8536
J . Phys. Chem. 1994,98, 8536-8542
Sum-Frequency Spectroscopy of Surfactants Adsorbed at a Flat Hydrophobic Surface Robert N. Ward,? David C. Duffy, and Paul B. Davies' Department of Chemistry, University of Cambridge, Lensfeld Road, Cambridge CB2 IEW, England
Colin D. Bain' Physical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, England Received: March 2, 1994; In Final Form: May 12, 1994"
Infrared-visible sum-frequency spectroscopy (SFS) has been used to obtain vibrational spectra in the C-H stretching region of surfactants adsorbed a t the solid-water interface. All of the surfactants formed monolayers a t a hydrophobic substrate with the hydrocarbon chains of the surfactant oriented away from the aqueous phase. Surfactants with a wide range of structures have been examined, including cationic, anionic, nonionic, and zwitterionic head groups and single, double, and branched alkyl chains. Features in the C-H stretching region originate principally from the hydrocarbon chains and not from the polar head groups. The packing density and the orientational order of the surfactant monolayers is deduced from the relative strengths of the chain methylene and terminal methyl modes. The structure of the monolayers adsorbed a t a flat, hydrophobic surface is compared to surfactant aggregates in bulk aqueous solution, a t the air-water interface and in the solid crystalline state.
Introduction The aggregation of surfactant molecules is of fundamental importance in a wide range of industrial processes. Micelles and emulsions play a central role in detergency, oil recovery, paints, personal products, and food chemistry.' An understanding of the structure of surfactant aggregates both in the bulk solution and at surfaces is essential. The structure of bulk surfactant phases is reasonably well understood through techniques such as optical microscopy, NMR, and light scattering2 In contrast, relatively few techniques exist for studying aggregation at surfaces, and, as a result, much less is known about the microscopic structure of surfactant films. In this paper, we report vibrational spectra of a variety of surfactants adsorbed at the solid-water interface, obtained by sum-frequency spectroscopy (SFS), and discuss the structural information contained in these spectra. Classical methods such as ellipsometry3 and the measurement of surface tensionla yield adsorption isotherms but provide little information on the microscopic structure of the adsorbed film. Techniques such as NMR,4neutron reflectivity5s6and vibrational spectroscopy are much more sensitive to local structure but are experimentally more demanding. Vibrational spectroscopy of molecules at the solid-water interface is particularly difficult due to the small number of molecules at the interface, the large IR absorption cross section of water, and the need to discriminate adsorbed molecules from the same species in bulk solution. In favorable cases, these obstacles may be overcome by surfaceenhanced Raman spectroscopy (SERS)' or attenuated total internal reflection infrared (ATR-IR) spectroscopy*and both of these techniques have been used to study surfactant adsorption at the solid-liquid interface. SFS is more generally applicable than SERS or ATR-IR as it is intrinsically surface specific and any flat surface can be studied.9 In this paper we focus on the adsorption of surfactants at a hydrophobic surface. We present vibrational spectra in the C-H stretching region of a range of anionic, cationic and nonionic surfactants adsorbed on a self-
'
Present address: Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, Mersyside L63 3JW, England. Abstract published in Aduance ACS Abstracts. July 15, 1994.
0022-3654194f 2098-8536$04.50f 0
assembled monolayer of octadecanethiol on gold.10 These spectra are very sensitive to the conformational order of the surfactant monolayer and highlight the effects of the different head groups and hydrocarbon chains on the structure of the monolayer. We compare the structure of the monolayers at the solid-liquid interface with our knowledge of the surfactants in the crystalline state, in bulk solution, and at the air-water interface.
