J . Phys. Chem. 1987, 91, 2938-2946
2938
k- (1.6 X 104 s-I) for Cu2+in EPE aggregate than in SDS micelles (1.2 X lo5 s-I).I6 Relative to EPE aggregates in solution phase, the micellar weight of EPE aggregates on silica (ca. 29 000) is close to that of SDS and it is more compact and contains less water; therefore, the k- value for Cu2+ (2.2 X lo5 S-I) is similar to that in SDS. For EPE aggregates on silica, the k+ (1.3 X 1O8 M-' s-l ) value is similar to that (1.4 X lo8 M-' s-') in solution phase; both are smaller than that in SDS (1.2 X lo9 M-' s-' ).I6 The smaller k+ for EPE on silica surface is possibly related to the smaller diffusion of Cu2+ due to the interaction between Cu2+ and silica.
Conclusion EPE block copolymers are adsorbed on silica, and the adsorption isotherm is similar to that for surfactants. The solubilization amount of pyrene in the polymer aggregates on silica is 4-10 times higher than that in the solution pahse. The micropolarity inside the aggregate on silica surface (Zl/Z3 = 1.54 for 0.2% EPE) is significantly smaller than that in the solution phase (Z,/Z3 = 1.84). The aggregation numbers of the polymer aggregates on silica surface (2-9 for [EPE] = 0.01-2%) are significantly smaller than those in the solution phase (35-600). the lifetime of pyrene (16) Hashimoto, S.; Thomas, J. K. Chem. Phys. Lett. 1984, 109, 115.
quenched by Cu2+in the aggregates on silica (193 ns) is higher than that in the solution phase (I32 ns). The shapes of the decay curves of pyrene quenched by Cu2+ show that the nature of polymer aggregates on silica is closer to that of SDS micelles than to the polymer aggregates in solution phase. These results show that the polymer aggregates on silica surface are smaller and more contracted and possess properties close to those of SDS micelles. On silica, the flat regions in the plot of aggregation number vs. [EPE] and of Zl/Z3 vs. [EPE] are the same, indicating that the polymer aggregates on silica is stable with an aggregation number of ca. 4. The values of k+ and k.. for Cu2+in the EPE aggregates on silica are 1.4 X lo8 M-' s-l and 2.2 X lo5 s-l, respectively, and those in the solution phase are 1.3 X lo8 M-' s-' and 1.6 X lo4 s-l, respectively. Relative to the k- of SDS (1.2 X lo5 s-l), the smaller k- for polymer aggregates in the solution phase is attributed to the big size and the high water-enriched interior.
Acknowledgment. The authors thank the Army Research Office and the National Science Foundation for their generous support of this research. The important advice and assistance of Professor P. Somasundaran and Dr. Prem Chandar of Columbia University's School of Engineering and Applied Sciences (Henry Krumb School of Mines) are gratefully acknowledged. Registry No. (EO)(PO)(block copolymer), 106392-12-5; SO2, 7631-86-9;Cu2*, 15158-1 1-9; pyrene, 129-00-0.
Microstructure of Formamide Microemulsions from NMR Self-Diff usion Measurements K. P. Das,*+A. Ceglie,l and B. Lindman* Department of Physical Chemistry 1. Chemical Center, University of Lund, S-221 00,Lund, Sweden (Received: December 3, 1986)
The microemulsion stability range and multicomponent self-diffusion data are presented for systems of formamide, alcohol, and sodium dodecyl sulfate in both the presence and absence of an oil, p-xylene; the alcohols used were 1-butanol, 1-pentanol, 1-hexanol, 1-heptanol, and 1-octanol. The results have been compared with the analogous water/alcohol/SDS systems. In the nonaqueous systems, high self-diffusion coefficients were observed for all the components in essentially all the regions studied. The results do not support any appreciable confinement of any component into closed domains. Rather the structure seems to be close to the structureless limit of simple solutions. The aqueous systems are structurally quite different from these nonaqueous solutions and show considerable surfactant organization in bicontinuous or droplet structures. Thus, distinct water droplets were found in the aqueous systems with primary alcohols having six or more carbons. The structure of the formamide system is, on the other hand, quite insensitive to the chain length of the alcohol. These conclusions about the nonaqueous and aqueous microemulsions remain valid in both the presence and absence of the oil, p-xylene. Structureless microemulsions have also been found by using as nonaqueous solvents N-methylformamide and N,N'-dimethylformamide in place of formamide. Self-diffusionmeasurements have also been extended to the two-component nonaqueous solvent/SDS systems. The results are again very different from those of the aqueous system and point to a quite insignificant aggregation with no evidence for distinct hard-core micelles. The results on organization in these nonaqueous systems are consistent with the recent findings of Rico and Lattes who have demonstrated that the organization in nonaqueous surfactant systems requires other choices of surfactant chain length, cosurfactant, and temperature than in aqueous systems.
Introduction It has been known for a long time that surfactants can form aggregates also in nonaqueous solvents.'-' Micelle formation of different surfactants has been reported in a number of solvents like various amides,2 dimethyl sulfoxides,2 glycol^,^*^ and even inorganic salt melt^.^.^ Since hydrophobic interactions are responsible for micelle formation in an aqueous medium, analogous solvophobic interactions were postulated for nonaqueous media. However, a recent study by Almgren et aL8 using a large number of experimental techniques has raised doubts as to the existence of real aggregates of surfactant molecules in such media and +On leave from the Department of Chemistry, Vidyasagar College, 39, Shankar Ghosh Lane, Calcutta 700 006, India. 'Present address: Dipartimento di Chimica, Universita degli Studi di Bari, Via Amendola 173, 70126 Bari, Italia.
