Langmuir 2001, 17, 5283-5287
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Self-Assembly of Hydrocarbon and Fluorocarbon Surfactants and Their Mixtures at the Mica-Solution Interface Tim W. Davey,† Gregory G. Warr,*,† Mats Almgren,‡ and Tsuyoshi Asakawa§ School of Chemistry, University of Sydney, New South Wales 2006, Australia, Department of Physical Chemistry, Uppsala University, Uppsala, Sweden, and Department of Chemistry and Chemical Engineering, Faculty of Engineering, Kanazawa University, Kanazawa 920, Japan Received January 16, 2001. In Final Form: May 14, 2001 The adsorbed layer structure of tetradecylpyridinium, hexadecylpyridinium, heptadecafluorodecylpyridinium, and tetradecyltriethylammonium chloride and their mixtures on mica has been determined by AFM imaging. In addition, the composition of the mixed adsorbed layers has been measured, showing a significant surface enrichment of the pyridinium surfactants, particularly the partially fluorinated species. Shape transitions in the adsorbed layer are correlated with surface and bulk compositions and explained by consideration of the adsorption mechanism.
Introduction The adsorption of surfactant mixtures onto solid substrates has been widely studied by adsorption isotherms, both deliberately and by studies of industrial surfactant preparations and systems containing adventitious impurities.1,2 The classic example is the adsorption behavior of polydisperse alkylphenol ethoxylates.3 Recently some effort has been made to understand the component adsorption isotherms of various mixtures, and this has been used to infer surface structure.4 The adsorbed layer structure of a number of surfactants at solid-liquid interfaces has been imaged in situ by atomic-force microscopy (AFM).5,6 The structures formed on hydrophilic substrates resemble bulk solution micelles and include spheres, rods, and bilayers. While adsorption isotherms of mixtures have been extensively studied, there is only one previous report where AFM was used to examine the adsorbed layer structures formed by aqueous mixtures of surfactants. Ducker and Wanless7 imaged the surface structures of mixtures of zwitterionic (dodecyldimethylammonio)propanesulfonate (DDAPS) and cationic dodecyltrimethylammonium bromide (DTAB). They observed that with increasing solution mole fraction of DTAB, the aggregates become more rodlike. Although DTAB was concluded to exhibit preferential adsorption to the surface, the adsorbed layer compositions were not known. In this work we examine the adsorbed layer structure of a series of cationic surfactant mixtures on mica by AFM * To whom correspondence should be addressed at the University of Sydney. E-mail:
[email protected]. † University of Sydney. ‡ Uppsala University. § Kanazawa University. (1) Huang, L.; Maltesh, C.; Somasundaran, P. J. Colloid Interface Sci. 1996, 177, 222. (2) Scamehorn, J. F.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1982, 85, 479. (3) Kira´ly, Z.; Borner, R. H. K.; Findenegg, G. H. Langmuir 1997, 13, 3, 3308. (4) Portet, F.; Desbene, P. L.; Treiner, C. J. Colloid Interface Sci. 1996, 184, 216. (5) Warr, G. G. Curr. Op. Colloid Interface Sci. 2000, 5, 88. (6) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409. (7) Ducker, W. A.; Wanless, E. J. Langmuir 1996, 12, 5915.
Figure 1. Structures and abbreviations of the surfactants used in this work.
imaging. Combining this with depletion adsorption studies on powdered mica, we also determine the composition of the adsorbed layer in equilibrium with its bulk solution. All experiments are carried out above the critical micelle concentration (cmc) of the mixed systems and hence describe the fully developed or saturated adsorbed layer. Two classes of mixed systems are examined. In the first, mixed adsorbed layers of tetradecyltriethylammonium chloride (TEC14‚Cl) and alkylpyridinium chlorides are compared. In the second TEC14‚Cl is mixed with a partially fluorinated pyridinium surfactant heptadecafluorodecylpyridinium chloride (HFDePC). The structures and abbreviations of the surfactants used in this study are shown in Figure 1. Aqueous mixtures of hydrocarbon and fluorocarbon surfactants are known to demix into hydrocarbon- and fluorocarbon-rich micelles at certain solution compositions and concentrations.8 There are few previous studies examining the mixing of hydrocarbon and fluorocarbon surfactants on surfaces. However, these do suggest nonideal mixing behavior similar to that exhibited by micelles. Langmuir-Blodgett monolayers of long-chain (8) Asakawa, T.; Hisamatsu, H.; Miyagishi, S. Langmuir 1995, 11, 478.
