A different view of structure-making and structure-breaking in alkali halide aqueous solutions through x-ray absorption spectroscopy Iradwikanari Waluyo, Dennis Nordlund, Uwe Bergmann, Daniel Schlesinger, Lars G. M. Pettersson, and Anders Nilsson Citation: The Journal of Chemical Physics 140, 244506 (2014); doi: 10.1063/1.4881600 View online: http://dx.doi.org/10.1063/1.4881600 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/140/24?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Vibrational and structural properties of amorphous n-butanol: A complementary Raman spectroscopy and X-ray diffraction study J. Chem. Phys. 138, 214506 (2013); 10.1063/1.4808159 The x-ray absorption spectroscopy model of solvation about sulfur in aqueous L-cysteine J. Chem. Phys. 137, 205103 (2012); 10.1063/1.4767350 Ultrafast conversions between hydrogen bonded structures in liquid water observed by femtosecond x-ray spectroscopy J. Chem. Phys. 131, 234505 (2009); 10.1063/1.3273204 The local structure of protonated water from x-ray absorption and density functional theory J. Chem. Phys. 124, 194508 (2006); 10.1063/1.2199828 Structural investigation of copper(II) chloride solutions using x-ray absorption spectroscopy J. Chem. Phys. 107, 2807 (1997); 10.1063/1.474638

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THE JOURNAL OF CHEMICAL PHYSICS 140, 244506 (2014)

A different view of structure-making and structure-breaking in alkali halide aqueous solutions through x-ray absorption spectroscopy Iradwikanari Waluyo,1,a) Dennis Nordlund,1 Uwe Bergmann,2 Daniel Schlesinger,3 Lars G. M. Pettersson,3 and Anders Nilsson1,3,b) 1

Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, P.O. Box 20450, Stanford, California 94309, USA 2 Linac Coherent Light Source, SLAC National Accelerator Laboratory, P.O. Box 20450, Stanford, California 94309, USA 3 Department of Physics, AlbaNova University Center, Stockholm University, SE-106 91 Stockholm, Sweden

(Received 13 January 2014; accepted 23 May 2014; published online 25 June 2014) X-ray absorption spectroscopy measured in transmission mode was used to study the effect of alkali and halide ions on the hydrogen-bonding (H-bonding) network of water. Cl− and Br− are shown to have insignificant effect on the structure of water while I− locally weakens the H-bonding, as indicated by a sharp increase of the main-edge feature in the x-ray absorption spectra. All alkali cations act as structure-breakers in water, weakening the H-bonding network. The spectral changes are similar to spectra of high density ices where the 2nd shell has collapsed due to a break-down of the tetrahedral structures, although here, around the ions, the breakdown of the local tetrahedrality is rather due to non-directional H-bonding to the larger anions. In addition, results from temperaturedependent x-ray Raman scattering measurements of NaCl solution confirm the H-bond breaking effect of Na+ and the effect on the liquid as similar to an increase in temperature. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4881600] I. INTRODUCTION

The role of water as a solvent is a crucial aspect of many chemical, biological, and environmental processes, where the simplest and most common solutes are salts that are dissolved into their constituent ions. The effect of ions on the structure of water is often categorized as either structure-breaking or structure-making, which property has been shown to depend on the ion size,1–3 as can be clearly seen in the alkali and halide ion series. From viscosity and hydration entropy data, only the smallest alkali cations (i.e., Li+ and Na+ ) are considered as structure-makers, while the rest (i.e., K+ , Rb+ , and Cs+ ) are structure-breakers.2, 4, 5 However, the role of Na+ in this classification is somewhat unclear as it is also often categorized as a structure-breaker. For instance, in a neutron diffraction study6 it was suggested that although Na+ binds water more tightly than K+ (i.e., Na+ is a local structure-maker), both cations disrupt water–water hydrogenbonding (H-bonding) beyond the first hydration shell. Similar to alkali cations, the relative strength of halide ions as structure-makers or structure-breakers depends on their size. This is corroborated by various experimental studies such as nuclear magnetic resonance (NMR) and dielectric relaxation spectroscopy1 as well as molecular dynamics (MD) simulations.7–9 We have also shown from a previous x-ray absorption spectroscopy (XAS) and small angle x-ray scattering (SAXS) study10 that fluoride, as the smallest anion, acts more a) Present address: Department of Chemistry, University of Illinois at

Chicago, 845 West Taylor Street, Chicago, Illinois 60607, USA.

b) Author to whom correspondence should be addressed. Electronic mail:

[email protected]. Telephone: (650) 926-2233. Fax: (650) 9264100. 0021-9606/2014/140(24)/244506/11/$30.00

like a structure-maker in the solution due to its strong interaction with water. The effect of alkali and halide ions on the structure of water has been extensively studied using experimental methods such as infrared11–13 and Raman14–17 spectroscopies, x-ray absorption spectroscopy,10, 18, 19 neutron and x-ray diffraction,6, 20–22 and femtosecond pump-probe spectroscopy,5, 23–28 as well as theoretical methods such as MD,7–9, 29–35 and reverse Monte Carlo fits to diffraction data.36–38 In particular, a recent series of studies of alkali halide solutions using femtosecond infrared pump-probe spectroscopy and dielectric relaxation spectroscopy has revealed interesting aspects of the dynamics of water in salt solutions and highlighted the different information content in the two techniques.26–28 In spite of the rather high concentration of 4 m salt solution, it was possible to distinguish the orientational dynamics of water solvating the halide anions from the dynamics of the bulk water and from water solvating the cations. In most cases, the influence of the ions on the dynamics did not extend beyond the first coordination shell, but for specific combinations of strongly hydrated ions, a cooperative effect was found affecting water also between the solvated ions.27 Furthermore, it was found that dielectric relaxation spectroscopy, sensitive to the reorientation of the permanent dipole of the coordinated water molecules, and femtosecond infrared pump-probe spectroscopy, sensitive to the reorientation of the transition dipole of the OD stretch (4% D2 O in H2 O), give complementary information on the dynamics of water in the salt solutions. In the present work we apply x-ray absorption spectroscopy (XAS), sensitive to H-bonding, to provide additional information targeting specifically the Hbond network of water in salt solutions.

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FIG. 1. O 1s T-mode XAS spectra of pure liquid water from three separate measurements. The important spectral features are labeled on the figure. All spectra have been normalized by area from 532 to 550 eV.

XAS is a technique that probes the unoccupied molecular orbitals of a system through the excitation of a core electron. XAS has been shown to be sensitive to the Hbonding environment of water in its bulk state as well as in solutions10, 18, 39–45 and when adsorbed at surfaces.46–48 In the O 1s XAS spectrum of water (Fig. 1), excitation into lowlying orbitals gives rise to the pre- and main-edge features that are signatures of weak or distorted H-bonds in water.39, 41, 42 On the other hand, the post-edge peak observed at a higher energy position gains intensity from intact or strong Hbonds.39, 41, 42 The main-edge feature, at around 537eV, is also enhanced upon formation of high-density amorphous ice49 and various crystalline high-pressure ice phases, such as III , IVI , IVII , and IVIII .50 This enhancement is related to the fact that the second shell collapses from the tetrahedral distance of 4.52 Å in ice Ih to much shorter distances leading to an overall higher density. As a consequence, H-bonds become less directional which allows the post-edge intensity to move into the main-edge region; note that this also slightly increases the pre-edge and that the reduced H-bond directionality is supported by the blue-shift of the OH-stretch band observed for the high-density ices.51 Through a combination of experimental and theoretical results, it was proposed that liquid water is primarily composed of asymmetrically H-bonded water with a smaller fraction of tetrahedrally H-bonded water.39, 40 This interpretation, which contradicts the conventional continuous tetrahedral H-bonding model of water, has generated quite a controversy.45, 52–62 Recently, this picture has been extended to make an analogy also with the high pressure ices49, 50 where the weakened H-bonded structure is related to high density liquid (HDL) structures.42, 63, 64 Independent of the controversy around the local structure of bulk water, experimental results indicate that XAS is indeed sensitive to changes in the H-bonding environment.10, 18, 42–44, 63, 65–67 In addition, XAS has been used to probe the electronic structure of hydrated cations to study the direct orbital interaction between the cation and water.68, 69

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XAS measured in the fluorescence yield (FY)65 and total electron yield (TEY)70, 71 detection modes has also been used to study alkali halide solutions, but with conflicting interpretations. Based on a FY-XAS study of NaCl and KCl solutions, Näslund et al.65 proposed that the spectral changes in XAS are primarily due to the effect of the cations, while chloride only elongates the water-anion H-bond distance. On the other hand, Cappa et al.70, 71 used TEY-XAS and suggested that halide anions electronically perturb the surrounding water molecules and that this gave rise to the observed spectral changes, while alkali cations have no effect. These conflicting interpretations are due to the experimental limitations in both FY and TEY-XAS in their application to liquid samples. Spectra of water and aqueous solutions obtained using FY and TEY detection modes show saturation-like effects that can change relative intensities in different spectral regions.55, 65 In FY-XAS, this saturation is caused by self-absorption and can be corrected mathematically.72 In theory, even spectra with minor saturation effects can be used to qualitatively observe trends in the spectral changes as long as they are reproducible and can be confirmed using other detection methods. TEYXAS spectra, however, have also shown reproducibility problems that further complicate data interpretation as it can be difficult to determine whether the spectral changes are real or caused by experimental artifacts. This can be clearly observed from the spectral changes measured by TEY-XAS as a result of temperature increase,54 which show a different trend from those measured by x-ray Raman scattering (XRS), which does not suffer from any saturation effects.42, 50, 55, 63 In the present work, we used XAS measured in transmission mode (T-mode) to study the effect of alkali and halide ions on the H-bonding structure of water. T-mode XAS, in theory, is the most straightforward method for obtaining xray absorption spectra; however, it has not been commonly used for liquid samples due to its impractical sample requirements. Due to the short attenuation length of soft x-rays in water, the sample thickness must be between 200 and 500 nm and it must be homogeneous. Previous studies have presented O K-edge spectra of liquid water measured in transmission mode,42, 72–76 including a recent pump-probe study75 and a unique flow cell set-up.76 Overall, the data in the present work represent the most extensive and systematic XAS study of a large range of alkali halide solutions. The analysis is further supported by XAS spectrum calculations. In Sec. IV, we suggest that the concepts of structurebreaking and structure-making can be connected to changing the balance between high-density liquid (HDL, broken-up Hbond network) and low-density liquid (LDL, strongly tetrahedral H-bonding) local structures around the first coordination shell; this is in line with conclusions from MD simulations of salt solutions down to supercooled conditions.30–33 Although we use the notation of HDL and LDL to connect with discussions in supercooled water and amorphous ice phases,64, 77 this should only be related to local structural properties such as if the local H-bonding is tetrahedral or if the 2nd shell has collapsed and there is weakened H-bonding in the first shell. The local density around the ions could be rather different from what is implied from a strict density notation in terms of HDL and LDL.

