Insight into Water Structure at the Surfactant Surfaces and in

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Insight into Water Structure at the Surfactant Surfaces and in Microemulsion Confinement Chayan Dutta,† Anton Svirida,† Muhammet Mammetkuliyev,† Marina Rukhadze,‡ and Alexander V. Benderskii*,† †

Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States Faculty of Exact and Natural Sciences, Ivane Javakhishvili Tbilisi State University, 3 I. Chavchavadze Avenue, Tbilisi 0128, Georgia

‡

S Supporting Information *

ABSTRACT: Interactions with surfactant molecules can significantly alter the structure of interfacial water. We present a comparative study of water−surfactant interactions using two different spectroscopic approaches: water at planar surfactant monolayers by sum frequency generation (SFG) spectroscopy and interfacial water confined in reverse micelles formed by the same surfactants using IR absorption spectroscopy. We report spectral features in the OH-stretching region (3200− 3700 cm−1) that are observed in both IR and SFG spectra, albeit with different relative amplitudes, for ionic surfactant sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT) and nonionic surfactant polyoxyethylene(4)lauryl ether (Brij L-4) reverse micelles in hexane and the corresponding monolayers at the air/water interface. A prominent feature in the SFG spectra of the OH stretch at 3560 cm−1 is attributed to water molecules that have a weak donor hydrogen bond to the surfactant headgroup. The same feature is observed in the IR spectra of reverse micelles after deconvoluting the interfacial versus bulk spectral contributions. We performed an orientational analysis of these water molecules utilizing the polarization-dependent SFG spectra, which shows an average tilt angle of the OH stretch of surfactantbound water molecules of ∼155° with respect to the surface normal.

I. INTRODUCTION Water is an omnipresent medium for many chemical and most of the biological processes. Water’s hydrogen-bonding interactions with a variety of interfaces are the key for a fundamental understanding of these systems. Biological membranes and biomolecules,1−6 micelles7 and reverse micelles,8−11 protein folding,12,13 and catalysis14,15 are a few examples in which confined water interacts strongly on the molecular scale with the interfacial molecules or ions. Biomembranes are complex structures formed by lipid bilayers with an array of aggregated lipids arranged in two dimensions, wherein the polar headgroups remain exposed to the aqueous phase and the lipid chains face each other. Confined water in reverse micelles provides a good model system for complex biological systems, such as biomembranes.2,6 Reverse micelles having one lipid/ water interface can be thought of as a simplified biomembrane model with similar types of interactions at the interface. The same interactions occur at the planar surfactant monolayer interfaces but without the geometric nanoconfinement effects (Figure 1). Ionic (sodium 1,4-bis-2-ethylhexylsulfosuccinate: AOT) and nonionic (polyoxyethylene(4)lauryl ether: Brij L-4) surfactants form spherical molecular aggregates in nonpolar organic solvents, known as reverse micelles. These reverse micelles form a nanometer-sized water pool confining a number of water molecules inside the cavity, where the charged hydrophilic © 2017 American Chemical Society

headgroups are in close contact with the enclosed water molecules and the organic tail groups are oriented toward the organic phase. The physical and chemical properties of these reverse micelles are highly sensitive to the w0 number (w0 = [H2O]/[surfactant]), the ratio of molarities of water and surfactants. Vibrational spectroscopy in the OH-stretching region is a convenient and powerful tool for studying water structure and dynamics in the bulk and at interfaces. Bulk water has been studied using a diverse range of linear and nonlinear techniques both theoretically and experimentally.16−19 The OH-stretching vibration of a water molecule is very sensitive to its hydrogenbonding environment in the bulk and can be delocalized over 10 or more surrounding water molecules.18,20 Intermolecular and intramolecular couplings between the OH stretches play a central role in determining the vibrational line shapes. OHstretching spectra also show a strong non-Condon effect, that is, the transition dipole strongly depends on the transition frequency.16,21 The inherent properties of water molecules at the interface are expected to be different from those of bulk water, primarily due to the asymmetry of the interface. Surfacesensitive nonlinear vibrational sum frequency generation Received: May 16, 2017 Revised: June 30, 2017 Published: July 6, 2017 7447

