464
Langmuir 1985,1, 464-468
the region of the respective kinks, 20-22 A2/mol and surface pressure ca. 15 dyn/cm, the viscosities increase 20-fold to ca. 20 (mdyn s)/cm. The viscosity displayed by stearic acid appears to arise principally from tail-group er1tang1ement.l~The viscoelasticityobserved with the fatty alcohols, with dipalmitoyl lecithin, and with stearic acid on water containing trivalent ions may be interpreted as showing the influence of the head group interaction mediated by the s u b ~ t r a t e . ~ JValinomycin ~J~ is disklike with very short pendant groups; neither stearic effects nor van der Waals attraction of the valyl groups appear to contribute to the viscosity. On the other hand, one would expect the polar group interaction might. Clearly, the factors contributing the viscoelastic behavior of monolayers are more subtle than expected. In conclusion, the work presented here establishes that monolayers of valinomycin have an anomalously low vis(17) Miyano, K.; Abraham, B. M.; Ketterson, J. B.; Xu, S. Q.J. Chem. Phys. 1983, 78,4776-4777.
cosity even at high areal density. The viscosity is comparable to that found for highly expanded films of stearic acid and the fatty alcohols. Further, the polar groups do not contribute to viscoelasticity as appears to be the case with the fatty alcohols and lecithin. The lI-A diagrams suggest that the valinomycin molecule exists in a single conformation in the monolayer regardless of the composition of the substrate studied. Pressure saturation occurs on substrates with either Na+ or K+ at an areal density of ca. 200 A2/mol, which is about the area of an expanded molecule; consequently, no conclusion can be reached about a unique molecular area in the monolayer.
Acknowledgment. We thank Prof. Robert MacDonald, Biochemistry Department, Northwestern University, Evanston, IL, and Prof. Ferenc Kezdy, Biochemistry Department, University of Chicago, Chicago, IL, for many helpful discussions. Participation of J. B. Ketterson in this research was made possible through DOE Grant DEFG02-84ER45125. Registry No. Valinomycin, 2001-95-8.
Droplet and Bicontinuous Structures in Microemulsions from Multicomponent Self - Dif fusion Measurements Paul Gugringl and Bjorn Lindman* Physical Chemistry 1, Chemical Center, S-221 00 Lund, Sweden Received December 14, 1984. In Final Form: March 1, 1985 The fourier transform pulsed-gradient spin-echo NMR method was used to provide multicomponent molecular self-diffusion data for three systems: (1)sodium dodecyl sulfate-butanol-toluene-water-sodium chloride, (2) sodium dodecyl sulfate-pentanol-toluene-water, (3) sodium dodecyl sulfate-butanol-toluenewater. The self-diffusion coefficients (0)were applied to deduce information on solution microstructure. For system 1D, >> Dtoluene at low salinitieswhile at high salinities Dtoluene >> Dwater.In an intermediate range Dwateri= Dtoluene. This provides evidence for a change in structure from oil droplets in water at low salinities to a bicontinuous structure at intermediate salinities and a structure of water droplets in oil at high salinities. Systems 2 and 3 were studied at low water volume fractions ($ ;5 0.3) along dilution lines. >> Dwater Dsurfaetant the latter corresponding to the droplet diffusion according to For system 2, Dtoluene light scattering data. A hard sphere water in oil type structure offers over a wide concentration range a good description of system 2. For system 3, Dtoluene>> Dwater >> Dsurfactant, the latter corresponding to droplet diffusion according to light scattering results. This can be interpreted in terms of water in oil type structure with incomplete confinement of water to droplets. The various structural implications complement and agree with previous studies on the same systems by other techniques.
