Langmuir 1996,11, 2931-2937
2931
Polymer-Surfactant Aggregates at a Hydrophobic Surface Studied Using Sum-Frequency Vibrational Spectroscopy David C. Duffy and Paul B. Davies" Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 lEW, England
Andrew M. Creeth" Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral L63 3JW, England Received February 7, 1995. I n Final Form: May 8, 1995@
The structure of polymer-surfactant aggregatesadsorbed at a hydrophobicsurface has been investigated using sum-frequency spectroscopy (SFS). The surface vibrational spectra of a cationic surfactant (tetradecyltrimethylammonium bromide) show that the surfactant molecules form an oriented monolayer at the surface, with the polymer electrostatically binding to the charged head groups of the surfactant at the interface with the aqueous phase. The polymer-surfactant interaction, however, does not appear to increase the maximum packing density of the surfactant layer above that of a monolayer of pure surfactant at its cmc. The general structural features of the surfactant layer remain unaffected by the presence of polymer, but steric and hydrophobicinteractions between the polymer and surfactant influence the detailed adsorbate structure. The sum-frequency spectra indicate that low concentrations of negatively charged polymers cause the adsorptionof surfactant at lower concentrationsthan from a solution of pure surfactant. The effects of pH, polymer concentration,and added electrolyte on the polymer-surfactant adsorbate have been examined. The SF spectrum of the poly(acry1icacid) and deuterated cationic surfactant adsorbate provides evidence that this polymer also adopts a preferential orientation at the surface under certain conditions.
Introduction The interactions between surfactants and polymers have been studied for many years.lP2 Those between ionic polymers and surfactants of opposite charge have been characterized for relatively low levels of polymer as follows.1p2 At low concentrations of surfactant there is no interaction; above a critical surfactant concentration, the critical aggregation concentration (cac), binding to form a polymer-surfactant (PS)aggregate begins. The surfactant association to the polymer is cooperative, highly so for polymers of high charge density. As the level of bound surfactant approaches that required to neutralize the polymer, precipitation occurs. Addition of excess surfactant can redissolve this precipitate if the polymer charge density is below a certain value3 or if nonionic surfactant is mixed with the charged ~ u r f a c t a n t . ~The ,~ forces which control the association are a combination of electrostatic and hydrophobic interactions between polymer, surfactant, and solvent molecules.lS2 The commercially important applications of PS aggregates often derive from their interactions at surfaces. For instance, they are used in the wetting and flotation of minerals, in enhanced oil recovery, as deposition aids, and for surface conditioning.lg6 However, there have been Abstract published in Advance A C S Abstracts, July 1, 1995. (1) Interactions of surfactants with polymers andproteins, Goddard,
@
E. D., Ananthapadmanabhan,K. P., Eds.; CRC Press: Boca Raton, FL, 1993. _. .. (2) Hayakawa, K.; Kwak, J. C. T. In Cationic Surfactants-Physical
Chemistry; Rubingh, D. N., Holland, P. M., Eds.; Dekker: New York, 1991; p 189. (3) Goddard, E. D.; Hannan, R. B. J.Am. Oil Chem. SOC.1977,54, 561. (4) Dubin, P. L.;Otari, R. J. Colloid Interface Sci. 1983, 95, 453. (5) Dubin, P. L.;Vea, E. Y.; Fallon, M. A.; The, S. S.;Rigsbee, D. R.; Gan, L. M. Langmuir 1990, 6,1422. (6) Goddard, E. D. J. Oil Chemists SOC.1994, 71,1.
relatively few studies of the adsorption of PS aggregates to interfaces and little is known about the structure of such a d ~ o r b a t e s . ~ This - l ~ is particularly true for the case of polymers and surfactants of opposite ~ h a r g e . l ~Here, -~~ we report the vibrational spectra, obtained by sumfrequency spectroscopy (SFS),of oppositely charged surfactant and polymer coadsorbed at a hydrophobic surface. Straightforward interpretation of these spectra provides information on the orientation and conformation of both the surfactant and polymer in the adsorbate. (7) Somasundaran, P. J. Colloid Interface Sci. 1989,31,557.
(8) Tadros, Th.F. J. Colloid Interface Sci. 1974, 46, 923.