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of the surfactant/water interface can provide additional information on the molecular orientation. Water-insoluble ionic and nonionic surfactants, when placed on the water surface, are expected to orient with the polar headgroups submerged in the topmost layer of water and the nonpolar organic groups pointing outward at the air phase. The water structure at these interfaces should mimic the environments of the water pools inside the reverse micelles. In general, as the number of hydrogen bonds around a water molecule increases, the stretching vibration of that water shifts toward a lower frequency. In this article, we present a comparative study of the IR absorption and VSFG of interfacial water in reverse micelles and Langmuir monolayer, respectively. Besides, we note that the VSFG and IR spectra are not directly comparable because they are governed by different selection rules (VSFG by orientation-averaged second-order nonlinear susceptibility and IR by the transition dipole moment). Nevertheless, spectral features at the same frequency are expected to be observed in the Fourier transform infrared (FTIR) and VSFG spectra if the same H-bonded species are present in both systems. In particular, our polarization-dependent VSFG experiments indicate the presence of water molecules with a central frequency around 3560 cm−1, similar to the interfacial feature in the FTIR spectra, which is likely an OH group weakly bound to a surfactant headgroup inside the water pool of ionic and nonionic reverse micelles. We have performed an orientational analysis on this type of water molecules to extract the average tilt angle of this particular type of water for a detailed understanding of the interfacial water structure.

Figure 1. Interfacial and bulk water in reverse micelles (top left) and surfactant monolayer/water interfaces (top right) used in this study. Structures of sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT, bottom left) and polyoxyethylene(4)lauryl ether (Brij L-4, bottom right).

(VSFG) spectroscopy has been successfully implemented to study structure and dynamics at the air/water interface.3,4,22−27 VSFG is a nonlinear spectroscopic technique with a monolayer surface specificity.28 The overall sum frequency generation (SFG) spectrum, which is polarization dependent, is formed by contributions from all of the different types of water hydrogenbonding classes possible at the interface, as calculated by Skinner et al.17,19 A reverse micelle in an organic phase can be directly studied by sum frequency scattering29,30 to obtain the structural and orientation information of water molecules in it; however, this is highly dependent on the size of the particles, and for small-radius particles, the scattering process will be highly inefficient. Analysis of the OH-stretching band of water in confined systems like reverse micelles provides information about the molecular interactions of water at the interface of nanoassemblies. According to the popular core/shell model31 of nanoconfined water in reverse micelles, there are two distinct regions inside the reverse micelle: the core water is of bulk character, which absorbs at the red side of the spectrum, and the interfacial water, which absorbs at the blue side due to hydrogen-bonding interaction with the surfactant headgroup. Sechler et al.32 determined the spectral contributions of these different water molecules from the core and interface of these reverse micelles to the overall absorption using volumetric analysis with IR absorption spectroscopy. The structure of interfacial water in the presence of soluble ionic and nonionic surfactants has also been studied extensively using VSFG spectroscopy.33−35 Because of the presence of charged surfactants at the interface, the surface waters become highly ordered, which in turn produces a stronger SFG signal from the surfactant/water interface compared to that from the pure air/water interface. Most of these studies are focused on the SSP polarization spectra, which probe water molecules and only have a component of transition dipole moment perpendicular to the surface plane. However, there are other polarization combinations (e.g., PPP) that can access water molecules with components both parallel and perpendicular to the surface plane. Hence, a polarization-dependent VSFG study

II. EXPERIMENT II.I. Preparation of the Reverse Microemulsion. AOT is an ionic surfactant with a negatively charged headgroup, whereas Brij L-4 is a nonionic surfactant. Structures of both the molecules are shown in Figure 1. Reverse microemulsion solutions were prepared with AOT and Brij L-4 surfactants in hexane (high-performance liquid chromatography grade) with varying water content. The molar ratio of water to surfactant was calculated by w0 = [H2O]/[surfactant]. Transparent onephase solutions of the surfactants in hexane were achieved through gentle sonication. All spectroscopy experiments were performed on stable, one-phase systems at room temperature. The reverse micelle diameter was measured using the dynamic light scattering technique, and the water pool radius for different solutions was calculated and is presented in detail in the Supporting Information. II.II. Spectroscopic Studies. II.II.I. Volumetric Analysis with IR. IR absorption spectra were recorded in the water−OHstretching band (3000−3800 cm−1) in a Bruker Vertex 80 IR spectrometer with vacuum capability equipped with a 0.05 cm path length calcium fluoride (CaF2) window. II.II.II. SFG Experiment. We used broad-band SFG spectroscopy to probe a wide range of vibrational frequencies. A schematic of our experimental setup has been described previously in detail. 36 Our broad-band mid-IR pulses (bandwidth ∼350 cm−1) were generated by an optical parametric amplifier (OPA) followed by a difference frequency generator with a GaSe crystal. The OPA was pumped with one portion of the Ti/sapphire amplifier output (∼50 fs, 1.8 mJ/ pulse) operating at a 5 kHz repetition rate. Another portion of the amplifier output (∼1.2 mJ/pulse) was passed through a 4f stretcher, and a narrow-band region (spectral resolution ∼20 cm−1) was selected spatially using a mechanical slit. The use of 7448