Introduction The solution chemistry of organized systems like surfactant systems is a field of much current interest2and one of the most intriguing problems is that of microemulsion ~ t r u c t u r e . ~micro - ~ emulsion^^^^ comprise a wide range of low-viscosity isotropic thermodynamically stable spontaneously formed solution phases composed of surfactant, water, oil, and often cosurfactant and/or electrolyte and one would, therefore, expect a considerable structural diversity. In particular for systems with a medium-chain alcohol (normally 1-butanol or 1-pentanol) as cosurfactant there has been considerable confusion, but some recent (1) Present address: Laboratoire de Spectroscopie Hertzienne de L'Ecole Normale Supgrieure, Paris. (2) Shinoda, K., unpublished results. (3) Shinoda, K. Prog. Colloid Polym. Sci. 1983, 68, 1. (4) de Gennes, P. G.; Taupin, C. J. Phys. Chem. 1982,86, 2294. (5) Kaler, E. W.; Bennett, K. E.; Davies, H. T.; Scriven, L. E. J.Chem. Phys. 1983, 79,5673. Kaler, E. W.; Davis, H. T.; Scriven, L. E. J. Chem. Phvs. 1983. 79. 5685. 16) Danielsson, I.; Lindman, B. Colloid Surf. 1981, 3, 391. (7) Friberg, S. Colloid Surf. 1982, 4, 201.
0743-7463/85/2401-0464$01.50/0
very significant work using inter alia small-angle X-ray scattering (SAXS)sheds much light on this p r ~ b l e m . ~It, ~ has, furthermore, been shown that multicomponent selfdiffusion studies provide sensitive tests of structural models of microemulsions.g-12 Multifield 13C NMR relaxation work has provided some insight intothe order and dynamics of surfactant molecules in micro emulsion^.^^ On the basis of a large number of experimental approaches it is now well appreciated that depending on the constituents (for example, cosurfactant vs. noncosurfactant systems), composition, salinity, and temperature (less well (8) Auvray, L.; Cotton, J. P.; Over, R.; Taupin, C. J . Phys. 1984, 45, 913. (9) Lindman, B.; Kamenka, N.; Kathopoulis,T. M.; Brun, B.; Nilsson, P. G. J. Phys. Chem. 1980,84,2485. (10) Lindman, B.; Stilbs, P.; Moseley, M. E. J. Colloid Interface Sci. 1981, 83, 569. (l!) Lindman, B.; Stilbs, P. In "Surfactants in Solution"; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; Vol. 3, p 1651. (12) Stilbs, P.; Lindman, B. Progr. Colloid Polym. Sci. 1984,69, 39. (13) Lindman, B.; Ahlnas, T.; %der", 0.; Walderhaug, H.; Rapacki, K.; Stilbs, P. Discuss. Faraday SOC.1983, 76, 317.
0 1985 American Chemical Society
Droplet and Bicontinuous Structures documented) distinct droplet structures (oil in water or water in oil) as well as bicontinuous structures are possible.3-5J2J4J5The above cited recent work has in particular emphasized that for cosurfactant (butanol or pentanol) systems, the microemulsions are bicontiquous over broad concentration ranges. It was furthermore demonstrated that there are surfactant “films” (or sheetlike surfactant regions) of a considerable order and that the bicontinuous structures are highly dynamic. Dynamic light scattering has been very extensively used to study microemulsions with a droplet-type structure and studies along dilution lines have been interpreted to provide information on droplet size and on interactions between microemulsion droplets.1621 The latter depend strongly on the microemulsion constituents and so, for example, a slight change in the cosurfactant length may change interdroplet interactions from repulsive to attractive. A dramatic effect of cosurfactant length is also observed in the self-diffusion studies which give changes in water diffusion by orders of magnitude as the cosurfactant length is varied.12s22 Although much progress results from current work there remain important controversial points as regards microemulsion structure. Confusion often results from different experimental methods being applied under different conditions (mainly different compositions). This concerns, for example, previous dynamic light scattering and selfdiffusion studies. The present work seeks to overcome this difficulty by obtaining multicomponent self-diffusion coefficients by the novel Fourier transform pulsed-gradient spin-echo nuclear magnetic resonance techniqueZ3yz4for cosurfactant microemulsions which are of different types as suggested by the dynamic light scattering ~ o r k . ’ ~InJ ~ particular, we investigate microemulsions in equilibrium with other phases, i.e., dilute aqueous solution, dilute hydrocarbon solution, and simultaneous equilibrium with these two. Furthermore, we choose constituents to cover both “repulsive”- and “attractive”-typemicroemulsions as given by the analysis of the dynamic light scattering work.