0022-365418712091-2938!$01.50/0
reported that unless there is the presence of sufficient water there is a poor organization of sodium dodecyl sulfate (SDS) in a nonaqueous solvent like formamide. Among other organized surfactant assemblies, liquid crystals have been identified in nonaqueous solvents, for quite some time. (1) Solution Behcwiour of Surfactants, Vol. 2, Part 111, Mittal, K. L., Fendler, E. J., Eds.; Plenum: New York, 1982; pp 743-886. (2) Singh, H. N.; Saleem, S. M.; Singh, R.P.; Birdi, K. S. J . Phys. Chem. 1980,84, 2191. ( 3 ) Ray, R. J . Am. Chem. Soc. 1969, 91, 6511. (4) Ionescu, L. G.; Fung, D. S. J . Chem. Soc., Faraday Trans. 1 1981, 77, 2907. ( 5 ) Evans, D. F.; Kaler, E. W.; Benton, W . J. J . Phys. Chem. 1983, 87, 533. (6) Evans, D. F.; Chen, S. H. J . Am. Chem. Sot. 1981, 103, 481. (7) Evans, D. F.; Yamauchi, A.; Rohan, R.; Casassa, E. J. J . Co//oid Interface Sci. 1982, 88, 89. (8) Almgren, M.; Swarup, S.; LBfroth, J . E. J. Phys. Chem. 1985,89,4621.
0 1987 American Chemical Society
Microstructure of Formamide Microemulsions A number of surfactants and lipids have been reported to form liquid crystals in different solvents and the nature of the interactions and the structure of these assemblies have been examined.”* Recently, very great interest has arisen in the study of microemulsions in general. Since microemulsions in certain respects have properties intermediate between those of micellar solutions and liquid crystals, it was predicted that microemulsions would be possible to prepare by replacing water by a wide variety of different solvents. Quite recently a number of nonaqueous microemulsion systems have been reported independently by different groups who replaced water by solvents like glycerol, ethylene glycol, formamide, other amides, etc. and used different surfactants. For example, Fletcher et aI.l3 reported a system glycerol/heptane/Aerosol-OT to give microemulsions. Friberg reported phase diagrams for the systems glycerol/xylene/triethanolammonium oleate and oleic acidI4 and ethylene glycol/decane/ 1 e ~ i t h i n . l ~Rico et al.I6q1’ reported on the formamide/butanol/cyclohexane/CTAB and formamide/butanol/isooctane/ CTAB systems and these authors also prepared a number of perfluorinated microemulsionsI8 using different perfluorinated alcohols, oils, and surfactants. Friberg later examined systems like glycerol alcohol/surfactant/oil using various surfactants and
alcohol^.'^^^
6
There seems to be a number of distinct advantages of nonaqueous microemulsion systems over the aqueous systems: (1) Nonaqueous systems often show much larger stability regions of isotropic solutions as compared to the analogous aqueous sytems.’8q21 (2) A large variety of different surfactants can be used to give nonaqueous microemulsions. For example, a long-chain phosphonium halide could be incorporated in nonaqueous microemulsions, while corresponding attempts to prepare aqueous microemulsions with this surfactant were unsuccessful.16 (3) These microemulsions can be used as good reaction m4ia.l’ They are, of course, particularly attractive for those reactants which react with water. While the phase behavior of these systems has been rather extensively investigated, little is known about the microstructures of these systems. From the similarities in phase behavior between aqueous and nonaqueous systems, Rico et a1.I6l8 suggested that the structures are similar. The authors tried to obtain support of their conclusion from complementary conductance measurements.1618 These properties are sometimes useful for structural elucidation but can also provide misleading information. For instance, it has been demonstrated* that a break in the conductance-concentration plot in binary surfactant/solvent systems is not necessarily indicative of the onset of micellization but can have a quite different origin. Thus, fluorescence quenching, surface tension, and self-diffusion measurements showed* that there was hardly any micelle formation of SDS in pure formamide although there was a break in the conductance-concentration plot. Light scattering studies of the glycerol/heptane/Aerosol-OT system by Fletcher et al.I3 were interpreted in terms of discrete glycerol droplets, much smaller than those in the corresponding aqueous system. Friberg,I9 however, concluded that the nonaqueous (9) Moucharafieh, N.; Friberg, S. Mol. Cryst. Liq. Cryst. 1979, 49, 231. (10) El Nokaly, M.; Ford, L. D.; Friberg, S.; Larsen, D. W. J . Colloid Interface Sci. 1981, 84, 228. (1 1) Larsen, D. W.; Friberg, S.; Christenson, H. J. Am. Chem. SOC.1980, 102, 6565. (12) Ganzuo, L.; El Nokaly, M.; Friberg, S. Mol. Cryst. Liq. Cryst. 1982, 72, 183. (13) Fletcher, P. D. I.; Galal, M. F.; Robinson, B. H. J . Chem. SOC., Faraday Trans. 1 1984.80, 3307. (14) Friberg, S . E.; Podzimek, M. Colloid Polym. Sci. 1984, 262, 252. (15) Friberg, S. E.; Wohn, C. S. Colloid Polym. Sci. 1985, 263, 156. (16) Rico, I.; Lattes, A. Nouv. J . Chim. 1984, 8, 429. (17) Samii, A. A.; Savignac, A,; Rico, I.; Lattes, A. Tetrahedron 1985, 41, 3683. (18) Rico, I.; Lattes, A. J . Colloid Interface Sci. 1984, 102, 285. (19) Friberg, S. E.; Liang, Y.-C. Colloids Surf., in press. (20) Friberg, S. E., Personal communication. (21) Oliveros, E.; Maurette, M. T.; Braun, A. M. Helv. Chim. Acta 1983, 66, 1183.