(9) Faucher, J. A.; Goddard, E. D. J.Colloid Interface Sci. 1976,55, 313. (10) Chibowski, S. J. Colloid Interface Sci. 1980, 76,371. (11) Chibowski, S.;Szczypa, J. Pol. J. Chem. 1982, 56,359. (12) Ma, Ch. -M. Colloids Surf. 1985, 16,185. (13) Ma, Ch. -M.; Li, Ch. -L Colloids Surf. 1990, 47,117. (14) Claesson, P. M.; Malmsten, M.; Lindman, B. Langmuir 1991, 7,1441. (15) Chari, K.; Hossain, T. 2.J. Phys. Chem. 1991, 95, 3302. (16) Kilau, H. W.; Voltz, J. I. Colloids Surf: 1991, 57,17. (17) Esumi, K.; Oyama, M. Langmuir 1993, 9, 2020. (18) Otsuka, H.; Esumi, K. Langmuir 1994, IO, 45. (19) Musabekov, K. B.; Omarova, K. I.; Izimov, A. LActaPhys. Chem. 1983,29,89 (20) Moudgil, B. M.; Somasundaran, P. In Fundamentals ofAdsorp-
twn, Proceedings of the Engineering Foundation Conference, 1983; Mayers, A. L., Belfort, G., Eds.; Engineering Foundation: New York, 1984; p 355. (21) Somasundaran, P.; Cleverdon, J. Colloids Surf 1986, 13,73. (22) Moudgil, B. J.; Somaaundaran, P. Colloids Surf: 1986, 13,87. (23)Amold,G. B.; Bruer, M. M. Colloids Surfi 1985, 13,103. Goddard, E. D.;Tirrell, (24) Ananthapadmanabhan,K P.;Mao, G. -2.; M. Colloids Su$ 1991, 61,167. (25) Esumi, K.; Masuda, A.; Otsuka, H. Langmuir 1993, 9, 284. (26) Shubin, V. Lungmuir 1994, 10,1093. (27) Knox, W. J.; Parshall, T. 0. J. Colloid Interface Sci. 1970, 33, 16. (28) Goddard, E. D.; Hannan, R. B. J.Colloid Interface Sci. 1976,55, 73. (29) Buckingham,J. H.; Lucassen, J.;Hollway, F. J.Colloid Interface Sci. 1978, 67,423.
0743-746319512411-2931$09.0010 0 1995 American Chemical Society
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For adsorption of oppositely charged PS aggregates to a solid phase, the presence of both components can reduce, have little effect on, or increase the adsorbed level of either component, depending on the charge of the solid phase, the pH, and solution condition^.^^-^^ The adsorption of PS aggregates at the air-water interface (of particular relevance to this study) has been examined using surface tension measurement^:^^-^^ a strongly synergistic effect was observed,with lower surface tensions occurring due to the PS adsorbate at much lower levels of surfactant than in the absence ofpolymer. These studies suggest that, at the air-water interface, a model for adsorption would be a layer of surfactant at the interface with polymer adsorbed to the surfactant layer. This was also the model used, at the solid-liquid interface, to explain the synergistic wetting of hydrophobic coal particles by anionic surfactants and poly(oxyethy1ene) (PE0).16 Sum-frequency spectroscopy is a surface specific technique which we have used to elucidate the structure of polymer and surfactant adsorbed at a hydrophobic solidliquid interface. In SFS, a visible laser beam of fixed frequency ( w ~and ~ )a tunable infrared laser beam (WIR) are pulsed simultaneously onto a surface and light is emitted at the sum of their frequencies The intensity of the light emitted is proportional to Ix@)12where x@) is the second-order nonlinear susceptibility. There are two contributions to x@): a nonresonant term (&k) from the substrate, which varies little with the infrared . ~ a~ result, when frequency, and a resonant term ( ~ f ) ) As OIR coincides with the resonant frequency of a vibrational mode, the intensity ofthe emitted light changes. Detecting the emitted light as a function of infrared frequency produces a vibrational spectrum. ~ ( is~ zero 1 in centrosymmetric media. This has two important consequences. First, a vibrational spectrum is produced only of molecules at an interface, where the centrosymmetry of the bulk phases is broken. Second, the interfacial molecules must have a net polar orientation-no SF emission results from molecules arranged with an equal number of opposite orientations or from a completely disordered surface structure. Solutions containing mixtures of polymer and surfactant are isotropic so any net polar orientation in either the surfactant or polymer adsorbed at a surface will result in a SF spectrum of the PS adsorbate. The SF spectrum contains information on the orientation and conformational order of the surfactant and polymer molecules adsorbed at the surface. Polar orientation is determined from the relative phase of the resonant and nonresonant ~ i g n a l ~and ~ a the ~ degree of conformational order is reflected in the strength of the resonance^.^^ We have used one cationic surfactant, tetradecyltrimethylammonium bromide (c14TAB), and examined the surface aggregates that it forms with a number of anionic polymers, uiz.,poly(styrenesulfonate) (PSS), poly(acrylicacid) (PAA), and a copolymer ofmaleic acid and ethylene (PMAE).The aggregates formed by these combinations of polymer and surfactant have been extensively studied in the bulk phase1,2,35-38 and therefore they provide a suitable system to study using SFS. (30) Shen, Y. R. Nature 1989, 337, 519. (31) Eisenthal, K. B. Annu. Rev. Phys. Chem. 1992,43, 627. (321 Bain, C. D.; Davies, P. B.; Ong, T.H.; Ward, R. N.; Brown, M. A. Langmuir 1991, 7, 1563. (33) Ward, R. N.; Davies, P. B.; Bain, C . D. J. Phys. Chem. 1993,97, 7141. (34)Ward, R. N.; D u m , D. C.; Davies, P. B.; Bain, C . D. J. Phys. Chem. 1994,98,8536.