Experimental Section Microemulsion Systems. The following three systems were investigated: (a) System 1is a mixture of oil, brine, surfactant, and cosurfactant of the followihg composition: oil, toluene 46.25% (w/w); brine, 48.8% ; surfactant, sodium dodecyl sulfate (SDS) 1.99% ; cosurfactant, butahol3.96%. By increasing brine salinity (S)a sequence of equilibria is observed.1s.26 S < 5.4 wt %, Winsor I, i.e., a dilute solution in toluene coexisting with a microemulsion phase; 5.4 wt % < S < 7.5 wt %, Winsor 111, i.e., a “middle phase” microemulsion coexisting simultaneously with aqueous and organic solutions which are both dilute; S > 7.5 w t %, Winsor 11, Le., microemulsion phase coexistihg with a dilute aqueous solution. The microemulsion phase was separated by decantation after (14) Talmon, Y.; Prager,S. J. Chem. Phys. 1978,69, 517. (15) Friberg, S.; Lapczynska, I.; Gillberg, G. J. Colloid Interface Sci. 1976, 56, 19. (16) Cazabat, A. M.; Langevin, D. J. Chem. Phys. 1981, 74, 3148. (17) Calj6, A. A.; Agterof, W. G. M.; Vrij, A. In “Micellization,Solubilization, and Microemulsions”;Mittal, K. L., Ed.; Plenum: New York, 1977; Vol. 2, p 779. (la) Cazabat, A. M.; Langevin, D.; Meunier, J.; Pouchelon, A. Adv. Colloid Znterface Sci. 1982, 16, 175. (19) Brunetti, S.; Roux, D.; Bellocq, A. M.; Fourche, G.; Bothorel, P. J.Phys. Chem. 1983,87, 1028. (20) Roux, D. Ph.D. Thesis, Bordeaux, 1984. (21) Bellocq, A. M.; Biais, J.; Bothorel, P.; Clin, B.; Fourche, G.; Lalanne, p.; Lemaire, B.; Roux, D. Adv. Colloid Interface Sci. 1984,20,167. (22) Stilbs,P.; Rapacki, K.; Lindman, B. J. Colloid Interface Sci. 1983, 95, 583. (23) Stilbs, P. J. Colloid Interface Sci. 1982, 87, 385. (24) Stilbs, P.; Moseley, M. E. Chen. Scr. 1980, 15, 176. (25) Winsor, P. A. Trans. Faraday SOC. 1948,44, 376.
Langmuir, Vol. 1, No. 4, 1985 465 Table I. Compositions of Microemulsions of System 1 brine salinitv. g of NaCl per mieroemulsion composition in wt % 100 cm3 of water SDS butanol toluene brine 3 4 4.5 5 5.7 6 6.3 6.7 7 8 8.5 9 9.5 10
3.6 3.6 3.4 3.4 4.5 4.9 5.2 5.3 5.1 3.1 3.2 3.3 3.3 3.3
3.5 3.4 3.4 3.4 4.1 4.3 4.6 4.9 5 5.3 5.4 5.5 5.6 5.7
8.2 9.6 12.3 14.3 27.4 33.6 38.8 46.1 51.7 74.8 76.1 77.3 78.5 79.2
84.7 83.4 80.9 78.9 64 57.2 51.4 43.7 38.2 16.8 15.3 13.9 12.6 11.8
*
1 month in a thermostated bath at 20 0.1 “C. It should be observed that qotnpositions above refer to total amounts of material. The self-diffusion work is done on the surfactant rich of the two or three phases present. The compositions of this phase are given in Table I. (b) System 2 (referred to as ATP) is hydrocarbon-rich mixture of toluene, pentanol, water, and SDS with a composition on a dilution line (located on a demixing surface). In ref 16 these microemulsions are assunied to contain droplets which consist of 1.25 cm3 of water and 0,83 cm3 of pentanol per g of SDS. The continuous phase is estimated to consist of 17 cm3 of pentanol and 0.4 cm3 of water per 100 cm3 of toluene. (c) System 3 (referred to as aTB) is a hydrocarbon-rich mixture of toluene, butanol, water, and SDS with compositions along a dilution line as for system 2. In ref 16 these microemulsions are assumed to contain droplets which consist of 1.0 cm3 of water and 0.50 cm3 of butanol per g of SDS. The continuous phase is estimated to consist of 12.3 cm3 of butanol and 0.34 cm3 of water per 100 cm3 of toluene. All chemicals were obtained commercially and used without further purification. Self-Diffusion Measurements. The self-diffusion coefficients were measured using the recently developed Fourier transform pulsed-gradient spin-echo method monitoring the ‘H NMR spectra.23 Measurements were performed on a Jeol FX-60 spectrometer which had been modified by Dr. Peter Stilbs to allow the FT self-diffusion measurements. All measurements were made a t 20 “C. Internal lock was provided by the 2H signal of heavy water (system 1)or deuterated toluene (system 2 and 3). Thus in systems 2 and 3 the toluene was a mixture of 50% C7Ds and 50% C,H& Evaluation of Self-Diffusion Coefficients. In a FT PGSE-NMR self-diffusion measurement sequence the peak heights hi of a signal from a molecule i is given by
