Liquid Interfaces: A Study by Sum-Frequency Vibrational

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FEATURE ARTICLE Liquid Interfaces: A Study by Sum-Frequency Vibrational Spectroscopy P. B. Miranda and Y. R. Shen* Department of Physics, UniVersity of California, and Materials Sciences DiVision, Lawrence Berkeley National Laboratory, Berkeley, California 94720 ReceiVed: NoVember 10, 1998; In Final Form: February 22, 1999

Liquid interfaces play a fundamental role in science and technology. The development of surface-sensitive probes capable of yielding molecular information about these interfaces is of great importance. This paper gives an overview of our and some others’ recent work on vibrational spectroscopy of liquid interfaces by the sum-frequency generation (SFG) technique. This technique, being highly surface specific and applicable to all interfaces accessible by light, has been proven to be a most powerful analytical tool for liquid interfaces. A wide range of systems have been studied including neat liquid surfaces, solid/liquid and liquid/liquid interfaces, surfactants at liquid interfaces, and electrochemical interfaces. In many cases, SFG is shown to be the only technique available that can provide detailed information about a liquid interface at the molecular level.

I. Introduction In modern science and technology, liquid interfaces are certainly as important as solid surfaces and interfaces. They play a key role in many problems that are intimately connected to our daily life, ranging from physics, chemistry, materials science, and engineering to life science and environmental science. For example, the air/water interface is directly involved in environmental problems such as acid rain and water pollution.1 Liquid/liquid interfaces are integral to many biological systems,2,3 while liquid/solid interfaces are highly relevant to such important processes as cleaning, wetting, adhesion, lubrication, etching, corrosion, electrochemical reactions, and oil recovery.4,5 It is expected that the interfacial structure of a liquid can be very different from the bulk liquid. This is because unlike in a solid, molecules in the liquid phase are more mobile. Accordingly, the interfacial structure of a liquid can be more easily perturbed by a disturbance at the interface. Since the physical and chemical properties of a liquid interface are completely governed by the liquid interfacial structure, the latter is what we must first determine in order to have a good understanding of the liquid interface. Unfortunately, this is not an easy task because of a lack of effective experimental probes. Most existing surface techniques that employ particle (electron, ion, or atom) scattering require a sample situated in high vacuum and therefore are not suitable for use on vapor/liquid interfaces or buried liquid interfaces. On the other hand, optical techniques such as attenuated total reflection spectroscopy, infrared spectroscopy, Raman spectroscopy, and ellipsometry are not intrinsically surface-specific. Consequently, although theoretical investigations of the structures at liquid interfaces have flourished through advances in computer simulation techniques,6,7 until recently most experimental studies of liquid interfaces have been limited to macroscopic thermodynamic * Corresponding author. E-mail: [email protected].

measurements.4 New probes capable of studying liquid interfaces at the microscopic level have been developed, notably X-ray diffraction8 and reflection,8,9 neutron reflection,10 atomic force microscopy,11,12 and nonlinear optical spectroscopic techniques such as second harmonic generation (SHG)13-18 and sumfrequency generation (SFG).13,17-19 The latter has appeared to be most successful and versatile. In this paper, we shall discuss how SFG vibrational spectroscopy can be used to study liquid interfaces and present some recent results that provide interesting structural information for a number of liquid interfaces. SFG is a second-order nonlinear optical process in which two input laser beams at frequencies ωIR and ωV, respectively, overlap in a medium and generate an output at the sumfrequency ωS ) ωIR + ωV. As a second-order process, it is forbidden in media with inversion symmetry in the electricdipole approximation but necessarily allowed at surfaces or interfaces where the inversion symmetry is broken.19 The process is therefore highly surface-specific and can be employed as a surface probe for systems with bulk inversion symmetry. When ωIR, ωV, or ωS approaches a surface resonance, the SFG output is resonantly enhanced. This yields a spectrum of the surface or interface. Here we will only be concerned with surface vibrational spectroscopy as it can provide useful information about molecular orientation and conformation at a surface and sometimes even surface structure.20 In this respect, SFG is complementary to X-ray diffraction or reflection. The latter provides information about ordered molecular arrangement at a surface but not much about molecular orientation and conformation or disordered surface structure. In the past decade, SFG surface vibrational spectroscopy has found many useful applications in various disciplines of surface science and has been proven to be a most powerful and versatile analytical tool.21 It can, in principle, be used to probe any interface accessible by light. Among the many interfacial systems that have been successfully studied, liquid interfaces