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Figure 2. Left panel: IR absorption spectra of OH-stretching band in AOT/hexane emulsions for multiple w0 values. Right panel: Absorption spectra normalized by total number density of the AOT molecules for the same w0 values. Z is the sample thickness, and N is the overall sample number density. The values of w0 and N for different reverse micelle solutions are tabulated according to the color of the corresponding spectra.

Figure 3. Left panel: IR absorption spectra of OH-stretching band in Brij/hexane emulsions for multiple w0 values. Right panel: Absorption spectra normalized by total number density of the Brij molecules for the same w0 values. Z is the sample thickness, and N is the overall sample number density. The values of w0 and N for different reverse micelle solutions are tabulated according to the color of the corresponding spectra.

a 4f stretcher gives us control over the power and bandwidth of the visible pulse. Both the visible and IR pulses were focused on the sample surface to a spot size of ∼160 μm. The laser powers at the sample surface were 1 and 12 μJ per pulse for the IR and visible pulses, respectively. The angle of incidence from the surface normal was 67° for the visible and 62° for the IR beams. All of the spectra were collected for SSP and PPP polarizations (SFG, visible, IR) and recorded with a 500 nm monochromator (Princeton Instruments, 1800 g/mm grating) and a liquidnitrogen-cooled charge-coupled device detector (Roper Scientific). The SFG spectra are corrected for the scattering of the 800 nm light (recorded by blocking IR light only and subtracted from the raw SFG data). To obtain clean water for the samples, we used 15 MΩ water from our Millipore system and distilled it through a sealed distillation apparatus cleaned with piranha solution before every measurement. AOT and Brij L-4 solutions in hexane were prepared, and a small volume of the solution was drop-cast on the pure water subphase so that the surface density was 1.3 × 1014 molecules/cm2 for both surfactants. Hence, the area per molecule of the surfactant was ∼75 Å2/molecules in both cases. SFG measurements on these surfaces were performed after allowing the hexane solvent to evaporate completely. To record the whole spectrum in the water-stretching region (3100−3800 cm−1), we have to tune the central frequency of the IR pulse in two different regions (3300 and 3600 cm−1

center frequency) to overcome the bandwidth limitation in our experiments. The two spectra recorded at the two different central frequencies were joined using MATLAB programming, details of which are given in the Supporting Information. Curve fitting of the actual measured spectra with the resonant and nonresonant parts is necessary to get the actual frequencies of different transitions. The second-order nonlinear susceptibility, χ(2), is expressed as a sum of Lorentzians, and the nonresonant part is fitted as a constant term with a phase difference in the real part, as shown in eq 1. χ (2) ∝ ANR e iΦNR +

∑ j

Bj ωIR − ωj + iΓj

(1)

where ANR is the nonresonant contribution to the overall spectra with a phase difference, ΦNR, from the resonant parts and Bj is the resonant amplitude with center frequency ωj and linewidth Γj.

III. RESULTS AND DISCUSSION We prepared a reverse micelle microemulsion of AOT and Brij L-4 in hexane with varying water content and performed a volumetric analysis to separate the bulk and interfacial water spectral responses in the OH-stretching region following the procedure developed by Sechler et al.32 The reverse micelle is made of a spherical pool of water surrounded by a surfactant 7449