hi = Ci exp[-(yG6)2Di(A- 6/3)]
(1)
where Ci is an amplitude factor (depending on the concentration of i as well as on the transverse relaxation time T2), y is the nuclear magnetogyric ratio, G is the gradient strength, A is the interval between the pulses (fixed to 140 mb in our experiments), and D, is the self-diffusion coefficient of the studied molecule. The field gradient pulse length 6 can be varied between 1and 100 ms. Prior to each experimental series, the factor ( Y G ) is ~ obtained by performing measurements on a sample with known self-diffusion coefficient (HDO in D20).26 For the case where one peak in the spectrum contains contributions from two molecular species (i and j) the data were analyzed as a double exponential h = Ci exp[-(yG6)2Di(A- 6/3)] + C, exp[-(yG6)2Dj(A- 6/3)] (2)
The toluene self-diffusion coefficient was deduced by a leasbsquares fit using eq 1on both the aromatic and methyl group (26) Mills, R. J. Phys. Chem. 1973, 77, 685.
Gudring and Lindman
466 Langmuir, Vol. 1, No. 4, 1985
7 D/m2s1
To Iuene
+
+
+
Toluene r
Do d r o p l e t 3
5
i
SALINITY
6
( g/
,
I
7
6
1
9
-
Wayr
10
100 m3 )
Figure 1. Self-diffusion coefficients (at 20 “C) of the constituents for microemulsions composed of sodium dodecyl sulfate (SDS) (1.99% by weight), butanol (3.96%), toluene (46.25%),and brine (46.8%). Brine salinity was varied between 3 and 10% whereby the system changed from Winsor I to I1 type. Between vertical lines there is a so-called “middle phase” microemulsion; i.e., the system is of type Winsor 111. signals. The alcohols have one distinct spectral signal and their self-diffusion coefficients could be obtained in the same way. The other alcohol peaks overlap with the main SDS peak. The SDS self-diffusion coefficient was deduced using eq 2 with the separately determined alcohol self-diffusion coefficient kept constant in the fit. Such a fit is facilitated by the very important difference between SDS and alcohol self-diffusion coefficients generally m2 s-’, respectively). observed (-lo-’’ and The evaluation of the water self-diffusioncoefficient requires special considerations when some of the other Constituents contain exchangeable protons. In general we observed fast proton exchange (on the relevant NMR time scale) between water and alcohol hydroxyl protons as evidenced by a single and narrow OH group peak. Under this condition the observed self-diffusion coefficient of these ‘H nuclei isz7
SDS 2.10411 I
I
0.1
I
1
a2
0.3
’
@
Figure 2. Constituent self-diffusion coefficients (at 20 “C) for microemulsions composed of SDS, pentanol, toluene, and water. Microemulsion compositions follow a dilution line and data are given as a function of volume fraction of dispersed phase (water droplet model assumed).’* Do is the dynamic light scattering diffusion coefficient extrapolated to infinite dilution.
-
D = -2 x
2x
+ 1 Dwater +
1 Dalcohol
Toluene
Butanol 6.1o-’oc 4 lo-+
Water
(3)
since proton exchange is slow enough not to contribute to the macroscopic,translationalmotion.2s Equation 3, where x is the molar ratio of water to alcohol, permits the calculation of the water self-diffusion coefficient since the alcohol self-diffusion coefficient could be obtained separately. Obviously, if the molar ratio of alcohol to water is large the contribution from water diffusion to the observed value is small. Therefore, the precision in Dwakr is low a t the lowest water contents and no values are given. It should also be pointed out that for system 2 we have observed evidence for proton exchange between water and alcohol outside the fast exchange limit for low water contents (broadening of the -OHsignal and a reduction of Tz).A corresponding observation has been made previously for the same type of systemaZ2In such a case the water self-diffusion coefficient is not accessible by the present technique.
