0 0)(1 1) being especially marked. The position of the first peak in the density profile of the divalent cations at the (1 1 1) surface is at a slightly shorter distance to the metal interface than in the case of Na+, (4.4 as opposed to 4.6 Å). Again, we propose that the driving force for the structuring of the ions at these interfaces is the nature of the first solvation shell around the different ions. In the case of Mg2+ the first solvation
shell is extremely tightly bound (as can be seen from Fig. S3 in the Supplementary Material), indicating that it is unfavourable for Mg2+ to lose a water molecule. This is suggested to promote a favourable interaction with the hex interfaces, where the ions can remain coordinated to six water molecules (three in the first water layer and three in the second water layer). However, our data indicate that it should not be as favourable for a Mg2+ adsorbed above a hollow site to coordinate to six water molecules on the squarepatterned (1 0 0)(1 1) surfaces, as suggested by the reduction in the coordination number of Mg2+ ions within 5.5 Å of the interface. In contrast, the first solvation shell of the Ca2+ ion is larger (both in radius and number of water molecules) and thus is spatially commensurate with the (1 0 0)(1 1) interfaces. In addition, we predict
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Fig. 9. Normalised density of water, Ca2+ and Cl at a concentration of 0.16 M at the (a) Au(1 1 1), (b) Ag(1 1 1), (c) Au(1 0 0)(1 1), (d) Ag(1 0 0)(1 1) and (e) Au(1 0 0)(1 1) interfaces.
that the water molecules in the first solvation shell calcium ions are not as tightly bound, compared to Mg2+. Indeed, in bulk solution there was a decrease in water coordination number of Ca2+ at 0.60 mol kg1, relative to 0.16 mol kg1 (see Table 1), due the fact that a water molecule was often displaced by a chloride ion. [55] In the 0.16 mol kg1 solution the calcium ions at the interface have a similar coordination number to the calcium ions in bulk solution (Table 1). At 0.60 mol kg1 the interfacial calcium ions have a larger water coordination number than those in the bulk, as the likelihood of the formation of Ca2+–Cl ions pairs was reduced. Obviously the ability of the FF to accurately describe the behaviour of the interaction of species is of paramount importance in molecular simulation. As such, it is important to recognise that
the fixed-charge, 12–6 LJ potential ion models used in CHARMM (and most biological FFs) have their limitations [56,57]. In the case of monovalent ionic species these models give reasonable agreement with experimental data [58,56,57] (if the simulation conditions of the system are not too different from those for which the FF was parametrised). However, in the case of divalent species the limitations become more apparent [55]. As such, while simulation can play an invaluable tool in helping to elucidate the structure of the EDL at metal interfaces, as with any method, its limitations need to be born in mind. By using molecular simulations in combination with theory and experiment, we can obtain a more accurate picture of the behaviour of water and ions at metal surfaces, which can in turn be used to help refine molecular FFs.
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0] direction of Au(1 1 1) (a and b), the [0 1 1] Fig. 10. 2-Dimensional density profiles of Mg2+ (a, c and e) and Ca2+ (b, d and f) at 0.60 mol kg1 concentrations, along the [1 1 direction of Au(1 0 0)(1 1) (c and b) and the [0 1 1] direction of Au(1 0 0)(5 1) (e and f). Crosses indicate the position of the surface metal atoms.
In summary, despite a long history of the investigation of the EDL, many unanswered questions remain. The GCS theory is known to have a number of limitations and the development/ refinement of new theoretical models will require data obtained from simulation and experimental studies. While the present study has only investigated charge-neutral interfaces, it has highlighted, in agreement with previous simulation studies [33,34,32], that the arrangement of the water molecules at the metal interface has a strong influence on the adsorption behaviour of species at the interface. These results concur with the development of newer theoretical models that explicitly consider the solvent molecules [19,23,26] and are to be favoured over the primitive model (PM) where the solvent is simply represented via a dielectric continuum. In addition, the fact that our results show that ions of the same valency behave differently suggests that modelling the water–ion interaction could be important. Unfortunately, at this time there is a general lack of direct structural experimental data on the metal–aqueous interface available for comparison, despite their importance in a wide range of areas. Recently-reported experiments have found that salt concentration can significantly influence the adsorption strength of molecules at the aqueous silica interface [17]. We anticipate that studies such as ours will
in future assist in evaluating such corresponding experimental data obtained for gold and silver surfaces.