Interpretation of Sum-Frequency Spectra Sum-Frequency Spectroscopy. In infrared-visible sumfrequency spectroscopy (SFS), a visible laser (frequency w,is) and an infrared laser (COIR) are pulsed simultaneously onto a surface and the light emitted at the sum of the two frequencies (wsum) is detected.]' The intensity, I,,,, of the light emitted by a material is proportional to the square of its second-order non-linear susceptibility lx(2)12.Since x ( ~is) zero in centrosymmetric or isotropic media, SFS is uniquely sensitive to the interface between bulk phases. x ( ~can ) be separated into two terms, and xf'. The nonresonant term, xgk, arises from the optical nonlinearity of the gold surface and varies little as the infrared frequency is scanned. The resonant susceptibility, xf), arises from molecules adsorbed at the interface. xf) can be expressed in terms of the Raman and infrared transition dipole moments of the molecules, M I , and A,,:
xgk
where Nis the number of molecules per unit area, w, the resonant frequency of a vibrational mode, and I'v the Lorentzian halfwidth of the mode, and the angle brackets indicate an average over the orientations of all the molecules at the interface.12 Equation 1 shows that &) will be zero unless the infrared laser is in resonance with a vibrational mode that is both infrared- and Raman-active. Scanning the infrared laser and monitoring the intensity of the sum-frequency emission produces a vibrational spectrum of molecules at the interface. 0 1994 American Chemical Society
Surfactants Adsorbed at a Flat Hydrophobic Surface
The Journal of Physical Chemistry, Vol. 98. No. 34, 1994 8537
The sum-frequency spectrum we detect depends on a convolution of the resonant and nonresonant susceptibilities:
Both these quantities are complex, so the shape of the spectrum depends on the relative phase of the two susceptibilities. In the spectra presented here, the dominant contribution to the S F resonant signal is the cross-term in eq 2.11 Infrared and Raman Spectra of Hydrocarbon Chains. To understand the sum-frequency spectraof thehydrocarbon chains of surfactants in the C-H stretching region, we must first l w k at some of the key features of their infrared and Raman ~pectra.1~19 The terminal methyl group of an alkyl chain gives rise to four peaks which occur at approximately the same frequencies in IR and Raman spectra.ls.20 Thesymmetricstretch issplit hy a Fermi resonance with an overtone of a bending mode to give two p e a k r+ at -2873 cm-l and r& at -2930 cm-I. The asymmetric stretch isdoubly degeneratein C3.symmetry, but issplit into two by interactions with the hydrocarbon backbone, which lower the symmetry to C,. The in-plane component, r;, occurs around 2963 cm-I, and the out-of-plane component, r;, about IO cm-' lower. Whereas these two peaks are readily resolved at cryogenic temperatures, they frequently merge at rwm temperature due to torsional motion about the terminal C-C bond.14 The frequencies and intensities of the methyl modes are relatively insensitive to the conformation of the hydrocarbon chain. Thestretching frequenciesof an extended polymethylenechain are complex and have been discussed extensively elsewhere.lc'8 In an all-trans chain, vibrations on adjacent carbons are coupled giving rise to modes that are delocalized over the whole chain. These modes are labeled d+(@)for the symmetric stretch and d-(@)for theantisymmetricstretch, where @isthe relative phase ofadjacent methylenegroups. Tbedispenion ind+ issmall, with botbtheRaman-active moded+(O) and the IR-actived+(r) falling between 2850 and 2845 cm-I. In contrast, the d- mode is highly dispersive with the Raman-active d-(0) near 2880 cm-l and the IR-active d-(r) at 2920 cm-I. The methylene modes are also complicated by Fermi resonance. The d+ stretches can interact with overtones and combinations of the in-plane deformation, S(4). In IR spectra, this interaction gives rises to broad bands centered around 2895 and 2925 cm-I, the latter being obscured bytheintensed-(r) band. InRamanspectra,a broad,asymmetric band stretching from 2860 to 2960 cm-I results. These features are illustrated in the low-temperature infrared spectrum ofC8H18 and the Raman spectrum of CzIHu in Figure 1. In disordered chains, such as those found in a liquid, the IRactive methylene bands increase in frequency by 6-8 cm-l.ll The most pronounced change in the Raman spectrum is a decrease in theintensityofthed-(0) modenear2890cm-1. For thisreason, theratiooftheRamanintensitiesat 2890and2850cm-Ihaslong been usedas a measureofconformationalorder in pbospholipids.21 Sum-Frequency Spectra of Hydrocarbon Chains. Now let us consider the sum-frequency spectra of the hydrocarbon