Mixed Adsorption of Fluorinated and Hydrogenated Surfactants

The 19F chemical shift of adsorbed perfluorooctanoate suggests that, for saturated surfaces, the two sorts of adsorbed surfactants form molecularly mi...
0 downloads 0 Views 143KB Size
Download PDF
Langmuir 2006, 22, 7969-7974

7969

Mixed Adsorption of Fluorinated and Hydrogenated Surfactants Lars Nordstierna, Istva´n Furo´,* and Peter Stilbs DiVision of Physical Chemistry and Industrial NMR Center, Department of Chemistry, Royal Institute of Technology, SE-10044 Stockholm, Sweden ReceiVed March 21, 2006. In Final Form: July 5, 2006 The adsorption isotherms of sodium perfluorooctanoate and sodium decyl sulfate and their 1:1 mixture on γ-alumina are recorded by depletion-type experiments with 1H and 19F NMR spectroscopy as the detection tool. The isotherms of the different surfactant species, obtained with and without added salt, closely resemble each other. Salt addition changes the isotherms from stepwise to the familiar S-shaped. After having reached saturation, a further increase of surfactant concentration in the mixed system leads to decyl sulfate desorption and increased perfluorooctanoate adsorption. The 19F chemical shift of adsorbed perfluorooctanoate suggests that, for saturated surfaces, the two sorts of adsorbed surfactants form molecularly mixed surface aggregates.

Introduction Over the past 50 years, fluorinated surfactants have drawn an increasing interest because of their exceptional chemical and physical properties which cannot be found in their more conventional hydrogenated counterparts. For instance, the impressive ability to decrease surface tension in not only aqueous but also organic media is exploited in many applications: various types of coatings, emulsions, cleaners, foams, and oil-fireextinguishing agents are products where fluorosurfactants have significant advantages. Their drawback is normally a high price and suspected environmental impact. For those reasons it is desirable to dilute them with hydrogenated surfactants. However, fluorinated/hydrogenated surfactant mixtures often exhibit nontrivial phase behavior that originates from the nonideal mixing of alkanes and perfluoroalkanes. Such phase behavior is, if any, further complicated by the presence of macroscopic surfaces. Of course, the surface activity of amphiphiles is the pivotal property, and surfactant properties at the liquid/solid interface are the key to wetting and cleaning applications. Although there exists a previous description of cooperative surfactant adsorption on a solid surface,1 it was in 1966 when the first detailed study presenting the nowadays classical S-shaped adsorption isotherm was published.2 Since then, many investigations have been aimed at understanding and modeling adsorption effects. For example, the effect of pH,3 the heat of adsorption,4 and different theoretical models5,6 have all been examined. For an ionic surfactant adsorbing on a charged surface there are four different regimes depending on the surfactant concentration. At low concentration, adsorption is exclusively governed by electrostatic interactions between the solid surface and the amphiphilic headgroups (region I). As the concentration increases, the hydrophobic interaction between the amphiphilic surfactant tails becomes a significant contributing factor (region II). The physical interpretation of this cooperative interaction is akin to * To whom correspondence should be addressed. Phone: +46 8 7908592. Fax: +46 8 7908207. E-mail: [email protected]. (1) Gaudin, A. M.; Fuerstenau, D. W. Trans. Am. Inst. Min. Eng. 1955, 202, 958-962. (2) Somasundaran, P.; Fuerstenau, D. W. J. Phys. Chem. 1966, 70, 90-96. (3) Fuerstenau, D. W.; Wakamatsu, T. Faraday Discuss. 1976, 59, 157-168. (4) Partyka, S.; Rudzinski, W.; Brun, B.; Clint, J. H. Langmuir 1989, 5, 297304. (5) Scamehorn, J. F.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1982, 85, 463-478. (6) Nagashima, K.; Blum, F. D. J. Colloid Interface Sci. 1999, 214, 8-15.

