Langmuir 1994,10,2060-2063
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In-Situ Sum-Frequency Spectroscopy of Sodium Dodecyl Sulfate and Dodecanol Coadsorbed at a Hydrophobic Surface Colin D. Bain* Physical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, United Kingdom
Paul B. Davies* Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 IEW, United Kingdom
Robert N. Ward Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral L63 3JW, United Kingdom Received February 22, 1994. I n Final Form: May 2, 1994@ The adsorption of sodium dodecyl sulfate (SDS) and dodecanol from solutions of the pure and mixed surfactants onto hydrophobic substrates was studied by infrared-visible sum-frequency spectroscopy. Pure films of dodecanol were found to be more densely packed and to contain fewer gauche defects than those of SDS. A marked increase in the conformational order of the SDS occurred upon the coadsorption of dodecanol. This effect was only observed below the critical micelle concentration (cmc)-above the cmc, the dodecanol only absorbed weakly and resided mainly in the SDS micelles.
Introduction Commercial formulations containing surfactants invariably involve mixtures of amphiphiles rather than just a single component. To develop a detailed understanding of how these mixtures affect interfacial properties, we must know the structure of the surfactant film adsorbed at the interface. We have used sum-frequency spectroscopy (SFS) to study the coadsorption of sodium dodecyl sulfate (SDS)and dodecanol from aqueous solutions onto a solid hydrophobic surface. Mixed films of SDS and dodecanol have been studied extensively because SDS is a widely used surfactant and is frequently contaminated with dodecanol. Previous work on adsorption at the airwater i n t e r f a ~ e l - showed ~ preferential adsorption of dodecanol, but yielded little detailed information on the structure of the surfactant film. In this letter, we demonstrate that mere traces of dodecanol in solution have a dramatic effect on the structure of the surfactant monolayer adsorbed at the solid-liquid interface. In sum-frequency spectroscopy, a surface is irradiated simultaneously with a pulsed visible laser, which is fxed in frequency (wyiS), and a tunable pulsed infrared laser ( W I R ) . Light emitted at the sum-frequency (oyiB + WIR) is d e t e ~ t e d .The ~ sum-frequency signal changes when the infrared laser is in resonance with a vibrational mode that is both infrared and Raman a ~ t i v e .A~vibrational spectrum is obtained by measuring the intensity of the emitted light as a function of infrared frequency. Abstract published in Advance A C S Abstracts, June 1, 1994. (l)Miles, G. D. J . Phys. Chem. 1945, 49, 71. (2) Nilsson, G.J. Phys. Chem. 1957,61,1135.Tajima, K.; Maramatsu, M.; Sasaki, T.Bull. Chem. SOC.Jpn. 1969,42, 2471. (3) Penfold, J.; Thomas, R. K.; Simister, E.; Lee, E.; Rennie, A. J. Phys. Condens. Matter 1990, 2, SA411. Crowley, T. L.; Lee, E. M.; Simister, E. A.; Thomas, R. K.; Penfold, J.; Rennie, A. R. Colloid Surf, 1990, 52, 85 (4) For a recent review see: Eisenthal, K. B.Annu. Reu. Phys. Chem. 1992, 43, 627. (5) Bain, C. D.; Davies, P. B.; Ong, T. H.; Ward, R. N.; Brown, M. A. Langmuir 1991, 7, 1563. @
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Within the electric dipole approximation, no SF signal arises from centrosymmetric environmentsS6As a result, SFS is highly surface-specific: the solutions in our experiments are isotropic and do not give rise to SF emission, whereas the surfactant films adsorbed at the interface are asymmetric and, hence, sum-frequency active. In addition, the strength of a sum-frequency resonance carries information on the degree of orientational order ofthe adsorbed molecules. Films with a high degree of polar order give strong features while a totally disordered film gives no spectrum a t all.
Experimental Section The laser system used for sum-frequency spectroscopy at Cambridge and the experimental geometry of the sample cell ~ , ~ film (-1 pm) have been described in detail e l ~ e w h e r e . ~A,thin of the aqueous surfactant solution was trapped between the hydrophobic substrate and a calcium fluoride prism, through which the laser beams passed. The surfactant solution and the substrate were allowed to equilibrate for 1h at 18 i 1"C before the sample was brought close to the prism. For concentrations of SDS below M, several milliliters of surfactant solution were passed through the cell during equilibration so that the dodecanol removed from solution by adsorptionwas replenished. The substrates were self-assembled monolayers of octadecanethiol or perdeuterated octadecanethiol (ODT and d-ODT) on a thin film of gold that had been evaporated onto chromiumprimed silicon wafer^.^ The mixed solutions were composed of
SDS and dze-dodecanol (d-ClzOH)in water. d-ODT and d-C~zOH have no resonances in the C-H stretching region (Figure 1 a) and so only C-H modes of adsorbed SDS were observed from the mixed solutionsin contact with d-ODTIAu. Conversely, ODT and SDS contain no D atoms, and so only d-C120H was detected in the C-D stretching region when the mixed solutions were in contact with ODT/Au. (6) Shen, Y. R. Principles of Nonlinear Optics; Wiley: New York, 1984. (7) Ward, R. N.; Davies, P. B.; Bain, C. D. J . Phys. Chem. 1993,97, 7141. (8) Ward, R. N.; Du@, D. C . ; Davies, P. B.; Bain, C. D. J. Phys. Chem.. in Dress. (9) For recent review see: Dubois, L. H.; Nuzzo, R. G. Annu. Reu. Phys. Chem. 1992,43, 437.
