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NORBERT MULLER AND TIMOTHY W. JOHNSON
Investigation of Micelle Structure by Fluorine Magnetic Resonance. 111. Effect of Organic Additives on Sodium 12,12,12-Trifluorododecyl
Sulfate Solutions1 by Norbert Muller and Timothy W. Johnson Department of Chemistry, Purdue University, Lafayette, Indiana
47907
(Received December 27,1968)
Sodium 12,12,12-trifluorododecyl sulfate has been prepared, and its fluorine chemical shift has been determined as a function of concentration in water and in mixed solvents including aqueous urea, glycine, glycerol, acetamide, methanol, acetone, ethanol, dioxane, and tetrahydrofuran. The critical micelle concentration of the fluorinated detergent depends on the solvent composition in much the same way as that of sodium dodecyl sulfate. Observed variations in the chemical shift for the monomeric species are not simply related to changes in the dielectric constant or refractive index of the solvent mixture. The additive effects on the monomer and micelle shifts together give an indication of the degree of penetration of additive into the micelles. Changes in the shapes of the dilution-shift curves show that additives strongly influence the micellar molecular weights, and in some cases the aggregation numbers are reduced to values less than five.
Introduction In the first two papers of this series we described a new approach to the study of micelle formation, using the fluorine nuclear magnetic resonance (nmr) spectra of solutions of appropriately labeled soaps or detergents.2J We presented data for several salts of the type CF3(CH2).COONa and showed that they behave very much like salts of ordinary carboxylic acids of similar chain length. We have now extended this work to sodium 12,12,12-trifluorododecylsulfate (F3SDS), an analog of the very extensively studied anionic detergent sodium dodecyl sulfate (SDS). Because F3SDS is the salt of a strong acid, the results reported here allow us to determine whether or not our earlier work stands in need of correction to allow for hydrolysis of the carboxylate ions. The ultimate aim of this research is to arrive a t a more detailed understanding of the interactions which drive micelle formation and which have lately received much attention because similar hydrophobic interactions play a part in stabilizing the tertiary structures of proteins in aqueous solutions. With the same end in view, other workers have investigated the effect of low-molecular-weight additives such as urea on the stabilities of micelles of SDS and other detergent^.^,^ Shortly after we obtained our first samples of FaSDS Professor Holtzer kindly sent us a prepublication copy of ref 5, and we decided to examine our detergent in the presence of some of the same additives, and subsequently one or two others, hoping to test and if possible to extend the conclusions presented there. We found changes in critical micelle concentration (cmc) which closely parallel those of ref 5 , showing again that introduction of the trifluoromethyl group does not The Journal of Physical Chemistry
greatly alter the behavior of a detergent. The nmr results convincingly support Emerson and Holtzer’s opinion that these cmc variations reflect an interplay of several factors including sometimes large changes in the size and composition of the micelles.
Experimental Procedures and Results 12,12,12-Trifluorododecylsulfuric acid was prepared from 12,12,12-trifluorododecanoicacid2by the reactions
CFs(CHJioCHz0H CFs(CH2)llOH
+ ClSOaH + CFa(CH2)iiOSOaH
+ HC1
(1) (2)
Neutralization of the alkylsulfuric acid with aqueous Na2C03 or ethanolic NaOH yields F3SDS. Purification was carried out in two ways. The first batch of material was Soxhlet extracted with hexane for 6 hr to remove unreacted alcohol and then recrystallized, first from methanol and then four times from water. The microanalytical results (% C, 41.84; % H, 6.65; % F, 16.52; % S, 9.11; % Na, 6.89) were in good agreement with calculated values (% C, 42.07; yo H, 6.48; % F, 16.66; % S, 9.37; % Na, 6.72), but apparently traces of the alcohol were formed by hydrolysis during the recrystallizations from water. A second batch was (1) Preliminary results of this work were presented a t the 155th National Meeting of the American Chemical Society, San Francisco, Calif., April 1968. (2) N. Muller and R. H. Birkhahn, J. Phus. Chem., 71, 957 (1967). (3) N. Muller and R. H. Birkhahn, ibid., 72, 583 (1968). (4) M.J. Schick, ibid., 6 8 , 3585 (1964), and references given there. (5) M. F. Emerson and A. Holtzer, ibid., 71, 3320 (1967).
