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Composition of Mixed Hydrocarbon and Fluorocarbon Surfactant Adsorbed Layers at Mica-Solution Interfaces Tim W. Davey,†,§ Gregory G. Warr,*,† and Tsuyoshi Asakawa‡ School of Chemistry, The University of Sydney, New South Wales 2006, Australia, and Department of Chemistry and Chemical Engineering, Faculty of Engineering, Kanazawa University, Kanazawa 920, Japan Received January 29, 2003. In Final Form: April 8, 2003 The compositions of the adsorbed aggregates at the mica-solution interface have been directly determined for mixtures of pyridinium chloride hydrocarbon and fluorocarbon surfactants by solution depletion measurements, including direct determination of the adsorbed layer composition for partially miscible surfactants. The measured adsorbed aggregate compositions are compared with predictions for bulk micelles using the group contribution method and regular solution theory. For fully miscible surfactants, there is good agreement between the models and experimental results. The group contribution method successfully predicts the existence of a miscibility gap for some mixtures, but adsorption experiments reveal some unusual features; the adsorbed layer consists of only one of the two coexisting micelle compositions, and its composition differs from that predicted. This effect is attributed to the substrate.
Introduction Mixed surfactant systems have been extensively studied for a wide range of applications. By tuning or engineering the interactions between the different components, one can exploit nonideal mixing effects to raise or lower mixed critical micelle concentrations (cmc’s), often referred to as synergism or antagonism.1 Micelle shape and size can also be dramatically altered by these interactions. Of particular interest are mixtures of hydrocarbon and fluorocarbon surfactants. Under certain conditions, these can demix into hydrocarbon- and fluorocarbon-rich micelles.2,3 This was first investigated by Mukerjee and Yang,4 who showed the existence of both a maximum in the mixed cmc (antagonism) and a “second” cmc as evidence for two kinds of coexisting micelles. The behavior of surfactant mixtures is usually described theoretically by a pseudophase separation model. Surfactants may be treated as an ideal mixture,5 or interactions may be included by using either a regular solution description6 or the group contribution method.7 Experimental verification of micelle demixing has proven to be difficult, even yielding contradictory results. The situation was recently summarized by Kadi et al.9 * To whom correspondence should be addressed. E-mail: g.warr@ chem.usyd.edu.au. † The University of Sydney. ‡ Kanazawa University. § Present address: Research and Development, Dulux Australia, P.O. Box 60, Clayton South, VIC 3168, Australia. (1) Hua, X. Y.; Rosen, M. J. J. Colloid Interface Sci. 1982, 90, 212. Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley: New York, 1978. (2) Funasaki, N. In Mixed Surfactant Systems; Surfactant Science Series No. 46; Marcel Dekker: New York, 1992; pp 145-188. Barthe´le´my, P.; Tomao, V.; Selb, J.; Chaudier, Y.; Pucci, B. Langmuir 2002, 18, 2557. (3) Asakawa, T.; Hisamatsu, H.; Miyagishi, S. Langmuir 1995, 11, 478. (4) Mukerjee, P.; Yang, A. Y. S. J. Phys. Chem. 1976, 80, 1388. (5) Clint, J. H. J. Chem. Soc., Faraday Trans. 1975, 17, 1327. (6) Holland, P. M.; Rubingh, D. N. J. Phys. Chem. 1983, 87, 1984. Rubingh, D. N. In Solution Chemistry of Surfactants [Proc. Sect. 52nd Colloid Surf. Sci. Symp.]; Plenum: New York, 1979; Vol. 1, p 337. (7) Asakawa, T.; Johten, K.; Miyagishi, S.; Nishida, M. Langmuir 1985, 1, 347. (8) Shinoda, K.; Nomura, T. J. Phys. Chem. 1980, 84, 365.