chains in OUT surfactant monolayers, first in an all-trans chain and then in the presence of conformational disorder. An all-trans chain is locally centrosymmetric (Figure 2a) and consequently the methylene modes are either IR- or Ramanactive, but not both.13 Thus no CHI peaks should appcar in the sum-frequency spectrum. The hyperpolarizabilities of the terminal methylenes are perturbed by the terminal functional group and therefore could be sum-frequency active, but this does notappeartobeamajoreffect inthemonolayerswebavestudied. The methyl modes are both 1R- and Raman-active and are therefore allowed in SFS. The intensity of the r+ mode depends only on the polar angle, 0, between the C3 axis of the methyl group and the normal to the surface. In our experiments, in
2800
2850
2900
2950
3
Wavenumber (cm-') Figurel. (a) lnfraredspcctrumintheC-Hstrctchingrcgionofcryxtalline n-CsHls at 93 K. (b) Raman spcctrum in the C-H stretching region of orthorhombic n-C21Hu at 110 K. (Redrawn from data of Snyder et a1.9
a
b
Figure 2. Schematic diagrams of (a) all-trans. and (b) terminal gauche conformations of alkyl chains.
which all the laser beams are p-polarized, two Cartesian components of ~ ( 2 make ) a contribution to the sum-frequency . "the r+ mode spectrum: x::! and x ~ ~ ~For
x::
u
1.42(cos 0) - (cos' 0)
(3a)
Under p-polarized light, the intensity of the r+ mode is relatively insensitive to orientation for small 9 but decreases to zero as 9 - r / 2 . Amoredetailedanalysisoftheorientationofthemethyl group requires spectra with other laser polarizations (usually involving s-polarized infrared, which is impractical on metal surfaces).'2 A key feature of eq 3 is that XI') changes sign if a molecule is inverted (0 T - 0)12 Thus the sign of ~ ( 2 yields ) the polar orientation of the surfactant. T h e r modes are moredifficult tomodel, partly due to rotation of the methyl group, and partly because ~ ( 2 1depends on both 9
-
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The Journal of Physical Chemistry. Vol. 98, No. 34, 1994
and +,the twist of the plane of the hydrocarbon backbone. For a freely rotating methyl group, xc2)is zero for 8 = 0 and r / 2 and a maximum at the magic angle of 0.96 rad. Conformational disorder signifies the presence of gauche defects,suchastheterminalgaucheconformationshown in Figure 2b (a common conformational defect in densesystems).’3 Gauche conformations have two important effects. First, they randomize theorientationsofthe terminal methylgroups. ~(2)dependsonly on averages over odd powers of cos 6, which vanish for randomly oriented configurations. Hence conformational disorder reduces the intensity of the methyl modes. This effect is particularly pronounced forther+ mode. Second,gaucheconformationslower the local symmetry of the polymethylene chain. For example, modes involving the terminal methylene group shown in Figure 2b will be both IR- and Raman-active. As a consequence, methylene modes may be sum-frequency active in the presence of chain disorder. The sharpest CH2 feature we observe is the d+ mode near 2850 cm-I, while the d& modes appear as a broad band between 2890 and 2930 cm-I. The d- mode is less likely to be a major contributor to the S F spectrum since, even in liquid alkanes, there is limited overlap between the infrared and Raman d- bands.”J% We note that for a film completely lacking any orientational order. the methylene modes will disappear again, since ~ ( 2 is) zero in isotropic media. It is useful to have a quantitative measure of conformational disorder when comparing a series of related compounds. A suitableparameter is theratioI(d+)/I(r+) where Iisthestrength of the resonance (see ref 13 for a detailed explanation of how I is derived from a spectrum). The I+ mode is chosen because it is easier to interpret than the r mode, and bccause its intensity appears to be more sensitive to orientation. The low-frequency d+ peak is sharp, whereas the broad band at 289&2930 cm-1 is difficult to fit accurately and overlaps the r& peak a t 2935 cm-1. For a perfectly ordered monolayer, I(d+)/I(r+) is zero. Conformational disorder increases I(d+) and decreases I(r+), thereby increasing this ratio. This sensitivity to conformation was demonstrated in a SFS study of a Langmuir-Pkkels film of pentadecanoic acid (PDA) at the air-water interface by Guyot-Sionnest et aLZ4 The SF spectrum of the liquid condensed phase of the PDA monolayer, which is known to be densely packed, contains intense methyl modes but no methylene modes; the liquid expanded phase, of lower packing density, has weaker methyl modes and stronger methylene modes, consistent with a substantial number of gauche defects in the chains.