Results Self-diffusion coefficients were obtained for the three different systems varying t h e microemulsion composition t o cover t h e different microemulsion characteristics. I n Figure 1 the self-diffusion coefficients of water, butanol, toluene, and surfactant are given for system 1as a function of salinity. Figure 2 presents t h e self-diffusion coefficients of toluene, water, and surfactant for system 2 as a function of volume fraetion of dispersed phase (estimated as in ref 18). Figure 3 gives t h e corresponding d a t a for system 3. It can be noted that t h e self-diffusion coefficients (given on a logarithmic scale) cover a range from t o above H.G . J. Phys. Chem. 1970, 74,3734.
2 10-10
Do droplet
01
0.2
0.3
0-
Figure 3. Constituent self-diffusion coefficients (at 20 “C) for microemulsions composed of SDS, butanol, toluene, and water. Explanations are as for Figure 2. m 2 s-l while t h e precision is generally (except when there are difficulties d u e to exchange and signal overlap) around 5% or better.
Discussion General. We have previously discussed at some length t h e relation between molecular transport over macroscopic distances and the microstructure of surfactant systems and also presented results and structural deductions for a range of different W e note here only t h a t t h e confinement of the molecules of a constituent t o closed domains (of nonmacroscopic extension) will give a very low self-diffusion coefficient of t h a t constituent while t h e molecules occurring in t h e continuous medium will be characterized by rapid diffusion (of t h e order of that of the pure liquid constituent). In fact we observe variations in t h e ratio of water-to-oil self-diffusion coefficients by several orders of magnitude as t h e system is varied from a pronounced O/W situation t o a W/O case.12 I n contrast
Droplet and Bicontinuous Structures
Langmuir, Vol. 1, No. 4 , 1985 467
to the droplet structural types where water and oil diffurapid which can be explained by the fact that in a biconsion 4 e very different, a bicontinuous structure is expected tinuous system the motion of surfactant molecules along to give high self-diffusion coefficients of both water and the surfactant films contribute to the macroscopic displacement; however, this motion is hindered. oil; surfactant molecules confined to interfaces in such a bicontinuous structure can, however, have quite low selfOn approach of the limiting salinities of the Winsor I diffusion coefficients. For a molecule disperse solution we and I1 regions we observe an increase in the self-diffusion expect the self-diffusion coefficients of all constituents to coefficients of the molecules of the dispersed pseudophase. be high. While the interpretation in these limiting situaThis can arise from either or both of two contributions: tions causes no problems there are, of course, also interThere can be a progressive change from a droplet to a mediate situations. There may be a droplet structure with bicontinuous microstructure so that there is some presence only partial confinement to the droplets; in that case one of large flexible aggregates and a highly polydisperse sitor more constituents will appear partly in droplets and uation or the droplets become increasingly attractive alpartly as molecules disperse in the medium. In cases where lowing the transfer of dispersed molecules between droplets. (The situation would be similar to that investigated the interfaces are very labile and interdroplet exchange occurs extremely rapidly (say of the order of nanoseconds), in some detail for nonionic surfactant micelle solution^.^^^^) this exchange will influence the results. Such rapid exAnother alternative is that small droplets still dominate change can be caused by the interdroplet interactions being but that there is an increasing partitioning of the droplet distinctly attractive. As regards the flexibility and lifetime molecules to the continuous medium. There is no sharp of the interfacial film we have elsewhere proposed the distinction between these alternatives and it is probable that the different effects appear simultaneously and constudy of multifield 13C or 2HNMR re1axati0n.l~ Such tribute to the observed effects. studies give order parameters that are roughly the same as those of lamellar liquid crystals and, therefore, give Cosurfactant (butanol) self-diffusion is throughout support for the existence of an interfacial surfactant film. rather rapid although always distinctly slower than that SDS-Butanol-Toluene-Water-Sodium Chloride of the continuous medium molecules. This suggests a System. In the Winsor I (salinity below 5.4%) and I1 picture with the cosurfactant molecules occurring both in (salinity above 7.5 % ) domains