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0 1994 American Chemical Society
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d-C120H
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its cmc and the dodecanol solutions were saturated. All the spectra show a large background signal away from resonance originating from the optical nonlinearity of the gold surface. Superimposed on this background are features due t6 the vibrational modes of surfactant molecules adsorbed to the substrate.12 In the C-H stretching region, the features a t 2874 and 2934 cm-l are assigned to the symmetric methyl stretch (r+),split by a Fermi resonance,13and the resonance a t 2962 cm-l to the antisymmetric methyl stretch (r-).13 In the C-D stretchingregion, the features a t 2070 and 2127 cm-l are assigned to the symmetric methyl stretch, again split by a Fermi resonance, and the one a t 2220 cm-l to the antisymmetric stretch. In addition to the methyl resonances, SDS and dodecanol also show two features a t about 2855 and 2900 cm-l, which we assign to the stretching modes of methylene groups (d).sJ4 d-Cl20H shows a weak feature a t approximately 2095 cm-’, which may be a CD2 resonance.15 In an all-trans chain, methylene groups are in a n environment which is locally centrosymmetric and, hence, are sum-frequency inactive. The methylene resonances are therefore assigned primarily to CH2 groups associated with gauche conformations, which reduce the local symmetry. We can deduce the polar orientation of the adsorbed surfactants by direct inspection of the SF spectra. The appearance ofthe methyl C-H resonances as “dipd’rather than “peaks” indicates that these groups are oriented toward the hydrophobic surface and away from the aqueous phase, as we have shown p r e v i ~ u s l y The . ~ degree of conformational order may also be deduced from the relative strengths of the methyl and methylene resonances. Disorder within the alkyl chains randomizes the orientations of the methyl groups and increases the number of gauche defects, weakening the methyl and strengthening the methylene r e s o n a n ~ e s . ~We J ~see from Figure 1 that the methyl resonances from h-Cl20H are stronger than those from SDS and the methylene resonances weaker. Thus, dodecanol forms a more highly ordered film than SDS a t the interface. The electrostatic repulsions between the sulfate head groups of adsorbed SDS molecules prevent close packing in the film and hence high order within the alkyl chains. Figure 2a shows SF spectra of SDS as a function of concentration. As the concentration is reduced below the cmc, the methyl resonances weaken, but the methylene resonances change little. This behavior indicates that the methylene chains become progressively more disordered as the concentration decreases, as might be expected from a decrease in the number density of adsorbed molecules.16 The intensity of the methylene resonances shows that there is significant adsorption even a t 1/30 cmc. However, by cmc/100 (not shown), the methylene resonances have largely disappeared as well. We now examine results from the mixed solutions (Figure 2b and c). Consider first those solutions below the cmc. Even when d-C120H is present a t a n impurity level of only 0.5% of the SDS concentration, the CD3
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Figure 1. Sum-frequencyspectra of pure surfactant solutions in contact with the hydrophobic substrates: (a)d-C120H,SDS, and C12OH in contact with d-ODT/Au; (b) SDS and d-C120H in contact with ODT/Au. The spectra are offset vertically for clarity-the zeros refer to the spectra of SDS. The sumfrequency, visible, and infrared beams were all p-polarized. All the mixed surfactant solutions contained dz6-dodecanou SDS in a molar ratio of 1:200. The most concentrated solution M) was above the critical micelle concentra([SDS] = 1.2 x tion (cmc 7 x 10-3 M),lO and so most of the dodecanol was
solubilized in the micelles. The other solutions were prepared by dilutingthis stock solution with water to concentrationsbelow the cmc. At these concentrations, the solubility of dodecanol should be approximatelythe same as in pure water (1.05 x M at 18 “C). Consequently, the dz6-dodecanol was above its M. solubility limit for [SDS]in the range 2.1 - 7 x Supersaturationdid not seem to be a problem since these solutions
gave the same spectra as solutions in which the dz6-dodecanol was exactly at its solubility 1imit.ll Evaporation ofd-ClzOHand hydrolysis of SDS were also found to be unimportant.
Results and Discussion Figure 1 shows SF spectra in the C-H and C-D stretching regions of pure aqueous solutions of dodecanol (h-ClZOH), &e-dodecanol, and SDS in contact with a hydrophobic substrate. In these spectra the SDS was a t (10)Estimated from light scattering measurements of mixed soluThe tions: Phillips, J. N.; Mysels, K. J. J. Phys. Chem. 1965,59,325. M: Flockhart, B. D. J . Colloid cmc of pure SDS solutions is 8.3 x Sci. 1961,16, 484. (11)Ward, R. N.Unpublished results.
