Article pubs.acs.org/Langmuir

Twist and Turn: Effect of Stereoconfiguration on the Interfacial Assembly of Polyelectrolytes Nicholas A. Valley, Ellen J. Robertson, and Geraldine L. Richmond* Department of Chemistry, University of Oregon, Eugene, Oregon 97403, United States S Supporting Information *

ABSTRACT: Understanding the conditions that promote the adsorption, assembly, and accumulation of charged macromolecules at the interface between aqueous and hydrophobic liquids is important to a multitude of biological, environmental, and industrial processes. Here, the oil−water interfacial behavior of stereoisomers of polymethacrylic acid (PMA), a model system for both naturally occurring and synthetic polyelectrolytes, is investigated with a combination of vibrational sumfrequency (VSF) spectroscopy, surface tension, and computations. Syndiotactic and isotactic isomers both show rapid adsorption to the oil−water interface with a net orientation indicative of a high degree of ordering. The stereoconfiguration is found to affect whether only a single layer or multiple layers assemble at the interface. Surface tension measurements show additional adsorption for syndiotactic PMA over time. The additional layers do not contribute to the VSF spectrum indicating disorder in all but the initial layer. The isotactic isomer shows no evidence of accumulation at the interface beyond the single ordered layer. Molecular dynamics calculations show marked differences between the two isomers in the orientation of their substituent groups at the interface. The hydrophilic and hydrophobic moieties in the isotactic isomer are easily partitioned to the water and oil phases, respectively, whereas a fair portion of hydrophobic groups remain in the water phase for the syndiotactic PMA. The available hydrophobic contacts in the water phase at the interface are credited with allowing further adsorption.

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INTRODUCTION The ability of polyelectrolytes to assemble at the boundary between water and a nonpolar fluid is essential to a wide variety of processes. Areas of application range from environmental1−7 to biological8 to industrial.9−15 Environmentally important processes that require a large reduction of interfacial tension, such as in enhanced oil recovery,11,16 water remediation,6,7,17,18 and emulsion stabilization,15 depend on a high degree of interfacial accumulation, achievable with both natural and synthetic polyelectrolytes. Also, whether polyelectrolytic humic substances accumulate at fluid interfaces can dictate the transport of natural organic matter.19−22 The accumulation of misfolded amyloid proteins (biological polyelectrolytes) on cellular membranes results in neurodegenerative diseases such as Alzheimer’s, but the mechanism for this accumulation is not well understood.23−29 For these applications, it is imperative to understand, on the molecular level, the specific characteristics of polyelectrolytes that dictate the adsorption, assembly, and accumulation at an oil−water interface. This work explores the molecular-level details related to how the chirality of successive monomers (tacticity) of a model polyelectrolyte polymer, specifically poly(methacrylic acid) (PMA), affects its adsorption and accumulation at the carbon tetrachloride-aqueous (CCl4−H2O) interface. Here, vibrational sum-frequency (VSF) spectroscopic data are shown, with © 2014 American Chemical Society

complementary information obtained from interfacial tension measurements and computational studies. VSF spectroscopy is a surface-selective technique that generates vibrational spectra of oriented interfacial molecules. Because the VSF signal intensity depends on both the number of interfacial molecules and their net orientation relative to the interfacial plane, complementary techniques are required to understand differences in VSF spectra. Interfacial tension serves as an appropriate technique because these measurements give information about the number of interfacial molecules and are not as sensitive to orientational effects. Computational studies can further deconvolve spectroscopic data by supporting a microscopic picture that is not obtainable with interfacial tension measurements. These combined techniques are used here to provide a detailed molecular-level description of how tacticity affects polyelectrolyte adsorption to an oil− water interface. PMA is a good example of a polyelectrolyte whose tacticity greatly affects its degree of hydrophobicity. For the isotactic isomer (iPMA), the chiral centers in successive monomer units all have the same configuration, whereas for the syndiotactic Received: September 19, 2014 Revised: November 3, 2014 Published: November 5, 2014 14226