Experimental Section The SF spectrometer has been described in detail elsewhere.25 Briefly, thevisiblelaser beam at thesample (0.6 mJ/pulse?6532 nm, 8 ns, 11.5 Hz) was provided by frequencydoubling theoutput of a Nd:YAG laser in KDP. Tunable infrared radiation (1-2 mJ/pulse, 270&3100cm-’) wasgenerated by stimulated Raman scatteringofa tunabledyelaserbeam in high pressure hydrogen.” The laser beams, both p-polarized. were overlapped on a 2 mm2 area of the sample and the resulting emission a t the sum-frequency (also p-polarized) was detected by a photomultiplier tube and gated integrator. Typical signals were 50 photons/pulse and the acquisition time for a spectrum was usually around 30 min. The substrate wasa self-assembled monolayerofperdeuterated octadecanethiol adsorbed on gold (dODT/Au). The gold was in the form of a 2000-A-thick film evaporated onto a chromiumprimed silicon (100) wafer?’ Atomic force microscopy showed that the surface of the gold resembled a series of shallow hills about 40 A high and several hundred angstroms across. These monolayers have been extensively characterised and are known to expose a hydrophobic surface28 composed predominantly of methyl groups at the aqueous interface. The octadecanethiol
Ward et al.
DDAPS
CMTAB
DPC
DDAB
SDS
AOT
A Fused silica
surlactant solution
Figure 3. Schematic diagram of the geometry of the laser beams at the
sample. was fully deuterated toeliminateSFsignal from theself-assembled monolayer in the C-H stretching region. The structures of the surfactants used are shown in Scheme 1. The surfactants were purified by repeated recrystallization: didodecyldimethylammonium bromide (DDAB, 9876, Aldrich), three times in ethyl acetate; tetradecyltrimethylammonium bromide (C14TAB, 99%. Aldrich), twice in 95% acetone/% methanol; dodecylpyridiniumchloride (DPC, 98% Aldrich), five times in ethyl acetate; sodium dodecyl sulfate (SDS, >99%, Polysciences), twice in ethanol; dodecyldimethylammoniopropane sulfonate (DDAPS, 98%, Aldrich), three times in 95% acetone/ 5% methanol. Dodecanol (C120H, 99%, B.D.H.), dodecyl propaoxyethylene ether (C12E3),CI4TABand C12E3with their hydrocarbonchainsdeuterated(all provided by Dr. R. K.Thomas. Oxford University) and bis(2-ethylhexyl)sodium sulfosuccinate (AOT, 99%, Aldrich) were used as received. The purity was assessed by measurements of surface tension. The absence of minima inthesurfacetension curvesconfirmedthatthesurfactants were free from surface-active impurities. Surfactant solutions were prepared in deionized, triply distilled water and stored, for no longer than I week, in glass jars that had been cleaned with chromic acid. The geometry of the laser beams a t the sample is shown in Figure 3. The sample housing is based on the design of a conventional electrochemical A thin film (-1 pm thick) of aqueous surfactant solution was trapped between a fused silica prism and a plane hydrophobic substrate. Before a spectrum was acquired, the surfactant was allowed to adsorb with the substrateapproximately IO mm from the prism. Adsorption was complete within 5 min for solutions with concentrations greater than lo-‘ mol dm-3; at lower concentrations adsorption was complete within I h. The intensity ofthe IR beam reflected from the sample indicated that the S F spectra were not distorted by absorption of infrared radiation by the C-H stretching modes of surfactant molecules in the bulksolution. However, theSFspectra were affected by the dispersion and absorption of the IR beam