10.1021/jp9843757 CCC: $18.00 © 1999 American Chemical Society Published on Web 04/09/1999

Feature Article are particularly notable. It has already been demonstrated that SFG vibrational spectroscopy can be applied to all types of liquid interfaces.22-25 In quite a number of cases, it has been found that SFG is the only spectroscopic technique applicable. For example, SFG is able to yield the vibrational spectrum of a neat liquid surface.26 It allows vibrational spectroscopic study of soluble molecular adsorbates at a liquid interface.22,25,27 Possible use of the technique for in situ electrochemistry studies28,29 and investigation of biological systems30 has also attracted much attention. There already exist in the literature several review articles dealing with SFG surface vibrational spectroscopy on liquid interfaces.18,22-25,31,32 In this paper, we shall focus on two topics studied by SFG, structures of neat liquid interfaces and surfactant conformations at liquid interfaces. It is known that molecules in a bulk liquid are randomly oriented with short-range ordering, but the same is probably not true at a surface. Probing of liquid surface structure has been attempted. However, except in special cases, X-ray scattering has not been very helpful in providing much information about surface structures of ordinary liquids. Atomic force microscopy has not yet produced clear microscopic images of liquid surfaces. Theoretical investigation of liquid surface structures, however, has been blooming in the wake of recent progress in numerical computation.6,7,33-36 If such theoretical calculations could be compared with experiments, a significant advance of the field would be likely. Here, we note that in the absence of any direct surface structural measurements, we can turn to the surface vibrational spectrum, which is closely related to surface structure.35 This is how SFG vibrational spectroscopy can come into play. We remark that SFG is currently the only technique that can yield a vibrational spectrum for a neat liquid interface. For vapor/liquid interfaces, we are interested in knowing whether the liquid surface structure can be more ordered than the bulk. Physically, this seems possible if the interaction between molecules in a preferred geometry is sufficiently strong, since then the interaction energy may dominate over the entropy contribution to the surface free energy. In a few cases, theoretical calculations have shown that this is indeed true.33-36 With SFG surface vibrational spectra, we can now provide experimental support for theoretical models. For solid/liquid interfaces, interactions between the liquid molecules and the solid substrate certainly will affect the liquid surface structure. It would be interesting to search for a better understanding of how this happens. Again, this can be achieved with SFG vibrational spectroscopy as a probe. We shall also use water, alcohols, and alkanes as examples in our later discussion on these subjects. In the same vein, liquid/liquid interfaces can also be studied by SFG spectroscopy. Here, unfortunately, residual infrared absorption in the bulk liquids in the spectral range of interest often makes the measurement difficult. Surfactants with long hydrocarbon chains are widely used in modern industry.4,5,37 They also appear frequently at interfaces of biological systems.2,3,38 A single monolayer of surfactants is able to completely alter the surface properties of a substrate.4 However, it is not generally clear how the monolayer affects such a change. The surface properties should depend on the surfactant chain conformation, but the latter could change with the liquid medium facing the chains. Despite its importance and relevance, the problem has not been well investigated experimentally, presumably because of a lack of suitable tools. In principle, infrared spectroscopy is sensitive enough to detect and analyze a surface monolayer. However, the observed

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Figure 1. Geometry for sum-frequency generation from an interface in the reflected direction. The polarization sheet generated at the interface by the input fields is imbedded in a thin interfacial layer of refractive index n′.