dynamic light scattering the dispersed pseudophases and as molecules disperse in studies along dilution lines have been p e r f ~ r m e d . ~ ~ . the ~ ~ medium. In the Winsor I region, Dbutanol is 60% of the These measurements were analyzed to provide two pavalue for butanol in water (0.70 X lo4 m2 indicating rameters, droplet size (from the diffusion coefficient exthat ca. 40% of the butanol molecules occur in the droplets trapolated to infinite dilution) and interdroplet interacand interfacial films. In the Winsor I1 region, Dbutanol is tions (from the concentration dependence of the diffusion more than 90% of the value for butanol in toluene (1.10 X m2 d),indicating that a very small fraction of the coefficient). Droplet size was found to increase slightly on approaching the limiting salinities of existence of the butanol molecules diffuse with the surfactant aggregates. Winsor I and I1 regions while interactions become inSDS-Pentanol-Toluenewater System. This system creasingly attractive on approaching these values. has been studied as a function of water content along a The self-diffusion coefficients give evidence for very dilution line. Light scattering data have been analyzed to different microemulsion microstructures in the three saindicate a droplet-type structure with hard sphere-type linity regions. In the Winsor I domain Dwater >> Dtoluene interaction between droplets.l6Js Various other techniques like conductivity, ultrasound absorption, and transient throughout, the difference approaching a factor of 100. electrical birefringence have suggested that a rather rigid, The structure can clearly be described in terms of oil compact interface is responsible for the hard sphere-type droplets in a water continuum. In the Winsor I1 domain behavior of these micro emulsion^.^^*^^^^^ the situation is reversed. Here Dtoluene >> D,, the ratio between the two being about 100 at salinities of ca. 100 The self-diffusion coefficients (Figure 2) provide a quite g/kg water. The structure becomes distinctly of the water clear-cut picture of these microemulsions which is in droplet in oil type. In the "middle phase" or Winsor I11 agreement with the previous work. Throughout the condomain a third type of behavior is encountered. Now D, centration range studied Dtoluene is 1-2 orders of magnitude = Dtoluene both being only a small factor from the values above Dwater correspondingto a structure of water confined for the pure liquids. This observation taken alone, which to droplets in oil. At not too high 4 values, D,,, DSDS, correspondsto a from the molecular mobility point of view, these values extrapolating to the value obtained by dyeffectively bicontinuous behavior, can as indicated above namic light scattering at low 4. have alternative interpretations, i.e., either a bicontinuous Since the precision in DsDs is rather low (low concenmicrostructure or a molecule disperse situation. There are tration, short T2) we do not emphasize its concentration approaches, however, such as X-ray scattering6r8 and dependence. However, we note that within experimental multifield NMR relaxation,13 which can provide direct error DsDsvaries linearly with 6 in the entire range studied, information on the presence of a surfactant interface or the slope being -2.2. The theory of the concentration film and which tend to confirm such a situation. In the dependence of the self-diffusion of interacting colloidal present case we observe throughout the surfactant selfparticles is a field of much current activity (e.g., ref 34-36 diffusion coefficient to be much lower than the diffusion and papers cited therein), but we here only note that our coefficients of the constituents occurring in the continuous medium. Therefore, the surfactant molecules occur to a (30) Nilsson, P.G.; Wennerstrom, H.; Lindman, B. J. Phys. Chem. very great extent in large aggregates, which argues strongly 1983,87,1377. Nilason, P.G.; Lindman, B. J.Phys. Chem. 1984,88,4764. against a molecule disperse situation. In the Winsor I and (31) Nilsson, P. G.; Wennerstrom, H.; Lindman, B. Chem. Scr., in press. I1 domains surfactant self-diffusion is very slow and only (32) Zana, R.; Lang, J.; Sorha, 0.;Cazabat, A. M.; Langevin, D. J . slightly dependent on salinity, a finding demonstrating the Phys. Lett. 1982, 42, L829. confinement of surfactant molecules to closed domains. (33) Gugring, P.;Cazabat, A. M. J. Phys. Lett. 1983, 44, L601. (34) Evans, G. T.; James, C. P. J. Chem. Phys. 1983, 79, 5553. In the middle phase region SDS diffusion is markedly more (29) Cazabat, A. M. J. Phys. Lett. 1983, 44, L593.
(35) Lekkerkerker, H. N. W.; Dhont, J. K. G. J. Chem. Phys. 1984,80, 5790. (36) van Megen, W.; Snook, I. J. Chem. Soc., Faraday Trans. 2 1984, 80, 383.