Chart 1
903
I
poly(acry1ate)
poly(styrene sulfonate)
PAA
PSS
Copolymer of maleate and ethylene PMAE
Experimental Section The SF spectrometer at Cambridge has been described in detail elsewhere.39 The pulsed visible laser beam at 532 nm was produced by doubling the output of a Nd:YAG laser in a KD*P crystal. Tunable infrared in the range 2700-3100 cm-l was provided by the stimulated Raman scattering of a dye laser beam in high pressure H2.39,40 The adsorption studies were carried out in a standard liquid cell.41An aqueous solution containing the surfactant and polymer was trapped between a prism (made of either fused silica or calcium fluoride) and a planar substrate. The solution was allowed to adsorb with the substrate 10 mm from the prism. Typically, no change in the spectrum was observed for adsorption times in excess of 10 min. The substrate was then moved to within l p m ofthe prism and the laser beams coupled in through the prism and overlapped on the substrate covering an area of about 2 mm2. The resulting SF emission was detected by a photomultiplier tube. The sum-frequency, visible, and infrared beams were all p-polarized. Acquisition of a spectrum took around ‘12 h and the spectra were subsequently corrected for the dispersion and absorption of the infrared beam by the prism.34 The substrate was a self-assembled monolayer of perdeuterated octadecanethiol adsorbed on gold (dODT/Au). These monolayers have been extensively c h a r a c t e r i ~ e dand ~ ~ are known to provide a well-defined and robust hydrophobic surface. The thiol was fully deuterated to ensure that it did not contribute to the SF spectra in the C-H stretching region. Perprotonated tetradecyltrimethylamonium bromide (hC14TAB; Aldrich, 99%)was recrystallized 3 times from 9 5 5 acetone/ methanol until no minimum was observed in its surface tension curve. The perdeuterated surfactant (dC14TAB) was used as received from Dr. R. K. Thomas (Oxford University). PSS (Aldrich, MW = 70 0001, PAA (Polysciences,MW zz 50 OOO), and PMAE (Fluka, MW = 50 000) were used as received. The structures of the anions of these polymers are shown in Chart 1. Polymer-surfactant solutions were freshly prepared in pure water (Elga “Maxima Ultrapure” deionized water) and allowed to equilibrate overnight. The most extensively studied system was hC14TABlPMAE. The majority ofthe spectra presented here were recorded using solutions well away from concentration regimes in which precipitates form in the PS mixtures. However, (35)(a) Hayakawa, K.;Kwak, J. C. T. J. Phys. Chem. 1982,86,3866. (b) Hayakawa, K.; Kwak, J. C . T.J. Phys. Chem. 1983, 87, 506. (c) Hayakawa, K.; Santerre, J.P.;Kwak, J. C . T.Macromolecules 1983,16, 1642. (d) Hayakawa, K.;Ayub, A. L.; Kwak, J. C. T.Colloids Surf 1982, 4,389. (e)Hayakawa, K.; Santerre, J. P.; Kwak, J. C . T.Biophys. Chem. 1983,17,175. (0Malovikova, A,;Hayakawa, K.; Kwak, J. C . T.J. Phys. Chem. 1984,88, 1930. (g) Santerre, J.P.; Hayakawa, K.; Kwak, J. C . T.Colloids Surf 1985,13,35.(h)Shimizu, T.; Seki, M.; Kwak, J.C . T. Colloids Surf 1986,20,289.(i) Gao, Z.; Kwak, J.C. T.; Wasylishen, R. E. J. Colloid Interface Sci. 1988, 126, 371. (j) Gao, 2.; Wasylishen, R. E.; Kwak, J. C . T.J . Phys. Chem. 1990,94, 773. (36) Kieffer, J. J.; Somasundaran, P.; Anathapadmanabhan, K. P. Langmuir 1993, 9, 1187. (37) Hansson, P.; Almgren, P. Langmuir 1994, 10, 2115. (38)Abuin, E. B.; Scaiano, J. C. J. Am. Chem. SOC. 1984,106,6274. (39) Ong, T.H.; Davies, P. B.; Bain, C . D. Langmuir 1993,9, 1836. (40) Rabinowitz, P.; Perry, B. N.; Levinos, N. J. IEEE J. Quantum Electron. 1986. QE-22. 797. (41) Bewick,’A:; Kunimatsu, R;Pons, S.; Russell, J.W. J. Electroanal. Chem. 1984,160,47. (42) (a) Bain, C. D.; Whitesides, G. M.Angew. Chem. 1989,101,522. (b) Nuzzo, L. H.; Dubois, D. L. Annu. Reu. Phys. Chem. 1992,43,437.