that of ordinary micellization. In region III, the adsorption rate decreases, as a consequence of surface neutralization. The adsorption comes to an end and a plateau is formed (region IV) when the bulk surfactant concentration reaches the critical micelle concentration (or if the surface becomes saturated). Since electrostatic interactions are heavily involved, it is perhaps not surprising that dissolved salt often changes the adsorption behavior. One remarkable demonstration of salt effects has been provided for dodecylpyridinium chloride adsorbing on silica.7-9 When that solution contains a high concentration of inorganic salts, the isotherm is a classically S-shaped one. In contrast, at low salt concentration the isotherm has a stepwise appearance with a significantly larger amount of adsorbed surfactant at low concentrations. The probable reason10 is that surface charges are unscreened at low salt concentration. Stronger electrostatic repulsion between headgroups at low salt concentration also inhibits cooperative aggregation in the adsorbed layer, which explains the lower observed slope in region II of the isotherm. This latter behavior should also depend on the length of the surfactant tail.10 On the other hand, adsorption of sodium nonylbenzenesulfonate on rutile, TiO2, has been shown to follow the classical S-shaped isotherm regardless of the salt concentration.7-9 In one of the first studies, adsorption on a solid surface in a mixed surfactant system concerned different alkylbenzenesulfonates and polyethoxylates11,12 and was studied by HPLC as a detection tool of surfactant depletion. Investigations of this type have thereafter multiplied as has been comprehensively reviewed.13 Adsorption of fluorinated surfactants in general and competitive adsorption of their mixtures in particular at the solid/ liquid interface has been less investigated.14-21 Although Davey (7) Koopal, L. K.; Goloub, T. P. ACS Symp. Ser. 1995, 615, 78-103. (8) Goloub, T. P.; Koopal, L. K. Langmuir 1997, 13, 673-681. (9) Goloub, T. P.; Koopal, L. K.; Sidorova, M. P. Colloid J. 2004, 66, 38-43. (10) Rosen, M. J.; Nakamura, Y. J. Phys. Chem. 1977, 81, 873-879. (11) Scamehorn, J. F.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1982, 85, 479-493. (12) Scamehorn, J. F.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1982, 85, 494-501. (13) Somasundaran, P.; Huang, L. AdV. Colloid Interface Sci. 2000, 88, 179208. (14) Esumi, K.; Ono, Y.; Ishizuka, M.; Meguro, K. Colloids Surf. 1988, 32, 139-147. (15) Esumi, K.; Sakamoto, Y.; Yoshikawa, K.; Meguro, K. Colloids Surf. 1989, 36, 1-11. (16) Esumi, K.; Sakamoto, Y.; Meguro, K. J. Colloid Interface Sci. 1990, 134, 283-288.

10.1021/la060757p CCC: $33.50 © 2006 American Chemical Society Published on Web 08/16/2006