468
Langmuir, Vol. 1, No. 4, 1985
value agrees with theoretical predictions for suspensions of hard spheres. At higher q!~values Dwab,becomes higher than DsDs and, retaining a hard sphere model, this observation indicates that water is then only partially confined to droplets. Hydrocarbon self-diffusion decreases linearly and slowly with 4. The decrease is almost twice that predicted for the obstruction effect of spherical particles pointing to non-hard-sphere interactions of the toluene molecules with the microemulsion droplet^.^' Cosurfactant self-diffusion is much slower than hydrocarbon self-diffusion and shows a stronger concentration dependence demonstrating the important presence of cosurfactant molecules in the microemulsion droplets. SDS-Butanol-Toluenewater System. This system, as the previous one, has been studied as a function of water content along a dilution line. The light scattering studies have been interpreted in terms of an attractive interaction between droplets. One possible model of this would be in terms of a droplet interpenetration during collisions. Theoretical work17,19~38 has explicitly taken into account this interpenetration to calculate the attractive potential and it has been suggested that there is a more flexible interface with butanol than with pentanol. The self-diffusion results also demonstrate a marked difference between the butanol and pentanol systems. A similarity is though that DsDsthroughout the range studied is quite low and corresponds to the droplet self-diffusion given by the extrapolated dynamic light scattering value. Water diffusion is much slower than hydrocarbon diffusion but in contrast to observations for the pentanol system considerably more rapid than droplet diffusion. This result shows that water is not confined into closed droplets, which can be taken to suggest a fast exchange of water between droplets during collisions due to a labile interface or that the structure becomes increasingly of a bicontinuous type as water content increases. The low SDS diffusion demonstrates that the surfactant molecules appear in large aggregates and that there seems to be little of a molecule disperse solution. Hydrocarbon and cosurfactant self-diffusion are both similar to those of the pentanol system. (37) Jonsson, B.; Wennerstrom, H.; Nilsson, P. G.; Linse, P., Colloid Polym. Sci., in press. (38) Lemaire, B.; Bothorel, P.; Roux, D. J. Phys. Chem. 1983,87,1023.
Gudring and Lindman
Conclusion Multicomponent self-diffusion studies provide a direct insight into microemulsion structure and demonstrate that the surfactant-cosurfactant-hydrocarbon-water systems can be either of a droplet-type structure (oil in water or watef. ion oil) or of a bicontinuous structure depending on the conditions. An increase in electrolyte concentration can bring about quite dramatic changes in microemulsion microstructure from an O/W droplet structure over a bicontinuous structure to a W/O droplet structure. Marked changes in structure are also observed on slight changes in cosurfactant chemical structure. As regards the bicontinuous microemulsions it seems difficult at present to propose any closer features of the structure. Surfactant D values of bicontinuous microemulsions are generally of the order of m2/s, which considering self-diffusion data of lamellar and cubic liquid crystals39would be consistent with a highly dynamic and flexible version of the structure of bicontinuous cubic phases.40 The dynamic nature of microemulsions is emphasized in the 13C NMR relaxation work (nanosecond time scale of fluctuations)13 and in recent theoretical However, more detailed discussions along these lines are required before we understand the structure of bicontinuous microemulsions. Acknowledgment. It is a pleasure to acknowledge helpful discussions with A. M. Cazabat, D. Chatenay, D. Langevin, and J. Meunier, E.N.S., Paris, and the use of their results. We are grateful to P. Stilbs for helpful advice on the PGSE technique and for its installation in our laboratory. The stay of P.G. in Lund was supported by grants from the Service Scientifique de 1’Ambassade de France 6 Stockholm (under the direction of Professor M. Durand) and the Swedish Natural Sciences Research Council. Registry No. NaC1, 7647-14-5; SDS, 151-21-3; l-butanol, 71-36-3; l-pentanol, 71-41-0; toluene, 108-88-3. (39) Erikmon, P. 0.;Johanseon, L. B. A.; Lindblom, G. In “Surfactants in Solution”; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; Vol. 1, p 219. (40) Scriven, L. E. In “Micellization, Solubilization, and Microemulsions”; Mittal, K. L., Ed.; Plenum: New York, 1977; Vol. 2, p 877. (41) Ruckenstein, E. In “Surfactants in Solution”; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; Vol. 3, p 1551.
