Published on Web 05/03/2006
Mixed Micelles of Fluorinated and Hydrogenated Surfactants Lars Nordstierna, Istva´ n Furo´ ,* and Peter Stilbs Contribution from the DiVision of Physical Chemistry and Industrial NMR Center, Department of Chemistry, Royal Institute of Technology, SE-10044 Stockholm, Sweden Received February 13, 2006; E-mail:
[email protected] Abstract: The model mixed surfactant system of sodium perfluorooctanoate and sodium decyl sulfate was carefully reexamined by a combination of nuclear magnetic resonance methods. Over a wide range of sample compositions, detailed 19F and 1H chemical shift data in combination with self-diffusion coefficients for the perfluorooctanoate and decyl sulfate ions are collected. All data are analyzed together in a framework that uses a minimal number of initial assumptions to extract the monomer concentrations of both surfactants and the micellar chemical shifts of 19F and 1H as a function of relative concentration. The main conclusion drawn from this analysis is that there exists neither complete demixing nor complete mixing on molecular or micellar levels. Instead, the experimental data favor a single type of micelles within which fluorinated surfactants are preferentially coordinated by fluorinated ones and hydrogenated surfactants by hydrogenated ones. The data are quantitatively interpreted in the framework of the first approximation of the regular solution theory (also called the quasi-chemical treatment) leading to an energy of mixing of ω ) W/kT ) 0.98 between the constituting surfactant types. These findings may help to resolve a long controversy about micellar mixing-demixing in this particular mixture and in its relatives.
Introduction
Fluorocarbons have a strong hydrophobic character, and liquid perfluoroalkanes show a very low solubility in water. However, at room temperature fluoroalkanes are also immiscible with hydrogenated alkanes. This simultaneous hydro- and oleophobic character is unique and is exploited in a variety of applications, for instance, refrigeration, filtration, lubrication, foams, surface protection, and plastic manufacturing. The hydrophobicity of hydro- and fluorocarbons makes them both suitable as constituents in amphiphilic materials such as surfactants. The general features of the aqueous phase behavior of fluorinated and hydrogenated surfactants are similar, albeit with a few systematic differences. Hence, fluorinated surfactants reduce the surface tension of an aqueous solution more than a hydrogenated surfactant of comparable length, and the critical micelle concentration, CMC in water is generally lower for fluorosurfactants. For a given CMC value and a common headgroup, a hydrogenated surfactant contains roughly 1.5 times more carbon atoms compared to its fluorinated counterpart.1 In the aggregated state, fluorosurfactants exhibit a lateral mobility2 comparable to that of hydrogenated ones although the hydrocarbon tails are far more flexible and disordered than the fluorocarbon ones.3,4 Many fluorosurfactants exhibit a high stability in extreme environments such as high temperatures and acidic conditions, largely due to the strongest single bond in organic chemistry, the C-F bond. (1) Kissa, E. Fluorinated Surfactants and Repellents; Marcel Dekker: New York, 2001. (2) Kadi, M.; Dvinskikh, S. V.; Furo´, I.; Almgren, M. Langmuir 2002, 18, 5015-5018. (3) Furo´, I.; Sitnikov, R. Langmuir 1999, 15, 2669-2673. (4) Dvinskikh, S. V.; Furo´, I. Langmuir 2000, 16, 2962-2967. 6704
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It has long been recognized that the mutual phobicity of fluorinated and hydrogenated surfactants can, together with the hydrophobicity of both species, lead to a nontrivial aqueous phase behavior.5-8 Despite intensive research, this nontriviality seems to be of persistent character. From a theoretical perspective, there are several possible descriptions with often conflicting predictions. This situation is both based on and fuelled by apparently contradicting experimental investigations. To a large part, experimental discrepancy results from observational and instrumental limitations and interpretational problems of sparse data sets. Nothing illustrates these issues clearer than the substantial research effort invested in clarifying the microscopic pseudophase behavior of the aqueous mixtures of sodium perfluorooctanoate, SPFO, and sodium decyl sulfate, SDeS. The main and recurring question is whether SPFO and SDeS do form mixed micelles? SPFO and SDeS are two common surfactants with approximately equal degrees of hydrophobicity. At 25 °C, their CMC values are close: 31 mM for SPFO9 and 33 mM for SDeS.10 Their degrees of counterion binding are somewhat different, 0.63-0.64 for SDeS11,12 and 0.54-0.56 for SPFO.13-15 (5) Funasaki, N. Coexistence of Two Winds of Mixed Micelles of Fluorocarbon and Hydrocarbon Surfactants. In Mixed Surfactant Systems (Surfactant Science Series); Ogino, K., Abe, M., Eds.; Marcel Dekker: New York, 1993; pp 145-188. (6) Esumi, K. Colloids Surf. A 1994, 84, 49-57. (7) Mukerjee, P. Colloids Surf. A 1994, 84, 1-10. (8) Miyagishi, S.; Asakawa, T.; Ohta, A. Microscopic Phase Separation in Mixed Micellar Solutions. In Mixed Surfactant Systems (Surfactant Science Series); Abe, M., Scamehorn, J. F., Eds.; Marcel Dekker: New York, 2005; pp 431-476. (9) Muller, N.; Simsohn, H. J. Phys. Chem. 1971, 75, 942-945. (10) Jo¨nsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; Wiley: Chichester, 1998. (11) Rathman, J. F.; Scamehorn, J. F. J. Phys. Chem. 1984, 88, 5807-5816. 10.1021/ja061029r CCC: $33.50 © 2006 American Chemical Society
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In their influential study by equivalent conductance, Mukerjee and Yang16 showed that the CMC for an aqueous mixture of SPFO and SDeS was higher than the CMCs of the individual components in water. The maximum CMC (roughly 40 mM at 25 °C) was found in the region of equimolar composition. On the basis of those data and inspired by the mutual phobicity between hydro- and fluorocarbon chains Mukerjee and Yang suggested the existence of two kinds of micelles, one rich in fluorosurfactants and one rich in hydrogenated surfactants. Their evidence was indirect: the measured CMC values were close to ones calculated for the case of complete demixing of SPFO and SDeS under the assumption of a logarithmic dependence between the CMCs and the concentration of counterions.17,18 The idea of two distinct kinds of micelles has been subsequently supported by others. Among those, Zhu and Zhao19,20 also relied on experimental CMC values (measured by surface tension) that coincided with values calculated under theoretical considerations similar to those of Mukerjee and Yang. In building perhaps the strongest case, Nagarajan claimed that within the framework of his sophisticated thermodynamic theory of micelle formation21 the conductivity- and surface-tension-derived CMC data of Wada et al.22 indicate micellar demixing. Two types of coexisting micelles were asserted: one rich in the fluorosurfactant with molar ratio xSPFO g 0.78 and the other one rich in SDeS. In a similar analysis, Aratono et al.23 reported the range 0.25 < xSDeS < 0.72 of sample compositions within which demixing occurred. Similar claims were sometimes based on partly contradicting experimental evidence: for example, Sugihara et al.24 found two apparent CMC values by measuring the specific conductivity versus concentration, where the second one was independent of the composition. Since such a behavior is inconsistent with mixed micellization, the finding was interpreted as a sign of demixing. Ever since the initial claim of demixing,16 others stated the opposite: no demixing in the aqueous mixture of SPFO and SDeS. The opponents generally relied on the regular solution theory, which was first applied to the SPFO/SDeS system by Shinoda and Nomura.25 Using the very CMC values of Mukerjee and Yang,16 Shinoda pointed out that the energy of mixing ω ) W/kT is below 2, and therefore complete mixing was expected. In this expression, W defines26 the increase of energy of the system upon replacing one molecule A by one molecule B and vice versa in two fully phase-separated domains of A and B. From the experimental perspective, Harada and Sahara27 (12) Lebedeva, N. V.; Shahine, A.; Bales, B. L. J. Phys. Chem. B 2005, 109, 19806-19816. (13) Sugihara, G.; Mukerjee, P. J. Phys. Chem. 1981, 85, 1612-1616. (14) Mukerjee, P.; Korematsu, K.; Okawauchi, M.; Sugihara, G. J. Phys. Chem. 1985, 89, 5308-5312. (15) Berr, S. S.; Jones, R. R. M. J. Phys. Chem. 1989, 93, 2555-2558. (16) Mukerjee, P.; Yang, A. Y. S. J. Phys. Chem. 1976, 80, 1388-1390. (17) Corrin, M. L. J. Colloid Sci. 1948, 3, 333-338. (18) Mukerjee, P.; Mysels, K. J.; Kapaun, P. J. Phys. Chem. 1967, 71, 41664175. (19) Zhu, B.; Zhao, G. Acta Chim. Sinica 1981, 39, 493-502. (20) Zhao, G. X.; Zhu, B. Y.; Zhou, Y. P.; Shi, L. Acta Chim. Sinica 1984, 42, 416-423. (21) Nagarajan, R. ACS Symp. Ser. 1992, 501, 54-95. (22) Wada, Y.; Ikawa, Y.; Igimi, H.; Makihara, T.; Nagadome, S.; Sugihara, G. Fukuoka UniV. Sci. Rep. 1989, 19, 173-187. (23) Aratono, M.; Ikeguchi, M.; Takiue, T.; Ikeda, N.; Motomura, K. J. Colloid Interface Sci. 1995, 174, 156-161. (24) Sugihara, G.; Nakamura, D.; Okawauchi, M.; Sakai, S.; Kuriyama, K.; Tanaka, M.; Ikawa, Y. Fukuoka UniV. Sci. Rep. 1987, 17, 31-40. (25) Shinoda, K.; Nomura, T. J. Phys. Chem. 1980, 84, 365-369. (26) Guggenheim, E. A. Mixtures; Oxford University Press: Oxford, 1952; p 38. (27) Harada, S.; Sahara, H. Chem. Lett. 1984, 1199-1200.