Surfactants Adsorbed at a Flat Hydrophobic Surface
P
e
a Y
The Journal of Physical Chemistry, Vol. 98, No. 34, 1994 8539
1
DDAB
“ O Oi DPC
SDS
2C 2800
2850
2900
2950
le AOT
3000
Infrared wavenumber (cm-’)
Figure 4. Sum-frequency spectra of (a) pure water, (b) a saturated aqueous solution of dodecanol, and aqueous solutions of (c) ClzE3, and (d) DDAPS, both at the cmc, all in contact with dODT/Au. The spectra are offset vertically for clarity; the zero intensity refers to (c).
by the prism and by the water. To correct this effect a smooth curve was fitted to the SF spectrum of the perdeuterated selfassembled monolayer under pure water and all subsequent spectra were normalized to this curve.
Results Figure 4 shows the sum-frequency spectra, in the C-H stretching region, of solutions of nonionic and zwitterionic surfactants in contact with a self-assembled monolayer of dODT on gold. Figure 5 shows the corresponding spectra of cationic and anionic surfactants. The surfactants were at their critical micelle concentrations (cmc), except for dodecanol, which was present as a saturated solution. Increasing the concentration above the cmc did not change the S F spectra, indicating that the packing density at the surface does not change above the cmc. It is also a useful test of purity: if any impurities were present in the monolayer below the cmc, they would tend to become solubilized in micelles above the cmc, changing the composition of the monolayer and hence its SF spectrum.’O The resonant features in the SF spectra are attributed to the C-H stretching modes of surfactant molecules (Table 1). At first sight, these features could arise from surfactant molecules dissolved in the bulk solution, adsorbed to the prism, or adsorbed to the self-assembled monolayer. As discussed above, the bulk solution is isotropic and therefore does not contribute to the S F spectra. Three observations suggest that surfactant adsorbed to the prism does not contribute significantly to the spectra. First, if the surfactant were adsorbed to the prism, the relative phase of the light emitted from the surfactant and the gold would vary as the separation of the prism and the gold was changed. However, the shapes of the spectra were found to be independent of the thickness of the film of solution. Second, in the SF spectrum of a solution of C12E3 in contact with a hydrophilic self-assembled monolayer (HO(CH2)lISH adsorbed to gold) there were no surfactant resonances. If the SF spectra arose from the prism surface, changing the self-assembled monolayer would have no effect. Third, we have studied SDS adsorption in a cell with a prism made of CaF2. SDS adsorbs strongly to CaFz31 but less strongly to silica, which carries a negative surface charge. The
2800
2850
2900
2950
3000
Infrared wavenumber (cm-’)
Figure 5. Sum-frequency spectra of solutions of (a) CI~TAB, (b) DDAB, (c) DPC, (d) SDS, and (e) AOT, in contact with dODT/Au. All the surfactants were at the cmc. The spectra are offset vertically for clarity; the zero intensity refers to (c).