10.1021/la010092l CCC: $20.00 © 2001 American Chemical Society Published on Web 07/24/2001
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surfactants were studied by AFM and shown to demix into two regions.9,10 Mixtures of ammonium perfluorooctanoate and ammonium decanoate at the air-water interface showed highly nonideal behavior by neutron reflectometry at certain compositions but seemed to behave ideally at others.11 The effect of hydrocarbon/fluorocarbon immiscibility on the composition and structure of the adsorbed layer on mica is also addressed in this work. Experimental Section Chemicals. 1H,1H,2H,2H-Perfluorodecylpyridinium chloride (HFDePC) was prepared as previously described.8 Tetradecyltriethylammonium chloride (TEC14‚Cl) was prepared by passage of a solution of tetradecyltriethylammonium bromide12 through a column containing Amberlite IRA-400 (Cl) ion-exchange resin (BDH). The eluent was freeze-dried to a white solid, which was recrystallized from ethyl acetate, and the resulting solid filtered and washed with diethyl ether. Microanalysis showed that the product was a hemihydrate. Dodecylpyridinium chloride (DPC) monohydrate (98%) was obtained from Aldrich and recrystallized 3 times from acetone. Tetradecylpyridinium chloride (TPC) was obtained by reaction of chlorotetradecane (Aldrich, 99%) with a large excess (9 equiv) of pyridine (Merck, 99.5%) for 1 week. The reaction was concentrated under vacuum, and the residue was recrystallized 3 times from ethyl acetate and shown by microanalysis to be a hemihydrate. Cetylpyridinium (CPC) monohydrate (98%) was obtained from Aldrich and was recrystallized twice from acetone:ethanol (4:1). 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 Brown C D Mica Co Pty Ltd. (Sydney, Australia). When this limited supply was exhausted a different batch was obtained from GMS Industrial Pty Ltd. (Melbourne, Australia). Both micas had surface areas of about 5 m2 g-1. All measurements were carried out with the mica powder from Brown C D Mica except for the TEC14‚Cl-DPC system. To confirm that the powder from GMS Industrial behaved in a similar manner, three solutions from the TEC14‚Cl-CPC system were mixed with the new powder as described below and similar equilibrium surface composition data were obtained, although the new powder adsorbed about 15% more surfactant. It has also been found that adsorption isotherms of other cationic surfactants measured with the different mica powders are very similar.13 Cmc Determination for TEC14‚Cl. A concentrated solution (ca. 12 x cmc) of TEC14‚Cl was added in progressive amounts from a 2-mL buret to a stirred thermostated flask containing 10 mL of water. The conductivity of the solution was measured after each addition with an EDT Instruments FE 280 conductivity meter. A plot of conductivity versus concentration showed a break, which was taken to be the cmc. Atomic Force Microscopy. Atomic force microscopy was performed with a Digital Instruments Nanoscope III in contact mode. The imaging method was to use the double-layer repulsion between the tip and the surface layer and fly the tip over the adsorbed film.6 The mica substrate was freshly cleaved before use with adhesive tape. Silicon nitride cantilevers with nominal spring constants of 0.3 N m-1 (DI) were cleaned by UV irradiation for 40 min prior to use. Surfactant solutions were injected into the liquid cell and thermally equilibrated for about 1 h before imaging. Scan rates were varied between 5 and 14 Hz. The solution was contained within the liquid cell by an O-ring which was rinsed with Milli-Q water and dried under a nitrogen flow before use. Solution Depletion Studies. The adsorption of surfactants on mica powder was determined by using a solution depletion technique. Solutions of surfactant mixtures (9 mL) were combined with mica (3 g) and mixed for 24 h on a roller-mixer (Ratek BTR5, (9) Zhu, B. Y.; Zhang, P.; Wang, R. X.; Liu, Z. F.; Lai, L. H. Colloids Surf. A Physicochem. Eng. Aspects 1999, 157, 63. (10) Imae, T.; Takeshita, T.; Kato, M. Langmuir. 2000, 16, 612. (11) Simister, E. A. Ph.D. Thesis, University of Oxford, 1994. (12) Buckingham, S. A.; Garvey, C. J.; Warr, G. G. J. Phys. Chem. 1993, 97, 10236. (13) Kovacs, L.; Warr, G. G., unpublished results.