The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 2939 systems he investigated were close to simple solutions with critical behavior. One possibility of getting some insight into the microstructure of microemulsions is based on multicomponent self-diffusion measurements.22-28 The self-diffusion behavior provides direct information on whether or not the different components occur in closed domains. Our previous study** on nonaqueous microemulsions with glycerol revealed clear differences in the structure between aqueous and nonaqueous microemulsions. The aqueous system was found to be quite structured whereas the nonaqueous system was far less structured and much closer to a structureless simple solution. However, because of the lack of structural information on a large number of nonaqueous microemulsion systems, it would be too early to attempt any generalization, in particular as glycerol is not a close analogue of water. For a more realistic comparison between aqueous and nonaqueous systems it is desirable to replace water by a solvent having polarity properties close to it, such as formamide. Formamide and its derivatives are known to give extensive isotropic regions in surfactant Structural studies on formamide systems could help in understanding reaction mechanisms and thus in establishing a better way of utilizing it as a reaction medium. We have, therefore, undertaken a study of the structure of formamide microemulsions using the Fourier transform N M R self-diffusion technique, since the self-diffusion behavior directly reflects microstructural features. In the previous study of the effect of the variation of the chain length of the components on the microstructure in the four-component aqueous cosurfactant systems, it was demonstrated that the most dramatic changes in microstructure result from a variation of the cosurfactant chain length. Particular attention has, therefore, been given to the structural situations in the presence of cosurfactants of different chain length. Solvents used in the present study to replace water are formamide, N-methylformamide, and N,N’-dimethylformamide, which will be referred to throughout the text as FM, NMF, and DMF, respectively. The self-diffusion behavior in these nonaqueous microemulsions will be compared with the analogous aqueous microemulsions.
Experimental Section Materials. The following chemicals were used in the present work: Formamide (analaR), N-methylformamide (analaR), N,N’-dimethylformamide (analaR), 1-butanol, 1-pentanol, 1hexanol, 1-heptanol, 1-octanol, and p-xylene were all from BDH, England. SDS was a specially pure biochemical grade from BDH, England. Tetramethylsilane was from E. Merck, Germany. Heavy water was from Ciba Geigy, Switzerland. Water was twice distilled. Determination of the isotropic Phase Boundary. For phase diagram construction, samples were prepared by weighing the components into glass ampules which were then flame sealed. All samples were left in a temperature controlled (2.5 “C) room for at least 7 days to reach equilibrium. The optical isotropy of each sample was checked under a crossed polaroid. For isotropic phase mapping of each system at least 80 samples were prepared. The (22) Lindman, B.; Stilbs, P.; Moseley, M. E. J . Colloid Interface Sci. 1981, 83, 569. (23) Lindman, B.; Ahlnas, T.; Siiderman, 0.; Walderhaug, H.; Rapacki, K.; Stilbs, P. Faraday Discuss., Chem. SOC.1983, 76, 317. (24) Lindman, B.; Stilbs, P. In Surfuctans in Solution, Vol. 3, Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; p 1651. (25) Stilbs, P.; Rapacki, K.; Lindman, B. J . Colloid Interface Sci. 1984, 95, 583. (26) Lindman, B.; Stilbs, P. In Physics of Amphiphiles, Micelles, Vesicles and Microemulsions, Degiorgio, V., Corti, M., Eds.; North-Holland: Amsterdam, 1984. (27) Ceglie, A,; Das, K. P.; Lindman, 9. J . Colloid Interface Sci. 1987, 115, 115. (28) Das, K. P.; Ceglie, A,; Lindman, B.; Friberg, S. E. J . Colloid Interface Sci., in press. (29) BergenstAhl, B.; JBnsson, A,; Sjbblom, J.; Stenius, P.; WBrnheim, T. Progr. Colloid Polym. Sci., in press.
2940
The Journal of Physical Chemistry, Vol. 91, No. 11, 1987
Das et al.
a. C50H
C70H
AAAA
FM
SDS FM
SDS FM
b.
C40H
WATER
SDS
FM
SDS
C60H
SDS
WATER
SDS
WATER
SDS
Figure 1. Isotropic regions in three-component nonaqueous (a) and aqueous (b) systems. Liquid crystalline phases are not shown for the sake of simplicity. Phase diagrams refer to weight percentage in compositions. Diagrams for the aqueous systems are from ref 20.
boundary of the isotropic region is supposed to be accurate to within f 2 wt %. Self-Diffusion Measurements. The pulsed field gradient spin echo N M R method used in the present study has been described earlier.22*30-32All self-diffusion measurements were done on a Jeol FX-60 FT N M R instrument operating at 60 MHz for protons. External D 2 0 lock was used for all samples. Temperature was adjusted to within f0.5 "C which was measured by a copper-constantan thermocouple. The time (A) between the 90" and 180" pulses used for measuring self-diffusion coefficients in the nonaqueous systems was chosen as 170 ms. This value of A makes the appearance of the solvent peaks in good phase in the spin-echo spectra. For the aqueous samples, the value of A used was 140 ms as Intensities (Z) of peaks arising from single components were fitted to single exponentials according to
I , = A exp[-GDi6'(A - 6/3)]
(1)
where 6 is the duration of the field gradient, Di is the self-diffusion coefficient of the ith species, G is a constant, and A is a parameter which was varied to give the best fit to the data. The constant G was determined from calibration experiments where the selfdiffusion coefficient34of trace HDO in D20was taken as standard. Often there was no separate peak from SDS,but the main -CH2 peak of SDS overlapped with that of the alcohol. Plotting the logarithm of the intensity of the -CH2 peak against the function 62(A - 6/3) showed two straight lines with different slopes. The self-diffusion coefficient of SDS was in such cases determined by fitting the intensities of the -CH2 peak to a double exponentialz2 according to the equation I= A[p exp(-GD,62(A - 6/3)
+ (1 - p ) exp(-GD26*(A - 6/3))] (2)
where D1 and D2 are the self-diffusion coefficients of the components whose peaks overlap, and p is the fitting parameter. The (30) Stilbs, P.; Moseley, M. E. J. Mugn. Reson. 1978, 31, 5 5 . (31) Stilbs, P.; Moseley, M. E. Chem. Scr. 1980, 15, 176. (32) Stilbs, P. Progr. N M R Spectrosc. 1987, 19, 1. (33) Stilbs, P. J . Colloid Interface Sci. 1982, 87, 385. (34) Mills, R. J . Phys. Chem. 1973, 77, 685.