spectrum is often complicated, and information about the monolayer chain conformation is rather indirect. The technique also has limited applicability since it is not intrinsically surface specific. As we shall see later, SFG vibrational spectra of hydrocarbon chains are very much simplified by the intrinsic selection rules and fairly sensitive to chain conformation. They are useful in providing qualitative information about chain conformation at an interface. In our study, we have seen drastically different surfactant chain conformations when they are exposed to different liquids.39 II. Theory The theory of SFG for surface studies has been described in detail elsewhere.18-20,40,41 Here, for the sake of a better understanding of our data analyses later, we present the bare essentials. Consider the beam geometry depicted in Figure 1. It can be shown that the SFG output signal at ωS ) ωIR + ωV generated by mixing of infrared and visible beams at frequencies ωIR and ωV, respectively, in terms of photons per pulse, is given by

S(ωS) ) 8π3ωS2 sec2 θS 3

|e(ωS)‚χ(2):e(ωV)e(ωIR)|2IVIIRAT (1)

pc n1(ωS)n1(ωV)n1(ωIR)

Here, IIR and IV are the intensities of the infrared and visible input beams, respectively, A is the beam overlapping cross section at the interface, T is the pulse width, θS is the reflection angle of the SFG beam from the surface normal, n1 is the frequency-dependent refractive index of medium 1, χ(2) is the effective surface nonlinear susceptibility, which is a rank-3 tensor, and e(Ω) ) L(Ω)‚eˆ (Ω), where eˆ (Ω) is the unit vector describing the polarization of the optical field at frequency Ω and L(Ω) are the Fresnel coefficients, which relate the input fields to the field in the polarization sheet. They are given by

Lxx(Ω) ) Lyy(Ω) ) Lzz(Ω) )

2n1(Ω) cos θ2 n1(Ω) cos θ2 + n2(Ω) cos θ1 2n1(Ω) cos θ1

(2)

n1(Ω) cos θ1 + n2(Ω) cos θ2 2n2(Ω) cos θ1

( ) n1(Ω)

n1(Ω) cos θ2 + n2(Ω) cos θ1 n′(Ω)

2

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Figure 3. Typical setup for sum-frequency vibrational spectroscopy. The generated SFG signal is filtered spatially and spectrally from the scattered visible light and detected by a photomultiplier (PMT). Figure 2. Possible line shapes of an SFG resonance with χNR complex and Aq real (see eq 3): (a) χNR is real and has the same sign as Aq; (b) χNR is negligible compared to Aq/Γq; (c) χNR is real and has the opposite sign as Aq; (d) χNR is imaginary and has the opposite sign as Aq/Γq, with |χNR| > |Aq/Γq|.

In the above equation, θ1 and θ2 refer to the incidence and refraction angles, respectively, for the beam under consideration and n′ is the refractive index of the polarization sheet. In general, χ(2) has contributions from both the interface and the bulk. However, in many cases, one can experimentally demonstrate that the bulk contribution is negligible.26,42,43 This is true for the liquid interfaces we shall discuss in this paper. We can then treat χ(2) as a quantity reflecting the intrinsic property of the interface. As governed by the symmetry of the interface, only a fraction of the total 27 elements of χ(2) are independent and nonvanishing. They can be deduced from the SFG measurements with proper input/output polarization combinations following eq 1. Also using eq 1, the SFG vibrational spectra of the interface can be transformed into χ(2) versus ωIR, which can be assumed to take the form

χ(2)(ωIR) ) χNR + χR(ωIR) ) χNR +

∑q ω

IR

Aq - ωq + iΓq

(3)