prism material had no effect on the SF spectra. From these experiments, we infer that the surfactant molecules we detect are adsorbed at the surface of the self-assembled monolayer. It is well-known that many surfactants adsorb to silica,32 so why is it that we do not detect this adsorption? There are two plausible explanations. First, adsorption of surfactants from concentrated solutions (-cmc) onto polar surfaces may give rise to b i l a y e r ~ , ~which I - ~ ~ are sum-frequency inactive by symmetry. Second, the fused silica prism and the gold surface are not exactly parallel. The path length, and hence the phasedifference, between the signal from the prism surface and the gold surface will vary across the illuminated area. If this variation in phase exceeds T , the contributions to the cross term involving and xf) would cancel out. A term in Ixf)12would still remain, but the resulting SF signal from the prism would be much weaker than the signal from the gold surface. The polar head groups of the surfactants do not appear to contribute significantly to the SF spectra. Figure 6 shows the SF spectra of C14TAB and C12E3 in which the chains are fully deuterated. The SF spectra of the fully protonated surfactants are included for comparison. Forthe chain deuterated surfactants, any resonant features in the C-H stretching region arise from the head group. Curve fitting of the SF spectrum of dCIzhE3 reveals a weak peak a t 2880 cm-I, assigned to a mode of the polyethoxy group.34 The spectrum of dCl4hTAB contains weak features at 2830,2900, and 2970 cm-I, probably due to N-methyl stretches.35 Since the resonant features from the head groups are so weak, it is reasonable to interpret Figures 4 and 5 in terms of the conformation of the hydrocarbon chains.
&i
Discussion General Features. All the surfactants we studied adsorbed at a hydrophobic surface. The resonant features in Figures 4 and 5 appear as “dips”, indicating that the cross-term in eq 2 is negative. As we have shown previously, destructive interference between
8540 The Journal of Physical Chemistry, Vol. 98, No. 34, 1994
Ward et al.
TABLE 1: Assignment of Resonant Modes Observed in the SF Spectra of Surfactants Adsorbed to dODT/Aua resonant assignment frequency (cm-1) 2873 terminal methyl (CH3) sym stretch split by Fermi resonance low frequency 2932 high frequency 2962 antisymmetric stretch chain methylene (CH2)
label r+
rk r
low frequency d+ high frequency d& a The frequencies are based on theoretical fitting of the SF spectra.13 The broad resonance in the region 2900-2930 cm-I has been assigned to dk, but it may gain intensity from the antisymmetric methylene mode, d- (see text).
2851-2858 2900-2930
1
a
2800
sym stretch split by Fermi resonance
2850
2900
2950
3000
Infrared wavenumber (cm")
Figure6. Sum-frequencyspectra of (a) Cl4TAB and (b) Cl2E3 solutions in contact with dODT/Au. Spectra of the chain deuterated surfactants
( C I ~ D ~ ~ N + ( Cand H ~C) I~~BD~~ ~ [ O C ~ Hrespectively) ~ ] ~ O H , are represented by crosses. The dots represent the spectra of the fully protonated surfactant. Zero intensity refers to the protonated Cl4TAB spectrum.