diffusion coefficient of the alcohol was known from the decay of the a-CH2 peak and was treated as a fixed parameter in the fitting to eq 2. Due to short transverse relaxation times, the amplitude of the alcohol a-CH,peak was too small to be measured accurately for the octanol systems and therefore the octanol and SDS selfdiffusion coefficients were not accessible. All numerical computations were done in a UNIVAC (Sperry 1100 OS) computer, using nonlinear least-squares fitting via Monte Carlo simulation. For details of the program subroutine and error calculations see ref 30, 32, and 35. All error limits in the calculation of diffusion coefficient correspond to the 80% confidence interval. In calculating water self-diffusion coefficients in the presence of alcohols, the exchange of the alcohol -OH protons with the water protons has been taken into account as was done previously.22 In the cosolubilization experiments, one drop (=lo mg) of tetramethylsilane (Me4Si) was added to each of the surfactant solutions or to the neat solvents (-2 g). The solutions were allowed to equilibrate overnight before carrying out the self-diffusion measurements.
Results and Discussion Phase Behavior of Three-Component Systems. The isotropic regions of ternary systems containing formamide, SDS, and a straight-chain alcohol having 5-8 carbon atoms in the chain are shown in Figure la. A single isotropic region extending from the alcohol corner to the formamide corner is observed in each case. A similar phase behavior was also noticed in the nonaqueous system glycerol/hexanol/SDS.'g~2*The isotropic area of the formamide system decreases progressively as the chain length of the alcohol increases, which is due to the decreased mutual solubility of alcohol and FM. These phase diagrams can be compared with the analogous aqueous systemsZo (see Figure Ib). With hexanol, two separate monophasic isotropic solutions are observed. This is probably also true with pentanol, as similar three-component systems with other surfactants like potassium oleate36or tetradecyltrimethylammonium bromide3' give such a behavior. (35) Chandler, J. P. "Program Manual for Subroutine STEPIT", program 66, Quantum Chemistry Program Exchange, Chemistry Department, Indiana University, Bloomington, IN 47401. (36) Warnheim, T.; Sjoblom, E.; Henriksson, U.; Stilbs, P. J . Phys. Chem. 1984, 88, 5420.
Microstructure of Formamide Microemulsions n-ENTA NOL
The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 2941 TABLE I: Self-Diffusion Coefficients ( D ) , Viscosities ( q ) , and Dielectric Constants (a) at 25 OC of the Neat Microemulsion Constituents
component FM NFM DMF H2O 1-butanol
FM
P-XYLENE
Figure 2. Isotropic region in the three-component nonaqueous surfac-
tantless system. Compositions are in weight percentage. As the alcohol chain length increases, the two isotropic regions move further apart and become less extended. The two isotropic areas in the aqueous system merge to form a single monophasic area only when the chain length of the alcohol is less than five. Thus one observes throughout a larger isotropic area in the formamide three-component systems than in the analogous aqueous systems. This is obviously due to a higher solubility of alcohols in F M than in water. For instance, it can be noted from Figure 1 that although butanol and water have a rather large immiscibility gap, pentanol (and hence all lower alcohols) is completely miscible with FM. The complete miscibility of pentanol and FM allows a hydrocarbon to be incorporated in the pentanol-FM mixture in appreciable amounts while maintaining the isotropic solution phase. This isotropic region is shown in Figure 2 for the system p-xylene/pentanol/FM. This type of surfactantless systems can be taken as a good reference system in discussions of the structure in four-component systems with added surfactant. Phase Behavior of Four-Component Systems. The isotropic region for the four-component systems consisting of FM, pxylene, SDS, and an alcohol is shown in Figure 3a for a f i e d ratio between alcohol and SDS (2:l). The corresponding phase diagrams of the are shown in Figure 3b. aqueous systems (with benzene as As in the three-component systems, a single isotropic region is found in the nonaqueous systems for all three alcohols, even for octanol. The shape of the isotropic regions is similar to that reported earlier by Rico and LattesI6 for the system FM/I-butanol/cyclohexane/cetyltrimethylammoniumbromide. For the aqueous systems a single isotropic phase appears only with butanol (or alcohols having less than four carbons) but when the carbon number in the linear alcohol exceeds 4, two separate isotropic phases appear. The monophasic areas decrease in extension with an increase in cosurfactant chain length both in the aqueous and the nonaqueous systems. One finds in both three- and fourcomponent systems that the isotropic areas are always larger in the FM systems than in the corresponding aqueous systems. This observation seems to be quite general and was found to be true also in some other nonaqueous systems, as for example, in perfluorinated microemulsions.IS Thus, with a short-chain alcohol as cosurfactant, the phase behavior of aqueous and nonaqueous systems turns out to be similar in nature, as was also observed by Rico et al.16J7 This is, however, not true in general and there are considerable differences which become apparent only when one uses alcohols having more than four carbons in the chain. Se[f-Diffusion Measurements in Two-Component Systems. Some controversy has recently arisen' as to the existence of surfactant micelles in nonaqueous solvents like FM. As a background to our studies of microemulsions with nonaqueous solvents, we have considered this problem and have done some self-diffusion measurements in two-component solvent/SDS systems. As sol(37) Friberg, s. E.; Vanable, R. L.; Raymond, L.; Kim, M.; Neogy, P. Colloids SurJ 1985, 15, 285. (38) Heil, J.; Clausse, M.; Peyrellasse, J.; Boned, C. Colloid Polym. Sci. 1982, 260, 93.