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INVESTIGATION OF MICELLESTRUCTURE BY FLUORINE MAGNETIC RESONANCE ~~
recrystallized once from absolute ethanol and then twice from water. A 10% solution of these crystals in 50% aqueous ethanol was extracted for 20 hr with pentane6 whereupon the aqueous phase was cooled to about -40" and the precipitate filtered and dried under vacuum at 0". The product was almost identical with the first batch but more nearly free from trifluorsdodecanol. The presence of traces of this alcohol was inferred from surface tension measurements carried out by Mr. Ronald C. Gamble by the pendant drop method. The plot of surface tension against concentration showed a shallow minimum a t 0.0135 M . Using analogous results for mixtures of SDS and known amounts of dodecanol' and the depth (about 1 dyn/cm) of the surface tension minimum, we estimated that the amount of alcohol in the second batch of FaSDS was less than 0.05%. Additional details will be given elsewhere.* The additives were commercial materials purified by recrystallization or fractional distillation before use. To prepare each set of samples, the appropriate amount of F3SDS was weighed into a volumetric flask and dissolved in water containing the selected additive a t a known molarity. Aliquots of the resulting stock solution were diluted with the same solvent mixture to bring the detergent concentration to the desired values. The molarity of additive is not strictly constant over such a set of samples but remains close to the nominal value, especially for detergent concentrations near the cmc. The nmr spectrometer was a Varian HA60-IL operated at 56.445 MHz with benxotrifluoride contained in capillary tubes as the source of the lock signal and as an external reference for chemical shift determ i n a t i o n ~ . ~Most of the spectra were recorded at an ambient temperature near 35". The reproducibility of the results showed that over several hours the temperature remained constant to within *0.5". Deviations as large as 0.05 ppm were sometimes found when examining the same sample on different days. Since the shifts have a temperature coefficient of about 0.017 ppm/deg, this can be attributed to long-term temperature changes of about 3". It is important to allow each sample to reach thermal equilibrium in the spectrometer before a measurement is made, which may require 15 min or longer, or to preheat the tubes to 35". To facilitate detection of the signals from the most dilute solutions, an NMR Specialties SD-6OB heteronuclear spin decoupler was used to irradiate the CH2 protons which give rise to the triplet structure of the fluorine resonance. The resulting singlet could be detected at concentrations down to 0.0025 M in a single slow scan. Typical data are presented in Figures 1, 2, and 3 as plots of chemical shift against the reciprocal of the detergent concentration. Solutions in 6.0 M urea, 3.0 and 6.0 M acetamide, and 2.0 M acetone gave results8 superficially similar to those in Figure 1, while solutions in 6.0 M acetone yielded a dilution-shift curves somewhat like that in Figure 2B for 6.0 M ethanol. These
Table I : Parameters for FsSDS in Water and Mixed Solvents
Water 6 . 0 M urea 2 . 0 M glycine 2 . 0 M glycerol 5.95 M glycerol 3 . 0 M acetamide 6 . 0 M acetamide 2 . 0 M methanol 6 . 0 M methanol 2 . 0 M acetone 1.75 M ethanol 6 . 0 M ethanol 2 . 0 M dioxane 2 . 0 M tetrahydrofuran
1.44 1.26 1.20 1.30 1.07 1.50 1.55 1.63 1.84 1.56 1.52 2.15 1.49 1.73
2.61 2.55 2.50 2.61 2.57 2.67 2.73 2.78 2.94 2.80 2.83 3.27 2.70 2.93
0.0146 ... 0 023 0.33 0.0061 0.46 0.0146 0.00 0.020 0.11 0.0215 1.0 0.045 1.1 0.0152 0.90 0.021 0.83 0.018 1.6 0.012 2.7 (0.049) (0.93) 0.022 1.8 (0,0085) (1* 1 ) I
plots were omitted for the sake of brevity, but parameters derived from them are included in Table I which is discussed below.