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the incident beams and a BK7 glass window facing the outgoing beams. The windows were sealed with Dupont Kalrez perfluoropolymer o-rings. The sample cell, o-rings, and BK7 window were all soaked in the NoChromix−sulfuric acid bath for at least 12 h and subsequently rinsed in water from the E-pure system for at least 20 min. The CaF2 window was soaked in the acid bath for no more than 15 min before being rinsed in water from the E-pure system. Daily, clean layers of CCl4−H2O were cycled through the cell to ensure cleanliness before spectra of the sample of interest were obtained. The laser system and experimental setup used in these studies have been described in detail elsewhere,37 except for the use of the sample cell described above. Briefly, a commercially available Nd:YAG laser (Ekspla) was used to generate 1064 nm pulses (∼26 ps, 10 Hz repetition rate). After amplification, a portion of these pulses was sent through an SHG crystal to generate a 532 nm beam. Part of this beam was sent to the interface to be used as the visible beam. The rest of the 532 nm beam along with the rest of the 1064 nm beam was sent into a standard OPO/OPG/DFG system (Ekspla) to generate a tunable IR beam (2−10 μm). When a total internal reflection geometry was utilized, the tunable IR beam (∼75° from the surface normal) and 532 nm visible beam (∼57° from the surface normal) were coherently overlapped at the CCl4−H2O interface. These angles precluded the overlap of the incident beams at the interface of the CaF2 window. Great care was taken to ensure that the IR energy in the CH/OH stretching region was kept well below energies that had been shown to cause heat-induced changes in adsorbed polymers. The generated sumfrequency signal was first sent to a monochromator (model MS2001) and then detected with a PMT (Hamamatsu R7899). VSF spectroscopy is a well-established surface-selective spectroscopy technique, and the theory behind it has been described in detail.38−40 A brief description is provided here as it pertains to these experimental studies. The detected sum-frequency signal is proportional to the intensities of the two incident beams (I(ωvis) and I(ωIR)) as well as the square of the second-order susceptibility (χ(2)) according to eq 1.

isomer (sPMA) the configuration alternates for every monomer. The chemical structures of the two isomers of PMA are shown in Figure 1. Both experimental30−34 and

Figure 1. Chemical structures of isotactic poly(methacrylic acid) (iPMA, left) and syndiotactic poly(methacrylic acid) (sPMA, right). For the experimental studies, n = 34.

theoretical30,35 studies have shown that the different arrangement of functional groups along the polymer chains for the different PMA isomers renders iPMA more hydrophobic than sPMA. The difference in hydrophobicity has the potential to affect the polyelectrolyte interfacial adsorption and assembly greatly. For the experimental work presented here, both isomers of PMA (molecular weight = 3 kDa, n = 34) were studied at pH 2, where the vast majority of the carboxylic acid groups are protonated.33 Under these solution conditions, previous studies have shown the bulk conformations of iPMA and sPMA to be somewhat different.30,31,35 Specifically, sPMA is in a compact coil structure in which its methyl groups reside more on the interior and its carboxylic acid groups reside more on the exterior, and iPMA is in a more extended conformation in which its methyl groups are more exposed to the aqueous phase than they are for sPMA.30,31,35 The differences in these isomers allow for exploring how the polyelectrolyte bulk conformation of PMA relates to its interfacial structure. Previous studies in this laboratory have shown such differences in the adsorption behavior of iPMA at the oil−water interface compared to that of the atactic isomer of PMA (aPMA).36 These studies demonstrated the ability of aPMA to form multilayers at the interface, whereas iPMA adsorbs only as a single monolayer. These studies, however, did not go into great detail concerning the structures or conformations of these polyelectrolytes that would lead to multilayer formation versus monolayer formation. The combined VSF spectroscopic and computational studies present here provide a detailed picture as to how polyelectrolyte structure at the oil−water interface can lead to very different adsorption processes. Specifically, this study clearly demonstrates that a simple change in the tacticity of the backbone configurations plays a major role in the oil−water interfacial adsorption and assembly of PMA.