who also provide some of the strongest evidence yet for two coexisting populations from a combination of solubilization, NMR, and cryogenic transmission electron microscopy (cryo-TEM) studies. A few studies have also examined hydrocarbon-fluorocarbon mixtures at the airwater interface,8,10-12 and some nonideal behavior has been reported. Adsorption isotherms of surfactant mixtures at solidsolution interfaces have also been studied extensively.12-14 However, there has been relatively little work on the adsorption of individual components of mixed surfactant solutions.15-17 There are many similarities between adsorbed layers at hydrophilic solid-solution interfaces and bulk micelle structure. This is exemplified by atomic force microscopy (AFM) studies showing micellelike aggregates on mica and other solid substrates,18-21 but even in the context of the bilayer picture, the conceptual similarities are still there; like bulk micelles, the adsorbed layer consists of a hydrophobic core region out of contact with the solution and the solid, coated on both sides by headgroups. (9) Kadi, M.; Hansson, P.; Almgren, M.; Furo´, I. Langmuir 2002, 18, 9243. (10) Zhu, B. Y.; Zhang, P.; Wang, R. X.; Liu, Z. F.; Lai, L. H. Colloids Surf., A 1999, 157, 63. (11) Imae, T.; Takeshita, T.; Kato, M. Langmuir 2000, 16, 612. (12) Simister, E. A. Ph.D. Thesis, University of Oxford, Oxford, U.K., 1994. (13) Huang, L.; Maltesh, C.; Somasundaran, P. J. Colloid Interface Sci. 1996, 177, 222. (14) Scamehorn, J. F.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1982, 85, 479. (15) Harwell, J. H.; Roberts, B. L.; Scamehorn, J. F. Colloids Surf. 1988, 32, 1. (16) Portet, F.; Desbene, P. L.; Treiner, C. J. Colloid Interface Sci. 1996, 184, 216. (17) Davey, T. W.; Warr, G. G.; Almgren, M.; Asakawa, T. Langmuir 2001, 17, 5283. (18) Manne S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409. Manne, S.; Gaub, H. E. Science 1995, 270, 1480. (19) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1999, 15, 1685. (20) Schulz, J. C.; Warr, G. G. Langmuir 2002, 18, 3191. Schulz, J. C.; Warr, G. G.; Butler, P. D.; Hamilton, W. A. Phys. Rev. E 2001, 63, 041604. (21) Velegol, S. B.; Fleming, B. D.; Biggs, S.; Wanless, E. J.; Tilton, R. D. Langmuir 2000, 16, 2548.
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Table 1. Structures and cmc’s of Surfactants Used in This Study
a
Conductivity, ref 3.
In a previous paper, we examined the composition and structure of a mixture of hydrogenous tetradecyltriethylammonium chloride (TEC14Cl) and fluorous heptadecafluorododecylpyridinium chloride (HFDePC; see Table 1) surfactants at the mica-water interface, showing that HFDePC formed rodlike aggregates on mica very similar to those reported for conventional cationic surfactants.17-19 In that work, the mixed adsorbed layers were found to be enriched in HFDePC to a much greater extent than in mixtures with hydrogenous surfactants of similar or even much lower critical micelle concentrations. Although solution demixing (predicted using the group contribution method3,7) was identified as a possible cause, the effects of the different headgroups resulting in different strengths of binding to the mica substrate, and that of surface aggregate shape transformations, could not be ruled out. We have extended this previous study to hydrocarbon and fluorocarbon cationic surfactants with identical pyridinium headgroups and chloride counterions, including systems in which nonideal mixing is expected both with and without solution demixing. The structures, abbreviations, and cmc’s of the surfactants used in this study are shown in Table 1. Using regular solution theory, Shinoda8 described the conditions for demixing into two populations of micelles of different compositions. HFOPC (1H,1H,2H,2H-perfluorooctylpyridinium chloride), which has a six-fluorocarbon chain, is expected to be insufficient to cause demixing, whereas HFDePC has eight fluorinated carbons and should not mix completely with hydrogenous surfactants.8 The focus of this work is on adsorbed layer compositions rather than morphology. Surface compositions are measured directly by solution depletion experiments, and these are compared with calculated aggregate compositions using both the group contribution method3,7 and regular solution theory.5 In making these calculations, we are assuming that the surface compositions are identical to those of the calculated bulk micelles. That is, the only effect of the surface is to lower the critical concentration for aggregate formation. Materials and Methods Chemicals. 1H,1H,2H,2H-perfluorodecylpyridinium chloride (HFDePC, C8F17(CH2)2N+C5H5 Cl-) and 1H,1H,2H,2H-perfluorooctylpyridinium chloride (HFOPC, C6F13(CH2)2N+C5H5 Cl-) were prepared as previously described.3 Dodecylpyridinium chloride (DPC), tetradecylpyridinium chloride (TPC), and cetylpyridinium (CPC) were the same as in a previous study.17 Water was obtained from a Milli-Q system and had a conductivity of 18 MΩ cm-1. All experiments were carried out at neutral pH. Mica powder was obtained from GMS Industrial Pty Ltd (Melbourne, Australia). This was the same as that used in our