where χNR and χR denote the nonresonant and resonant contributions, respectively, and Aq, ωq, and Γq are the strength, resonant frequency, and damping constant of the qth resonant mode, respectively. We note that since the SFG signal is proportional to |χ(2)|2, the spectral line shape of SFG is often affected by interference between various terms in eq 3. Therefore, to extract the vibrational frequencies, line widths, and mode strengths, one generally has to fit the SFG spectrum to eq 1 using an appropriate expression for χ(2) such as eq 3. Figure 2 shows several possible line shapes for different values of χNR and Aq. We realize that the connection of χ(2)(ωIR) to an interfacial structure is equivalent to that of a linear optical dielectric constant (ω) to the structure of a bulk medium. In both cases, theoretical calculations are often needed to help relate the measured spectrum to the structure. Without such calculations, one must resort to physical pictures and simple models to find an answer. The latter is unfortunately what we do most often because surface vibrational spectroscopy is still in its infancy. In the case where the vibrational resonances are expected to be only weakly affected by intermolecular interactions, the simplest model is to assume that the surface is composed of a collection of noninteracting molecules. We then have

χ(2) ijk ) N

∑ 〈(ıˆ‚ξˆ )(jˆ‚ηˆ )(kˆ ‚ζˆ )〉R(2)ξηζ

ξ,η,ζ

(4)

where N and R(2) are the surface density and nonlinear polarizability of the molecules, respectively, and the angular brackets refer to averaging over the molecular orientational distribution. According to eq 4, the ratios of various elements of χ(2) at certain resonant peaks can often provide information about the orientations of selected functional groups in the molecules, assuming a reasonable model for the molecular hyperpolarizability R(2).44-46 One feature implicit in eq 4 is that if the molecules at the interface are inverted (reoriented by 180°), χ(2) changes sign. The sign of χ(2) can be measured by interference techniques, thus allowing the absolute orientation of molecules at interfaces to be determined.47,48 Examples will be discussed in later sections. III. Experimental Considerations The experimental setup (see Figure 3) for our SFG spectroscopic measurement has also been described elsewhere.42,49,50 Briefly, an active-passive mode-locked Nd:YAG laser producing 20 pulses/s with a ∼ 25 ps pulse width was used to pump an optical parametric generation/difference frequency generation system to produce tunable infrared radiation with a wavelength up to 9 µm. The infrared beam was then overlapped with a frequency-doubled output beam from the laser (532 nm) at the sample surface as illustrated in Figure 3. The SFG output in the reflected direction was detected and recorded by a photomultiplier/gated integrator system. Typically, with the infrared and visible pulse energies at 100 µJ and 1 mJ, respectively, focused to a several hundred micrometer spot on the sample, the SFG signal was around 100 photons/pulse. Because the intensity of the infrared beam varied with frequency, the measured SFG spectrum must be normalized against a sample with a known dispersion in the tuning range. A comment on a possible geometry for SFG spectroscopic measurements is in order. It is known that the input field strength at an interface can be strongly enhanced in total internal reflection geometry, i.e., Lii(Ω) in eq 2 is greatly enhanced.40,51-56 One would then be tempted to set up an SFG measurement with both input beams totally reflected at the interface. This indeed will enhance the output signal if the input intensity at the interface is not limited by optical damage.57-59 Otherwise, the maximum signal one can get is determined only by the local maximum input intensity the interface can tolerate, and no gain will be expected from the total reflection geometry. It is also important to know that with the infrared beam in the total reflection geometry, the SFG vibrational spectrum may be severely distorted.57-59 As shown in Figure 4, Lii(ωIR) peaks sharply near the critical angle, which is determined by the infrared refractive indices of the bounding media. If one of the

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Figure 5. SFG spectrum for the neat water/vapor interface at 40 °C with polarization combination SSP (S-, S-, and P-polarized sumfrequency output, visible input, and infrared input, respectively). The solid line is a fit to eq 3.