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II. METHODS

Aqueous solutions of alkali halides at various concentrations were prepared using commercially available salts (>99% from Fisher Chemicals and Acros Organics) dissolved in Millipore-purified H2 O (∼18 M cm). Transmission mode (T-mode) XAS spectra were obtained at beamline (BL) 10-1 at Stanford Synchrotron Radiation Lightsource (SSRL). The endstation consisted of two sections separated by a 200 nm Si3 N4 window: the ultrahigh vacuum (UHV) I0 section with a base pressure lower than 1 × 10−9 Torr and a mechanically pumped sample chamber with a pressure lower than 10 mTorr. The sample chamber was equipped with a 25 μm diameter molybdenum pinhole mounted on a vertical linear transfer rod, a manipulator with a rod on which up to four sample cells can be mounted, and a silicon p-n junction photodiode (International Radiation Detectors, Inc.). The sample cells for T-mode XAS were prepared at the Stanford Nanofabrication Facility (SNF) and consisted of two silicon nitride windows supported on Si(100) chips that can be sandwiched with a droplet of liquid sample in between the windows. Initially, Si(100) wafers (Silicon Specialists Inc., Hayward, CA) were coated on both sides with 200 nm Si3 N4 (performed by International Wafer Services, Santa Clara, CA). On one side of the wafers, an Au layer of 300 nm thickness was deposited to form spacers to satisfy the thickness requirement for T-mode XAS. The wafers were then wet-etched in 22.5% KOH solution to create 500 × 500 μm2 Si3 N4 areas as the x-ray windows. A thin layer (10 nm) of SiO2 was deposited using plasma-enhanced chemical vapor deposition to improve the hydrophilicity of the membranes, and the finished wafers were cut into 5 × 5 mm2 pieces. Comparison between water spectra measured in oxide-coated windows and Au-only coated windows showed that the oxide coating does not affect the spectra in any way after normalization with the empty cells. As the photon energy was scanned between 520 and 570 eV, the transmitted beam intensity for both an empty cell (I0 ) and filled cell (I) was measured separately. The absorption coefficient as a function of photon energy μ(ω) of a sample with thickness x can be determined from the standard expression for transmission, I(ω) = I0 exp(−μ(ω)x). The beamline was operated at 0.1 eV energy resolution. In addition, we also measured XRS spectra of 4 m NaCl solution at 4 ◦ C, 25 ◦ C, 60 ◦ C, and 90 ◦ C at SSRL BL 6-2 equipped with a Si(311) double-crystal monochromator and a high-energy-resolution 14-crystal spectrometer. The setup selects 6.46 keV photons with an energy resolution of ∼0.5 eV and momentum transfer of Q = 2.6 ± 1 Å−1 . The monochromator energy was scanned from 6980 to 7032 eV, corresponding to an energy transfer from ∼520 to 572 eV. The overall energy resolution was 0.55 ± 0.02 eV full width at half maximum (FWHM). The solution was flowed through an aluminum cell connected to a Huber Ministat 125 circulating heater/chiller and a 5 × 5 mm2 Si3 N4 window of 1 μm thickness was attached to one side of the cell. A more detailed description of the XRS experimental setup can be found in Ref. 18. To illustrate the effects of H-bond distance and orbital relaxations, XAS spectrum calculations were performed on a

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model water dimer using the transition-potential, half-corehole approach45, 78 as implemented in the StoBe-deMon density functional code. For the core-excited oxygen in the Hbond donor, the IGLO-III basis79 was used in order to allow core relaxation upon creation of the core-hole. The oxygen in the H-bond acceptor had the 1s level replaced by an effective core potential80 to make the O 1s level on the donor water unique. Spectra were generated as described in Ref. 45 and broadened using Gaussian functions linearly increasing from full-width-at-half-maximum 0.4 eV before the edge up to 3 eV at 12 eV beyond the edge.