7970 Langmuir, Vol. 22, No. 19, 2006

et al.20,21 measured the structure (by AFM) and the composition (by HPLC detection) of saturated layers of hydro- and fluorocarbon surfactants at the mica/water interface, they did not study the adsorption isotherm. It was Esumi et al. who first measured the adsorption isotherms in hydrogenated/fluorinated surfactant mixtures in combination with different solid surfaces. By isotachphoresis and UV spectrophotometry as detection tools, their concentration ranges were, with some exception,19 rather narrow. This is clearly a disadvantage since important features of adsorption isotherms appear as sudden changes of slope on a logarithmic scale and can clearly be identified by inspecting the concentration dependences over a wide, typically 10-5 to 1 M concentration range. Indeed, the authors18 emphasized the need for other detection methods to determine the adsorption isotherms of individual components in mixtures. Although optical methods,22,23 for example, ellipsometry or, recently, dual polarization interferometry,24 are the premier techniques for measuring adsorption, they are often less suitable for identifying the molecular composition of the layer adsorbed from a mixed solution. In a previous paper,25 we demonstrated the advantages of using NMR spectroscopy in depletion mode for exactly this purpose. NMR spectroscopy has also been demonstrated as a useful tool for characterizing surface-adsorbed surfactants in the dry state.26,27 Here, we proceed with a quantitative and comprehensive investigation of surfactant adsorption in both pure and mixed surfactant systems. We use the well-known alumina as the solid. On the surfactant side, sodium perfluorooctanoate, NaPFO, is one of the most investigated anionic fluoroamphiphiles. While sodium dodecyl sulfate, SDS, is probably the most studied anionic hydrogenated surfactant, its aqueous cmc (∼8 mM) is much lower than that of NaPFO (31 mM) at 25 °C.28 For this reason, we selected as the hydrogenated surfactant sodium decyl sulfate, SdeS, whose cmc is 33 mM29 at 25 °C. We have previously30 studied the bulk aqueous mixture of NaPFO and SDeS and demonstrated that the two surfactants form one mixed micelle within which each surfactant type has a slight preference for its own species to be surrounded with. Experimental Section NaPFO was prepared by neutralizing pentadecafluorooctanoic acid (Aldrich, 96%) with aqueous sodium hydroxide (Riedel-de Hae¨n, g99%). The solution was first freeze-dried and then redissolved in water, followed by recrystallization at ca. 0 °C from the water phase and final freeze-drying. All other chemicals, sodium decyl sulfate (Fluka, g99%), sodium chloride (Merck, g99.5%), hydrochloric acid (Merck, 37%), and deuterium oxide (Isotec, 99.9 atom % D) were used as received. Porous active acidic aluminum oxide (Merck (17) Esumi, K.; Otsuka, H.; Meguro, K. J. Colloid Interface Sci. 1991, 142, 582-588. (18) Esumi, K.; Otsuka, H.; Meguro, K. Langmuir 1991, 7, 2313-2316. (19) Esumi, K.; Tokui, Y.; Nagahama, T.; Meguro, K. J. Colloid Interface Sci. 1991, 146, 313-319. (20) Davey, T. W.; Warr, G. G.; Almgren, M.; Asakawa, T. Langmuir 2001, 17, 5283-5287. (21) Davey, T. W.; Warr, G. G.; Asakawa, T. Langmuir 2003, 19, 5266-5272. (22) Denoyel, R. Colloids Surf., A 2002, 205, 61-71. (23) Zhmud, B.; Tiberg, F. AdV. Colloid Interface Sci. 2005, 113, 21-42. (24) Lu, J. R.; Swann, M. J.; Peel, L. L.; Freeman, N. J. Langmuir 2004, 20, 1827-1832. (25) Evena¨s, L.; Furo´, I.; Stilbs, P.; Valiullin, R. Langmuir 2002, 18, 80968101. (26) Pawsey, S.; Yach, K.; Halla, J.; Reven, L. Langmuir 2000, 16, 32943303. (27) Pawsey, S.; Reven, L. Langmuir 2006, 22, 1055-1062. (28) Muller, N.; Simsohn, H. J. Phys. Chem. 1971, 75, 942-945. (29) Jo¨nsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; Wiley: Chichester, U.K., 1998. (30) Nordstierna, L.; Furo´, I.; Stilbs, P. J. Am. Chem. Soc. 2006, 128, 67046712.

Nordstierna et al. 90, particle sizes 0.063-0.200 mm, activity stage I, for column chromatography) was used. Its pore diameter and the specific surface area were specified by the manufacturer to 90 Å and 120 m2/g, respectively. In quantitative NMR experiments, described further down, H2O must be avoided as a solvent for protonated molecules of interest. Accordingly, D2O was used for preparing all samples. The pD in the samples was obtained by the well-known corrective factor31 pD ) pH(nominal) + 0.4 where pH(nominal) is the standard pH meter reading yielded here by an Orion SA 520 meter connected to an Orion Ross 8103 electrode. In the following, we revert to the standard pH notation with this in mind. Samples were prepared in duplicate for each surfactant composition with two sets of the aqueous phase: one with added 0.1 M NaCl and one without. The surfactant composition was pure NaPFO or pure SDeS or a 1:1 mixture of these two, up to a 200 mM total surfactant concentration. Exceptions were the solutions of pure NaPFO, which has a low (∼25 mM) aqueous solubility in 0.1 M NaCl at 20 °C. This value was set as the maximum amount of surfactant in the initial suspension when that sample series was prepared. (Note that this limits cbulk to 6.6T1, which provides a contribution to the intensity error of 3 × 10-2 M). The major differences compared to Figure 1 are the shape of the isotherm, especially at low surfactant concentrations, and the magnitude of Γ. A comparison to the results by Koopal and Goloub7,8 is inevitable. Akin to their (52) Roberts, B. L.; Scamehorn, J. F.; Harwell, J. H. ACS Symp. Ser. 1986, 311, 200-215. (53) Lopata, J. J.; Harwell, J. H.; Scamehorn, J. F. ACS Symp. Ser. 1988, 373, 205-219. (54) Lai, C. L.; Harwell, J. H.; Orear, E. A.; Komatsuzaki, S.; Arai, J.; Nakakawaji, T.; Ito, Y. Colloids Surf., A 1995, 104, 231-241. (55) Mukerjee, P.; Sharma, R.; Pyter, R. A.; Gumkowski, M. J. ACS Symp. Ser. 1995, 615, 22-35.