PS Aggregates at a Hydrophobic Surface
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I
*
I
1 1 1 1
-3
Langmuir, Vol. 11, No. 8, 1995 2933
I +I
-2
cmc log,, (surfactant concentration/ mol dm3)
Figure 1. Schematic phase diagram of the C14TAB/Pw system at pH 7. The hatched area indicates the narrow region over which neutral polymer-surfactant complex was seen to
precipitate from the solution-in all other regions the solutions were clear. The cmc of C14TAB is marked on the x-axis. to ensure that the spectra were not affected by the adsorption of solid PS complexes, the phase behavior of solutions of hC14TAB and PMAJ3 was investigated. A schematic phase diagram of this system (in the concentrationregions examined)is shown
in Figure 1. This diagram illustrates that cloudy solutions only formed in a narrow range of surfactant concentrations near the cmc, allowing us to interpret the SF spectra in terms of the adsorption of soluble PS aggregates. Polymers20s21 and surfactants@are known to adsorb at silica-it was therefore important to ensure that the SF spectra did not containresonant features from molecules adsorbed at the prism. Although it is unlikely that signal will arise from these molecules,34 several control experiments, described in detail elsewhere,34were carried out for each new system studied to confirm this. These experiments unambiguously showed that the resonant features in the SF spectra presented here arise only from molecules adsorbed at the hydrophobic substrate.
Results Orientational and Conformational Order of the PS Adsorbate. Figure 2 shows the SF spectra in the C-H stretching region of solutions containing hC14TAB and 500 ppm of the three different anionic polymers, adsorbed at the hydrophobic surface, as a function of surfactant concentration. The polymer concentration was chosen to ensure suflFicient interaction between the polymer and surfactant at the surface. The spectra of hC14TAEi in the absence of polymer are included for comparison (Figure 2a). The concentration of C14TAB in each solution is expressed as a fraction of the critical micellar concentration (cmc = 3.5 x M).44 The corresponding SF spectra to those in Figure 2 with the alkyl chains of the surfactant molecules fully deuterated contained no resonances; i.e., the CH2 groups of the adsorbed polymer molecules did not contribute to the SF spectra. Therefore these spectra can be interpreted in terms of the structure of adsorbed surfactant molecules alone: only in one case did the polymer contribute to the SF spectra (videinfra). Resonant signal in the SF spectra in Figure 2 indicates that the surfactant forms an oriented monolayer in both the presence and absence of polymer. The resonances in the SF spectra in Figure 2 arise from the C-H stretches of the surfactant hydrocarbon chains and they have been fully assigned elsewhere:34methyl modes (originating from the terminal CH3 groups) are labeled r in Figure 2a, and methylene modes (from the CH2 groups of the chain) are labeled d. (43) Rumrecht. H.: Gu. T. Colloid PoZvm. Sci. 1991.269.506. (44) Rosin, M.’J. Surfaces a n d Inte$aciaZ Phenomena,’ 2nd ed.; Wiley: New York, 1989.