Davey et al. Ratek Instruments, Australia). The solutions were allowed to sit for 1 h to allow the mica to settle, and the upper liquid was then centrifuged (Hettich Universal centrifuge) at 6000 rpm for 20 min and the supernatant removed for analysis by HPLC. Final solution concentrations were calculated from the ratio of the areas of the conductivity traces of the before and after solutions. Initial concentrations of surfactant mixtures were selected such that after adsorption the TEC14‚Cl solution concentration would be close to 10 mM. An exception to this was for the TEC14‚Cl-TPC system where the HPLC does not separate the two surfactants. In this case the final solution concentration of TPC was determined by measurement of the absorbance intensity for the solutions at λmax (259 nm) using a UV-visible spectrophotometer and relation of the intensity to concentration from a Beer-Lambert plot. When the concentration of TPC was above 3 mM the solutions were diluted due to the high extinction coefficient of the pyridinium group. The TEC14‚Cl concentration was then obtained from the equation Κ ) λTEC14‚Cl[TEC14‚Cl] + λTPC[TPC], where Κ is the area of the conductivity trace for the mixture measured by HPLC and λTEC14‚ Cl and λTPC are the molar conductivities of TEC14‚Cl and TPC, determined previously for pure solutions of the surfactants by HPLC from plots of conductivity area versus concentration. The differences between the surfaces of crushed and sheet mica have been examined by Lyons et al.14 They found that both substances leached aluminum ions but that this effect was not significant at the pH at which we carried out our measurements. High-Performance Liquid Chromatography. A Waters HPLC system was used fitted with a 431 conductivity detector and a Symmetry C18 (5 µm, 3.9 x 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. UV-Visible Spectroscopy. A Cary 1E UV-visible spectrophotometer was employed. Samples were contained in quartz cells with a path length of 1 mm. Micellar Pseudophase Diagrams. The group contribution method was used to determine the cmc’s for mixtures of TEC14‚ Cl with DPC, TPC, and CPC. A Microsoft Excel 97 Visual Basic program was written based on the group contribution method developed by Asakawa et al.8,15 For TEC14‚Cl the following variables were used: cmc ) 4.65 mM, Kg ) 0.6, group assignment ) 1CH3, 13CH2, 3NEt3. The molecular structure data (Rk and Qk values) for the NEt3 group was taken from Gmehling et al.16 The group interaction parameters for the NEt3 group was taken as the same as for the NMe3 group.