Nordstierna et al.

Figure 3. Individual adsorption of NaPFO (9) and SDeS (b) in their 1:1 mixture on γ-Al2O3 at 20 °C in 0.1 M NaCl aqueous solution at pH 6.2. The horizontal axis corresponds to the total bulk concentration. The gray field indicates the plausible region for the cmc (16 mM < cmc < 26 mM) of the 1:1 mixture in 0.1 M NaCl aqueous solution.32

observations for dodecylpyridinium chloride adsorption on silica at different salt concentrations, at low concentrations we find stronger adsorption as a consequence of unscreened surface charges and thereby stronger electrostatic interactions. Note that the previously25 studied ammonium perfluorooctanoate (APFO)/ γ-Al2O3 system exhibits the same type of salt dependence of the adsorption isotherm (data not shown). We also note that our previous study25 concerned a smaller range of concentrations (down to 10-4 M < cbulk) without added salt. Hence, the four different regions of adsorption were less clearly identified there. Those experiments were performed at pH 4, which significantly increased the amount of adsorbed surfactant as a result of higher surface charge. As can be seen in Figures 1 and 2, the two surfactants show almost identical affinity for the γ-alumina in water suspension. Hence, it is not surprising that the total adsorption of the 1:1 mixture follows the same trend as well. The onset of the plateau regions is in agreement with the known cmc values; hence, the onset with high salt concentration is at lower cbulk because of the lower cmc at high ionic strength.54 At the air/water interface, each PFO headgroup occupies 4142 Å2 in a monolayer.56-59 The corresponding number for decyl sulfate is 55 Å2.60 With those numbers, our plateau region in Figure 2 corresponds, approximately and on average, to monolayers (∼0.8 monolayer for pure NaPFO and ∼0.9 monolayer for pure SDeS). Note that, like for adsorption on graphite,55 NaPFO has higher Γ values at the plateau than SDeS. This is primarily a consequence of the smaller headgroup area for NaPFO. At this point it might be worthwhile to recall that we did not determine the actual structure (monolayer) but merely the average adsorbed amount consistent with that assumed structure. In Figures 3 and 4, the adsorption of the individual species in the mixed system is visualized with the total equilibrium concentration on the horizontal axis. From both plots, we may conclude that the adsorbed amount is almost identical for the two surfactants in cooperative regions II and III. There seems (56) Shinoda, K.; Hato, M.; Hayashi, T. J. Phys. Chem. 1972, 76, 909-914. (57) Simister, E. A.; Lee, E. M.; Lu, J. R.; Thomas, R. K.; Ottewill, R. H.; Rennie, A. R.; Penfold, J.; Webber, S. E.; Zhulina, E. B. J. Chem. Soc., Faraday Trans. 1992, 88, 3033-3041. (58) Downes, N.; Ottewill, G. A.; Ottewill, R. H. Colloids Surf., A 1995, 102, 203-211. (59) Duvoisin, S.; Kuhnen, C. A.; Ouriques, G. R. THEOCHEM 2002, 617, 201-207. (60) Karakashev, S.; Manev, E. J. Colloid Interface Sci. 2002, 248, 477-486.

Fluorinated/Hydrogenated Surfactant Mixed Adsorption

Langmuir, Vol. 22, No. 19, 2006 7973

Figure 4. Individual adsorption of NaPFO (9) and SDeS (b) in their 1:1 mixture on γ-Al2O3 at 20 °C in aqueous solution at pH 6.2. The horizontal axis corresponds to the total equilibrium concentration. The arrow indicates the concentration of the samples that provide the 19F NMR spectra recorded in the sediment and presented in Figure 6. The dashed line indicates the cmc of the 1:1 mixture ()40 mM) in aqueous solution.33