(12)The signal does not arise from surfactant adsorbed to the CaFz prism. If it did, the relative phase ofthe signals from the surfactant and the gold and hence the appearance of the spectra, would depend on the thickness of the trapped film of solution. This is not the case. (13)MacPhail, R. A,; Straws, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1984,88, 334. (14)Guyot-Sionnest, P.; Hunt, J. H.; Shen, Y . R. Phys. Rev. Lett. 1987,59,1597. (15)CD2 stretching resonances are observed at 2105 and 2198 cm-’ in the Raman spectra of perdeuterated fatty acids: Hsi, S. C . ; Tullock, A. P.; Mantsch, H. H.; Cameron, D. G. Chem. Phys. Lipids 1982,31,97. (16)The strength of sum-frequency resonances depends on both the orientation and the surface density of the adsorbates. The extent to which these two quantities can be separated is discussed in ref 8.
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Figure 2. Sum-frequencyspectra at various concentrationsfor (a)pure SDS in contact with d-ODT/Au, (b)SDS with 0.5% d-C120H in contact unth d-ODT/Au, and (c) SDS with 0.5% dE-C120Hin contact with ODT/Au. The spectra are offset vertically-the zeros M. The beams were all p-polarized. refer to the spectra where [SDSI = 2.7 x resonances of d-C120H are 30-60% of the intensity observed with the pure dodecanol s01ution.l~A substantial fraction of a monolayer of dodecanol is clearly adsorbed at the interface. The presence of dodecanol in the monolayer has a remarkable effect on the spectrum of the coadsorbed SDSmolecules-the methyl resonances become much stronger while the methylene resonances decrease substantially in intensity (compare Figure 2a and b). Thus, the mixed monolayers are more densely packed with a higher degree of orientational order in the terminal methyl groups and have far fewer gauche conformations than the pure films of SDS. This increase in packing density reflects the ability of dodecanol to fill the free volume available in the monolayer without increasing the electrostatic repulsions between the SDS molecules (Figure 3). The sum-frequency spectra suggest that the mixed films of SDS and dodecanol between the cmc and 8.2 x M are densely packed and are conformationally well-ordered. At sufficiently low concentration (2.7 x M) the mixed films once again become disordered, reflecting a decrease in the total amount of adsorbed surfactant. Our data are consistent with previous measurements of the surface viscosity of films on water,18which indicated a condensed overlayer on a solution of 2.7 x M SDS containing 0.5% dodecanol a t 18 "C. Above the cmc the picture changes. Compare the spectra with [SDS] = 5.5 x M (below the cmc) and 1.2 x M (above the cmc). While the spectra at these two concentrations are identical for the pure solutions (Figure 2a), they are very different for the mixed solutions (Figure 2b and c). The resonances of the d-C120H disappeared as the concentration was raised above the cmc: dodecanol was no longer present in the monolayer. Above the cmc, the methyl resonances of the SDS (17) The strengths of the resonances in the spectra of d-C120H at and 2.7 x M were less reproducible than at other concentrations.C-D resonanceswere, however,always observed. (18)Ross,J. J . Phys. Chem. 1958, 62,531.
[SDSI = 8.2 x
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Figure3. Schematicillustration of possible structures for films adsorbed at the surface of the hydrophobic substrates: (a) solutions of pure SDS; (b)mixed solutions of SDS and dodecanol. weakened and the methylene resonances strengthened, suggesting that the low orientational and conformational order characteristic of pure SDS films had returned. The decrease in the adsorption of dodecanol above the cmc is caused by the competitive uptake of dodecanol by the micelles in solution. Some dodecanol was probably still
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adsorbed in the monolayer (below our detection limit in the C-D stretching region) because the SDS molecules were more conformationally ordered than in a solution of pure SDS a t the same concentration. Similar behavior has been observed a t the surface of the mixed solutions. Surface tension,l radio-tracer measurementsY2and neutron reflection3 all point to a decrease in the amount of adsorbed dodecanol above the cmc of the solution.
monolayer a t a hydrophobic surface whereas SDS yields a conformationally disordered monolayer. Solutions of SDS contaminated with 0.5%dodecanol produce densely packed monolayers below the cmc, in which both the SDS and dodecanol molecules contain few gauche defects. Above the cmc, dodecanol dissolves in the micelles and a disordered monolayer of SDS forms a t the solid surface, containing only small amounts of dodecanol.
Conclusion We have used sum-frequency spectroscopy to study the adsorption of a mixture of surfactants a t the solid-liquid interface. Dodecanol alone produces a densely packed
Acknowledgment. We thank Unilever Research, Port Sunlight Laboratory, and the SERC for support and Malkiat Johal and David Duffy a t the University of Cambridge for experimental assistance.