Figure 2 shows that solutions of pure surfactant give detectable SF resonances down to Vlo0 cmc. However, in the presence of polymer, resonances appear at much lower concentrations: down to VIOOOO cmc for CI~TAEWMAE. The effect of the polymer on the surfactant SF spectra is most strikingly demonstrated by comparing the spectra of at ‘1300 cmc, with and without PMAE: pure surfactant does not form an oriented monolayer (there are no resonances) but PMAE causes the surfactant to orient at the surface, producing a spectrum very similar to that of pure surfactant at l/10 cmc. The SF spectra in Figure 2b-d clearly show that the synergistic interaction between the polymer and the surfactant causes the surfactant to adsorb at lower bulk concentrations than the surfactant alone.45 This mirrors the reduction in the cac observed in the bulk solution (vide supra): in both cases the aggregation of surfactant molecules is being made more favorable by the interaction with polymer. The shape of the resonances in the SF spectra indicate the orientation of the surfactant in the PS adsorbates. There is destructive interference between the SF emission from the gold and from the surfactant molecules;33i.e., the resonances appear as dips, in both the presence and absence of polymer. We have already shown that the appearance of methyl and methylene resonances as dips in the spectra of pure surfactant indicates that they form monolayers with their alkyl chain pointing toward the hydrophobic surface and their charged head group toward the aqueous p h a ~ e . The ~ ~ surfactant ,~~ molecules in the PS adsorbate therefore also appear to form a monolayer with this net polar orientation. The relative strengths of the methyl and methylene modes indicate the conformational order of the adsorbed ~ u r f a c t a n t .Strong ~~ methyl modes are indicative of a monolayer with a high degree of orientational order, and weak methyl modes arise from a disordered monolayer. In an all-trans chain, the CH2 groups are locally centrosymmetric and consequently SF inactive.32 Chain disorder, which arises from gauche defects, disrupts this centrosymmetry and consequently methylene resonances are observed in the SF spectrum. The ratio of the intensities of the methylene and methyl symmetric stretches, Z(d+)/Z(r+),can be a useful indicator of conformational order.34 For example, it is clear that pure c14TAB forms a relatively disordered monolayer-the rf mode is weak and the d+ strong.34 Comparison of the relative intensities of these modes in Figure 2 reveals that the surfactant molecules in the PS adsorbate have a maximum conformational order similar to that of a pure monolayer of c14TAB at bulk concentrations in the range ‘/30-l/10 cmc. It is clear that the interaction between polymer and surfactant never results in an increase in conformational order and, by inference, in the packing density, above that of a “saturated” monolayer of pure surfactant. Above the cmc, the SF spectra of all of the PS adsorbates are comparable to that of a pure surfactant monolayer indicating the same packing density in each case. A similar observation has been made in PS adsorbates at the air-water i n t e r f a ~ efrom ~ ~ ~surface ~~ tension measurements-at concentrations greater than the cmc, the surface excess of the PS solutions converged with that (45) We have also examined the interactions between these anionic polymers and other cationic surfactants (dodecylpyridinium chloride and didodecyldimethylammonium bromide). These surfactants behaved in a similar fashion to C14TABwhen adsorbing at a hydrophobic surface in the presence of polymer, and subsequent experiments concentrated on C14TAB. Preliminary experiments on a mixture of anionic surfactant and positively charged polymer (sodium dodecyl sulfate (SDS)/cationically modified poly(acry1amide)) indicated that the results presented here were representative ofthe general behavior of systems of oppositely charged surfactant and polymer.
2934 Langmuir, Vol. 11, No. 8, 1995
Duffy et al.
b) C,,TAB
/
c) &,TAB I PSS
PMAE
d) C,,TAB / PAA
Infrorod mvmumber (cm-')
Figure 2. SF spectra of solutions of hC14TAB (a) without polymer and with 500 ppm of (b)PMAE, (c) PSS, and (d) PAA, all at pH 7, in contact with dODT/Au. In all cases, the concentrationof surfactant in solution is given in terms of the cmc of pure C14TAB. The spectra are offset for clarity and the spectrum of the pure surfactant at l/loo cmc refers to the zero intensity.
L
1
I
o! . . . . , . . . . , . . . . , . . . . 2800
2850
2900
2950
I
3000
Infrared wavenu mber (cm" ) Figure 3. SF spectra of solutions of (a) 500 ppm PAA at pH 5 and 500 ppm PAA and l/30cmc dC14TAB at (b) pH 7 and (c) pH 5, in contact with dODT/Au. Spectrum a refers to zero intensity. ofpure surfactant solutions. Goddard has suggested that this is due to the desorption of polymer from the surface into bulk solution as micelles are formed.6 Our results are suggestive of a similar process at the solid-liquid interface but cannot directly confirm it. The SF spectra in Figure 2 demonstrate that the structure of the surfactant layer is dependent on the polymer in the adsorbate. The surfactant layer in C14TABPMAE! has a maximum packing density comparable to cmc pure surfactant. C14TAl3PSS has a less well packed surfactant layer-the r+ mode is much weaker, and at '/~oo cmc a very disordered monolayer seems to form. More compelling evidence for the dependence of the surfactant structure on the polymer is provided by Figures 3 and 4. Figure 3 shows the SF spectra in the C-H stretchingregion of the c14TAB/pAA adsorbate with the surfactant chains deuterated, under various conditions. In the absence of surfactant no resonances appear (Figure 3a), indicating that, on its own, the polymer does
2800
2850
2900
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3000
Infrared wavenumber (cm-') Figure 4. SF spectra of solutions of hC14TAB and 500 ppm of PAA at pH 5,in contact with dODT/Au. The spectrum at l / 1 ~ cmc refers to zero intensity.