previous study and had a surface area of approximately 5 m2 g-1 as determined by methylene blue adsorption. Atomic Force Microscopy. AFM images of the adsorbed surfactant layer were measured as described previously.17 Solution Depletion Studies. The adsorption of surfactants on mica powder was determined using a solution depletion technique as described previously.17 Equilibrium concentrations of both surfactants in the depleted mixture were measured using a Waters HPLC system fitted with a 431 conductivity detector and a Symmetry C18 (5 µm, 3.9 × 150 mm) column and operating with Maxima 820 software. The solvent system was a mixture of HPLC-grade methanol (Riedel-de Hae¨n) and water (85:15) with 0.2 M NaCl (Univar, 99.9%). Sample injection volumes were 50 and 200 µL. Micellar Pseudophase Diagrams. The group contribution method3,7 was used to calculate the cmc’s for mixtures of HFOPC, HFDePC, DPC, TPC, and CPC as described previously.17 To determine micellar mole fractions for concentrations above the mixed cmc, a procedure similar to that of Rubingh6 was used that includes a consideration of counterion dissociation. According to Shinoda et al.,8 the following relationships hold at the cmc:
C1m(C1m + C2m)Kg ) C11+Kg1x1F1
(1)
C2m(C1m + C2m)Kg ) C21+Kg2(1 - x1)F2
(2)
where C1m and C2m are the monomer concentrations of surfactants in the mixed systems, C1 and C2 are the cmc’s of the pure surfactants, and Kg is the fractional micelle counterion binding, while the subscripts 1 and 2 refer to surfactant 1 and 2. (For all the surfactants used here with chloride counterions, Kg was between 0.57 and 0.60.) x1 is the micellar composition, and F is the activity coefficient. F1 and F2 are calculated using the group contribution method. Combining eqs 1 and 2 gives
Cm1+Kg ) C11+Kg1x1F1 + C21+Kg2(1 - x1)F2
(3)
Above the cmc, mass balance requires
x1 ) (RCt - C1m)/(Ct - C1m - C2m)
(4)
where R is the mole fraction of surfactant 1 in the overall binary mixture and Ct is the total surfactant concentration. By substituting for C1m and C2m from eqs 1 and 2 into eq 4, one obtains a quadratic expression, the solution for which is
x1 ) [(E - Ct) + x{(Ct - E)2 + 4RCtE}]/2E
(5)
E ) [C21+Kg2F2 - C11+Kg1F1]/CmKg
(6)
where
Equations 3 and 5 were solved by iteration to provide a solution for x1 given R and Ct (10 mM in all cases). The initial input value of x1 was taken to be equal to R.
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Figure 1. Mixed cmc’s predicted for (a) TPC-CPC, (b) HFOPC-DPC/TPC/CPC, (c) HFDePC-TPC, and (d) HFDePC-CPC, using the group contribution method. The dotted line indicates the total surfactant concentration used in the experiments (10 mM). Regions containing tie lines in (c) and (d) indicate the bulk composition range over which micelle coexistence is expected. For certain compositions, R, above the mixture cmc, hydrocarbon- and fluorocarbon-rich micelles may coexist with compositions XH and XF, respectively. XH and XF were determined using the group contribution method3,7 as described previously.
Results and Discussion Prediction of Mixed cmc’s. Figure 1 shows the cmc’s for the surfactant mixtures examined in this study as a function of bulk mole fraction, R, calculated using the group contribution method.3,7 This approach has previously been shown to successfully reproduce experimentally determined mixed cmc’s for surfactant mixtures, including the hydrocarbon and fluorocarbon pyridinium surfactants studied here.22 Partial miscibility in surfactants is more often described by regular solution theory,6,23 in which interactions between the two components are characterized by a single interaction parameter, β, derived from the interchange energy. β may be positive indicating repulsions between unlike components, zero in the case of ideal mixing, or negative if there is attraction. β values in excess of 2 lead to a miscibility gap and the coexistence of two different micelle compositions.23 Calculation of micelle compositions in this description follows the same general procedure as described above. In this description, the activity coeffecients F1 and F2 are calculated from the interaction parameter β:6
F1 ) exp[β(1 - x1)2]
(7)
F2 ) exp[βx12]
(8)
Best fit β values from a regular solution model incorpo(22) Asakawa, T.; Ishikawa, K.; Miyagishi, S. J. Colloid Interface Sci. 2001, 240, 365-367.