1-pentanol 1-hexanol 1-heptanol 1-octanol p-xylene
1 0 9 ~m2 , s-l 0.521" 0.848" 1.62' 2.27b 0.43b 0.31b 0.23b 0.18 0.14 2.17'
n, CP
td
3.30 1.65 0.796 0.891 2.95 (20 OC)e 3.6@ 5.20' 6.80' 8.50' 0.65 (20 oC)c
109.5 182.4 36.71 78.8
"From this work. bFrom ref 25. 'From ref 22. dFrom ref 45. 'From ref 46. 'Estimated at 25 O C from the data at the different temperatures reported. Reference 46. vents we have taken FM, NFM, and DMF. In Figure 4 are presented the SDS self-diffusion coefficients as a function of the SDS concentration. For the sake of comparison, the self-diffusion results of the SDS/water system taken from the literature39are also included in the figure. One finds from the figure that there is a sharp fall of the SDS self-diffusion coefficient in the aqueous system within 1 wt % SDS, while the fall is not at all so sharp with D M F and is indeed very small in the case of FM and NFM. The self-diffusion coefficients in the nonaqueous solvents are considerably higher than in the aqueous solvent at SDS concentrations above 2 wt % which is roughly 10 times the cmc of SDS in water. Since the viscosities of FM and N F M are considerably higher than that of water (see Table I), one would expect the self-diffusion coefficients in these nonaqueous solvents to be appreciably lower than those in water, if the aggregates had a similar size as in water. This is contrary to our findings. One, furthermore, finds a big difference (a factor of about 7) between the SDS diffusion coefficients in water and D M F at higher concentrations (>2 wt %) although the viscosities of the two solvents are not much different. This clearly points to a significant difference in the association behavior of SDS in the aqueous and nonaqueous media and the question arises if there is any aggregate at all in these nonaqueous solutions. To shed further light on this problem the following qualitative analysis is made. The value of the SDS self-diffusion coefficient at infinite dilution m2 s-I. From viscosities we estimate the in water39is 6.2X infinite dilution self-diffusion coefficients in the nonaqueous solvents to be 1.62X m2 s-I in FM, 3.24 X m2 s-l in NMF, and 6.72X 10-Iom2 s-l in DMF. The observed ranges of diffusion coefficients are (1.5-1.1) X lo-'' m2 s-I in FM, (3.1-2.2)X m2 s-l in NMF, and (6.6-4.5)X m2s-I in DMF; these are all within a factor of 1.5 of the expected infinite dilution values. The concentration range over which self-diffusion coefficients have been measured is much higher than the reported cmc's in these solvents.2 It is apparent from these simple considerations that these solutions have a self-diffusion behavior close to that expected for monomeric solutions; any larger aggregates are highly unlikely. The small decrease in self-diffusion coefficient with increasing SDS concentration can to a large extent be referred to ion-ion interactions. The qualitative difference between SDS self-association in water and the nonaqueous solvent is perhaps best illustrated by the relative self-diffusion coefficients presented in the inset of Figure 4. To deduce more quantitative information about the micelle diffusion coefficient, fraction of free monomer present, counterion binding, etc., a usual practice is to introduce trace amounts of a very hydrophobic solute inside the aggregate.m2 In the case (39) Lindman, B.; Puyal, M.-C.; Kamenka, N.; Rymdbn, R.; Stilbs, P. J. Phys. Chem. 1984,88, 5048. (40) Stilbs, P. J. Colloid Interface S C I .1983, 94, 463. (41) Lindman, B.; Kamenka, N.; Puyal, M.-C.;Brun, B.; Jonsson, B. J . Phvs. Chem 1984. 88. 53 '(42) Lindman, E.; Puyal, M.-C.; Kamenka, N.; Brun, B.; Gunnarsson, G. J . Phys. Chem. 1982, 86, 1702.
2942
The Journal of Physical Chemistry, Vol. 91, No. 11, 1987
Das et al.
a.
XYLENE
FM
FM
XYLENE
XYLENE
FM
b. C,OH/SOS
SOH/SDS
WATER
BENZENE
WATER
BENZENE
Figure 3. Isotropic regions in four-component nonaqueous (a) and aqueous (b) systems. The nature of the isotropic phases of the aqueous systems with heptanol, octanol, or higher alcohols is the same as that with hexanol. Diagrams for the aqueous systems are from ref 38. Sample composition for diffusion measurements are indicated by dots in (a).
( L I D S (NFY)
7 .
e x
::
M
I
1 10
I 5 X,
1
45
SDS
Figure 4. SDS diffusion coefficients in different solvents as a function of SDS concentration (wt %). The data for water is taken from ref 39. The inset shows self-diffusion coefficients relative to those at infinite dilution of SDS. The symbols are uniform throughout the figure.
