17 Anomalies of Partially Fluorinated Surfactant Micelles* PASUPATI MUKERJEE School of Pharmacy, University of Wisconsin, Madison, Wis. 53706 KAROL J. MYSELS 8327 La Jolla Scenic Drive, La Jolla, Calif. 92037
Introduction The behavior of amphipathic compounds such as ordinary sur factants having a hydrophobic hydrocarbon moiety and a hydro philic polar one or of the perfluoro surfactants in which a l l the hydrogens of the hydrophobic moiety are replaced by fluorines is now reasonably well understood. Partially fluorinated sur factants, in which the hydrophobic moiety contains both hydrogens and fluorines, present, however, anomalies to which we wish to call attention. Our main point is that these anomalies can be readily understood once it is realized that the partially fluorocarbon parts, though both hydrophobic, are not mutually philic but phobic and tend to exclude each other. The lipo philic hydrocarbon part is fluorophobic and the fluorocarbon part is lipophobic. The anomalies of the partially fluorinated surfactants involve both macroscopic phenomena such as their critical micellisation concentrations (c.m.c.) and microscopic ones as revealed by fluorine-NMR results. Both are in accord however with the bulk behavior of fluoro- and hydrocarbons and with the mixed micelle formation of ordinary and perfluorosurfactants. The Mutual Phobicity of Hydrocarbons and Fluorocarbons Though often neglected, this has been known and well docu mented since the early days of fluorocarbon development. In 1950 Simons and Dunlap (V) reported that n-pentane and nperfluoropentane were completely miscible only above 265.5 Κ *The main conclusion of this paper was presented at the Fall 1970 ACS meeting in Chicago in connection with a paper by Mukerjee, P. and Cardinal, J . R. (unpublished work). The principal arguments were submitted in a paper received July 19, 1971, but not accepted by the Journal of Physical Chemistry. This material has been re organized and amplified for presentation at the Voids symposium. 239
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whereas regular solution theory would predict 62°K for their consolute temperature. The corresponding carbon compounds form two phases up to 40°C (2). Hildebrand, Prausnitz, and Scott devote several pages of their book (3) to this tendency of perfluorocarbons and hydrocarbons to unmix. Even though completely miscible at room temperature, the C* compounds form extremely non-ideal solutions, the activity coeffi cient of the perfluorobutane reaching the value of 10 at infinite dilution (4). In other words, the tendency of an isolated per fluorobutane molecule to escape from η-butane is ten times greater than when i t is surrounded by its own kind. For η-butane the activity coefficient reaches 5 (4). Thus clearly hydrocarbons and fluorocarbons, though they will mix i f the temperature is high enough, show a considerable reluc tance to do so and a considerable mutual phobicity. Mixed Micelle Formation Between Ordinary and Perfluoro Surfactants Klevens and Raison (5_) had published about 20 years ago what seems to be the only study thus far of the micellization of such a mixed system, namely sodium dodecyl sulfate (SDS) and perfluorooctanoic acid (F0A). Figure 1 shows their results for surface tension measurements on the pure systems (though the SDS itself contains minor impurities as shown by the minimum) and at four intermediate mole fractions, plotted as a function of total sur factant concentration. They noted the peculiar character of their results but an interpretation had to wait t i l l a better under standing of solubilization and mixed micelle formation of ordinary surfactants had been gained. Details of such an interpretation will be given elsewhere (6) and only the feature pertinent to the present discussion will be brought out here. It may be noted that ordinary surfactants generally form mi celles which are mutually completely miscible. Thus commercial non-ionic surfactants which are generally mixtures of many homo logues behave very much as their average, and the phase diagram of sodium decyl and dodecyl sulfates (7) (Figure 2) shows only minor deviations from ideality. If the results of Klevens and Raison for systems containing 1.0, 0.75 and 0.20 mole fraction F0A are plotted against the con centration of the F0A as shown in Figure 3, several points become apparent. All three systems reach substantially the same low and constant surface tension after a sharp change of slope which indi cates the formation of F0A micelles after a definite c.m.c. for them. For the 1.0 and 0.75 mole fraction systems, the values of the c.m.c. are indistinguishable at about 8.5 mM showing that the solubility of the SDS in these micelles is sufficiently small to be undetectable. In the 0.2 mole fraction system these micelles do not form until the F0A concentration reaches 30 mM, i.e. 21.5 mM higher than in the other two. This system also shows a change of slope and perhaps a small minimum around 2 mM F0A where
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SURFACTANT, mM
Figure 1. Surface tensions of sodium dodecyl sulfate, perfluoro octanoic acid and their mixtures as a function of the total surfactant concentration. This is Figure 4 of reference (5) replotted from the data given in that reference.
mole % NaDS
Journal of Colloid Science
Figure 2. The equilibrium compositions of micelles and monomers in the sodium decyl and dodecyl sulfate system. Solid lines represent the behavior of ideal solutions of both monomers and micelles. X = experimental c.m.c!s. = the composition of mixed micelles as deduced from conduc tivity data (7).