Figure 4. Variation of Fresnel factors Lii as functions of the incident angle for a representative case of 1 ) 1, ′ ) 2, 2 ) 10: (a) incoming beam from medium 1; (b) incoming beam from medium 2.

refractive indices changes significantly, the critical angle and hence Lii(ωIR) will change accordingly. This could happen when the infrared frequency scans over a resonance. As an example, the refractive index of water around its OH stretch resonance changes from 1.46 to 1.12. If the input angle of the infrared beam is set near the critical angle for a selected frequency, Lii(ωIR) can vary appreciably across the resonances and the SFG spectrum will be modified accordingly. To find the spectrum of χ(2) versus ωIR, we must divide S in eq 1 by |Lii(ωIR)|2 to eliminate the frequency dependence due to Lii(ωIR). We now want to make some general comments about sample preparation. Contamination is always an important issue in surface science, and cleanness in sample handling and preparation is of paramount importance. The liquids used in our experiment (except water) were purchased from Aldrich or J. T. Baker with purities of >99% (reagent grade or higher) and used without further purification. Water was distilled and purified by a deionizer/ultrafiltration system (Barnstead Easy Pure RF) and had a resistivity higher than 18 MΩ‚cm and total organic carbon (TOC) content less than 10 ppb. For experiments on liquid/solid interfaces, the solid substrates used were infraredgrade fused quartz (Esco Products). They were thoroughly cleaned in hot chromic acid for several hours and extensively rinsed with purified water. Sample cells were made of Teflon and/or glass, and every component contacting the sample was cleaned in strongly oxidizing solutions. For specific sample preparation procedures, the reader is referred to the original references. IV. Neat Liquid/Vapor Interfaces The first surface vibrational spectrum of a neat liquid ever taken was obtained by SFG on the methanol/vapor interface.26 The spectrum of methanol in the CH stretch region was observed. Experimental tests were made to show that the SFG spectrum was indeed dominated by contribution from the liquid surface. Measurement of the phase of the SFG output wave indicated that the CH3 group of methanol points away from the liquid at the interface, as predicted by theory.34 Spectra obtained with different input/output polarization combinations allowed us to conclude that the CH3 groups have a rather broad orientational distribution with its average along the surface normal.

Figure 6. Temperature dependence of the SFG spectrum for the neat water/vapor interface. The polarization combination is SSP.

Water is the most important liquid on earth and therefore was the subject of our more recent studies. Figure 5 presents the SFG spectrum of the water/vapor interface in the OH stretch region.43 The polarizations used for the SF output, visible input, and infrared input are S, S, and P, respectively, denoted by SSP. The SPS spectrum is very weak and shows hardly any discernible features. The spectrum in Figure 5 exhibits three prominent peaks at 3680, 3400, and 3200 cm-1; they can be assigned to OH stretches associated with dangling OH bonds, OH with both O and H hydrogen-bonded to neighbors in a relatively disordered structure, and OH hydrogen-bonded in a well-ordered icelike structure, respectively. The presence of the 3680 cm-1 peak is particularly significant. It indicates that the spectrum indeed comes from the surface water monolayer with the dangling OH bonds pointing out of the liquid. The other two peaks can find their counterparts in infrared and Raman spectra of bulk water and ice,60 with the 3400 cm-1 one being characteristic of water and the 3200 cm-1 one characteristic of ice. As expected, the former reduces and the latter increases in strength when the temperature decreases and the water surface structure supposedly becomes more ordered, as shown in Figure 6. To extract structural information from the surface vibrational spectrum, we analyzed the spectrum more quantitatively. It was