Discussion Aqueous F3SDS without Additives. The aqueous solution data in Figure 1 define a curve very similar to those found earlier2 for sodium trifluorocarboxylates. The monomer shifts, S(S), and the micelle shifts, S(S,), are nearly the same for both types of surfactant. F3SDS also resembles the carboxylates in that its cmc is approximately twice as large as that of the unfluorinated analog. These observations show that allowance for possible hydrolysis effects reflecting the low acid strengths of the carboxylic acids would not materially change the conclusions in ref 2 and 3. Solutions of sodium alkyl sulfates do undergo slow hydrolysis of the type
ROSOI-
+ H2O +ROH + HOSOa-
(3)
and we investigated this by using aged samples and samples doped with CF3(CH2)110H. Small amounts of but do reduce the alcohol do not change 6(S) or S(S,), the sharpness of the break in the plot of shift against reciprocal concentration in the cmc region. This finding is not surprising, since the alcohol should disturb the monomer-micelle equilibrium most noticeably when the micelle concentration is finite but small, and it indicates that reliable monomer and micelle shifts can be obtained even with slightly contaminated samples. Between 0.013 and 0.005 M the chemical shift of F3SDS is independent of concentration. It has been (6) S. P. Harrold, J . Colloid Sci., 15, 280 (1960).
(7) G. D. Miles and L. Shedlovsky, J. Phys. Chem., 48, 57 (1944). (8) T. W. Johnson, Ph.D. Thesis, Purdue University, in preparation. (9) It was eventually found that the fluorine signal of CFaCCI=CC12
is sharper and more conveniently located than that from benzotrifluoride, and this substance was used as the reference in the measurements for the tetrahydrofuran solutions. The shifts were converted into the benzotrifluoride scale by subtracting 2.26 ppm. Volume '73,Number 6 June 1969
NORBERT MULLER AND TIMOTHY W. JOHNSON
2044
I1 I
25
KXI
75
50
103
Ix) 150
203
I/S (Liters /Mole)
xo
4
I/So (Liters / Mole)
Figure 1. Fluorine chemical shift of FaSDS plotted us. the reciprocal of the detergent concentration in: A, water; B, 2.0 M methanol; C, 6.0 M methanol.
Figure 3. Fluorine chemical shift of FaSDS plotted us. the reciprocal of the detergent concentration in: A, 2.0 M tetrahydrofuran; B, 2.0 M dioxane; C, 2.0 M glycine.
1
30
I
a2
40
€3
60
Kx,
l/So (Liters/Mole)
I 25
50
75
KYJ
130 150
l/So (Liters /Mole)
Figure 2. Fluorine chemical shift of FsSDS plotted us. the reciprocal of the detergent concentration in: A, 1.75 M ethanol; B, 6.0 M ethanol; C, 2.0 M glycerol; D, 5.95 M glycerol.
suggested that SDS dimerizes appreciably below its cmc,l0 but the present results provide no evidence for such dimerization of FBDS. Effect of Additives on Aggregation Numbers. Figures 1, 2, and 3 show that the change of slope of the plot of shift against 1/ [So] near [So] = cmc is less abrupt in the mixed solvents than in water, and in several cases the plot remains curved over the entire accessible concentration range. This brings to mind Emerson and Holtser’s findings that it becomes very difficult t o determine cmc’s from conductivity data in the presence of some additives. It is almost certainly a consequence of the fact that micelles formed when different additives are present generally contain different numbers of ions. Although it is difficult t o draw quantitative conclusions The Journal of Physical Chemistry
Figure 4. Chemical shift plotted us. reciprocal of the concentration for model systems consisting of monomers and monodisperse micelles for selected values of the aggregation number, m.
in this regard, the nmr data are considerably more revealing than conductivity data, especially when the aggregation numbers become very small. Dilution curveb for model systems of the type mS
Z8,;
K,
=
[S,I/[SIm
are easy to calculate for selected values of m and K,. Such results are instructive even though, because the counterions are ignored, the models resemble the real system less closely than one would wish. Figure 4 shows calculated curves for systems with 6(S) = 0, 6(S,) = 1, K,’s chosen so that m[Sm]= [SI when [SI = 0.02 M , and several values of m. A change of even 10 units in m has a barely detectable effect when m E 100, (10) P. Mukerjee, Advan. C o l l o ~Interface Sci., 1, 248 (1967), and references given there.