Results and Discussion Pure Surfactant Systems. Figure 2 shows the morphology of the adsorbed layer on mica of four cationic surfactants used in this study. No alkylpyridinium surfactants have previously been examined at the solutionsolid interface by AFM, but numerous adsorption isotherms have been determined on a range of substrates.17,18 AFM images of the pyridinium surfactants TPC, CPC, and HFDePC all show stripes, implying adsorbed cylinders. Repeated attempts to image DPC were unsuccessful. The cylindrical micelles formed by HFDePC on mica are very straight in all images, especially when compared with the meandering cylinders of TPC. The images of the hydrocarbon-chained pyridinium surfactants are comparable with those of alkyltrimethylammonium surfactants such as DTAB and TTAB, which tend to be quite wavy.19 (14) Lyons, J. S.; Furlong, D. N.; Healy, T. W. Aust. J. Chem. 1981, 34, 1177. (15) Asakawa, T.; Johten, K.; Miyagishi, S.; Nishida, M. Langmuir 1985, 1, 347. (16) Gmehling, J.; Rasmussen, P.; Fredenslund, A. Ind. Eng. Chem. Process Des. Dev. 1982, 21, 118. (17) Aubourg, R.; Bee, A.; Cassaignon, S.; Monticone, V.; Treiner, C. J. Colloid Interface Sci. 2000, 230, 298. (18) Zhu, B.; Gu, T.; Zhao, X. J. Chem. Soc., Faraday Trans. 1 1989, 85, 3819.
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Figure 2. AFM images of self-assembled structures formed by cationic surfactants at the mica-solution interface. All images are 200 x 200 nm. (a) HFDePC (3.7 mM) showing parallel cylindrical micelles; (b) TPC (7.1 mM) showing meandering cylindrical micelles; (c) TEC14‚Cl (7.9 mM) adsorbed on mica showing spherical micelles; (d) CPC (10 mM) showing parallel cylindrical micelles.
This difference may be due to the rigidity of the perfluoroalkyl chain,20 although such effects were not evident in a recent cryo-TEM studies of threadlike micelles of HFDePC and related compounds.21 In contrast with the pyridinium surfactants, the TEC14‚ Cl adsorbed layer consists of spherical micelles, as previously reported for the bromide salt of several quaternary ammonium surfactants with large headgroups.19 Table 1 lists nearest-neighbor center-to-center spacings for each of the adsorbed layer morphologies observed. It is at first glance puzzling that the spacings for HFDePC are the same as for TPC and TEC14‚Cl and also for tetradecyltrimethylammonium chloride, TTAC.19 This dimension equals the adsorbed micelle diameter plus intermicellar distance, which should largely be governed by the range of the electrostatic repulsion between adsorbed micelles, and hence by the critical micelle concentrations, as they determine the Debye length at (19) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1999, 15, 1685. (20) Bastiansen, O.; Hadler, E. Acta Chim. Scand. 1952, 6, 214. (21) Wang, K.; Karlsson, G.; Almgren, M.; Asakawa, T. J. Phys. Chem. B 1999, 103, 9237.
Table 1. Critical Micelle Concentrations and Nearest Neighbor Spacings on Mica of Cationic Surfactant Chloride Salts. Adsorbed Structure Is Stripes (Cylinders) unless Otherwise Noted surfactant TEC14‚Cl DPC TPC CPC HFDePC dodecyltrimethylammonium chloride tetradecyltrimethylammonium chloride hexadecyltrimethylammonium chloride
cmc/mM 4.65a 16.0b 3.6b 0.9b 2.5b 20c 5.4d 1.4e
nearest-neighbor distance/nm 5.8 (spheres) 5.8 7.1 5.8 4.7f 5.4g (spheres) 5.7g 6.5 ( 0.3h
a Conductivity, this work. b Conductivity.8 c Conductivity.28 Surface tension.29 e Surface tension.30 f Reference 31. g Reference 19. h Reference 32.
d
the cmc in solution. Figure 3 confirms the dependence of spacing on Debye length, which was also shown to affect the spacing of sodium dodecyl sulfate hemicylinders adsorbed on graphite in the presence of electrolytes.22,23 (22) Wanless, E. J.; Ducker, W. A. J. Phys. Chem. 1996, 100, 3207. (23) Wanless, E. J.; Ducker, W. A. Langmuir 1997, 13, 1463.
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Figure 3. Nearest-neighbor spacing of adsorbed cylindrical micelles ([) of various chloride surfactants (see Table 1) on mica as a function of Debye length (calculated from critical micelle concentration). Also shown is the nearest-neighbor spacing for TEC14‚Cl spheres (9).