of a micelle having a distinct hydrophobic core the diffusion coefficient of the solubilized molecule equals the micelle diffusion coefficient and becomes less than the surfactant diffusion coefficient; it usually decreases by an order of magnitude from the diffusion coefficient in the absence of surfactant. We have tried to solubilize tetramethylsilane (Me4Si) in the nonaqueous surfactant solutions. The diffusion of Me4Si was found to be only slightly slower than that in absence of surfactant but was always higher than the surfactant diffusion coefficient. For example, the Me$i diffusion coefficient (DMe&,)in N M F (no SDS) is 7.8 X
1O-Io m2 s-l and DNMF is 8.3 X m2 s-l. In a 15 wt % SDS solution in NMF, DMyr"p-= 5.0 X Dsos = 2.1 X and hMF = 5.3 X m s It can be noted that both N M F and T M S diffusion fall by the same (36%) fraction. This observation strongly indicates that all the Me4Si is free in the N M F medium and that no micelle having distinct hydrophobic core exists. The fall of the Me$i and N M F diffusion coefficients can be attributed to the increased viscosity of the medium due to the presence of the surfactant. Results for the other solvents, DMF and FM, are basically the same as that for NMF. Our observation is consistent with a recent study by Rico and L a t t e who ~ ~ ~have demonstrated that the Krafft temperature of the ionic surfactant SDS in formamide is much higher than that in water (Le., 55 O C in FM as compared to 16 O C in H 2 0 ) and hence cooperative aggregation of SDS in FM at the present measurement temperature of 25 O C is unlikely. Note Added in Proof: After the submission of this manuscript the high-temperature self-diffusion behavior of SDS in F M has been examined. These studies provide evidence for micellization of SDS in FM at 57 O C in accordance with the observations of Rico and Lattes. From the results a detailed picture of micellization in FM emerges which will be presented at a later date. It might be mentioned here that attempts have previously been made to relate the aggregation behavior of the surfactants in these nonaqueous solvents with the dielectric constants and internal structure of the solvents.2 However, the self-diffusion behavior in, for instance, N M F and D M F indicates practically the same picture of SDS self-association although the dielectric constants are vastly different (see Table I) and hence the idea of micellization, determined largely by the dielectric constant of the solvent, receives no support. The present results also tend to show that the role of structredness of these solvents in determining the reported cmc's is probably somewhat overemphasized. Self-Diffusion Coefficients in Three-Component Microemulsions. In Figure 5 we present the self-diffusion coefficient data of the system pentanol/FM/SDS. Measurements have been carried out at two different surfactant concentrations (kept constant in the series), namely 5 and 15 wt %, and also without surfactant (binary system). The diffusion coefficients of FM and pentanol in this binary system are clme to their respective diffusion coefficients in the pure liquids; thus, in the entire miscibility range they are molecularly disperse solutions. For 5 wt % ' SDS, FM (43) Rico, I.; Lattes, A. J . Phys. Chem. 1986, 90, 5870.
The Journal of Physical Chemistry, Vol. 91 No. 11, 1987 2943
Microstructure of Formamide Microemulsions 10
~
-
m2 s-I which is slightly less than the constant around 2.5 X value for the pure liquid (DIDo 0.8). The surfactant diffusion coefficient is less than the values of both FM and pentanol and varies between 0.71 X 1O-Io and 1.1 X m2 s-l. The value of DsDs at low FM content is about 50% less than that in pure FM. Such relatively high values give no support for any extensive association of surfactant molecules to droplets. Possibly there may be aggregates with limited spatial extension and highly flexible interface and with a large fraction of free monomers. For a bicontinuous microemulsion structure DsDs has been found to be m2 s-I for a number of aqueous systems, i.e. the ca. 1 X relative (to infinite dilution) value of SDS, (D/DO)sDs, is ca. 0.16. For the FM systems we observe throughout, even at 15% SDS, (D/DO)SDS to be above 0.36 which would suggest that also this type of microemulsion structure is inapplicable. However, since the conversion from aqueous to nonaqueous systems is much less obvious for a bicontinuous than for a droplet-type structure and information on surfactant diffusion in nonaqueous lamellar liquid crystalline systems is not available, we would not stress this point too much at present. The results with 15 wt % surfactant are basically the same as those at 5 wt % although the self-diffusion coefficients of all the components are slightly lower. From the surfactant diffusion coefficient one can infer that the association does not change appreciably. The constancy of DsDs ((D/Do)SDs 0.36) with composition shows that the degree of association remains the same even at higher F M contents. For aqueous systems the degree of organization and structure are known to depend strongly on the length of the alcoholz5and we will now examine the self-diffusion behavior of the threecomponent nonaqueous systems when the alcohol chain length is increased. In Figure 6 results are presented for the corresponding hexanol, heptanol, and octanol systems as a function of FM concentration. The surfactant concentration is kept fixed at 10 wt % for all the systems. FM diffusion coefficients in all cases m2 s-l which are within a factor lie in the range (1.0-3.5) X of 2-5 of the value for neat FM in the entire range for all the three alcohols. Although there is a slight decrease in FM diffusion coefficients with increasing alcohol chain length at low FM contents (e.g. at 25 wt % FM, DFM= 1.6 in the hexanol, 1.25 in the heptanol, and 1.0 in the octanol system, in units of 1O-Io mz s-l), this does not point to any significant confinement of FM in closed domains. Thus, (D/DO)FM > 0.2 while a droplet model should give ((0.1. Even for FM droplets of 1.5-nm hydrodynamic radius (which should include the surfactant molecule length) we estimate the D of droplets to be 2.8 X lo-" and 1.7 X lo-" mz
2 -1
DxlO m s 4.0 -
3.0
5%)
2.0
-
4 20
40
60
80
-
100
% FORMAM1DE
Figure 5. Self-diffusion coefficients of the isotropic solution phase in the FM/pentanol/SDS system against the FM content. The SDS content in each series is indicated on the different curves.