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I(ωSFG) ∝ |χ (2) |2 I(ωvis) I(ωIR )

(1)

which is χ is the sum of a nonresonant component dependent upon the hyperpolarizability of the interfacial material, and the sum of all resonant components (χ(2) RV ) that arise when the frequency of the IR beam is coincident with an allowed sum-frequency vibrational transition (eq 2). In these studies, χ(2) NR was found to be negligible. (χ(2) NR),

(2)

(2) χ (2) = χNR +

∑ χR(2)v

(2)

v

tensor, different To probe different components of the χ polarization combinations of the incident and detected beams can be employed. For these studies, the ssp polarization combination (s, sum frequency; s, visible; and p infrared, respectively) was used to probe vibrational modes having components of their IR transition moment normal to the plane of the interface. All spectra shown are an average of at least 300 laser shots per data point, taken from at least 3 different spectra obtained on 3 different days, and were normalized by dividing the raw spectra by nonresonant gold spectra in the corresponding spectral regions. This accounts for changes in timing and overlap as the IR beam frequency is tuned. The spectra were fit to a convolution of a Gaussian and a Lorentzian distribution as described by Bain et al.41 (eq 3). Fitting parameters and errors for all spectra can be found in the Supporting Information. (2)

EXPERIMENTAL SECTION

Materials and Sample Preparation. Syndiotactic poly(methacrylic acid) (3 kDa, PDI = 1.16) and isotactic poly(methacrylic acid) (3 kDa, PDI = 1.3; 28 kDa, PDI = 1.25) were purchased from Polymer Source and used as received. Samples were prepared in glassware that had been soaked in a NoChromix−sulfuric acid bath for at least 12 h and was subsequently rinsed extensively with water (18.2 MΩ cm) that had passed through an E-pure water filtration system. The 100 ppm polymer stock solutions were prepared by dissolving 0.01 g of either iPMA or sPMA in 100 mL of water from the E-pure system with 150 μL of 1 N NaOH (volumetric standard, purchased from Aldrich). Solutions were adjusted to pH 2 using 37% HCl (ACS grade, purchased from Sigma-Aldrich). CCl4 (HPLC grade ≤99.9%, purchased from Sigma-Aldrich) was twice distilled before use. Sum Frequency Experiment and Background. The sample cell was machined from a solid piece of Kel-F with a CaF2 window facing

|χ (2) (ωSF)|2 = (2) iϕNR χNR e

+

∑∫ v

∞

−∞

2

A v e iϕve−[(ωL − ωv)/ Γv] dω L ωL − ωIR − iΓL

2

(3)

Interfacial Tension Measurements. Interfacial tension measurements were performed using a pendant drop tensiometer (KSV) that consisted of a camera and an LED backlight to ensure good 14227

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photograph resolution. For the measurements, an aqueous drop containing the sample of interest was dispensed from a 1 mL Hamilton gastight syringe with a Hamilton repeating dispenser. The formed drop was suspended from a 22 gauge hooked Kel-F hub needle and placed in a 1 × 1 cm2 quartz cuvette filled with approximately 4 mL of CCl4. Samples were measured by recording a photograph of the drop every minute until the interfacial tension no longer changed with time. Using the software provided with the instrument, we first fit the drop shape to the Laplace−Young equation to determine the shape factor, β, which was then used to calculate the interfacial tension, γ, according to eq 4, in which Δρ is the density difference between the two liquids, g is the gravitational constant, and R02 is the square of the radius of the drop curvature at the drop apex. γ=

ΔρgR 0 2 β

(4)

Molecular Dynamics Methods. Classical molecular dynamics (MD) calculations were performed using the Amber 12 suite of programs.42 Starting configurations were created using the PACKMOL43 program. PMA polymers 12 or 24 monomer units long were respectively placed in a 30 or 40 Å cube of water adjacent to a 30 or 40 Å cube of carbon tetrachloride. A slab system was created by an extension of the box dimension perpendicular to the water−oil interface and the application of periodic boundary conditions. Simulated annealing was performed to generate multiple starting polymer conformations. Neither the polymer length, the solvent box size, nor the starting conformation had a significant effect on the behavior calculated. The calculation parameters and force field are the same as those detailed in earlier studies.44,45 Quantum Mechanical Methods. Density functional theory (DFT) calculations were performed using the NWChem46 and Gaussian47 program packages. Full geometry optimization and frequency calculations for PMA monomers (capped with H atoms with a nuclear mass of 12.01) were performed using the B3LYP exchange-correlation functional and a 6-311++G(2d,2p) basis set. Polarizabilities and dipole moments at displaced geometries were calculated using the same level of theory. Anharmonic corrections to vibrational frequencies were afforded by second-order vibrational perturbation theory. Calculated vibrational frequencies are not scaled. Vibrational sum-frequency intensities were calculated by inspecting the second-order nonlinear susceptibility tensor. The tensor was constructed using polarizability and dipole moment derivatives with respect to the vibrational normal coordinates, combined according to χijk(2) ∝