Table 2. β Values Determined from Fits of a Regular Solution Model to Mixed cmc Curves Calculated Using the Group Contribution Methoda system: component 1-component 2
β
TPC-CPC HFOPC-DPC HFOPC-TPC HFOPC-CPC HFDePC-TPC HFDePC-CPC
0 1.4 1.5 1.5 2 2
XH
XF
miscible miscible miscible miscible 0.41 0.65 0.40 0.72
a X and X denote the mole fractions of fluorinated surfactant H F in the hydrocarbon- and fluorocarbon-rich coexisting micelles, respectively, calculated using the group contribution method.
rating counterion binding8 to the mixed cmc’s calculated using the group contribution method are listed in Table 2. Thus for the TPC-CPC system (Figure 1a) ideal mixing is expected, whereas nonideal mixing is generally predicted between fluorocarbon and hydrocarbon surfactants and is exhibited by all other systems studied here (Figure 1b-d). This observation is substantiated by experimental cmc measurements.22 A miscibility gap is predicted by the group contribution method for HFDePC/TPC and HFDePC/CPC mixtures. Coexistence regions of hydrocarbon-rich and fluorocarbon-rich micelles with coexisting micelle compositions XH and XF (Table 2), respectively, calculated using the group contribution method are shown by the tie lines in Figure 1c,d. The best regular solution fit to the group contribution cmc’s of the HFDePC systems is β ) 2, which is the minimum necessary to obtain a miscibility gap and yields (23) Mixed Surfactant Systems (Developed from a Symposium Sponsored by the ACS Division of Colloid and Surface Chemistry at the 65th Colloid and Surface Science Symposium, Norman, Oklahoma, June 17-19, 1991); Holland, P. M., Rubingh, D. M., Eds.; ACS Symposium Series, Vol. 501; American Chemical Society: Washington, DC, 1992.
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Table 3. Surfactant Adsorbed Layer Morphologies as Imaged by AFM surfactant
aggregate morphology on mica
HFOPC HFDePC DPC TPC CPC
straight rods straight rods (ref 17) globules flexible rods (ref 17) straight rods (ref 17)
two coexisting micelle populations of virtually identical composition. This differs somewhat from the group contribution prediction. Direct determination of the compositions of coexisting micelles is difficult and rarely undertaken, so this quantity is usually inferred from the cmc(R) curve. In the following, we directly measure the composition of the adsorbed layer of these fully and partially miscible surfactants by solution depletion of both adsorbing components. Adsorbed Layer Morphology. The adsorbed layer structures of the pure-component surfactants on mica at twice their cmc’s are listed in Table 3. All images showed surfaces to be fully covered with the aggregates described. As noted in our earlier work,17 changes in surface composition may be accompanied by structure transformations. However, many of the mixtures studied here contain components with similar or identical adsorbed layer morphologies. We have not examined the adsorbed layer structure of mixed systems in this work. The observation of globular aggregates for DPC is consistent with expectations based on previous studies of quaternary ammonium surfactants adsorbed on mica;19 dodecyltrimethylammonium chloride (DTAC) also forms globular aggregates on mica, although the bromide salt DTAB forms cylinders. Similarly, TPC forms cylinders as does tetradecyltrimethylammonium chloride. The cylindrical structure in the HFOPC adsorbed layer is also broadly consistent with the patterns observed here and in previous work for the adsorbed layer structure of hydrogenous surfactants, and with the greater hydrophobicity and rigidity of fluorous chains. Shortening the alkyl chain of poly(ethylene oxide) nonionic surfactants from dodecyl- to decyl- disrupts the hemicylindrical aggregates24 and leads to laterally unstructured adsorbed layers on graphite.25 However, the shorter hydrophobic chain of lithium perfluorooctanesulfonate still produces short hemicylinders.26 The salient point for this work is that the combinations of hydrocarbon and fluorocarbon surfactants examined involve mixing different aggregate morphologies. Aggregate shape appears to have little influence on the adsorbed layer compositions. Indeed, some of the most striking effects occur for mixtures of components which both adsorb as cylinders. Adsorbed Layer Compositions. We have studied the adsorption of binary micellar surfactant mixtures onto mica by solution depletion. Initial concentrations were chosen such that after mixing with mica powder, the total final bulk concentration was 10 mM, which in most cases is above the cmc of the mixture concerned. We consider the composition of the surface layer in equilibrium with the bulk solution for a mixture of two hydrocarbon, or hydrocarbon and fluorocarbon, surfactants. We also compare differences between the composition measured (24) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1997, 13, 4349. Patrick, H. N.; Warr, G. G. Colloids Surf., A 2000, 162, 149. (25) Grant, L. M.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. B 1998, 102, 4288. (26) Lamont, R.; Ducker, W. J. Colloid Interface Sci. 1997, 191, 303.