TABLE 2 Ratio of the Strengths of the d+ and r+ Modes surfactant I(d+)/I(r+)" surfactant I(d+)/I(r+)" 3.2 f 0.05 0.2f 0.05 C14TAB C12OH 3.9 f 0.4 0.8 f 0.1 DPC DDAB DDAPS 7fl Ci2E3 2.5 f 0.05 1.4 f O.lb 2.6 f 0.1 AOT SDS Strengths are derived from theoretical fitting" of between three and eight spectra. Errors are at the la level. * Not directly comparable with other values (see text). (1
xci
xf) and for the methyl modes indicates that the surfactants are adsorbed with the terminal methyl group directed toward the gold surface and, by inference, with their polar head groups toward the aqueous phase.22 This polar orientation has been assumed in previous studies of surfactant adsorption at hydrophobic surfaces36-38 but is explicitly demonstrated by the SF spectra. This orientation minimizes hydrophobic contacts between the water and the hydrocarbon chains and between the water and the surface, while retaining polar interactions between the head groups and the aqueous phase. The adsorbed layer is almost certainly monomolecular-in a bilayer the SF emission from the two sheets of molecules would cancel out. The relative intensities of the d+ and r+ modes provides a useful measure of the conformational order of the hydrocarbon chain and the values of (Z(d+)/Z(r+)) for the surfactant monolayers are presented in Table 2. The wide variation in this ratio indicates that the degree of orientational and conformational ordering within the monolayer varies greatly among the surfactants. The conformational order can be interpreted in terms of the various forces controlling the aggregation of surfactants. Aggregation is promoted by attractive hydrophobic interactions between surfactant molecules and by favorablevan der Waals interactions between densely packed hydrocarbon chains. Aggregation is inhibited by repulsive interactions between the head groups. These repulsions may be electrostatic, steric or entropic in nature. In
addition, the all-trans conformation characteristic of densely packed systems, though favored energetically, is entropically highly unfavorable. The structures adopted by the surfactant films result from a balance between these influences. At one extreme lies dodecanol, with a value of Z(d+)/Z(r+) = 0.2. In the absence of head-group repulsions, the attractive forces between the hydrocarbon chains dominate, resulting in a densely packed monolayer with few gauche defects. All the other surfactants have a much higher ratio of Z(d+)/Z(r+), indicating considerably more conformational disorder and lower packing density in the monolayer. In the extreme case of DDAPS, repulsive forces between the head groups result in such a low coverage that the terminal methyl groups are almost totally disordered. The individual surfactants are discussed below. C I ~ O HIn . the SF spectrum of dodecanol the methyl modes are strong-comparable to a monolayer of ODT on gold22-and the methylenemodes weak. We infer that the monolayer is densely packed with few gauche conformations. Since the polar head group (OH) is small and uncharged, there are no head-group repulsions to prevent close-packing of the molecules in the monolayer. The area per molecule of the shorter homologues at the airwater interface indicates the formation of a densely packed and possibly tilted monolayer at the air-water interface as well.39 For example, decanol has an area per molecule of 27-28 A2, compared with 20-25 A2 expected for densely packed hydrocarbon chains oriented at right angles to the surface of the water.Ia C14TAB and DPC. The weakness of the r+ mode in the SF spectra of C14TAB and DPC indicates a lack of orientational order in the methyl groups at the end of the hydrocarbon chain. The strong methylene modes imply the presence of considerable conformational disorder. Strong repulsions between the bulky, charged head groups disfavor lamellar structures: in dilute solutions CldTAB and DPC form spherical micelles. C14TABab and DPC4&do form planar bilayers in thecrystalline phase, but only with interdigitated chains to compensate for the mismatch between the cross-sectional area of the head group and the hydrocarbon chain. The surfactants cannot interdigitate with the densely packed monolayer of ODT. Consequently, the packing density of the chains is low and a liquidlike monolayer forms. A similar structure has emerged in studies of related surfactants at the air-water interface. Neutron reflectivity5 studies of C14TAB and molecular dynamics (MD) simulations of C16TAC4I monolayers reveal a highly disordered film with broad density profiles for the trimethylammonium groups and the chain termini. Surface tension measurements on C14TAB42 and DPC43 yield areas per molecule around 60 A2-about 3 times greater than densely packed hydrocarbon chains. SDS. SDS formed a more densely packed monolayer at the surface of dODT than C I ~ T A or B DPC (SDS has a lower value ofZ(d+)/Z(r+)) but one that was still conformationally disordered. This observation is consistent with the behavior of SDS at the air-water interface. Neutron reflectivity5b gives an area per molecule of 45 A2-less than the 56 A2 of C14TAB, but much greater than expected for a densely packed film. SDS has a large, charged head group and, like C14TAB and DPC, forms