∑ Cabc a,b,c

∂αab ∂μc ∂Q q ∂Q q

Figure 2. (A) VSF spectra (ssp polarization) of 5 ppm sPMA (black) and iPMA (red) at pH 2 in the carbonyl stretching region. The solid lines are fits to the data. (B) Cartoon depicting the two different solvation environments of the carbonyl groups, which in either isomer may point into the water phase (right) or the oil phase (left), corresponding to the peaks near 1730 cm−1 (left) and 1790 cm−1 (right).

This higher-energy peak has not been observed in previous studies of poly(acrylic acid) at the oil−water interface,36,53,54 indicating that the greater degree of hydrophobicity of PMA allows for a larger fraction of carbonyl groups to reside in the oil phase compared to the number residing in poly(acrylic acid). Figure 2B shows a cartoon of the two different carbonyl solvation environments. The larger amplitude for the peak near 1790 cm−1 for iPMA compared to that for sPMA indicates that there are more oriented carbonyl groups in the oil-rich environment for the adsorbed iPMA than there are for sPMA. This is consistent with bulk studies that suggest that iPMA is overall more hydrophobic than sPMA30,31,33−35 and therefore may sit further into the oil phase than sPMA. Results of the molecular dynamics studies further support this picture. The calculated density profiles of iPMA and sPMA from two separate calculations at the oil−water interface are overlaid and shown in Figure 3. On average, iPMA sits nearly an angstrom further into the oil phase than does sPMA. The positions of the individual monomers show similar behavior. The spectra in the CH/OH stretching region for each PMA isomer, shown in Figure 4, also confirm the adsorption of highly ordered polymer layers to the oil−water interface. The structure of the polymer that is adsorbing to the interface, therefore, is not the disordered coiled conformation of PMA that has been shown to exist in bulk solution.33,34 If such disordered conformations did adsorb to the interface, then they

(5)

where α is the molecular polarizability, μ is the dipole moment, Qq is the normal coordinate of mode q, and C is a geometrical factor relating the molecular and laboratory reference frames. Derivatives were calculated using three-point finite differentiation.

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RESULTS AND DISCUSSION VSF spectra obtained for both sPMA and iPMA at pH 2 in the carbonyl stretching region are shown in Figure 2. Given the selection rules for VSF spectroscopy, the presence of signal for each isomer indicates that both iPMA and sPMA adsorb to the CCl4−H2O interface and that their carbonyl functional groups are highly ordered normal to the plane of the interface. The spectra were fit to two out-of-phase peaks, one near 1730 cm−1 and one near 1790 cm−1. As shown previously in this laboratory,48 the lower-energy peak near 1730 cm−1 is attributed to carbonyl groups that point into the aqueous phase and are in a more water-rich, hydrogen-bonded environment,49−51 whereas the higher-energy peak centered at 1790 cm−1 is attributed to carbonyl groups that point into the oil phase and are thus in a more oil-rich environment.52 14228

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for the peak Gaussian widths and amplitudes. Assignments are made comparing spectral fits to computational results and literature precedent.49 Peaks due to the adsorbed polymers specifically appear near 2527 cm−1 for the carboxylic OH stretching mode and near 2886, 2930, 2933, and 2978 cm−1 for the methyl and methylene stretching modes. Additional peaks appear near 3100 and 3400 cm−1 and are assigned to the OH stretching modes of the coordinated interfacial water molecules.57 A comparison with calculated spectra results is presented in Table 1. The peaks are assigned to the methyl and Table 1. Experimental and Calculated CH Peak Frequencies and Intensities for sPMA and iPMA exp int