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the concentration of SDS is about 8 mM. Hence this is where SDS micelles must begin to form. Thus i t appears that between these concentrations some of the FOA is solubilized by the SDS micelles but the rest continues to increase its monomer activity t i l l its c.m.c. is reached. The 21.5 mM FOA difference indicates upon closer analysis that the SDS micelle dissolves about 15 moles% of FOA. The resulting phase diagram is shown schematically in Fig ure 4. The contrast with Figure 2 is striking and clearly can only be ascribed to the mutual phobicity of hydro- and perfluorocarbons. Thus this characteristic of macroscopic behavior is also found in the micellar realm. Following presentation of this paper an interesting confirma tion of this point was reported by Tiddy and Wheeler (8) who found that the solubilization of octanol by ammonium perfluorooctanoate was about 4 times less than that by sodium octanoate. C.m.c.'s of Perfluorinated Surfactants Perfluoro alkanes are more hydrophobic than the hydrocarbon ones. Though comparative solubility data are sparse, the effect is clear. CF4 is almost seven times {9) less soluble in water on a mole basis than CH* and water is also almost seven times (10) less soluble in perfluoroheptane than ordinary heptane on a weight basis and this becomes a factor over 25 on a mole basis. This greater hydrophobicity leads to a more pronounced amphipathic character of the perfluoro surfactants as shown by their well known greater surface activity both in terms of surface tension lowering at same concentration, and by the ultimate value attained which goes below 20 dyne/cm for many perfluoro surfactants. This is also reflected in their lower c.m.c.'s. Figure 5 shows the available comparisons (11 ). The best documented values are those for the potassium octanoates. Here the perfluorocompound has a 13-fold lower c.m.c. than the ordinary one. At lower chain!engths the difference seems to decrease as shown by the only three-fold lower values for the very poorly documented potassium hexanoate and for the better established butyric acids. In this last case the c.m.c.'s are of the order of 1 M so that the intermicellar liquid is no longer simply aqueous. Since in the following comparisons we will consider only compounds having 8 or more carbon atoms in the chain, we can conservatively take 13 as the ratio of c.m.c.'s in hydrocarbon and perfluorocarbon systems. C.m.c.'s of Partially Fluorinated Surfactants If partially fluorinated chains were to behave ideally, one would expect the c.m.c.'s of these compounds to l i e between those of the corresponding perfluoro and ordinary surfactants varying approximately linearly with the fraction of hydrogens replaced by fluorines as indicated by the solid line of Figure 6. As shown
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Figure 3. Data for selected systems of Figure 1 as a function of the concentration of perftuorooctanoic acid present
tshSDS (MICELLES/
— •> .
C S0 Na n
4
.
.
1.
-C OOH 8
Figure 4. Schematic of the equilibrium compositions of micelles and monomers in the sodium dodecyl sulfate and perfluoro octanoic acid system. Note the presence of two types of micelles having limited mutual solubility. Based on Ref. (6).
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Figure 5. Critical micelle concentrations of perfluoro and corresponding ordinary surfac tants as a function of carbon atoms. Data from Ref. (11).
Figure 6. Relative critical micelle con centrations of ordinary, perfluoro, and partiallyfluorinatedsurfactants as func tion of the F/H ratio in the molecule. Based on Figures 5, 7, and 8. Solid line indicates ideal behavior.
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also on that figure, the real situation is quite different. The two groups of intermediate points of Figure 6 correspond to the ω-hydro perfluoro and the ω - t r i f l u o r o compounds. The com parison of c.m.c.'s on which these points are based are shown in Figures 7 and 8. It should be noted that the replacement of a single fluorine by a hydrogen increases the c.m.c. by a factor of about 3 or 5, i.e. brings i t within a factor of 3 or 4 of the ordinary surfactant. Replacement of three ω-hydrogens by fluor ines in the ω-trifluoro surfactants actually raises the c.m.c. instead of reducing i t as would be expected from the simple mix ture rule. These anomalous c.m.c.'s of the partially fluorinated surfac tants are readily understandable, however, in terms of the mutual phobicity of the hydrocarbon and fluorocarbon portions of their chains which counteracts the aggregating tendency due to the hy drophobic character of the hydrocarbon portion and the s t i l l more hydrophobic character of the fluorocarbon part. Additional fac tors reinforcing the mutual phobicity at the water interface include the dipole moment of the partially fluorinated chains (12J and the possible tendency of the ω-hydrogen to form hydrogen bonds with water (13). Fluorine NMR Results The NMR line of an atom shifts generally with the solvent and thus can be used to investigate the surroundings of an atom within the micelle. Muller (14) developed the concept of the "degree of hydrocarbon-like character," Z, of the environment of such an atom. Ζ = ( a ~ £ ) / ( ô - ô ) where δ is the line shift and the sub scripts indicate m, tne micelle, h hydrocarbon, and w, water, respectively. This can be readily generalized to a "degree of organic character" by replacing 6u by δ for the shift in the appropriate organic solvent as Muller has effectively done later (15). m
h
h
0
Perfluoro Surfactants. There are data for two perfluorosurfactants, the C4 carboxylic acid reported by Bailey and Cady (16) and the Cg sodium carboxylate studied by Muller and Simsohn (loT. The former authors did not interpret their results in terms of environment effects but their data permit the calculation of Ζ factors using the anhydrous perfluorobutyric acid itself as the basis for the δ value and obtaining the 6 values by extrapola tion of the linear portion of their graph. Table I shows the Ζ values so obtained as well as those given by Muller and Simsohn. It is apparent from Table I that Ζ is higher, i.e. the en vironment is more organic for the ω-carbon than for the /8-carbon and this is higher than for the