Surfactants Adsorbed a t a Flat Hydrophobic Surface spherical micelles in dilute solution." SDS does, however, form planar monolayers more readily than these two cationic surfactants. For example, crystalline SDS contains bilayers that are n~ninterdigitated.~& C12E3. The nonionic surfactant, ClzE3, has the structure of Cl20H with three additional ethylene oxide (EO) groups in the head group. The effect of these EO groups on the packing of the surfactant is dramatic: Z(d+)/Z(r+) increases more than 10 times reflecting extensive conformational disorder in the monolayer. Despite the similarity between the SF spectra of Ci2E3 and the single-chain, ionic surfactants, the bulk phase behavior is different. C12E3 forms a lamellar structure in solution above its whereas the ionic surfactants form spherical aggregates. C12E3 also forms a more densely packed monolayer at the air-water interface (36 A2/molecule).6 The poly(ethy1ene oxide) chain appears to be disordered at both the air-water (from neutron reflectivity measurements)6 and solid-water interface (no headgroup resonances in SFS). In contrast, the alkyl chains of C12E3 are more disordered a t the surface of dODT than would be expected from its behavior at the air-water interface and in bulk solution. DDAPS. The film formed by DDAPS was the most disordered of the surfactants we studied-the r+ mode is virtually absent from its spectrum. The strong CH2 modes, however, confirm the presence of the surfactant at the surface in a film containing some net polar orientation. The large size of the polar head group, N+(CH3)2(CH2)3SO3- prevents close-packing of the hydrocarbon chains. The phase behavior of aqueous solutions of DDAPS is in line with the SF data, with spherical micelles existing over much of the phase diagram.46 DDAB and AOT. DDAB and AOT have two chains per molecule and consequently the head group and the hydrocarbon chains have comparable cross-sectional areas, which favors planar structures. For example, a closely related surfactant, ditetradecyldimethylammonium bromide (DTAB), is known to crystallize in flat sheets containing noninterdigitated bilayer^.^" Both DDAB and AOT form lamellar aggregates in dilute solutions, with areas per molecules of 68 A* 47 and 65 A2,48respectively. It is therefore not surprising that DDAB forms a more highly ordered monolayer at the surface of dODT than its single-chain relation Ci4TAB. Compared to dodecanol, however, DDAB is still disordered, with a higher value of Z(d+)/Z(r+)-more akin to the liquid-crystalline phase of lipids than a closely packed crystal.49 This is consistent with the formation of liquid-crystalline aggregates of DDAB in solution at room t e m p e r a t ~ r e . ~ ~ The SF spectrum of AOT cannot be interpreted quite as simply as the other surfactants, since the branched chains lack the local centrosymmetry of the linear chains. From the strength of the r+ modes in Figure 5 it is nevertheless clear that the monolayer formed by AOT at the surface of ODT is oriented but less densely packed than dodecanol, as would be expected from the molecular structure. A similar deduction has been made from adsorption isotherms of AOT on hydrophobic carbon black.37
Conclusion Sum-frequency spectroscopy has been used to study surfactants adsorbed at the solid-liquid interface. All of the amphiphilic surfactants studied formed monolayers at a hydrophobic surface with the hydrocarbon chains of the surfactant oriented away from the aqueous phase. The relative intensities of the CH3 and CH2 stretching modes depend sensitively on the orientational and conformational order in the adsorbed surfactant film. The observed structures span a wide range from the compact, highly oriented monolayer formed by dodecanol to the loosely packed, conformationally disordered monolayers formed by single-chain surfactants with bulky or charged head groups, such as C I ~ T A B , DPC, and DDAPS. In between lie molecules such as DDAB, in
The Journal of Physical Chemistry, Vol. 98, No. 34, 1994 8541 which the sizes of the chains and the head groups are more closely matched, and which may be viewed as forming liquid crystalline monolayers. With the exception of Ci2E3, which forms a more disordered monolayer than expected, the structures adopted by the surfactants a t a solid hydrophobic surface are broadly consistent with the behavior of the surfactants a t the air-water interface and in bulk phases.
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methylene peaks associated with gauche conformers near the head group, which are inevitable if the two chains are to lie parallel to each other.