Figure 3. Density profiles of iPMA (red) and sPMA (black) overlaid from two separate calculations at the carbon tetrachloride (green)/ water (blue) interface. Water and carbon tetrachloride density profiles are from the iPMA calculation.

calcd int

exp frq (cm−1)

sPMA

iPMA

calcd frq (cm−1)

sPMA

iPMA

2886 2930 2933 2978

1.6 5.2 2.0 1.4

1.9 5.4 3.3 0.02

2890 2916

0.1 7.7

0.04 11

2977

11.8

2.7

methylene groups of the polymer end-caps (2886 cm−1) and the methyl (2930 and 2933 cm−1, symmetric stretches) and methylene (2978 cm−1, symmetric stretch) groups of the polymer backbone. The experimental fits show that the amplitudes for the peaks near 2527, 2930, and 2933 cm−1 are significantly larger for iPMA but the peak near 2978 cm−1 is significantly larger for sPMA. The calculated VSF spectra show a similar trend, with iPMA having a significantly larger peak near 2916 cm−1 that is due to the methyl symmetric stretching mode. This intense peak is due the highly ordered methyl groups that are solvated by the oil phase. Conversely, sPMA has a significantly larger peak near 2980 cm−1, which is due to the methylene symmetric stretching mode. The backbone methylene groups are more oriented into the oil phase than the methyl groups because some of the methyl groups are oriented into the aqueous phase. The overestimation of the intensity of the peak near 2980 cm−1 in the calculations and the minor differences in peak position are attributed to numerous factors, including the vibrational coupling between adjacent monomers that is not accounted for in the current implementation. An analysis of the molecular dynamics trajectories indicates that the backbone configurations of iPMA allow the hydrophobic methyl moieties to be completely solvated by the oil phase and the carboxylic acid OH groups to be completely solvated by the water phase. For sPMA, however, to solvate the carboxylic acid OH groups, the preferred backbone configuration causes some of the methyl groups to point into the aqueous phase. These differences are consistent with the differences seen in the VSF spectra of the two isomers in the CH/OH stretching region. Figure 5 displays the orientational distributions of the CO and C−CH3 bonds of iPMA and sPMA monomers near the interface. For iPMA, two strong groupings are found. In both, the distribution of the methyl orientation is centered between 30 and 45° from the surface normal, pointing strongly into the oil phase. The CO bond is found to lie in the interfacial plane (∼90°) or to point strongly into the aqueous phase (∼150°). In the case of sPMA, there are also two major distributions. In contrast to iPMA, the CO bond in both of the major distributions is mainly in the interfacial plane whereas the methyl group points either

Figure 4. VSF spectra (ssp polarization) of 5 ppm sPMA (black) and iPMA (red) at pH 2 in the CH/OH stretching region. The solid lines are fits to the data.

would produce an insignificant VSF signal. The intense signal observed for both PMA isomers is attributed to highly ordered polymers at the interface and indicates that the adsorbed polymer chains are highly extended rather than existing in more disordered coiled conformations. Specifically, the broad peak near 2500 cm−1 is assigned to the OH stretching mode of the polymer carboxylic acid groups55 and indicates the high degree of orientation of the carboxylic acid groups at the interface that was also observed in the carbonyl stretching region. The sharp peaks between 2800 and 3000 are assigned to the CH stretching modes of the polymer methyl and methylene groups49 and indicate a high degree of order of the polymer backbone and side-chain groups. Additionally, the absence of the free-OH mode near 3670 cm−1 in the spectra of both isomers signifies that the interfaces are completely covered by polymer.38,56 Even though both sPMA and iPMA show a high degree of order at the oil−water interface, is clear from the CH/OH stretching region spectra in Figure 4 that differences in interfacial behavior exist for the different PMA isomers. To determine the significant differences between the iPMA and sPMA CH/OH stretching region spectra, spectra were fit using a global routine in which all variables were held constant except 14229