Figure 2. Aggregate versus bulk solution compositions of mixtures of tetradecyl- and hexadecylpyridinium chloride surfactant systems at Ct ) 10 mM. Points show experimental compositions in the surface layer, and the solid line shows the predicted composition of solution micelles calculated using the group contribution method. This curve is indistinguishable from the ideal mixing or regular solution calculations; see Table 2.
Figure 3. Adsorbed layer versus bulk solution compositions of mixtures of HFOPC-DPC ([) at Ct ) 10 mM. Lines show predicted solution micelle compositions at the mixed cmc, calculated using the group contribution method (solid line), regular solution theory (long dashes), and ideal solution theory (short dashes).
for the micelles adsorbed at the surface and predicted for the micelles in solution. Figure 2 shows the measured mole fraction of CPC in the adsorbed layer for the (ideally mixing) CPC-TPC system as a function of the bulk composition. CPC is enriched in the surface layer due to its higher hydrophobicity (lower cmc) and therefore tends to partition preferentially into the adsorbed aggregates from aqueous solution. The observed surface compositions are very similar to those predicted for the solution micelles by either the group contribution method or ideal mixing. The equilibrium concentration of 10 mM is well above the cmc’s of all TPC/CPC mixtures studied (see Figure 1a), so the solution composition is a weighted average of both micelle and monomer compositions. In these calculations and all those following, we have used the bulk cmc’s for the pure components to calculate micelle compositions. We have not attempted to measure critical concentrations for surface aggregation or micellization from, for example, adsorption isotherms. As these are lower than bulk cmc’s, this should lead to aggregate compositions closer to the overall composition at any particular total surfactant concentration. An exception might occur if the surface cmc’s were more different from each other than the bulk cmc’s. Figure 3 shows the adsorbed layer compositions in mixtures of HFOPC and DPC as a function of bulk
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Figure 4. Adsorbed layer versus bulk solution compositions of mixtures of HFOPC-TPC (9) at Ct ) 10 mM. Lines show predicted solution micelle compositions at Ct ) 10 mM calculated using the group contribution method (solid line), regular solution theory (long dashes), and ideal solution theory (short dashes).
Figure 5. Adsorbed layer versus bulk solution compositions of mixtures of HFOPC-CPC (2) at Ct ) 10 mM. Lines show predicted solution micelle compositions at Ct ) 10 mM calculated using the group contribution method (solid line), regular solution theory (long dashes), and ideal solution theory (short dashes).
composition at a total concentration of 10 mM. These solutions are all below the bulk cmc’s of the mixtures, which amplifies the difference between the bulk (monomer) and surface (aggregated) compositions. Also shown for comparison are the predicted micelle compositions using the group contribution method, regular solution theory, and ideal mixing, all calculated at their mixed cmc’s. Ideal mixing fails to describe the results even qualitatively, as would be expected both by inspection of the cmc versus composition of this system (see Figure 1b and ref 22) and from the fitted β ) 1.4. An inherent limitation in the use of a phase separation model for calculating micelle compositions is that there are simply no micelles below the cmc with which to equilibrate. We therefore assume that the surface acts only to effectively lower the “surface cmc” and that the composition of the surface aggregates is the same as in bulk. Micelles have certainly been shown to exist on solid surfaces in which the bulk concentration was below the cmc.21 Even with these assumptions, this approximation will have some effect on calculated compositions. The micelle compositions are derived from equating the chemical potentials of the micellized and monomer surfactants, which in turn depend on their bulk concentration. However, this is only expected to be small in the systems examined, where