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IR studies show that there are two distinctly different regions of water with various degrees of hydrogen bonding inside the reverse micelle. The core water molecules behave more like bulk waters and absorb more on the red side of the spectrum, whereas the water molecules that are in close proximity to the surfactant headgroups absorb more on the blue side. Detailed structural and orientational information of these interfacial waters in close proximity to the surfactant headgroups can be obtained by surface-selective SFG spectroscopy on surfactant monolayers. Usually, pure air/water spectra give a double-hump structure in the water stretch (3200−3600 cm−1) band of the SFG spectra. Several other groups have performed SFG experiments (homodyne 26,37−40 and phase sensitive25,27,35,41) on the air/water interface and fitted the spectra with four Lorentzians. The sharp spectrally separated peak on the blue side is attributed to “free water” molecules having one −OH dangling at the interface. The other three frequencies are well separated from the free −OH bond due to stronger Hbonding interactions. It is difficult to assign these broad regions to a particular kind of molecules. Skinner and co-workers calculated the spectral densities of different types of H-bonding molecules for pure air/H2O23 and air/D2O22,24 interfaces. They also defined different H-bonded classes of water molecules, according to the total number of donor and acceptor hydrogen bonds. In general, as the H-bonding interactions between adjacent water molecules become stronger, the OH-stretching frequencies decrease. The spectral intensity around 3700 cm−1 is attributed to “free-OH” molecules, specifically to 2S and 1N types of molecules, according to Skinner’s definition, where 2S represents water molecules having two hydrogen bonds with a single donor hydrogen and 1N represents a type of water having only one hydrogen bond without any donor hydrogen. The other OH-stretching frequency in the same free water molecule at the air/water interface is assigned25,37,42 to be around 3550 cm−1. Figure 5 shows the SFG spectra of the surfactant/water interface in SSP (red) and PPP (blue) polarizations and the separated IR response (green) of the interfacial water obtained from volumetric analysis. Both the AOT/water (Figure 5, left panel) and Brij/water (Figure 5, right panel) interfaces show four resonant contributions in the hydrogen-bonded region in both SSP and PPP polarizations, but spectral fitting in the Brij/ water interface is somewhat complicated due to the strong nonresonant background signal. However, we have identified the main types of water molecules at both surfaces that resemble the interfacial component in the IR spectra of reverse micelles. At the surfactant interfaces, the free −OH peak is suppressed due to interactions with the hydrophilic surfactant headgroup, so we do not observe a sharp peak in the far blue side of the spectrum (∼3700 cm−1 for pure air/water interface). The other part of the spectra of the AOT/water interface is fitted with four main resonant features at 3230, 3430, 3560, and 3640 cm−1 for both polarizations. For the Brij/water interface, the SSP spectra show two peaks around 3200 and 3400 cm−1, but the 3560 cm−1 peak is buried in the strong background signal. On the other hand, PPP spectra in the Brij interface show most prominently at the 3560 cm−1 peak. Figure 6 compares the spectra for SSP (left panel) and PPP (right panel) polarizations for all three interfaces: AOT, Brij, and air/water. The PPP polarization spectra from the ionic and nonionic surfactant interfaces show a prominent peak at 3560 cm−1, in clear resemblance to the interfacial component in the IR spectra obtained from the volumetric analysis.

headgroup, with two distinct water phases. For large values of pool radius, water molecules at the center of the pool are more homogeneous and surrounded by other molecules as in bulk water. Water molecules at the edges where they interact strongly with the surfactant are of a truly interfacial character and have a distinctly separate spectral signature. Figure 2 shows the IR spectra of AOT (left panel) and reverse micelle with different w0 values. As the water content increases, the size of the water pool inside the micelle increases and the stronger hydrogen-bonding interactions from the bulk water become dominant, which shifts the overall spectra toward lower frequencies. This effect of increasing bulk hydrogen-bonding interactions can be better understood from the normalized absorption spectra, as depicted in Figure 2 (right panel), where the bulk frequencies dominate the overall spectra as w0 increases. Figure 3 shows the IR absorption spectra for multiple microemulsion solutions of Brij L-4 with varying w0 and the normalized absorption spectra in the left and right panels, respectively. Brij L-4 reverse micelles behave similarly to those of AOT, and the overall water spectra are interface-water-like for smaller micelles and bulk-water-like for larger micelles. Details of the volumetric analysis adopted from Sechler et al.32 are provided in the Supporting Information. Figure 4 shows the spectral responses of the bulk and interfacial water regions determined from the fits of A/zN

Figure 4. Separated spectral responses of bulk (dotted) and interfacial (solid) water regions for AOT (blue) and Brij (red) reverse micelles determined from the fits of A/zN vs r, where r is the radius of different microemulsion solutions. The solid black line shows the pure bulk water IR absorption spectra.

versus R, where z is the sample thickness, N is the number density of the surfactant, and R is the radius of the water droplet inside the reverse micelle. The bulk water responses for AOT and Brij L-4 (blue and red dotted lines, respectively) are very similar to the pure bulk water IR absorption spectrum (solid black line). Even though the interfacial counterpart is blue-shifted from the bulk due to different degrees of hydrogenbonding interactions, as described previously, the interface spectra for the ionic (AOT: blue solid line) and nonionic (Brij: red solid line) micelles are different. Because of stronger charge−dipole interactions with the ionic surfactant headgroup, the AOT/water interface contains a large ensemble of molecules with various degrees of hydrogen bonding. On the other hand, the Brij/water interface has weaker dipole−dipole interactions and most of the hydrogen-bonding interactions resemble those of the bulk water interactions, making it redshifted compared to the ionic interface. 7450

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Figure 5. SFG spectra of OH-stretching band in AOT/water interface (left panel) and Brij/water interface (right panel) for SSP (red) and PPP (blue) polarization combinations of the SFG, visible, and IR pulses. Spectral responses of the interfacial water from IR spectra of the AOT and Brij reverse micelles are plotted in the same graphs for comparison (green lines).