III. RESULTS A. Reliability of T-mode XAS

A subset of the samples was characterized using scanning transmission x-ray microscopy (STXM), revealing that the water sandwiched between Si3 N4 windows does not form a film of homogeneous thickness.81, 82 We observed a nonnegligible thickness inhomogeneity (of tens of nm) over a typical length scale of 10 μm. Previously, it was demonstrated that STXM generates the most reliable spectra of liquid water since the 50 nm beam focus in STXM is much smaller than the length scale of the thickness variations in the sample.42 In the T-mode XAS experiments presented here, the pinhole only reduces the beam spot size to about 25 μm. Therefore, the beam probes areas of the sample with inhomogeneous thickness, which is manifested in the spectra as a slight saturation effect.42 This effect of thickness inhomogeneity has also been reported by Schreck et al.76 As we have previously mentioned, the reproducibility of XAS spectra is crucial in order to observe trends in spectral changes, especially if the spectra display saturation effects. Fig. 1 shows T-mode XAS spectra of liquid water measured over three different experimental runs separated by several months. The spectra are nearly identical, thereby demonstrating the applicability and reliability of T-mode XAS, as performed in the present work, as a quantitative technique for O K-edge XAS in solutions. Further evidence for the reliability of T-mode XAS is presented in Fig. 2, which compares the difference spectra for some alkali halide solutions measured by both T-mode XAS and XRS. XRS is a hard x-ray alternative to XAS in which a high-energy photon (∼7 keV) is inelastically scattered and a fraction of the energy is used to excite a core electron to an unoccupied orbital corresponding to an x-ray absorption process. The advantage of XRS is that it is not affected by any saturation effect and it has a much simpler sample-handling procedure. However, XRS has a low scattering cross-section and a slightly poorer resolution (∼0.5 eV) compared to soft x-ray methods (∼0.1 eV). The black lines represent the water-subtracted T-mode XAS spectra of 1 m NaCl, 4 m NaCl, 4 m KF, and 8 m KF, while the corresponding XRS spectra are shown as the red lines (the thickness of the lines represents the error bars). The T-mode XAS spectra were broadened to 0.5 eV resolution to match the energy resolution of the XRS spectra. Although the inhomogeneous sample thickness causes a small but noticeable saturation-like effect in T-mode XAS spectra,42 it is clear from Fig. 2 that,

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FIG. 2. The water-subtracted O 1s T-mode XAS (solid black line) and XRS (red line) spectra of (a) 1 m NaCl, (b) 4 m NaCl, (c) 4 m KF, and (d) 8 m KF solutions. The T-mode XAS spectra have been broadened to 0.5 eV resolution to match the resolution of the XRS spectra.

within error bars, the difference spectra obtained by T-mode XAS reproduce those obtained by XRS.

B. T-mode XAS spectra of alkali halide solutions

The pre-edge (535 eV), main edge (537–538 eV), and post-edge (540–541 eV) spectral features are affected when ions are dissolved in water. Fig. 3 shows the spectra for NaCl and KI solutions as an example. A complete set of concentration-dependent O 1s XAS spectra of water in alkali halide solutions can be found in the supplementary material.83 The difference spectra, i.e., solution minus water, are displayed above each set of spectra. The solvation of NaCl has the effect of increasing the pre- and main edge and decreasing the post-edge, which is indicative of an increased fraction of distorted or broken H-bonds in the solution. This is consistent with the analogy between adding salt and the effect of temperature and pressure increase,84 in agreement with previous experimental results from FY-XAS,65 XRS,18 as well as neutron diffraction analyzed using empirical potential structure refinement (EPSR).20 The spectral changes caused by KI solvation, shown in Fig. 2(b), show a similar trend, i.e., intensity redistribution from the post-edge to the pre- and main edge, albeit with a larger magnitude compared to NaCl.

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FIG. 3. O 1s T-mode XAS spectra showing the concentration dependence for (a) NaCl aqueous solutions compared to pure water, and (b) KI aqueous solutions compared to pure water. All spectra have been normalized by area from 532 to 550 eV. The difference spectra (i.e., the spectra of solution minus the spectrum of water) are displayed above each set of spectra.

We note that, in spite of the high concentrations of ions (1–6 m NaCl and 1–4 m KI) with a majority of water molecules directly interacting with an ion, the resulting spectra are still clearly very similar to that of bulk water. This indicates that the electronic structure of water is not significantly affected by the ionic charges in the solution. Evidently no new states appear where, e.g., charge-transfer from water to a cation would lead to unoccupied valence states on water which would manifest as low-energy excitations before the pre-edge (535 eV) in XAS. Such low-energy states are observed for water solvating transition metal cations due to mixing with d-states,66 but are completely absent for the alkali halides. The water-anion interaction is furthermore expected to have a similar effect on the spectra as the water-water interaction. The latter leads to rehybridization which can be viewed in terms of charge transfer from the water oxygen lone pair to the OH antibonding orbital and corresponding backtransfer, maintaining charge neutrality.85 Net charge transfer from an anion to a water molecule could only occur through partial filling of the OH σ * antibonding states where the lowest is associated with the pre-edge feature; charge transfer would thus lead to a decreased pre-edge intensity where instead an increase is observed.