2045
INVESTIGATION OF MICELLE STRUCTURE BY FLUORINE MAGNETIC RESONANCE but the curves are much more sensitive to changes in m when m is small. The aggregation number of SDS in water” is about 62, and comparison of Figures 1 and 4 suggests that the size of the F3SDS micelles is not greatly different from this. It appears from the other experimental curves that micelles formed in any of the mixed solvents are substantially smaller than those formed in water. If the micelles remain spherical, reducing the aggregation number increases the surface area per ionic head group, and it is surprising that this is apparently favored regardless of the effect of the additive on the solvent dielectric constant. I n most of the mixed solvents, the aggregation numbers seem to fall roughly in the range 10 < m < 25, with values depending both on the nature of the additive and on its concentration. Any attempt to evaluate these numbers at all precisely would require more accurate data and more sophisticated model calculations. The solutions in 2 M tetrahydrofuran, 6 M ethanol, and 6 M acetone show by far the largest reductions in micelle size, with apparent aggregation numbers of 4 or less. A simple model which closely approximates the data in 6 M ethanol is one which includes only monomers and trimers. The solid curve in Figure 2B is for such a system with S(S) = 2.15, S(S3) = 3.27, and Ka = 156.5 (l./mol)2. The curve through the tetrahydrofuran data in Figure 3A represents a monomertetramer system with 6(S) = 1.73, 6(S4) = 2.93, and K4 = 3.9 X lo6 (l,/mol)2. Attempts to fit the 6 M acetone data with a curve based on a simple model were less successful and suggested that both dimers and trimers should be included. The agreement between the calculated and observed shifts in 6 M ethanol and 2 M tetrahydrofuran may be in part fortuitous, but it seems beyond doubt that micelles in these solutions are very different from those formed in water or even in most of the other mixtures. The available data give no indication as to the number of additive molecules which may be incorporated into these very small micelles. It is worth noting that molecules of ethanol, tetrahydrofuran, or acetone could orient themselves at an interface with a polar group on one side and only hydrocarbon groups on the other. This is not true for otherwise rather similar molecules, like dioxane or glycerol, which apparently do not stabilize very small micelles. The difference in behavior of F3SDS in 2 M solutions of the two cyclic ethers, dioxane and tetrahydrofuran, where the dielectric constants should be rather similar, is particularly striking (see Figure 3). A few additional experiments were carried out in 6 M ethanol to find whether the apparently trimeric species might be NaSz-. I n the range 0.006 M< [F3SDS]< 0.02 M , solutions containing equal concentrations of FBDS and NaCl gave essentially the same shifts as those without added NaC1. The absence of a common ion effect
suggests that the principal associated species is (SaEtOH,)3-, where S represents the detergent anion. Ej’ect of Additives on Cmc, Monomer Shift, and Micelle Shift. Table I gives the cmc, S(S), and S(S,) values obtained graphically as in ref 2. For the solutions in 6 M acetone, 6 M ethanol, and 2 M tetrahydrofuran, the graphic procedure fails, and indeed the concept of a cmc is no longer meaningful. No parameters are given for the 6 M acetone solutions since no simple model was found which fits the data. For the other two solvent mixtures the table includes S(S) and S(S), values for the respective model systems mentioned in the preceding section and “cmc” values, in parentheses, equal to the monomer concentration in a solution in which half of the detergent is associated. A “cmc” thus defined is of some use in making comparisons because it approaches the actual cmc in the limit of large aggregation numbers. The effects of the additives on the cmc’s closely resemble those reported for F3SDS by Emerson and Holtzer,6 who discussed at length the difficulties involved in rationalizing such results. They arise from a combination of factors including the changes in electrostatic forces entailed by changes in the dielectric constant, the ability of some additives to stabilize the monomers either by modifying the water structure or by favorable hydrophobic interactions between monomer and additive, and changes in the size and composition of the micelles, especially when a penetrating additive is used. By themselves, our cmc values contribute little new information except that the fluorine-labeled detergent ions behave very much like the unlabeled species. The chemical shifts, on the other hand, provide independent evidence showing that the additives differ widely in their ability to penetrate into the micelles. It must be borne in mind, however, that the additive may profoundly change the micelle size, so that it is not permissible to think of the micelles as essentially constant entities differing only with regard to the presence or absence of incorporated molecules of additive. Previous studies of solvent effects on fluorine chemical shifts suggest that the monomer shifts should be correlated with solvent refractive indexI2 or dielectric constant,I8 but our results fit into no such simple pattern. Ethanol and water have nearly the same refractive index but yield shifts differing by more than 2 ppm. The dielectric constant (E) of water is lowered by heating or by adding glycerol and raised by adding urea or glycine, but any one of these changes reduces S(S). Moreover, adding ethanol lowers e while adding N-methylacetamide raises E , but each of these causes S(S) to increase.* Apparently the monomer shifts are affected by specific solvent-solute interactions, perhaps (11) K. J. Mysels and L. H. Princen, J . Phys. Chem., 63, 1696 (1959). (12) D.F. Evans, J. Chem. SOC.,877 (1960). (13) J. W.Emsley and L. Phillips, Mol. Phys., 11, 437 (1966).