Figure 6. Comparison of the SFG spectra of air/water, AOT/water, and Brij/water interfaces for SSP (left) and PPP (right) polarization combinations of the SFG, visible, and IR pulses.

Figure 7. Isotopic dilution study at the HOD/AOT interface. Homodyne-detected SFG spectra of OD stretch at the HOD/AOT interface. D2O and H2O were mixed at a 1:3 ratio so that D2O/HOD/H2O is 1:6:9. SSP and PPP polarization spectra are plotted in the left and right panels, respectively. Spectra were fit with three resonant Lorentzians with center frequencies at 2540, 2616, and 2670 cm−1.

number density of water molecules at different sites in the zwitterionic lipid interface. They have assigned the lowfrequency band to water molecules bonded to the polar phosphate and the choline group of the POPC molecule, whereas the other high-frequency band was said to be associated with water molecules near the carbonyl oxygens. Neither AOT nor Brij contains any highly polar nitrogen or phosphate centers, but we still observe high-intensity peaks in the low-frequency side of the spectrum in SSP polarization,

This indicates that similar hydrogen-bonding structures may be present at both surfactant interfaces. A detailed study of water/phospholipid interface in homodyne VSFG43,44 and heterodyne-detected VSFG45,46 experiments in SSP polarization demonstrated the presence of two distinct positive bands: a strong band around 3300 cm−1 and a weak band around 3580 cm−1. Recently, Ishiyama et al.47 have calculated the Im[χ2] spectra at the water/POPC (3-palmitoyl-2-oleoyl-Dglycero-1-phosphatidylcholine) interface and provided the 7451

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our orientational analysis. Table S3 shows the β values and the corresponding “C” and “D” parameters for the vibrational mode under consideration in the SSP and PPP polarizations. We simulated the orientational angle dependence of the OH group for our experimental geometry assuming a “δ-distribution” for the tilt angle for the 3560 cm−1 peak. In accordance with the recently published results by Nojima et al.,46 we believe that the water molecules above the negative surfactant headgroup will be oriented downward. Figure 8 shows the change of SFG

with the high-frequency peak more pronounced in the PPP polarization spectra. The position of the 3560 cm−1 peak is constant in the ionic or nonionic surfactant surface, whereas the other two peak positions change. This suggests that this peak originates from a water molecule having one donor hydrogen weakly bound to the surfactant headgroup, as recently suggested by Nojima et al.46 This assignment of the 3560 cm −1 peak is further investigated in the isotopic dilution study using the HOD/ H2O mixture. We measured the SFG spectra from a HOD/ H2O surfactant interface, where the OD stretching frequency will be decoupled from the rest of the system. SFG spectra from the HOD/AOT interface in SSP and PPP polarizations are plotted in Figure 7. Three Lorentzian fitting schemes have been applied to both the spectra with three frequencies at 2540, 2616, and 2670 cm−1 (also shown in Figure 7). The first two peaks appear in the same region as that of the pure air/water interface (peaks at 3430 and 3560 cm−1) when scaled (divided by the isotopic factor ∼1.36)41 for an OD stretch instead of an OH stretch. A low-intensity peak at 2670 cm−1 is observed in both polarizations, as previously reported by Stiopkin et al.25 This peak had been tentatively assigned to D2O molecules with two donor hydrogen and one acceptor hydrogen bonds; however, its contribution to the overall spectra is negligible, and it will not be discussed any further for our present study. The main contribution in the PPP spectra is from the 2616 cm−1 peak, which is from a molecule with one donor “D” and one donor hydrogen bond probably due to the surfactant headgroup. Orientational analysis for this −OH/−OD stretch is performed for both the water/AOT and HOD/AOT interfaces, as discussed in the next section.