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Figure 6. MD snapshots of the interfacial conformations of the monomers of iPMA (left) and sPMA (right). Polymers are represented with a space-filling model with red oxygen, green carbon, and white hydrogen atoms. Stick bond structures represent water (red) and carbon tetrachloride (green). Images were generated with VMD.58

the methyl groups pointing toward the oil phase. The natural backbone bend in the syndiotactic isomer prefers to be perpendicular to the interfacial plane with the methylene CH bonds pointing into the oil and both methyl and carboxylic moieties pointing into the water phase. As discussed previously, differences in VSF peak amplitudes can be due to a difference in interfacial populations, orientations, or both factors. To decouple these factors and better interpret the spectra in Figure 4, interfacial tension measurements were obtained for each isomer and are shown in Figure 7. As seen in our previous studies of iPMA and aPMA,36

Figure 7. Surface pressure vs time data for 5 ppm sPMA (black) and iPMA (red) at pH 2. The error bars represent the standard error.

the surface pressure behavior is markedly different for each PMA isomer. For iPMA, the surface pressure increases only slightly to ∼0.5 mN/m in the first few seconds, and then does not noticeably change with time. For sPMA, the surface pressure increases to ∼16 mN/m over ∼800 s and then does not noticeably change with time. The time dependence observed in the surface pressure data of sPMA is not reflected in the VSF spectroscopic data of either isomer. The VSF signal is seen within a minute of interfacial preparation and does not significantly change with time. Because VSF spectroscopy can detect only molecular functional groups with a net orientation, the changes that appear in the surface pressure over time for sPMA are attributed to the adsorption of disordered polymer to the interface. For iPMA, where no large changes in surface pressure are seen over time, polymer does not appear to adsorb beyond the initial monolayer.

Figure 5. Coupled angular distributions of the C−CH3 and CO bonds relative to the surface normal pointing into the oil phase. Data for monomers within 1.5 Å of the interface.

strongly into the oil phase (∼15°) or moderately into the aqueous phase (∼135°). Snapshots from the dynamics calculations shown in Figure 6 display representative structures of iPMA (left) and sPMA (right) at the interface. The monomers in these structures have orientations that correspond to points near the maxima in Figure 5. The short polymer chains in the calculations show a strong preference for an extended zigzag backbone conformation when at the interface. The isotactic isomer prefers to have the natural bend of the backbone in the interfacial plane with 14230

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seems counterintuitive that sPMA continues to adsorb to the interface over time yet iPMA does not. An explanation of this phenomenon is that the characteristics of the initially adsorbed layer dictate whether multilayer formation can occur. This is consistent with the theoretical calculations as well as with the fact that the VSF spectra in the CH/OH stretching region for sPMA and iPMA appear to be very different from each other, showing that the initially adsorbed polymer layers are very different. A picture consistent with the spectroscopic, interfacial tension, and computational results is one in which iPMA initially adsorbs to the interface with its methyl groups highly solvated by the oil phase and its carboxylic acid OH groups highly solvated by the water phase. This is supported by the relatively large peak due to the carboxylic acid OH groups seen in the CH/OH spectrum of iPMA. Because such highly hydrophilic groups point into the aqueous phase, it is unlikely for favorable hydrophobic interactions to occur between this initially adsorbed polymer layer and the polymer that remains in bulk solution. sPMA, however, initially adsorbs to the oil− water interface in a manner such that some of the methyl groups point into the aqueous phase. These hydrophobic moieties are exposed to remaining polymer in bulk solution, promoting hydrophobic interactions between the initially adsorbed sPMA and the sPMA that remains in solution, allowing sPMA to adsorb to the interface over time.

A combination of the VSF spectroscopic and interfacial tension data shows a quick initial adsorption and elongation of the polymers from the coiled structures in the bulk. The resulting highly ordered structures give rise to the strong VSF signal corresponding to CH, carbonyl, and carboxylic acid OH stretching modes. This process is too fast to be observed with spectroscopy. For sPMA, polymer in a disordered conformation, potentially similar to its bulk conformation, continues to adsorb to the interface over time. For iPMA, no further accumulation occurs. Whether further accumulation occurs cannot be detected with VSF spectroscopy but is detected with the surface pressure measurements. A cartoon depicting these differences is shown in Figure 8.