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measured the mean partial molar volumes of a 1:1 mixture and arrived at only one CMC value, suggesting a completely mixed micelle with equal amounts of both surfactants. Reapplying the regular solution theory Kamogawa and Tajima28 concluded that the CMC data derived from surface tension and conductance measurements are consistent with complete mixing over the whole compositional range. Those authors also presented supporting results from a molecular technique: electron spin resonance, ESR. Experiments monitored a nitrosyl radical incorporated into the micelles. The obtained probe rotational correlation times varied continuously over the entire composition range at 45 and 100 mM total surfactant concentration, which was interpreted as direct indication of complete mixing. Arguments for this finding and some indications of partial demixing (among micelles when 100 mM NaCl is added to the system, or into separate intramicellar regions) are weakened, though, by a serious drawback of the ESR method, namely the addition of and measurement on a third non-native molecular component. A third and also a fourth probe were added by Asakawa and Miyagishi29 in a fluorescence quenching study. The fluorescence of alkyl- and perfluoroalkylpyridinium ions was monitored in 0-50 mM surfactant solutions with 200 mM NaCl added. The results suggested “a rather miscible mixed micelle”. NMR spectroscopy is a prominent molecular technique that uses native molecular probes. In the first such study in this context, Carlfors and Stilbs30 exploited 1H and 19F self-diffusion coefficients and spin relaxation data. Conclusions at that time were that demixing did occur, and that separate micelle types were formed in mixed alkanoate systems. Asakawa et al.31 measured 19F NMR chemical shifts and interpreted the data within the regular solution theory as signature of complete mixing. In contrast to the majority of previous studies, they also investigated the system at higher surfactant concentrations. Unfortunately, the extrapolative fashion they derived their micelle-specific chemical shifts remained unjustified. Guo et al.32 instead used surface tension and 1H and 19F chemical shifts to obtain the CMC values for the mixed system. Within the regular solution theory, those results confirmed complete mixing. However, the chemical shifts did not give distinct CMC values at all compositions: both the 19F shift at low SPFO content and the 1H shift at low SDeS content resulted in curved intercept shapes. Unfortunately, this interesting point was ignored, and the conclusions were drawn from only a few reliable CMC values. Some other weaknesses of thermodynamic approaches, such as uncertainties in experimental CMC values and the counterion effect have been pointed out by Kamrath and Frances.33,34 Both have large impact on the decisive parametersthe energy of mixing. Kamrath pointed out that these effects may altogether render the results based on apparent CMCs inconclusive. De Lisi et al.35 used the CMC values from the literature23 and (28) (29) (30) (31)
Kamogawa, K.; Tajima, K. J. Phys. Chem. 1993, 97, 9506-9512. Asakawa, T.; Miyagishi, S. Langmuir 1999, 15, 3464-3468. Carlfors, J.; Stilbs, P. J. Phys. Chem. 1984, 88, 4410-4414. Asakawa, T.; Miyagishi, S.; Nishida, M. J. Colloid Interface Sci. 1985, 104, 279-281. (32) Guo, W.; Fung, B. M.; Christian, S. D.; Guzman, E. K. ACS Symp. Ser. 1992, 501, 244-254. (33) Kamrath, R. F.; Franses, E. I. Ind. Eng. Chem. Fundam. 1983, 22, 230239. (34) Kamrath, R. F.; Franses, E. I. J. Phys. Chem. 1984, 88, 1642-1648. J. AM. CHEM. SOC.