Figure 8. Simulated SFG spectral intensity of the 3560/2616 cm−1 bond at different tilt angles (between the OH bond and surface normal) with δ-function orientational distribution for SSP and PPP polarizations at the air/H2O interface (left) and the air/HOD interface (right). The gray line represents the experimental intensity ratios in PPP and SSP polarizations (IPPP/ISSP), which are 2.46 and 1.82, respectively

IV. ORIENTATIONAL ANALYSIS Orientation of a particular molecular group on the surface can be determined from the amplitude ratio of SFG spectra in PPP and SSP polarizations of the same molecular transitions. Gan et al.37,38 studied the orientation of water molecules at the air/ water interface in detail and analyzed the symmetry properties of the four main peaks of the OH-stretching region. Their polarization analysis reveals that the sharp peak at 3700 cm−1 and the broad peak at 3550 cm−1 have C∞v symmetry, whereas the other two broad hydrogen-bonded peaks in the red side belong to the C2v symmetry group. The peak at 3550 cm−1 was assigned to the hydrogen-bonded OH of the interfacial water molecule with a free −OH bond. This peak also appears in the IR spectra of water dimer,48 where the frequency for the donor OH bond is around 3550 cm−1, which in turn supports the peak assignment. In a surfactant/water interface, there is little chance of existence of a free water molecule on the surface or the concentration of those molecules will be so low that it could not be detected by SFG spectroscopy. However, dangling OD was detected in lipid monolayer interfaces by Ma et al.,49 assigned to water molecules in the hydrophobic region in the long hydrocarbon tails of the lipid monolayer. Other VSFG studies provide evidence of dangling OH bonds of water at the water/hydrophobic surfaces.50,51 On the basis of this, we suggest that the peak at 3560 cm−1 in the present study arises due to water molecules having a donor hydrogen weakly bound to the surfactant headgroup. This allows us to perform an orientational analysis on the 3560 cm−1 peak using C∞v symmetry and use hyperpolarizability tensor β values similar to those of the free −OH group.52,53 We have followed the general equations as provided by Gan et al.37,38 for

spectral intensity with tilt angle for both SSP and PPP polarizations. The gray line shows the experimentally observed SSP/PPP ratio. Orientational analysis for both H2O/AOT and HDO/AOT interfaces (3560 cm−1 peak for the OH stretch and 2616 cm−1 peak for the OD stretch) yields the same tilt angle, ∼155 ± 5°, from the surface normal, indicating that the hydrogen of this group is on average pointing down into the water phase. We have also performed orientational analysis assuming a Gaussian distribution for the distribution function and found that the average tilt angle is nearly insensitive to the assumed distribution. This may be due to the fact that the surfactant interface interaction between the polar headgroup and one of the OH groups could restrict the orientational motion; also, there is evidence of reduced rotational mobility of water at the interface.54 The assignment is in general agreement with the recently proposed assignment of Nojima et al. of the weakly H-bonded peak observed in phospholipid monolayers.46

V. CONCLUSIONS In conclusion, our study found similarities between the interfacial water structure inside reverse micelles and at surfactant/water interfaces, as evidenced by the same spectral features observed in the IR spectra of the reverse micelles and the SFG spectra at the surfactant interfaces. Lower-frequency bands (3200−3400 cm−1) are dominant in the SSP SFG spectra at both ionic and nonionic surfactant monolayers. The 3560 cm−1 peak is more pronounced in the PPP polarization SFG spectra, and it is observed for both surfactants. The same peak is also observed in the IR spectra of interfacial water in reverse micelles. Orientational analysis on the 3560 cm−1 peak provides the average tilt angle of the surface water molecules 7452

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close to the surfactant surface to be around 155° with respect to the surface normal. Our present study suggests the tentative assignment of this spectral feature to water molecules associated with the surfactant headgroup, with the OH bond pointing down into the water phase. A more rigorous theoretical analysis with more precisely calculated values of molecular hyperpolarizability would be desirable for future studies refining the spectroscopic assignment of the hydrogenbonding species present at various water interfaces.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b04733. Volumetric analysis of FTIR spectra, particle size determination by dynamic light scattering measurements, SFG experimental details, and spectral fitting of the SFG data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Chayan Dutta: 0000-0003-4839-2245 Alexander V. Benderskii: 0000-0001-7031-2630 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by AFOSR grant No. FA9550-151-0184 (C.D. and M.M.) and ARO grant No. W911NF-14-10228 (A.V.B.). M.R. gratefully acknowledges the support of the Fulbright Program, Council for the International Exchange of Scholars. A.S. was partially supported by the USC REU program.

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REFERENCES

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