Mixed Surfactant Systems. Figure 4 shows the solution phase behavior of the surfactant mixtures studied in this work, calculated using the group contribution method.8,15 Mixtures of TEC14‚Cl and CPC, TPC, and CPC (Figure 4b-d) are predicted to form mixed micelles at all compositions, with mixed cmc’s well described by an ideal mixing model.24 In contrast, TEC14‚Cl-HFDePC mixtures (Figure 4a) show a highly nonideal mixed cmc curve as a function of mole fraction, R, with a cmc maximum at the azeotropic composition RHFDePC ) 0.27, cmc ) 5.25 mM. Micellar demixing occurs, shown as a “two-phase” region in Figure 4a, for solution mole fractions of RHFDePC between 0.20 and 0.79 at concentrations far above the cmc. This behavior is similar to that calculated for mixtures of HFDePC and alkylpyridinium chloride.8 The effect of partial fluorination is most conveniently seen by comparing parts a and c of Figure 4, as HFDePC, TPC, and TEC14‚Cl all have approximately equal critical micelles concentrations (Table 1). In terms of cmc, a fluoromethylene is equivalent to 1.5 methylene groups,25 and thus the effective HC chain length of HFDePC is (1.5 x 8 CF2) + 2CH2 ) 14CH2. In an ideally mixing system with similar cmc’s, the micellar and monomer composition are expected to be similar, which is clearly not the case for TEC14‚Cl-HFDePC mixtures. Compositions of the mixed adsorbed layers as a function of their equilibrium bulk solution composition are shown in Figure 5. It should be noted (see Figure 4) that the equilibrium bulk solution is far above its (mixed) cmc in all systems studied. With the exception of pure pyridinium surfactants and one high concentration sample of TEC14‚ Cl-HFDePC, all solutions contain TEC14‚Cl at an approximately constant concentration of 10 mM. The preferential adsorption of HFDePC observed thus corresponds to a difference in composition between the adsorbed layer and bulk micelles. If we considered hydrophobicity alone as the determining factor for composition of the adsorbed layer, then all mixtures would be expected to have the same surface and micelle compositions; i.e., all data in Figure 5 would lie on a line of slope 1. The adsorbed layer compositions show (24) Clint, J. H. J. Chem. Soc., Faraday Trans. 1 1975, 71, 1327. (25) Shinoda, K.; Hato, M.; Hayashi, T. J. Phys. Chem. 1972, 76, 909.
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instead a strong preferential adsorption of all pyridinium surfactants over TEC14‚Cl at the mica/solution interface. This is also true for HFDePC, which is the most strongly adsorbed of all pyridinium surfactants studied. When the solution mole fraction of HFDePC is above 0.27 in the TEC14‚Cl-HFDePC system, the surface mole fraction is already at 100%. The preferential adsorption of the pyridinium headgroup is due to an interaction with the mica surface and hence primarily affects the “bottom half” of the adsorbed layer. The steric bulk of the triethylammonium group probably prevents the positive charge from approaching as close to the surface as the pyridinium group. Therefore the bonding between the residual negative charge on the mica is stronger with the pyridinium surfactants than with TEC14‚Cl. This interaction is sufficient to enrich the adsorbed layer with the pyridinium surfactant. Considering the alkylpyridinium surfactants first, Figure 5 shows that longer alkyl chains lead to greater enrichment of the mixed adsorbed layer by the pyridinium surfactant. Adsorption isotherms of surfactants on many hydrophilic solids (mica, kaolinite, silica) clearly show a cooperative component due to associations between alkyl tails, often referred to as region 2.26 In addition to the preferential adsorption of the pyridinium headgroup, the lower cmc of longer alkyl chains and hence greater tendency to aggregate together is sufficient to explain the trends in our observed surface compositions. Once again, however, the hydrophobicity explanation fails when applied to the fluorocarbon/hydrocarbon mixed system. HFDePC has a much higher cmc than CPC, yet the enrichment