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calculated the excess free energy for mixed micelle formation at all compositions. The work indicated no demixing region but instead a so-called critical point that is not in agreement with either of the two contrasting theories described above. Again, those results highlighted the extreme sensitivity of the obtained mixing scenario to the accuracy of CMC data. Considering the wealth of investigations, it is not surprising that the SPFO/SDeS aqueous mixture is often presented as a prime and thoroughly reviewed5 model system of surfactant mixing/demixing. The surprise is, indeed, that the basic question is still opensi.e., are the micelles mixed or demixed? In this paper, we try to resolve this long-debated issue. In a direct molecular (in contrast to thermodynamic) approach we exploit chemical shift and self-diffusion coefficient data by 1H or 19F NMR of the corresponding molecular species at varying concentrations. We want to point out that many other fluorinatedhydrogenated surfactant systems present interesting challenges to which the NMR approach outlined below could contribute positively. Experimental Section The total surfactant concentration (c) ranged from far below the CMCs up to 250 mM, and the mole fraction xh of SDeS in the mixed system from 0 to 1. For all samples, both 19F and 1H NMR spectra were acquired and the peak positions recorded with the spectrometer frequency locked to the D2O signal. All chemical shift data presented in Figure 1 are relative to the chemical shifts in the monomeric (c ) 10 mM) SPFO and SDeS samples, with 0 ppm for 19F assigned to the trifluoromethyl C(8)F3 signal. The 1H shift data instead refer to shift differences between the C(10)H3 and the C(1)H2 peaks, relative to the same shift difference in the monomer state. This approach is necessitated by the smaller value of the 1H shift changes compared to those of 19F. This condition makes the absolute chemical shift scale of 1H more sensitive to minor (in the order of a Hz) errors in setting the external shift reference. Further experimental details are given as Supporting Information.
Results and Discussion
Chemical Shifts in Mixed Systems. The NMR chemical shift is a parameter that in part depends on the intermolecular surrounding of the nucleus in question. For a surfactant in a micellar solution, typically there are two such surroundings, an aqueous one in the monomer state and a hydrophobic one consisting of close-packed surfactant tails in the aggregated state. The exchange time between these two states is usually (with the exception of polymeric surfactants36,37) short with regard to the NMR time scale that is the inverse of the difference between the state-specific NMR frequencies. In that case, one can observe only the population average of the instantaneous chemical shifts
δobs )
(
)
cmon mon cmon mic δ δ + 1c c
(1)
where δmon represents the shift in the monomer state, δmic the shift of the aggregated state, cmon the monomer concentration and c the total surfactant concentration. Let us first consider the chemical shift of an atom in the hydrophobic surfactant tail. At low concentrations, the chemical (35) De Lisi, R.; Inglese, A.; Milioto, S.; Pellerito, A. Langmuir 1997, 13, 192202. (36) Furo´, I.; Iliopoulos, I.; Stilbs, P. J. Phys. Chem. B 2000, 104, 485-494. (37) Iliopoulos, I.; Furo´, I. Langmuir 2001, 17, 8049-8054. 6706 J. AM. CHEM. SOC.
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Figure 1. NMR chemical shifts in aqueous SPFO/SDeS mixtures at different molar fractions xh ) 0 (9), xh ) 0.25 (1), xh ) 0.50 (f), xh ) 0.75 (2), xh ) 1 (b) and as a function of the inverse total surfactant concentration. (a) The 19F chemical shift of the trifluoromethyl group in SPFO. (b) The 1H chemical shift difference between methyl and R-methylene groups of SDeS. (Inserts) Application of eq 2 to the shift data in the 150-250 mM concentration range.
shift is independent of concentration up to the CMC, as a consequence of the unaffected aqueous chemical environment. At CMC, micelles start to form, and the atom may become embedded in the micellar core, with the surrounding there significantly different from that in water. Upon increasing concentration, more and more surfactants form micelles while the monomer concentration remains close to the CMC value, yielding the functional dependence of eq 1. The further the atom from the headgroup, the larger the change of environment. Hence, the end of the hydrophobic tail should be most sensitive to micellar aggregation. Under the assumption of a constant cmon above CMC and concentration-independent micellar interior, δobs becomes a linear function of c-1 and the intercept of this dependence and the constant δmon line defines the CMC. The method outlined above has been used countless times for measuring CMCs by NMR. However, the validity of assumptions underlying eq 1 is limited even in simple surfactant systems:38,39 for example, above the CMC, cmon is not independent of concentration.40-42 For a variety of reasons, the chemical shift of 19F nuclei is more than 1 order of magnitude more sensitive to the intermo(38) Drakenberg, T.; Lindman, B. J. Colloid Interface Sci. 1973, 44, 184-186. (39) Persson, B.-O.; Drakenberg, T.; Lindman, B. J. Phys. Chem. 1979, 83, 3011-3015. (40) Teubner, M.; Diekmann, S.; Kahlweit, M. Ber. Bunsen - Ges. Phys. Chem. 1978, 82, 1278-1282. (41) Stilbs, P.; Lindman, B. J. Phys. Chem. 1981, 85, 2587-2589. (42) Baumgardt, K.; Klar, G.; Strey, R. Ber. Bunsen - Ges. Phys. Chem. 1982, 86, 912-915.
Fluorinated−Hydrogenated Mixed Micelles
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lecular environment than that for 1H nuclei. Hence, the 19F chemical shift difference between aqueous and dense fluorocarbon environments is roughly 2-3 ppm, while the 1H chemical shift difference between aqueous and hydrocarbon environments is