to be some preference for NaPFO (higher Γ values) in region I, where the adsorption is due to monomer-surface interaction. The behavior is apparent when the amount of adsorbed surfactant is small (Figure 3). On the other hand, we observe a novel type of behavior, illustrated more clearly in Figure 5, where we compare the adsorbed amount of a surfactant in the mixed sample relative either to the total amount of adsorbed surfactant (because of the limited solubility of NaPFO in 100 mM NaCl solution, Figure 5a) or to the adsorbed amount of the same surfactant in the pure system (Figure 5b). For mixtures, we see that the amount of adsorbed NaPFO increases remarkably in the plateau region: that of SDeS decreases, while the total amount of adsorbed surfactant remains roughly constant. Recall that, up to the plateau, the adsorptions of NaPFO and SDeS in the mixed or pure systems are almost identical. The desorption of SDeS and the parallel increase of the adsorbed amount of NaPFO, as shown in Figure 5, are not consistent with classical theories of adsorption. Our explanation is based on the nonideality of mixing of these two surfactants.30 In a bulk solution NaPFO and SDeS form mixed micelles: from the point of view of NaPFO there is less free energy penalty for getting involved in an SDeS micelle than for remaining in solution or for forming a separate type of aggregate. However, with the alumina surface present, there is another alternative. Hence, with increasing concentration SDeS drives NaPFO from the bulk micelles to the alumina surface and vice versa. Consistently with this interpretation, the start of the buildup of surface excess seems to coincide (within bounds defined by the experimental error and the density of concentration points) with corresponding cmc values marked in Figure 5. On the basis of this scenario, either surfactant could exhibit the surface excess. However, as shown in the behavior in region I, the PFO- ions seem to have a higher affinity for alumina than the DeS- ions, which explains why the fluorosurfactant enriches at the surface. To be able to characterize the state of the surfactants on the alumina surface, we also recorded some 19F spectra in the sediment. This was done at 65 mM total surfactant concentrations without added NaCl (see the Experimental Section). Hence, the samples were located directly below the plateau region (see the arrows in Figures 2 and 4) with roughly identical amounts of NaPFO and SDeS. As we have shown previously,25 porous Al2O3 grains are large and of regular shape, and therefore, the liquid

Figure 5. Adsorbed amounts of NaPFO (9) and SDeS (b) in their 1:1 mixture (a) relative to the total adsorbed amount in 0.1 M NaCl aqueous solution and (b) relative to the amounts in corresponding solutions of the pure substances in aqueous solution. The scaling in (b) is not available in (a) because of the limited solubility of NaPFO in 0.1 M NaCl aqueous solution. The data were obtained from the adsorbed amounts presented in Figures 1-4 and refer to adsorption on γ-Al2O3 at 20 °C and at pH 6.2. The gray field and the dashed line indicate corresponding cmc’s in 1:1 mixtures as given in Figures 1-4.

volumes between the grains are roughly in the size range of the grains. Therefore, the molecular exchange time between intergrain and intragrain liquid volumes is slow (.1 ms) on the time scale of the time-domain NMR signal. Consequently, the spectral contributions from within the pores and from the intergrain volume can be separated. On the other hand, the exchange between the surfactants within the pore volume (of ∼9 nm diameter) and adsorbed on the pore surface is fast. Hence, we can expect two 19F spectral lines. Indeed, as illustrated in Figure 6 this is the case for the trifluoromethyl spectral region recorded in samples either with pure NaPFO or with the 1:1 mixture. The two lines are at different chemical shifts, which is a consequence of the welldocumented sensitivity of the 19F chemical shift to the intermolecular environment.30,61 The lines in the observed spectrum are far broader than those in the supernatant (not shown; the line widths there are a few hertz). This is a consequence of susceptibility broadening by the grains and, for the adsorbed peak, slow molecular dynamics by surface diffusion. The ratio of the two peaks corresponds roughly to the estimated relative amounts of surfactants in the two environments. The known intergrain volume is filled by a monomer solution with a surfactant concentration of cbulk; this amounts to roughly 15-20% surfactant in the sediment. On the other hand, adsorbed surfactant on the known total pore surface25 is roughly 75%, while a solution with a surfactant concentration (61) Furo´, I.; Iliopoulos, I.; Stilbs, P. J. Phys. Chem. B 2000, 104, 485-494.