4. Conclusions In conclusion, we have predicted the interfacial solution structuring at the Au/Ag(1 1 1), native Au/Ag(1 0 0) and reconstructed Au(1 0 0) (5 1) facets for four common saline solutions, at two solution concentrations. Unlike many previous molecular simulations of such systems, the model we have used employs both an explicit water model as well as atomistically-detailed polarisable metal surfaces, as opposed to a flat wall, thus keeping much of the chemistry of the system. For a given combination of metallic surface and facet, we found the largest variations in density profile for the cations in solution, with these density profiles showing little appreciable change across the concentration range considered here. The divalent metal ions were found to exhibit the most variable response to the different metallic surface and facets. Mg2+ was the only ion for which we predicted materials-dependent differences in interfacial solution structuring between the Ag and Au. Our findings were rationalized in terms of the structuring of liquid
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water at the aqueous metallic interfaces, considered in conjunction with our predictions for the coordination spheres of each type of cation in solution. Our results agree with previous studies in that the structuring of liquid water at the metal interface appears to be a major factor in the behaviour of the ionic species in solution at the interface. There is also good agreement with previous data on the structuring of the cationic species. However, the force-fields used appeared to underestimate the strength of the interaction of the chloride anion with the metal surfaces. To address this limitation, modification of these force-fields is identified as an area of future work. In addition, the adaptation of our polarisable forcefields to describe metal surfaces that carry an overall charge is another area for future investigation. Our findings also provide a basis for future studies to predict the impact of ion structuring at the aqueous metal interface on biomolecular adsorption at the interface, particularly in high-salinity and low-salinity cases. Acknowledgements The authors thank the Victorian Life Sciences Computational Initiative (VLSCI) and the National Computing Infrastructure (NCI) for provision of computational resources. ZEH and TRW thank veski for research funding, and TRW thanks veski for an Innovation Fellowship. This work was partially supported by the Air Force Office for Scientific Research (Grant #FA9550-12-1-0226). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.08.045. References [1] B.E. Conway, J. Electrochem. Soc. 138 (1991) 1539–1548. [2] C.K. Harnett, J. Templeton, K.A. Dunphy-Guzman, Y.M. Senousy, M.P. Kanouff, Lab Chip 8 (2008) 565–572. [3] J.K. Nørskov, T. Bligaard, J. Rossmeisl, C.H. Christensen, Nat. Chem. 1 (2009) 37– 46. [4] S.R. Whaley, D.S. English, E.L. Hu, P.F. Barbara, A.M. Belcher, Nature 405 (2000) 665–668. [5] R. Braun, M. Sarikaya, K. Schulten, J. Biomater. Sci. Polym. Ed. 13 (2002) 747– 757. [6] H. Heinz, B.L. Farmer, R.B. Pandey, J.M. Slocik, S.S. Patnaik, R. Pachter, R.R. Naik, J. Am. Chem. Soc. 131 (2009) 9704–9714. [7] Y. Fang, N. Poulsen, M.B. Dickerson, Y. Cai, S.E. Jones, R.R. Naik, N. Kroger, K.H. Sandhage, J. Mater. Chem. 18 (2008) 3871–3875. [8] J.W. Han, Top. Catal. 55 (2012) 243–259. [9] M. Hnilova, C.R. So, E.E. Oren, B.R. Wilson, T. Kacar, C. Tamerler, M. Sarikaya, Soft Matter 8 (2012) 4327–4334. [10] J. Schneider, L. Colombi Ciacchi, J. Am. Chem. Soc. 134 (2012) 2407–2413. [11] L. Ruan, H. Ramezani-Dakhel, C.-Y. Chiu, E. Zhu, Y. Li, H. Heinz, Y. Huang, Nano Lett. 13 (2013) 840–846. [12] Z. Tang, J.P. Palafox-Hernandez, W.-C. Law, Z.E. Hughes, M.T. Swihart, P.N. Prasad, M.R. Knecht, T.R. Walsh, ACS Nano 7 (2013) 9632–9646. [13] A. Calzolari, G. Cicero, C. Cavazzoni, R. Di Felice, A. Catellani, S. Corni, J. Am. Chem. Soc. 132 (2010) 4790–4795. [14] L.B. Wright, P.M. Rodger, S. Corni, T.R. Walsh, J. Chem. Theory Comput. 9 (2013) 1616–1630. [15] L.B. Wright, P.M. Rodger, T.R. Walsh, S. Corni, J. Phys. Chem. C 117 (2013) 24292–24306.
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