not adopt a net orientation a t the surface. This is also the case for the adsorbed polymer in solutions containing dC14TAB at pH > 5 (Figure 3b). However, at pH = 5 , the SF spectrum of dC14TABPAA has a dip at 2916 cm-l (Figure 3c). This mode is assigned to a CH2 stretch of the polymer,46indicating that the CH2 groups of PAA adopt (46) Allara, D.L.; Atre, S. V.; Parikh, A. N. In Polymer Surfaces and Interfaces II; Feast, W . J., Munro, H. S., Richards, R. W., Eds.; John Wiley & Sons Ltd.: Chichester, 1993. Kaji, K. J . Appl. Polym. Sci. 1986, 32, 4405.
Langmuir, Vol. 11, No. 8, 1995 2935
PS Aggregates at a Hydrophobic Surface
0 2800
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2950
3000
Infrared wavenumber (cm-') Figure 5. SF spectra of solutions of hC14TAB at l/300cmc and various concentrations of PMAE in contact with dODT/Au. The spectrum at 1ppm refers to zero intensity.
a net orientation at the surface. Resonant signal was not observed from the polymer in any other PS adsorbate at any pH. Figure 4 shows the effect on the conformational order of the alkyl chains of the surfactant of the orientation of PAA: the spectra were taken with both the polymer and surfactant perprotonated. Theoretical curve fitting32 of the spectra in Figures 2d and 4 reveals a significant increase in the strength of the d+ mode (ranging from 10 to 30%) of those spectra recorded at pH 5. This suggests that the conformationalorder of the surfactant is decreased on the orientational binding of PAA at pH 5. In order to examine the factors affecting the formation of PS adsorbates, the C14TABPMAE system was studied in some detail. Effect of Polymer Concentration o n the C14TAB/ PMAE Adsorbate. Figure 5 shows the SF spectra of hC14TAB at 1/3W cmc with various concentrations ofPMAE. The spectra show that very low polymer concentrations are required to cause the enhanced adsorption of surfactant: the effect starts as low as 0.75 ppm. The small range of polymer concentrations over which surfactant adsorption is completed further illustrates the strong cooperativityin the PS interaction. It was also found that at polymer concentrations between 10 and 1000 ppm, the concentration of surfactant at which an oriented monolayer started to form was constant. A similar observation has been made from a surface tension study of the anionic surfactant SDS, and poly-L-lysine, where it was found that the cac was independent of polymer concentration once a minimum value had been reached.29 Effect of Added Electrolyte on the CI~TAB/PMAE Adsorbate. The addition of relatively low concentrations ( M) of nonassociative salt (NaC1)to solutions containing surfactant and 500 ppm PMAE did not change the surfactant concentration at which an oriented adsorbate started to form at the surface. This is perhaps surprising given the increase in the cac of the cationic
surfactantPMAE system on the addition of 0.005 M NaC1.36h The only salt effect on the PS adsorbate that was detected by SFS is shown in Figure 6, which demonstrates the effect of high concentrations of KBr on the surfactant monolayer with and without PMAE being present. It is clear that at l/3W cmc, very high concentrations of added salt have a marked effect on the SF spectra of hC14TAB. The large increase in the strength of the r+ methyl mode suggests that the packing density of the monolayer has increased. This is likely to be caused by the shielding of repulsive forces between the surfactant head groups. However, when 5 M KBr is added to a solution of C14TAB/PMAE, the increase in the packing density in the surfactant layer is not as marked-the r+ mode is clearly weaker. It would appear that the polymer is so strongly bound to the surfactant that it is not fully displaced by small counterions even at extremely high concentrations of added Br-. The obvious implication of this observation is that there is a significant hydrophobic element to the binding of polymer to surfactant. Effect of pH on the ClrTAB/PMAE Adsorbate. Figure 7 shows the effect of pH on the hC14TABlPMAE system. There is very little variation in the conformational order and packing density of the surfactant between pH 5 and 9. Given that the degree of dissociationof the acidic groups on the polymer increases linearly with increasing pH35hand that the polymer and surfactant associate largely through electrostaticinteractions, this observation is perhaps surprising. However, the net charge density of the polymer in the absence of surfactant remains essentially constant throughout this pH range due to counterion c ~ n d e n s a t i o n .The ~ ~ structure of the polymer layer above the surfactant could also change within this pH range but would remain undetected by SFS. However, it appears that any such changes in the polymer do not affect the structure of the surfactant layer. The changes in the SF spectrum at pH 3 are much clearer: the resonant strength is obviously weaker indicating