3296 J. Phys. Chem. B, Vol. 103, No. 17, 1999 found that the strength of the 3680 cm-1 peak comes from 2030% of water molecules in the surface monolayer, with each molecule contributing one dangling OH bond. In a separate experiment,61 it was observed that the peak strength decreased when methanol was mixed into the water and completely disappeared when the bulk methanol concentration reached 11%. In an earlier surface tension measurement on methanol/water mixtures, it was reported that 11% of methanol in the bulk corresponds to 25% of methanol at the surface.62 Knowing that each methanol molecule at the surface can eliminate one dangling OH bond, we can then conclude that there should be about 25% of water molecules in the surface monolayer carrying a dangling bond. This is what one would find at a truncated hexagonal ice surface.63 The result therefore suggests that the water surface is icelike, in the form of a tetrahedrally coordinated hydrogen-bonding network. However, unlike ice, the network must be significantly disordered, as evidenced by the presence of the strong peak at 3400 cm-1. This agrees well with theoretical prediction.36 Physically, to minimize the surface energy, the water surface would like to retain as many hydrogen bonds between molecules as possible. One finds that the best it can do is to break one out of the four tetrahedral bonds per molecule residing at the surface. In a recent theoretical paper by Benjamin,35 the calculated surface vibrational spectrum of the water/vapor interface matches fairly well with the observed spectrum. The above results indicate that the water/vapor interface has a more ordered structure than the bulk because of the existence of strong hydrogen bonding between water molecules. This then suggests that other liquids with strong hydrogen bonding between molecules would also have a more ordered structure at the liquid/vapor interface than in the bulk. This was found to be true for alcohols.64 As seen in Figure 7, the surface SFG vibrational spectra of methanol and ethanol in the OH stretch region appear to resemble more those of the bulk solids than liquids. The SFG spectra in the CH stretch region also indicate that the surface of alcohols is more ordered than the bulk. Figure 8 presents the SSP spectra for a series of alcohols from methanol to octanol (CnH2n+1OH, with n ) 1-8). In each spectrum, except ethanol, the two prominent peaks arise from the stretch vibrations of the terminal CH3 group, with the lower frequency one attributed to symmetric stretch (CH3-s, at 2875 cm-1) and the higher frequency one to a Fermi resonance between the symmetry stretch and the overtone of the bending mode (CH3FR, at 2940 cm-1).65,66 The feature that appears as a shoulder or small peak at ∼2850 cm-1 can be assigned to the symmetric stretch of the CH2 groups (CH2-s).65,66 In all cases (except ethanol), the CH3 modes dominate the spectrum. The CH2-s mode is expected to be weak if the chains are almost all-trans because of near inversion symmetry of the CH2 groups on the chains. The result then indicates that the alcohol chains at the surface have little trans-gauche defects and are better ordered than those in the bulk. In fact, as shown in Figure 8, the amount of defects does not seem to increase very much with increase of the chain length. This is contrary to what one would expect if we assume that the chains are not interacting. We can therefore conclude that the chain-chain interaction between surface alcohol molecules must be very effective in keeping the chains in the nearly all-trans conformation and in helping establish the more ordered surface structure of long alcohols. One would then raise the question whether long-chain alkanes may also have a surface structure more ordered than the bulk, even though they are not hydrogen bonding liquids. Figure 9 depicts the SFG spectra of the n-eicosane (n-C20H42)/

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Figure 7. SFG spectra for the liquid surface of (a) methanol and (b) ethanol in the OH stretch region. The dotted curves are the bulk liquid infrared spectra, while the dashed lines are the bulk crystalline infrared absorption spectra. The polarization combination is SSP.

vapor interface at two different temperatures with the SSP polarization combination.42 They indeed show that the alkane surface is highly ordered. If the surface were as disordered as the bulk, we would expect from a symmetry argument hardly any observable spectrum. The spectra are characterized by three peaks. Again, the ones at 2850 and 2875 cm-1 are attributed to the CH2-s and CH3-s, respectively.65,66 The third broad peak at higher frequency has contributions from two resonances at 2920 and 2940 cm-1, assigned to the CH2 antisymmetric stretch (CH2a) and the CH3-FR, respectively.65,66 The weak CH3 antisymmetric stretch mode (CH3-a) at ∼2960 cm-1 in the spectrum indicates that the chains are oriented more or less along the surface normal.49 The strong CH2 peaks in the higher temperature spectrum means that there are significant amounts of trans-gauche defects in the chains.49 When the temperature of the liquid was lowered to less than 3 K above the bulk freezing temperature, the spectrum suddenly changed to that shown in Figure 9b. Now the CH2 modes are significantly suppressed, indicating that the chains have assumed the all-trans conformation with their orientation nearly parallel to the surface normal. This sudden change of surface structure of the alkane liquid at ∼3 K above the bulk freezing temperature was first observed by X-ray reflection and diffraction, ellipsometry, and surface tension measurements and is known as the surface freezing transition.67,68 The SFG vibrational spectra here permit a better understanding of the nature of this transition.