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Water dimer 9

Gasphase Acceptor Frozen Relaxed R_OO 3.6

8

7

6

5

4

3

2

1

0 530

535

540

545

550

FIG. 4. Computed XAS spectra for the H-bond donor in a water dimer model (O–O distance 2.8 Å) illustrating general spectroscopic effects due to Hbonding. Gas phase monomer (black) is shown as reference. Core-excitation with the H-bond accepting water (green) with frozen density (i.e., orbitals frozen from the gas phase molecule) differs insignificantly from the spectrum where the acceptor orbitals are allowed to relax (blue). Computing the spectrum (red) for a separation of 3.6 Å (similar to O–I− ) reduces the repulsion between the core-excited 2b2 state and the acceptor molecule leading to this state being less pushed up in energy and moving towards the gas phase position.

Thus, rather than a change in the ground state electronic structure of the water molecules it is the wave function of the excited state, which mostly is on the hydrogen atoms,40–42, 86 that becomes modified through Pauli repulsion if it overlaps with either another water molecule or an anion. In Fig. 4 we show a simple example based on calculated spectra for the H-bond donor in water dimer at 2.8 Å, i.e., the O–O distance typical of the liquid. As reference we show in black the computed spectrum of the gas phase monomer. The asymmetric H-bonding, with one free OH and the other H-bonded, leads to a pre-edge state close to the 4a1 position of the gas phase water which is localized along the free OH; in the condensed phase, this gets slightly pushed up in energy compared to the gas phase due to excluded volume effects. The 2b2 state, on the other hand, localizes along the H-bonded OH directly pointing to the H-bond acceptor.41 Since it contains significant Rydberg character it gets pushed up in energy into the post-edge region. We show two spectra where we either freeze

the orbitals on the H-bond acceptor or allow them to relax in the field of the core-excitation resulting in very minor differences; the main effect is the Pauli repulsion between the excited orbital and the closed-shell acceptor molecule where the excited orbital is significantly more polarizable and thus gets modified through the repulsion. Since the magnitude of the repulsion is strongly dependent on distance, we find that, if we pull the H-bond donor out to the distance (3.6 Å) corresponding to H-bonding to I− , the repulsion becomes significantly reduced and the 2b2 state moves down in energy towards the main-edge position. The spectral changes thus mainly reflect changes in spatial overlap of the excited state with various electron densities and are less dependent on charge transfer. To facilitate the following discussion, the spectra of solutions with the same cations (i.e., KCl, KBr, KI, and NaCl, NaBr, NaI) and those with the same anions (i.e., LiCl, NaCl, KCl, RbCl, CsCl, and KF, RbF, CsF) at the same concentration are shown in Figs. 5 and 6. The concentration-dependent spectra (Fig. 3) show that the spectral changes are consistent over the whole range of concentrations, even at 1 m, indicating that ion pairing,69 which occurs at high concentrations, does not significantly affect the XAS spectra. We thus show the spectra of solutions at 4 m concentration in Figs. 5 and 6 specifically because the spectral changes are more pronounced than at lower concentrations while being the highest common concentration for all of the solutions. The effect of F− has been discussed extensively in a previous work as being similar to the effect of a temperature decrease, which increases the fraction of tetrahedral structures in the solution due to the formation of strong H-bonds between water and the anion.10 In Fig. 5, we focus the discussion on the effect of the larger halide anions (i.e., Cl− , Br− , and I− ). The X–O distances between the halide ion X− and the oxygen atom of a water molecule in the first hydration shell are, on average, ∼3.15 Å for Cl− , ∼3.3 Å for Br− , and ∼3.6 Å for I− , which are longer than the average O–O distance in bulk water (∼2.8 Å).87 Although the anions should have a similar effect as H-bond elongation, it has been proposed that the spectral changes caused by Cl− anion are actually small when compared to the effect of the cation,18 which is also supported by a MD study.88 From Fig. 5, it is clear that, regardless of the cation, the spectra of Br− containing solutions closely resemble those of solutions containing Cl− ; there is only a very small shift of the main edge to lower energy and small intensity variations that can be attributed to uncertainties in the concentration. For I− solutions, however, the spectra change dramatically, especially the main-edge peak, which becomes very sharp and shifts to lower energy, while the pre-edge is slightly increased. The origin of the main-edge feature in the O 1s XAS spectrum of liquid water has not been as well understood as those of the pre- and post-edge. A recent study of highpressure crystalline ices demonstrated, however, that the main-edge intensity is linearly related to the density in highpressure ices which in turn is related to the collapse of the second shell to shorter distances.50 This leads to distortions from the typical tetrahedral bond angle of hexagonal ice resulting in less directional H-bonds and a shift of the 2b2 molecular state down in energy towards the onset of the main-edge. Based on

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FIG. 5. O 1s T-mode XAS spectra of salt solutions at 4 m concentration with the same cation and different anions: (a) NaCl, NaBr, and NaI, (b) KCl, KBr, and KI. The spectrum of pure water is shown as the black line. All spectra have been normalized by area from 532 to 550 eV. The difference spectra are displayed above each set of spectra.

this, we hypothesize that the sharp increase of the main-edge peak in KI and NaI solutions can be explained by the weak Hbonding to I− ions at the long interatomic distance resulting in a situation effectively similar to non-H-bonded water with more local molecular orbital character. Therefore, while Cl− and Br− have immeasurable effects (see Fig. 5) on the XAS spectra, I− as the largest anion likely significantly changes the coordination with the surrounding molecules resulting in less tetrahedral bonding, thereby also enhancing the H-bond breaking effect of Na+ and K+ . In Fig. 6, the effect of alkali cations is studied by comparing the spectra of solutions with the same anions (Cl− in

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FIG. 6. O 1s T-mode XAS spectra of salt solutions at 4 m concentration with the same anion and different cations: (a) LiCl, NaCl, KCl, RbCl, and CsCl, (b) KF, RbF, and CsF. The spectrum of pure water is shown as the black line. All spectra have been normalized by area from 532 to 550 eV. The difference spectra are displayed above each set of spectra. The insets show the enlarged pre-edge region.