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SUMMARY AND CONCLUSIONS The combined VSF spectroscopic, interfacial tension, and computational studies presented in this work clearly demonstrate how polyelectrolyte backbone structure affects the adsorption behavior and thus degree of accumulation of PMA at the oil−water interface. These results specifically depict a picture in which sPMA and iPMA initially adsorb to the oil− water interface as highly ordered polymer layers, with a net orientation of the carboxylic acid, methyl, and methylene functional groups normal to the interfacial plane. The extended nature of the polymers at the interface exists in contrast to the bulk coiled and disordered conformations of these polymers, as is especially true for sPMA. The process of adsorption and spreading of the initial thin polymer layer at the oil−water interface occurs so quickly that it cannot be monitored with spectroscopy. This rapid adsorption of highly ordered polymer to the oil−water interface is in part attributed to the inherent interfacial field that assists in the assembly of the polyelectrolyte.37,55 The interfacial tension studies indicate that sPMA continues to adsorb to the interface over several minutes in a disordered manner because the field is likely unable to penetrate deeply enough into the interface to cause ordering of this later-adsorbing material. iPMA, however, does not appear to accumulate at the interface over time. This is attributed to favorable hydrophobic interactions existing between the initially adsorbed sPMA layer and the sPMA that continues to adsorb to the interface, which are not as prevalent in the iPMA system. Importantly, computational results, which are consistent with the experimental data, indicate that the conformation of the initially adsorbed sPMA allows for further polymer to adsorb to the interface over time but that the conformation of iPMA does not. These results have implications for research aimed at either utilizing natural polyelectrolytes or designing synthetic ones that have the ability to accumulate at oil−water interfaces, either for the purpose of stabilizing emulsions, recovering oil to

Figure 8. Cartoon representing the differing interfacial behavior of sPMA (top) compared to iPMA (bottom). The black coils represent PMA with bulklike conformations. The accumulation of additional disordered polymer at the interface after the first ordered layer is seen for sPMA but not iPMA.

Multilayer formation has been observed in other polyelectrolyte systems at the oil−water interface.36,48,53,54 In these previous studies, the initial quick adsorption of a highly ordered and extended polymer layer to the interface was attributed to the inherent CCl4−H2O interfacial field. It was demonstrated that this field assists in the quick adsorption of highly oriented material to the oil−water interface. This field does not penetrate deep enough into the bulk water to orient the more slowly adsorbing material, in part due to neutralization by the polymer that initially adsorbs to the interface, causing the more slowly adsorbing polymer to adsorb to the interface in a disordered manner. That the initial layer does not significantly increase the surface pressure was concluded to be due to the entropy loss associated with the restricted conformation of the extended polymer at the interface.59 For these previous studies and also for studies of protein adsorption to the oil−water interface,60 the ability of macromolecules to adsorb as multilayers was attributed to favorable hydrophobic interactions between the initially adsorbed layer and the layers that subsequently adsorb to the interface. These same conclusions can be applied to the systems under study here, in which the interfacial field assists in the quick adsorption of ordered polymer layers to the interface for both PMA isomers but favorable hydrophobic interactions occur only between the initially adsorbed sPMA layer and the sPMA that continues to adsorb to the interface over time. As discussed previously, bulk studies have shown that iPMA is overall more hydrophobic than sPMA.30,31,33−35 Thus, it 14231

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be used as a fuel source, or remediating contaminated water supplies. As shown here, not only does the chemical nature of the monomer play a role in the ability of a polyelectrolyte to adsorb to the interface but also the specific stereochemistry of monomers along the polymer chain and the resulting interplay of the structure with the interface dictate whether the adsorption is that of a signal layer or multilayers. Incorporating stereocenters into synthetic polyelectrolytes or selecting natural polyelectrolytes with specific stereochemistries can thus serve as a means to control the degree of polyelectrolyte accumulation at an oil−water interface.

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ASSOCIATED CONTENT

S Supporting Information *

Fitting parameters and associated errors in VSF spectral fits. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 541-346-3422. Phone: 541-346-0116. Author Contributions

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Author Contributions

N.A.V. and E.J.R. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The computational work has been supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under award DE-FG02-96ER45557, and the initial PMA experimental studies have been supported by the National Science Foundation (CHE-65231). We thank Professor Fred Moore of Whitman College for helpful discussions.

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REFERENCES

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