the concentrations are never far below half of the cmc. The group contribution and regular solution models both describe the sigmoidal experimental results for HFOPCDPD mixtures equally well. Regular solution theory suggests slightly less segregation of the two components. These results are typical of moderate repulsions between the two components. However, we note that the experimental surface compositions are richer in the partially fluorinated surfactant than either model predicts over much of the composition range studied. This may reflect a shortcoming of the theory or may be due to the mica surface. Figure 4 shows adsorbed layer compositions for mixtures of HFOPC and TPC, together with the predicted compositions of the three models. In this case, the mixed cmc exceeds 10 mM above a bulk HFOPC mole fraction of RHFOPC ) 0.83 (slightly higher for the ideal mixing approximation; see Figure 1). The calculated compositions above this point are, like those of Figure 3, for micelle compositions at the mixed cmc and should be interpreted with the same caveats. Both group contribution and regular solution models describe these results well over the entire composition range. Even the ideal mixing model captures the essential
qualitative feature, that the (surface) aggregates are substantially enriched in the more hydrophobic TPC over a wide range of bulk compositions, although it is by no means in quantitative agreement with experiment. As with the HFOPC-DPC mixtures, repulsions between unlike components increase the difference between aggregate and overall compositions, excluding the less hydrophobic HFOPC from the TPC aggregates. In this case, the surface aggregates remain almost pure TPC until its bulk mole fraction is below 0.2, consistent with calculations of both regular solution and group contribution models. Little can be concluded regarding the models from surface compositions above RHFOPC ) 0.83, however. The surface composition must increase from xHFOPC ) 0.1 to 1 as RHFOPC increases from 0.83 to 1, and it is expected that the surface compositions would change abruptly when the solution concentration intersects the mixed cmc curve (see Figure 1). This effect is even more apparent with HFOPC-CPC mixtures, shown in Figure 5. The adsorbed layer contains less than 5% of the fluorinated compound even at bulk compositions as high as 98% HFOPC, and this is once again in good agreement with the predicted aggregate compositions based on either group contribution or regular solution models. As in Figure 4, the compositions change abruptly when the total concentration intersects the mixed cmc. In the HFDePC-TPC and HFDePC-CPC systems, a significant miscibility gap is predicted by the group contribution calculation, which should be experimentally evident at 10 mM (see Figures 1c,d). However, regular solution theory places these systems on the cusp of coexistence. Figure 6 shows the measured surface compositions for HFDePC-TPC and HFDePC-CPC, together with the predictions of the three models. Unlike HFOPC-TPC and HFOPC-CPC, these systems are above their mixed cmc’s at all compositions studied (see Figure 1). The striking feature of both of these systems is the dramatic enrichment of the adsorbed layer with HFDePC relative to the bulk compositions, which include coexisting micelle populations. This selectivity for fluorinated chains is unexpected, as the mica substrate is hydrophilic and adsorption is driven by electrostatic and ion-exchange mechanisms.27 None of the models for aggregate compositions seem to adequately describe the experimental results for these systems, which both show a fluorocarbon-rich adsorbed (27) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160.
Composition of Mixed Surfactant Adsorbed Layers
Figure 6. Adsorbed layer versus bulk solution compositions of mixtures of (a) HFDePC-TPC (9) and (b) HFDePC-CPC (2), all at Ct ) 10 mM. Solid lines show predicted micelle compositions at Ct ) 10 mM using the group contribution method, and dashed rectangles show the predicted miscibility gap. Long dashes show the calculated micelle compositions using regular solution theory, and short dashes show ideal mixing.