Fig. 6(a) and F− in Fig. 6(b)). From Fig. 6(a), it is apparent that all alkali cations increase the fraction of weak/distorted H-bonds as evidenced by the intensity redistribution from the post-edge to the pre- and main edge; this is true even for Li+ , which is generally considered as a structure-maker due to its small size.1, 19 However, the magnitude of the spectral changes varies for different cations. The pre-edge peaks for these spectra are shown enlarged in the inset of Fig. 6(a). From LiCl to RbCl, the intensity of the pre-edge clearly shows a dependence on cation size; however, this effect is reversed for CsCl, where the pre-edge is between those of NaCl and KCl. The

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same reversal effect is also observed when the Cl− anion is replaced with F− , as shown in Fig. 6(b). In our previous work,10 we proposed that F− has the opposite effect from K+ , thereby resulting in a total spectrum that is largely similar to the spectrum of pure water; however, the residual effect of K+ could be observed as a slightly higher pre-edge that increases with concentration. Similarly, the pre-edge peaks shown in the inset of Fig. 6(b) can be used to compare the relative effect of the cations, where it is seen that the pre-edge intensity of CsF is lower than that for KF and RbF, confirming that these spectral changes are real, reproducible, and independent of anion (F− or Cl− ). The experimental and theoretical TEY-XAS study of alkali halide solutions by Cappa et al.70, 71 proposed that alkali cations have no observable effect on the O 1s XAS spectra. However, in Fig. 6, we can directly observe from the experimental data that there are cation-dependent spectral changes associated with H-bond weakening. Cappa et al.70, 71 suggested, based on their DFT calculations, that the changes in the XAS spectra of alkali halide solution are, instead, caused by the electronic perturbation of water molecules by the anions. However, the anion-water distance used in their spectrum calculations was too short (Cl–O distance of 2.818 Å instead of 3.15Å),70, 71 leading to the distortion of the unoccupied orbitals of water due to strongly exaggerated Pauli repulsion in their computational model. In general, our observation that alkali ions have H-bond breaking effects on water is consistent with the existing literature. Although Li+ is generally classified as a structuremaker due to its small size, a reverse Monte Carlo study using neutron and x-ray diffraction data has proposed that Li+ is a structure-breaker,37 in agreement with our results. In addition, from a gel-sieving chromatography study,89 the apparent dynamic hydration number (ADHN) of Li+ is less than one, whereas structure-making ions such as F− , Mg2+ , and Al3+ have ADHNs of five, six, and nine, respectively; ADHN is the number of water molecules bound to an ion as it elutes through the chromatographic column. Therefore, despite its small size, Li+ clearly does not have a strongly bound hydration shell. Our results for halide ions slightly contradict the literature, where the structure-breaking properties of halides show a systematic dependence on size.1 We observe only small differences between Cl− and Br− , and only I− potentially induces a fraction of significantly perturbed water similar to HDL with a collapsed 2nd shell and weakened Hbonding. Cs+ as the largest alkali cation has been proposed to behave like a neutral hydrophobic particle where the electrostatic interaction between ion and water becomes less important than the water-water interaction, leading to the formation of a hydration cage around the Cs+ ion.90 Indeed, water molecules are known to form ice-like cages around small hydrophobic molecules;91 this H-bond enhancing effect has recently been confirmed experimentally for small hydrophobic solutes using XRS.92 This may explain the reversal effect observed in Fig. 6, where the pre-edge systematically increases from Li+ to Rb+ but decreases when the size is further increased to Cs+ . Water molecules in the first hydration shell of Cs+ may form H-bonds with each other, thereby compensat-

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ing for some of the structure-breaking effect. For smaller alkali cations, the ion-water electrostatic attraction dominates; therefore, water cannot form a hydration cage around these cations. From a purely electrostatic viewpoint, the interaction energy between a dipole and a positive charge is completely equivalent to that with a negative charge if the dipole is allowed to reorient. It is thus interesting to compare the two largest ions, Cs+ and I− , where the former behaves as a hydrophobic entity enhancing H-bonding while I− rather disrupts the H-bond network through less directional H-bonds in its solvation shell. We ascribe this difference to the higher polarizability of the anions and the possibility for the negative charge to delocalize more towards the water in the hydration shell, thus providing improved bonding which can compete with water-water H-bond formation while still not overcoming it, as indicated by the non-directionality of H-bonding to I− as deduced from the main-edge intensity.