m2 s-l, diffusion coefficients vary in the range (2.4-3.8) X which corresponds to a relative (to neat solvent) self-diffusion of 0.46-0.73; the ratio becomes still higher coefficient, when the reference state is taken to be that in the binary mixture of FM and pentanol. With F M confined to droplets, DFM can = kT(l - 1.84)/6rsr, i.e. the be estimated from DFM = Ddroplet Stokes-Einstein equation with a correction for droplet-droplet hard-sphere interactions (volume fraction of droplets, $), For a droplet radius of 30 A, as an example, we estimate with 11 = Q~~~~~~ = 3.30 CPa DFM value of 2 X lo-" m2s-l. The observed DFM is thus an order of magnitude above that predicted even for small with 4 is not indicated droplets; furthermore, a decrease in Ddmplet in the data. Therefore, FM is apparently not confined in closed domains to a detectable extent. The pentanol diffusion Coefficient is rather
10 2-1 Ox10 m s
10 2 -1 0x10 m s
10 2 -1 DxlO ms
C70H -
4.0-
4 .O
3 .O
2 .o
1 .o
'.Ot"
=
O
SDS
L
25 45
65 85
Yo FM
2 .o
alcohol v
u
I
I
25 45
n
SDS I
I
65
85
'10 F M
1.o
n
l
25
45 o/'
65 85
h
FM
Figure 6. Self-diffusion coefficients in three-component nonaqueous systems with hexanol, heptanol, or octanol against the FM content of the microemulsion. The SDS content is 10 wt % in all samples. In the octanol case, octanol and SDS diffusion coefficient data were inaccessible due to short transverse relaxation times and signal overlap.
2944
The Journal of Physical Chemistry, Vol. 91, No. 11, 1987
Das et al.
TABLE II: Sample Comwsitions and Self-Diffusion Coefficients in the Three-Comwnent Surfactantless Svstem Shown in Finure 2 sample composition, wt % self-diffusion coeff, m2 s-l formamide xylene pentanol formamide xylene pentanol 40 30 20 10 20 20
10 12 13 15 20 30
50 58 67 75 60 50
D /Do
3.86 3.47 3.94 3.96 4.74 4.40
6.28 6.17 7.42 8.75 8.72 9.49
f 0.1 1
f 0.06 f 0.13 f 0.07 f 0.14 f 0.11
I
t
function of alcohol chain length. Weight ratios of aqueous samples are H20:alcohol:SDS= 20:70:10 and of nonaqueous samples FM:alcohol: SDS = 25:65:10. The ordinate is in log scale. even neglecting droplet-droplet interactions, in hexanol and octanol, respectively. FM is thus essentially continuous in all the composition ranges for all the alcohols studied. The hexanol and heptanol diffusion coefficients are of the same order of magnitude as in the pure liquids. It is interesting to note that there is essentially no change in the SDS diffusion coefficients on going from hexanol to heptanol. The value of DsDs is quite high (0.65 X 1O-Io m2 SKI)compared to the infinite dilution value in FM (1.62 X 1O-Io m2 s-'), ruling out any confinement of SDS into droplets or in fact any predominant location in aggregates. Unfortunately, due to a short transverse relaxation time, SDS diffusion data could not be obtained in the octanol system. However, by comparing the FM diffusion coefficients it can be safely argued that there is no significant structural change on going from hexanol to octanol in these three-component systems, the structures of which are probably not far from a molecularly disperse solution. It is illuminating to compare these results with those of the corresponding aqueous systems. As an example, the relative diffusion coefficients of water and FM in three-component solvent/SDS/alcohol systems are presented as a function of the alcohol chain length in Figure 7 . In both systems, samples have a fixed weight composition. Compositions of aqueous samples are water:alcohol:SDS = 20:70:10 and of nonaqueous samples FM:alcohol:SDS = 25:65:10 in terms of weight. Although the weight compositions are slightly different in the two systems, this will have no consequences for our attempts to obtain a general picture of microstructure. One finds that there is a sharp fall in the relative diffusion of water on increasing the alcohol chain length while the effect is very much smaller for FM. With a higher alcohol like octanol, DID, of FM is about 0.2 whereas that of water is about 0.02. There is thus an order of magnitude difference between the relative diffusion of water and FM and while water is dinstinctly confined into droplets in octanol, FM in the analogous nonaqueous system does not show any appreciable confinement. In other words, the effect of increasing the chain length of the SI,
3.73 3.43 3.89 4.32 4.46 4.75
f 0.04 f 0.1 1 f 0.09 f 0.10 f 0.16
0.10 0.04 0.04 0.07 f 0.12 f 0.10
f f f f
DxlO m s A A I O 2 - ' \
5*0
ALCOHOL CHAIN LENGTH Figure 7. Relative self-diffusion coefficients ( D I D o ) of water and FM in water/alcohol/SDS and FM/alcohol/SDS microemulsions as a
f 0.06
20
\
XYLENE
40
'A FM BY
60
80
WEIGHT
Figure 8. Self-diffusion coefficients in the FM/p-xylene/pentanol/SDS
microemulsions as a function of the FM content. Sample compositions are shown in Figure 3a. alcohol has a minimal effect in changing microstructure in the nonaqueous three-component systems while dramatic effects are seen in the corresponding aqueous three-component systems. Self-Diffusion in Surfactantless Solutions. It seems interesting at this point to consider the self-diffusion behavior of solutions of oil, alcohol, and FM in the absence of surfactant. The weight composition and the self-diffusion coefficient data of the samples within the isotropic region of Figure 2 are shown in Table 11. We note that all the self-diffusion coefficients are quite high. That of FM is close to that in the neat liquid. The p-xylene self-diffusion coefficient is about 2-4 times smaller than that in pure xylene. This is expected because the viscosity of pure xylene is quite small in comparison with the viscosity of FM or pentanol (see Table I). Pentanol diffuses faster than in pure pentanol. It may be mentioned that higher (than in neat liquid) self-diffusion of medium chain alcohols in multicomponent systems were also observed before.2,28 This was attributed to the breakage of hydrogen bonds between alcohol molecules in the microemulsion.2 Clearly there is no structure in these solutions which are molecularly disperse. Self-Diffusion in Four-Component FM Microemulsions. Self-diffusion data for the system FM/xylene/pentanol/SDS is presented in Figure 8 as a function of the FM content. With an increase in FM concentration the xylene and pentanol self-diffusion decreases while FM self-diffusion increases. The minimum FM self-diffusion coefficient is within a factor of 3 of that of pure FM. This factor becomes still less (=2) if one compares the observed self-diffusion coefficients in the four-component system with the corresponding three-component surfactantless system at the same ratios among the nonsurfactant components. The p-xylene self-diffusion coefficient falls by a factor of about 6; this fall is comparately moderate recalling that, in the ternary surfactantless mixture, the xylene self-diffusion coefficient is already less than