7974 Langmuir, Vol. 22, No. 19, 2006

Figure 6. 19F NMR spectra of the trifluoromethyl spectral region of NaPFO in the sediment for samples prepared with pure NaPFO (full line) and with a 1:1 mixture of NaPFO and SDeS (dashed line) at the concentrations indicated in Figures 2 and 4. The smaller peaks at higher chemical shift correspond to monomers; for simplicity, their shift is set to 0 ppm.

of cbulk in the pore volume is ∼5% of the total surfactant in the sediment. Hence, we expect two lines with an intensity ratio of ca. 1:5, which is roughly the case despite the fact that the spinecho detection (with a 2 ms total echo time) somewhat suppresses the larger peak. The spin-echo pulse sequence with a delay time of ∼1 ms was used here instead of the conventional 90° detection since that detection mode advantageously separates the two peaks due to faster transverse relaxation of the adsorbed surfactant (we stress that all quantitative measurements in the supernatant samples were made by single 90° pulse experiments). As shown in Figure 2, cbulk ) 20 mM, which means that the NaPFO molecules (cmc ) 31 mM) are monomeric in both solutions. In accordance, the smaller intergrain peaks in Figure 6 are at the same chemical shift, signifying an aqueous environment in both solutions. On the other hand, the signal that originates from the adsorbed layer shifts as the system changes from NaPFO to the 1:1 mixture. In the case of pure NaPFO, the observed 2.5 ppm is in agreement with having the CF3 groups in a dense fluorinated environment. This clearly excludes PFO monolayers with the CF3 groups oriented toward the water as the surface structure. Instead, surface aggregates are indicated. In the 1:1 mixture we find a 1.7 ppm shift difference between the two peaks, which corresponds exactly to the value expected in molecularly mixed surface aggregates of NaPFO and SDeS.30

Conclusions We report here the adsorption isotherms of sodium perfluorooctanoate and sodium decyl sulfate and their mixture on γ-alumina in aqueous solution at different electrolyte concentra-

Nordstierna et al.

tions. We use NMR spectroscopy in depletion-type measurements. Since NMR is a chemically selective and quantitative technique, we can readily obtain the adsorption isotherms of individual components in the case of multicomponent adsorption. We find that in the investigated system the adsorption behavior changes dramatically upon addition of salt. In 0.1 M NaCl aqueous solution the adsorption isotherm has the typical S-shaped form, while without salt the isotherm has a stepwise appearance. It is at low concentration we find the strongest quantitative difference: at 10-4 M bulk surfactant concentrations, the amount of adsorbed surfactant in the absence of salt is 102 times higher than that with 0.1 NaCl added. On the other hand, the adsorption is of the same magnitude for any strength of electrolyte at bulk surfactant concentrations higher than 10-3 M. Comparing the adsorption of fluorinated and hydrogenated surfactants, the fluorinated one has a somewhat higher affinity for alumina. Despite this, at intermediate concentrations the adsorbed amounts of the two surfactants are roughly identical. Hence, we can conclude that in the regime where surfactantsurfactant interactions are the main driving force of adsorption the surfactant-surface interactions become comparatively less important. In this context, it is surprising that after the bulk concentration reaches the critical micelle concentration decyl sulfate desorbs from the surface and becomes replaced by perfluorooctanoate. The mutual phobicity between fluoro- and hydrocarbons is certainly a contributing factor to this behavior. The fluorocarbon/hydrocarbon phobicity also presents a question about the structures formed by the adsorbed surfactants: are they molecularly mixed or not? Mixed adsorbed monolayers of long perfluorinated and hydrogenated surfactants have previously been shown to micro-phase-separate caused by their mutual phobicity.62 In contrast, molecularly mixed surface aggregates are shown to form for the shorter surfactants of the present study, as decisively demonstrated by the 19F chemical shift of the adsorbed perfluorooctanoate. Monolayer structures are excluded by the magnitude of the 19F shift for the terminal trifluoromethyl groups, which indicates instead that those groups reside in a nonaqueous environment. Further studies starting from the low-concentration regime are in progress to characterize the evolution of the surface structures by increasing the bulk concentration. Acknowledgment. This work has been supported by the Swedish Research Council (VR) and the Knut and Alice Wallenberg Foundation (KAW). LA060757P (62) Kato, T.; Iimura, K. I. Micro-Phase Separation in Two-Dimensional Amphiphile Systems. In Mixed Surfactant Systems; Abe, M., Scamehorn, J. F., Eds.; Surfactant Science Series; Marcel Dekker: New York, 2005; pp 59-91.