IV. DISCUSSION

We have discussed the effects of alkali halide salts on water in terms of the traditional picture of structure-breakers and structure-makers. However, there are several indications that a more general picture is possible in terms of the added ions affecting the balance between two different types of coordination in the liquid. From neutron diffraction studies, the paircorrelation functions of water in salt solution and water under pressure have been found to be very similar, leading to the suggestion that adding salt is equivalent to exerting pressure on the liquid resulting in an inward collapse of the 2nd shell.84 Interestingly, from the dielectric relaxation spectroscopy and femtosecond infrared pump-probe studies discussed in the Introduction, it was found that, except for certain combinations of strongly hydrated ions, where cooperative effects became important, a rather small number of water molecules had their dynamics significantly affected by ions;26–28 it thus becomes relevant to discuss the effects in terms of the properties of water. Of particular importance in the present context are the thermodynamical response functions κ T (isothermal compressibility) and CP (isobaric heat capacity) which depend on, respectively, fluctuations in density and entropy93 and show minima in pure water at 46 ◦ C and 35 ◦ C, respectively. Upon further cooling, κ T and CP increase anomalously indicating that fluctuations increase as thermal energy is removed, and in the supercooled regime, below the melting point, the anomalous behavior is further enhanced such that both can be fitted to a power-law diverging towards a temperature of −45 ◦ C at ambient pressure.94 In the supercooled regime, this behavior is commonly explained in terms of structural fluctuations between two types of local environments as HDL, with broken H-bonds allowing closer packing and favoring entropy, and LDL with tetrahedral H-bonds minimizing enthalpy.93 However, the anomalous behavior of κ T and CP is apparent already under ambient conditions, which would indicate that such structural fluctuations should be important already in this regime.

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The early studies by Myneni et al.39 and Wernet et al.40 using XAS and XRS on ambient to hot water concluded that at ambient conditions most molecules are not tetrahedrally Hbonded, but have distorted H-bonds; this is now recognized to be similar to HDL.64 Recently,63, 95 XRS, x-ray emission spectroscopy (XES), and SAXS were combined to take this interpretation one step further by concluding dynamical structural heterogeneities in ambient liquid water due to fluctuations between local HDL and LDL environments. The HDL species, with its weakened and distorted H-bonds, maximizes entropy.63, 64 The tetrahedrally H-bonded LDL species is of lower enthalpy, and the mutual exclusivity of weakened Hbonds giving close-packed HDL and intact H-bonds giving LDL generates fluctuations between the two. We propose that structure-making and structure-breaking can be viewed as the ions affecting the balance between the two types of preferred local structures: HDL with broken-up H-bond network, and LDL with strongly tetrahedral local H-bonding giving lower density. In this picture, structure-breakers enhance the fraction of HDL species while structure-makers promote LDLlike species. Indeed, SAXS studies96 of supercooled NaCl solution demonstrate a reduced correlation length compared to supercooled neat water, which implies that the size of tetrahedral, LDL-like regions in the liquid is decreased, fully consistent with the equivalence between adding salt and increasing the pressure; note that, even if only water molecules in direct contact with ions are affected, it will be enough that those water molecules are locked into HDL-like, H-bond-disordered environments to affect the balance between HDL and LDL since only the remaining water molecules are free to participate in fluctuations into LDL-like environments. Further sup-

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port for this concept is obtained from MD simulations where Gallo and co-workers in a series of papers30–34 have investigated the effects on the thermodynamical properties of simulated water from adding salt. Going to deeply supercooled conditions and high pressure in the TIP4P model used, they find a coexistence line between HDL and LDL which terminates in a critical point at Tc = 190 K and Pc = 150 MPa. The addition of salt in the simulations was found to shift the critical point to higher temperatures and lower pressure; the magnitude of the shift was found to depend on the concentration. The interpretation was in terms of salt solution changing the balance between HDL and LDL, and a connection to structure-making and structure-breaking was suggested.31 Investigating the temperature dependence of effects on the experimental O K-edge spectrum of water in salt solutions provides additional insight. XRS spectra of 4 ◦ C and 90 ◦ C 4 m NaCl solutions are compared to the spectra of water at the same temperatures (from Ref. 63) in Fig. 7. The complete set of temperaturedependent XRS spectra of NaCl (including 25 ◦ C and 60 ◦ C) can be found in the supplementary material.83 In Fig. 7(a), the spectra are grouped by temperature while in Fig. 7(b), they are grouped by species. It is clear that the spectral changes due to NaCl solvation are much larger at 4 ◦ C than at 90 ◦ C. This can be more clearly observed in Fig. 8(a), which shows the difference spectra with respect to species (i.e., NaCl minus H2 O) for each temperature. The difference spectra at 25 ◦ C and 60 ◦ C are in between those at 4 ◦ C and 90 ◦ C, but they are omitted for clarity. Previously, we concluded that Na+ breaks the H-bonding network of the surrounding water molecules, thereby increasing the fraction of distorted HDL species in

FIG. 7. O 1s XRS spectra of 4 m NaCl solution compared with pure water, both measured at 4 ◦ C and 90 ◦ C. In (a), the spectra are grouped by temperature while in (b), the spectra are grouped by species. All spectra have been normalized by area from 532 to 550 eV.