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Figure 9. SFG spectra of the air/n-eicosane interface at (a) 40 °C and (b) 37 °C, taken with the SSP polarization combination.

Figure 8. SFG spectra for the surface of liquid 1-alcohols in the CH stretch region. The spectra were taken with SSP polarization combination, and each panel is labeled with the name of the corresponding alcohol.

SFG spectra of other types of neat liquid/vapor interfaces can also be obtained, but without a simple model structure of the liquid surface in mind, they are difficult to interpret. As an example, we show in Figure 10 the SFG spectra of the neat acetone surface in the CH stretch range, taken with SSP and SPS polarization combinations.69 The SPS spectrum is very weak, with hardly any discernible features. The SSP spectrum however shows a strong peak at ∼2925 cm-1. Fitting the spectum to eq 3 suggests that another weaker peak at ∼2965 cm-1 is also present. The peak at 2925 cm-1 can be assigned to the CH3 symmetric stretch and the one at 2965 cm-1 to the out-of-plane CH3 antisymmetric stretch.70 We recall that if the surface molecules were randomly oriented, no signal should be observed in the SFG spectra. These preliminary measurements

Figure 10. SFG spectra for the liquid/vapor interface of pure acetone ((CH3)2CO) with (O) SSP and (0) SPS polarization combinations. The dashed line is a fit to eq 3 including only one peak at ∼2925 cm-1, and the solid line is the fit including another peak at ∼2965 cm-1.

then indicate that molecules at the surface of non-hydrogenbonding liquids are also polar ordered in orientation. A quantitative analysis of the spectra could yield the average orientation of the CH3 groups, and combining with measurements in the CO stretch range, a determination of the average orientation of surface acetone molecules would be possible. Such results could then be compared to molecular dynamics calculations, if available. Other examples will be discussed in the next section concerning the surface structure of liquid mixtures.

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Figure 11. SFG spectra of (a) pure water and (b-f) sulfuric acidwater mixtures (with different acid concentrations as indicated) at the liquid/vapor interface at 20 °C, taken with the SSP polarization combination.

V. Liquid Mixture/Vapor Interfaces SFG vibrational spectroscopy has also been used to study liquid/vapor interfaces of mixtures such as alcohol-water,71-73 alkylnitrile-water,74,75 acid-water,76-79 and aqueous salt solutions.79,80 The latter two are relevant to a number of environmental problems such as ozone depletion in the stratosphere.81,82 It was found that if a mixture is composed of hydrogen bonding molecules, the surface always appears to be more ordered than the bulk. Impurities at the surface may disrupt the ordered network, but charged impurities could enhance the ordering via the surface electric field effect. For methanol-water mixtures,71,72 it was found that methanol molecules are more ordered at lower concentration than at the neat methanol surface. The surface structure becomes less ordered with an increase of methanol concentration presumably because of the reduced number of hydrogen bonds between the surface molecules. Acetonitrile-water mixtures have been studied by Eisenthal and co-workers.74 The SFG spectra suggest the occurrence of a surface orientational phase transition when the bulk CH3CN concentration reaches a critical value. The authors believed that the molecular reorientation necessarily occurs in order to reduce the dipole-dipole repulsion energy between molecules in the surface monolayer when the CH3CN density reaches a critical value. Huang and Wu72 however have proposed an alternative interpretation that the observation may be due to phase separation of mixtures at the surface. The same phenomenon has also been observed on propionitrile (CH3CH2CN)-water mixtures, but not with alkylnitriles longer than propionitrile.75 Our laboratory has recently studied the surface of sulfuric acid-water mixtures.76 Figure 11 shows the SFG spectra of sulfuric acid-water mixtures at the liquid/vapor interface for various bulk acid concentrations. Comparing the surface spectra at low acid concentrations (