layer of constant composition across much of the bulk composition range studied. Considering Figure 6a more closely, the group contribution method predicts a coexistence rectangle shown by the dashed rectangle. That is, between bulk HFDePC mole fractions of 0.39 and 0.53, two different populations of micelles coexist, each with constant compositions of XHFDePC ) 0.41 and 0.65. The solid line traces the average composition of the two micellar “phases” as the relative population of each changes with the overall composition of the mixture. (The regular solution model with β ) 2 has a coexistence rectangle of zero area.) However, in contrast with partially miscible insoluble layers,28-30 these aggregates are in equilibrium with bulk
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solution and one of the two coexisting micelle populations may preferentially adsorb onto the mica substrate. This leads to an adsorbed layer composition which follows the perimeter of the coexistence rectangle in one direction or the other, depending on which of the two coexisting micelle populations is preferentially adsorbed (Figure 6). Both paths yield a constant adsorbed layer composition over the entire coexistence region and a step change at one end or the other of the rectangle. The experimental results are thus consistent with an even larger miscibility gap than that predicted by the group contribution calculation, together with the preferential adsorption of fluorocarbon-rich aggregates onto the mica surface. The situation is depicted schematically in Figure 7. This interpretation is also consistent with results reported previously for mixed HFDePC-TEC14Cl adsorbed layers on mica,17 suggesting that this phenomenon is quite general. The difference in observed and calculated coexistence compositions may point to a shortcoming in the group contribution model, although it qualitatively does better than the simpler regular solution approach. Why is an adsorbed layer rich in fluorinated chains preferred throughout the coexistence region? In these systems, the headgroups of both surfactants in the mixture are the same, so little difference in their ability to compete for the surface should be expected. We are therefore obliged to revise our previous conclusion that differences in the electrostatic attraction between the mica surface and triethylammonium or pyridinium headgroups cause preferential adsorption of HFDePC.17 One might argue that HFDePC, with its shorter and more rigid alkyl chain, could pack more densely on the mica surface than TPC and CPC and lead to exclusion of hydrocarbon surfactant through fluorocarbon-hydrocarbon repulsion; however, this is at odds with the observed total adsorbed amount of surfactant, which is invariant with RHFDePC,surf (within error; data not shown). Note also that HFDePC, TPC, and CPC all form cylindrical aggregates on mica, which should yield similar adsorbed amounts, as observed. Hydrophobicity is again ruled out as a simple explanation. In the HFOPC-TPC and HFOPC-CPC systems, the adsorbed layer is enriched in the more hydrophobic hydrocarbon chain, but CPC also has a lower cmc than HFDePC. In any case, the micelles present in bulk adequately accommodate the hydrophobic effect. Selective adsorption based on headgroup effects is ruled out by this work, yet our previous study suggests that the effect is qualitatively the same for all headgroups. The reason for the selectivity must be a general effect common to all fluorinated surfactant chains. We conclude therefore
Figure 7. Schematic diagram of the adsorbed layer composition of a surfactant mixture such as HFDePC + CPC or TPC in equilibrium with coexisting micelles, in which the fluorocarbon-rich micelle is preferentially adsorbed. This yields a constant composition for the adsorbed layer as the bulk composition is varied across the coexistence region.
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that the fluorinated surfactant preferentially adsorbs because of stronger dispersion force attractions to the mica. This would be resolved by comparing adsorbed layer compositions in the coexistence region for a range of different substrates. The difference between measured adsorbed layer compositions and calculated micelle compositions for HFDePC-TPC and HFDePC-CPC in the coexistence region may be a further consequence of dispersion forces, leading to adsorbed layers enriched in fluorocarbon relative to bulk micelles at all compositions. Neither the regular solution model nor the group contribution model considers the effect of the substrate. As noted above, the fully miscible HFOPC-DPC system (Figure 3) also shows enrichment of the adsorbed layer in the fluorinated surfactant with respect to both models, especially near R ) 0.5, again suggesting some selectivity for fluorinated surfactants by this hydrophilic substrate. Conclusions The composition of the adsorbed layer at the micasolution interface for miscible pyridinium chloride hydrocarbon and fluorocarbon surfactants is well described (28) Suga, K.; Yamada, N.; Fujihira, M. Colloids Surf., A 2002, 198200, 127. (29) Overney, R. M.; Meyer, E.; Frommer, J.; Guentherodt, H.-J.; Fujihira, M.; Takano, H.; Gotoh, Y. Langmuir 1994, 10, 1281. (30) Yagi, Kazuto; Fujihira, M. Appl. Surf. Sci. 2000, 157, 405.
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by group contribution and regular solution models. The surface is, like a bulk micelle, generally enriched in the more hydrophobic component, but intermolecular interactions can lead to enhanced segregation of the two components. In systems for which partial miscibility is predicted by the group contribution method, the adsorbed layer shows behavior consistent with the existence of two discrete micelle types. We believe this is the first direct measurement of the compositions of coexisting surfactant aggregates. Although qualitatively correct, current theories are unable to predict the experimentally measured compositions of the coexisting aggregates. In addition, one kind of micelle is preferentially taken up into the adsorbed layer, leading to a significant difference between the bulk and surface compositions. We conclude that this is largely due to dispersion forces. Acknowledgment. T.W.D. acknowledges the receipt of a Henrie Bertie and Florence Mabel Gritton Research Fellowship from the University of Sydney. This work was funded by the Australian Research Council. We thank Ms. Annabelle Blom for her assistance with some of the AFM studies. LA034150+