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shown in Fig. 8(b). By the same reasoning as for Fig. 7(a), this is consistent with the H-bond breaking effect of Na+ . As more HDL is already present as a result of Na+ solvation, raising the temperature from 4 ◦ C to 90 ◦ C does not significantly increase the fraction of the HDL species; the shift of the pre- and main-edge to lower energy, however, shows the increasing disorder in the HDL species due to thermal excitation. Overall, the temperature-dependent study of NaCl solution supports our previous conclusion that Na+ breaks the water-water H-bonding network and increases the population of HDL species.18 V. CONCLUSION

FIG. 8. Difference spectra of temperature-dependent O 1s XRS spectra of 4 m NaCl solution and pure water. In (a), the spectra of water at 4 ◦ C and 90 ◦ C are each subtracted from the spectra of 4 m NaCl solution at the same temperatures. In (b), the spectra of 4 ◦ C water and NaCl solution are subtracted from the spectra of 90 ◦ C water and NaCl solution.

the picture of fluctuations between HDL and LDL.18 Similarly, a temperature increase also has the effect of converting the LDL to the HDL species, but with an additional thermal excitation of the existing HDL species to be discussed below.63 Therefore, at a higher temperature where most of the water molecules already exist as HDL species, the addition of NaCl to the solution has a comparatively smaller effect. Conversely, at a low temperature where more LDL species are present, though not dominant, the H-bond breaking effect of Na+ can have a more significant influence. This observation is in agreement with a MD study that proposes that the structure-breaking character of NaCl is enhanced at lower temperatures.97 It has also recently been pointed out that the O–H stretch Raman spectra in water changes in a similar manner between adding NaCl and changing the temperature.17 In XAS, the pre- and main-edge peaks are observed to shift towards the gas phase position with increasing temperature,42 indicating thermal excitation of the HDL species contributing to pre- and main-edge and, as a result, a continuous loosening up of the H-bonding in this species. In Fig. 7(b), in addition to the intensity redistribution from the post-edge to the pre- and main-edge, indicating an increasing fraction of HDL species, the pre-edge and main-edge shift to lower energy is also observed for NaCl similar to what is found for water at higher temperature. For NaCl, however, the changes as a result of temperature are much smaller compared to water, which can also be observed in the difference spectra with respect to temperature (i.e., 90 ◦ C–4 ◦ C)

In conclusion, T-mode XAS has been shown to be a reproducible and reliable method for measuring the x-ray absorption spectra of water and aqueous solutions. We propose, as supported by recent MD simulations,30–34 an interpretation of the structure-making and structure-breaking effects of the various ions in terms of shifts in the balance between HDL and LDL species in water, where HDL corresponds to a broken-up H-bond network while LDL corresponds to strongly tetrahedral H-bonding; the former is enhanced by structure-breakers while the latter is favored by structuremakers. A recent femtosecond pump-probe and dielectric relaxation study28 demonstrated that only water molecules in the proximity of the ions had their dynamics significantly affected, but it cannot be ruled out that structural effects extend further into the solution as recently suggested in a MD study.35 However, even if only water molecules in the first solvation shell are affected, the HDL/LDL balance is still affected when these are locked into more HDL-like, disordered local H-bond environments. By comparing the O 1s XAS spectra of various alkali halide solutions, we show that Cl− and Br− do not significantly alter the H-bonding network of water in the solution, thereby confirming our previous hypothesis.18 However, the sharp increase in the mainedge feature of the spectra of I− solutions indicates a dramatic increase in the population of species with broken or significantly weakened H-bonds of liquid water as a result of I− solvation. Previous FY-XAS and XRS studies have concluded that Na+ has an H-bond breaking effect18, 65 resulting in a higher fraction of HDL species of water, similar to the effect of a temperature increase. Our temperature-dependent XRS spectra confirm this assertion by demonstrating the enhanced structure-breaking effect at a lower temperature compared to at higher temperature. Finally, the T-mode XAS spectra of a series of alkali chloride salts show a similar trend in the spectral changes, i.e., increased pre- and main-edge as well as decreased postedge, indicating that all alkali cations break water H-bonds and increase the population of HDL species in the solution. This effect is generally enhanced by increasing the ionic radius from Li+ to Rb+ . However, an interesting reversal is observed for Cs+ , which, due to its large size, may exhibit hydrophobic properties, thereby inducing the formation of a water cage around the cation which compensates the H-bond breaking effect. This is, on the other hand, not observed for

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the largest anion I− , which we suggest is due to its greater polarizability and availability of excess negative charge that can delocalize towards the solvating water molecules providing additional bond strength which balances the water-water H-bonding propensity.

ACKNOWLEDGMENTS

This work was supported by the National Science Foundation (U.S.) CHE-0809324 and the Swedish Research Council. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. Work was performed in part at the Stanford Nanofabrication Facility (a member of the National Nanotechnology Infrastructure Network) which is supported by the National Science Foundation under Grant No. ECS-9731293, its lab members, and the industrial members of the Stanford Center for Integrated Systems. 1 Y.

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