that less surfactant is adsorbed at the surface. At this pH the polymer is effectively uncharged in solution;36thus it might be expected that no oriented adsorbate would form at all. However, it is known that surfactant can displace protons from polyacrylate at low pH, and that PS aggregation is favored by the greater hydrophobic nature of the polymer at low charge densities.36 Discussion It is believed that polymers and surfactants in solution interact to form micelle-like surfactant aggregates with the polymer associated at the micelle surface. Generally, the micelles appear to be similar in structure and size to those formed in the absence of Thus although the polymer nucleates the formation of micelles at much lower concentration than the cmc, it does not necessarily impose significant structural changes on the micelle. The SF spectra presented here indicate that this behavior is repeated at the planar hydrophobic solid-water interface. In both the presence and absence ofpolymerthe SF spectra show that the surfactant molecules adsorb as a monolayer with their charged headgroups pointing toward the aqueous phase and that generally the adsorbate conformational order and packing density differ little from that in the absence of polymer. Thus a likely structure, with the polymer adsorbed electrostatically to the surfactant monolayer, is shown in Figure 8. This proposed structure confirms that inferred from surface tension measurements at the air-water i n t e r f a ~ and e ~ postulated ~ ~ ~ ~ at the solidwater interface.I6 At the solid-liquid interface, the polymer can be envisaged as a large counterion bound to the surfactant
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2936 Langmuir, Vol. 11, No.8, 1995
.-, , I
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30
Infrared wavenumber (cm-') Figure 6. SF spectra of solutions of hC14TAB at l/300cmc and various concentrations of KI3r (a)without polymer and (b) with 10 ppm PMAE, in contact with dODT/Au. The spectrum at 1M KBr refers to zero intensity. aqueous phase
+
bound anionic polymer "counterion"
600
? tu
C',TA+ monolayer
W
.-E v)
hydrophobic surface
E
$! 400
.E
SI C
Figure 8. Schematic diagram of the polymer-surfactant adsorbate deduced from the SF spectra. The cationic surfactant adsorbs to the surface as a monolayer and the negatively charged polymer binds to its headgroups as an extended counterion. Details of the polymer layer, for instance whether further surfactant micelles are located in the overlayer, cannot be determined by SFS: complementary data from other surface techniques are required.
0)
3
u
t"I
200
5
v)
0 2800
2850
2900
2950
3000
Infrared wavenumber (cm" )
Figure 7. SF spectra of solutions of hC14TAB at ~ / ~cmc O Oand O 500 ppm PMAE at varying pH, in contact with dODT/Au. The pH 7 spectrum refers to zero intensity.
monolayer, as visualized in Figure 8. The polymer "counterion" is unusual in that it probably binds to many surfactant molecules which enhances surfactant-surfactant interactions thereby promoting aggregation, whereas a monomeric counterion binds only to one surfactant molecule. This means that very low concentrations of polymer are required to dramatically reduce the surfactant concentration at which bulk aggregation and surface adsorption commence, as we observe experimentally (Figure 5). As stated above, the general structural features of the adsorbed Surfactant layer do not change substantially on addition of polymer. However, the detailed structure can
be significantly perturbed. This is illustrated by several observations. First, the addition of polymer does not increase the maximum packing density above that of pure surfactant. This is presumably because the steric limitations of the large polymer molecule places an upper limit on the closest approach of the surfactant molecules bound along the polymer chain. This also seems to be the case at the air-water interface: the surface tension measurements of B u ~ k i n g h a mand ~ ~ Goddard28indicate that the maximum packing density of a surfactant in a PS adsorbateis less than that of a pure surfactant monolayer. Second, the SF spectra of C14TABPSS and C14TABI PAA indicate that steric and hydrophobic interactions between the surfactant and polymer can have a significant effect on the surfactant structure. From Figure 2c, it appears that PSS is less effective than PMAE in causing the adsorption of surfactant a t low concentrations. At first glance this would appear to contradict the findings of several other studies35aj*37*38 which demonstrated that PSS is the most effective polymer in lowering the cac of alkyltrimethylammonium surfactants. However, these
Langmuir, Vol. 11, No. 8, 1995 2937