of the adsorbed layer with HFDePC is far greater than with CPC. In this system the preferential adsorption of the pyridinium group onto mica forms a fluorocarbon-rich environment for further adsorption, which will obviously preferentially adsorb more HFDePC and tend to exclude any hydrocarbon surfactant. In that sense we are seeing the effect of partial miscibility on the surface, which ultimately manifests itself as complete exclusion of TEC14‚Cl from the adsorbed layer at RHFDePC ) 0.27 in solution. One consequence of this enrichment by the alkylpyridinium surfactant is a change in the mean curvature of the adsorbed aggregate, favoring rods over spheres. For all the systems examined, a sphere-to-rod transition occurs at a surface mole fraction of pyridinium surfactant between 0.4 and 0.7. Figure 5 shows the compositions at which either spheres or rods were observed in each system by AFM. At intermediate surface compositions in the TEC14‚ Cl-TPC and TEC14‚Cl-CPC systems, spheres or rods were observed in different experiments, which we presume to be due to slight changes in conditions such as temperature or composition or possibly due to the system not having fully equilibrated. In all such cases these lie between regions which can clearly be identified as either spheres or rods. In no cases did we observe sphere + rod coexistence. The sphere-to-rod transition is driven by surfactant packing considerations.19,27 To a first approximation one (26) Fan, A. X.; Somasundaran, P.; Turro, N. J. Langmuir 1997, 13, 506. (27) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (28) Anthony, O.; Zana, R. Langmuir 1996, 12, 1967. (29) Hayami, Y.; Ichikawa, H.; Someya, A.; Aratono, M.; Motomura, K. Colloid Polym. Sci. 1998, 276, 595. (30) van Os, N. M.; Haak, J. R.; Haak, L. A. M. Physico-chemical Properties of Selected Anionic, Cationic, and Nonionic Surfactants; Elsevier: New York, 1993. (31) Manne, S. Prog. Colloid Polym. Sci. 1997, 103, 2226.
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Figure 4. Micellar pseudophase diagrams for the (a) TEC14‚Cl-HFDePC, (b) TEC14‚Cl-DPC, (c) TEC14‚Cl-TPC, and (d) TEC14‚ Cl-CPC systems. Solid lines show the locus of mixed cmc’s and, in the TEC14‚Cl-HFDePC system, the micelle demixing region. Also shown are experimental compositions studied by depletion adsorption (2) and AFM imaging ([). Concentrations are in mM.
the alkylpyridiniums all have almost identical packing parameters. The data shown in Figure 5 are consistent with this but are too coarsely spaced to distinguish small differences in the surface transition composition due to more subtle packing effects. Because of the different degrees of surface enrichment for different pyridinium surfactants, the shape transition condition of (approximately) constant surface composition leads to a very wide range of bulk solution compositions. For both TPC and CPC, a solution mole fraction of 0.8 is required to generate rods; for HFDePC a bulk mole fraction of only 0.07 is sufficient to induce the shape transition.
Figure 5. Surface mole fraction versus solution mole fraction of the pyridinium surfactant in the systems TEC14‚Cl-HFDePC (2), TEC14‚Cl-DPC (b), TEC14‚Cl-TPC ([), and TEC14‚ClCPC (9) determined by solution depletion experiments. Surface structures observed by AFM imaging are indicated by the dashed lines and labels (see text).
might expect the surface transition composition for mixtures of a sphere-former (TEC14‚Cl) and any rodforming alkylpyridinium surfactant to be the same, as (32) Aksay, I. A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P. M.; Gruner, S. M. Science 1996, 273, 892.
Conclusions Combining component adsorption isotherm data with AFM imaging yields a detailed picture of the adsorbed layer of mixed surfactant systems on solid substrates. Differences in surfactant-surface and surfactant-surfactant interactions lead to differences in composition between bulk micelles and the adsorbed layer, which also controls adsorbed layer morphology. 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. LA010092L