The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 2945
Microstructure of Formamide Microemulsions 10
10 2 -1 0x10 m s
2 -1
DxlO m s
D x ld0Jg1
8.06.0 4.0
-
2.0
-
h -u a
-
5
6
SDS
4
25
35
45
55
25
35
45
55
'le F M '1, F M Figure 9. Self-diffusioncoefficients in the FM/p-xylene/alcohol/SDS microemulsions with hexanol or octanol as alcohol against the FM content. The SDS content is kept fixed at 10 wt % for all the samples. SDS and octanol diffusion data could not be obtained due to short transverse relaxation times and signal overlap. that in pure xylene by a factor of 2-4. Pentanol diffusion coefficients are not much different from those in either the neat liquid or in the surfactantless solution. Therefore, these results do not indicate any appreciable confinement in closed domains of any of these components. The surfactant diffusion coefficient is practically constant at ca. 0.8 X lo-'' m2 s-I over the entire composition range, i.e. only a factor of ca. 2 below the value at high dilution in FM. Thus, well-defined aggregates or rigid interfaces are not suggested. If there is any aggregation of SDS in these solutions, the aggregates must be small and there should be a high monomer concentration. Results of the four-component systems with higher alcohols, i.e. with hexanol and octanol, are shown in Figure 9. Sample compositions correspond to a line having constant amount of alcohol and surfactant (23.4 and 11.6 wt %, respectively) while the FM and oil contents are varied. In general, the results follow the same trend as the system with pentanol. The surfactant diffusion (0.65 X m2 SKI) in the case of hexanol is just slightly lower than in the pentanol system. Xylene self-diffusion is essentially the same as in the pentanol system, while FM diffusion is slightly slower. Even with octanol as cosurfactant, the FM self-diffusion coefficient is throughout within a factor of 3 of its value in neat FM. The slight fall of surfactant self-diffusion on going from pentanol to hexanol is as expected for molecule disperse solutions and does not indicate any appreciably higher degree of association. There is certainly not any dramatic change in the structure in the four-component system on passing from pentanol to octanol. This is in sharp contrast to previous observationsz5 for the four-component aqueous microemulsions. Compare, for instance, parts a and b of Figure 10 where self-diffusion results for aqueous (with toluene as oil) and nonaqueous systems are presented for different chain lengths of the alcohol, while keeping the weight ratios among the components fixed (see legend of Figure 10). Although the weight ratios in the two systems are not exactly the same, they are quite close to each other. Note how dramatic is the effect of alcohol chain length on water diffusion in the aqueous system. In the nonaqueous system, the FM self-diffusion coefficient decreases from 2.4 X to 1.2 X lo-'' m2 s-' on going from butanol to octanol which is very much less than the decrease from 5 X to 2.0 X lo-" m2 s-l, i.e. more than one order of magnitude, observed for the aqueous system.25 We also note that the SDS self-diffusion coefficient in the nonaqueous system is fairly constant on increasing the cosurfactant chain length from 4 to 7, whereas in the aqueous system one observes a steady decrease.25 The aqueous system is thus far more sensitive to cosurfactant chain length than the nonaqueous system. The aaueous system changes dramatically to a water discontinuous situati'on witcan increak of cosurfactant chain length while no
7
8
alcohol chain Length
1 2 3 4 5 6 7 8 alcohol chain Length
Figure 10. Comparison of self-diffusion behavior in nonaqueous (a) and aqueous (b) four-component microemulsions as a function of the cosurfactant chain length. The oil used is p-xylene in the nonaqueous systems and toluene in the aqueous systems. Weight ratios of the components in the nonaqueous systems are FM:p-xy1ene:alcohol:SDS = 35.0:20.030.0:15.0 and in the aqueous system water:toluene:alcohoI:SDS = 35.0:12.5:35.0:17.5. Data for the aqueous system is from ref 25.
structural change can be invoked for the nonaqueous system. Comparison with Previous Studies of FM Systems. There is very little work reported so far on the structural aspects of these microemulsions. However, it is of relevance to consider the work of Rico and Lattes1"18 in this context. From their studies on the system FM/ l-butanol/cycIohexane/CTABl6and FM/ l-butanol/i~ooctane/CTAB'~ they concluded that the structure of these systems is the same as their aqueous analogues. This conclusion is based on the similarities in phase and conductance behavior. These authors reported the presence of droplets of FM in the oil-rich region, the main argument coming from changes in the electrical conductance of the microemulsions. The change of conductance observed is, however, often within the same order of magnitude near the transition region, and it is not clear if this change can be attributed to "percolation". The percolation behavior reported for the analogous aqueous systems involved changes by no less than 4-5 orders of magnitude.44 The conclusion concerning the nonaqueous perfluorinated microemulsionsI8 is similarly based on observations of conductance behavior. However, perfluorinated surfactants can have quite different aggregation behavior from the perhydrogenated surfactant^.^^ The region where F M droplets were inferred for the perhydrogenated ~ y s t e m ' ~contains ,'~ very small amounts of FM (volume fraction 4 of FM < 0.04, which corresponds to