PS Aggregates at a Hydrophobic Surface studies also found that the aggregation number of surfactant molecules in aggregates with PSS was significantly lower than with other polymer^.^^^^* The explanation of this given by Kwak was that hydrophobic contacts between the styrene group of the polymer and the surfactant chains reduce the surfactant-surfactant interactions along the polymer chain thereby decreasing the number of surfactant molecules in the aggregate. This hypothesis has been verified by NMR data which show that the styrene group of PSS penetrates into the hydrocarbon region of the bulk phase surfactant aggregate.35i It would appear that the strong influence on micelle structure imposed by PSS is also reflected in the relatively disordered adsorbate structure at the solidwater interface. The SF spectra of the C14TABPAA adsorbate at pH 5 (Figures 3 and 4)indicate that the polymer adopts a net polar orientation and that there is an accompanying disruption in the conformational order of the surfactant layer. The phase of the polymer resonance (Figure 3c) suggests that the CHZgroups of the polymer are oriented toward the gold surface and we infer that it is likely that the hydrophobic part of the polymer penetrates into the hydrophobic part of the monolayer. We have drawn a similar conclusion from the phase of the SF resonances of small organic counterions bound to a charged monolayer: 47 these ions are known to be located in the hydrophobic region of the surfactant aggregate.48 Penetration of the hydrophobic backbone of the polymer into the surfactant layer would provide an explanation for the resultant conformational disordering of the surfactant chains that is evident from Figure 4. Two questions arise from the observation of a SF-active vibrational mode in bound PAA: first, why does the polymer only show a net orientation at pHs of 5 or less and, second, why is the CHZ mode of PAA SF active whereas those of PMAE are not? Some understanding as to why the polymer only orients at low pHs is provided by a recent study using a surfactant-sensitiveelectrodewhich found that the cac of C14TA13was reduced more by PAA of low charge density.36 This was attributed to a change in the conformation ofthe polymer from extended to coiled on reduction of the ionization of the chain. A coiled conformation might result in the penetration of the polymer into the monolayer and allow it to adopt a net polar orientation. The fact that PMAE does not have any SF-active C-H modes could be due to several factors.First, (47) Duffy, D. C.; Davies, P.B.; Bain, C. D. Submitted t o J. Phys. Chem. (48)Fendler, J . H.; Fendler, E. J . Catalysis in Micellar and Macromolecular Systems; Academic Press: New York,1975.
P W , with its longer hydrocarbon backbone, is likely to adopt more conformationsthan PAA, thereby randomizing the orientations of the CH2 groups and resulting in an overall cancellation of the SF signal. Second, if P W binds to C14TA+through only one ofits carboxylate groups, with the other pointing into the aqueous phase, then there would be an equal number of oppositely oriented CH2 groups in the polymer yielding no net SF signal. Third, if the polymer binds through both ofits carboxylate groups (as is postulated in the bulk phase aggregates of C14TAB/ PMAE3,S6then the CHZgroups will be lying flat in the plane of the surface and their modes will consequently be SF inactive.
Conclusion Sum-frequency spectroscopyhas been used to probe the structure of a polymer-surfactant adsorbate at a hydrophobic surface. The SF spectra show that the interaction between a polymer and a surfactant of opposite charge results in enhanced adsorption of the surfactant at low concentrations. The phase of the resonant signal from the surfactant indicates that the structure ofthe adsorbate is composed of a monolayer of surfactant molecules, adsorbed with their headgroups pointing toward the aqueous phase and with the polymer binding to the headgroups. We envisage the polymer as a large countenon, binding to many surfactant molecules thereby increasing surfactant-surfactant interactions and promoting adsorption. Unlike monomeric counterions, such as bromide, the polymer counterion did not increase the maximum packing density of the surfactant layer above that of a pure surfactant monolayer. The detailed monolayer structure in the PS adsorbate can be significantly perturbed by specific interactions between the polymer and the surfactant-both PAA and PSS seem to penetrate into the hydrocarbon region of the monolayer. This was strikingly illustrated by the preferential orientation adopted by PAA in the C14TABPAA adsorbate; i.e.,there appears to be structure in the polymer layer as well as the surfactant layer. Further experiments are underway to determine the effect of the nature of the hydrophobic side chain on the orientation of the bound polymer.
Acknowledgment. We thank the EPSRC and Unilever Research (Port Sunlight Laboratory) for equipment grants and a CASE studentship for D.C.D. and Dr. Robert N. Ward (Unilever Research) for helpful comments on the manuscript. Dr. R. K. Thomas kindly supplied perdeuterated C14TAB. LA950091R