Volume 7.9*Number 6
June 1969
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NORBERT MULLERAND TIMOTHY W. JOHNSON
involving F-H-0 hydrogen bonding or the formation of clathrate-like cages around the trifluoroalkyl groups. We hope to elucidate the nature of these interactions in subsequent work. The effect of additive on S(S,) has the same sign as that on 6(S) but differs in magnitude. It is convenient to use the notation =
Aa(Sm)
- a(S)water solvent - a(Sm)wster
S ( S ) m i x e d solvent a(Sm)mixed
R
=
(4)
AS(S,)/AS(S)
If the CF, groups of micellized detergent were entirely out of contact with the solvent, A6(S,) and hence also
R would be zero for a nonpenetrating additive. However, at any instant some of the CFa groups in the surface region of the micelles seem to be exposed to the surrounding medium.2 Moreover, the additive may induce changes in the size, extent of hydration, or counterion binding ability of the micelles. Nonzero values of AS(S,) thus need not indicate that the additive enters the micelles, but nevertheless one expects to find R < 1 for nonpenetrating additives, especially a t concentrations low enough to assure that the micelle size is not drastically altered. If the additive is hydrocarbon soluble, it should dissolve in the micelles and may prefer a micellar to an aqueous environment. As long as the aggregation number is not too greatly changed this will tend to make AS(S,) larger than AS(S), or R > 1. At higher additive concentrations, the micelles may become saturated with the additive, as suggested in ref 5. Then S(S,) will tend t o level off as more additive is introduced, and R will tend to decrease. However, increasing the additive concentration may lead t o such an enormous drop in the aggregation number that the concept of penetration of the additive into a preexisting micelle loses its utility. I n such cases R values cannot be even approximately predicted, nor are the observed values readily rationalized. Experimental values of R are included in the last column of Table I. The accuracy of these values is rather low, especially when Aa(S) and AS@,) are small, because of the temperature variations mentioned in the
The Journal of Physical ChemiEtTy
Experimental Section. Nevertheless, glycerol, glycine, and urea, which should be nonpenetrating, give the lowest R’s, between 0 and 0.5. Methanol and acetamide are less insoluble in hydrocarbons and give R’s near 1.0, approximately independent of additive concentration. The largest R’s are found for 1.75 M ethanol, 2 M acetone, and 2 M dioxane which are typical penetrating additives. The values found in 6 M ethanol and 2 M tetrahydrofuran are probably not significant since exceptionally small micelles form in those solutions. It appears that the R parameter can indeed be used to distinguish among nonpenetrating, weakly penetrating, and strongly penetrating additives, but one should not at this stage attempt to interpret small differences in R.
Conclusions The results presented here vividly demonstrate that the effects of additives on detergent cmc’s cannot be understood simply on the basis of the ability of the additive to stabilize or destabilize the monomeric species. The nmr technique reveals in more detail than methods previously used that additives may affect the composition of the micelles and bring about drastic changes in micellar molecular weights. A more suitable system for following such changes quantitatively would comprise water, additive, and nonionic detergent, eliminating variables representing repulsions between charged groups and counterion binding. Even for urea, glycine, and glycerol, the least penetrating additives, the demicellizing effectiveness is not simply related to their effect on the dielectric constant. The chemical shifts for the monomeric ions in the various mixed solvents also fail to show any obvious regularity. FBSDS does not appear to dimerize below its cmc in water solutions. Although this finding does not rule out the possibility of such dimerization for SDS, our results further support our earlier conclusion that solution properties of trifluoroalkyl detergents are closely similar to those of the related alkyl detergents.
Acknowledgment. This research was supported by the National Science Foundation through Grant G.P. 8370 and by the National Institutes of Health through a predoctoral fellowship awarded to T. W. J.
