Phase Behavior and Microstructure of Nonaqueous Microemulsions. 2

Langmuir 1995,11, 779-784

779

Phase Behavior and Microstructure of Nonaqueous Microemulsions. 2 A. Martinot and E. W. Kaler*>f Fuel Sciences Division, Sandia National Laboratories, Albuquerque, New Mexico 871 85-0710, and Department of Chemical Engineering, Colburn Laboratory, University of Delaware, Newark, Delaware 19716 Received July 15, 1994. In Final Form: December 15, 1994@ The microstructure of nonaqueous microemulsionsformed with propylene glycol,glycerol,three different alkanes, and pentaethylene glycol mono-n-dodecyl ether (C12E5) is probed with NMR self-diffusion measurements and small angle neutron scattering (SANS). At low oil concentrations, both NMR selfdiffusion and SANS results can be modeled in terms of a microstrucure of ellipsoidal oil-rich droplets with only excluded volume interactions. These droplet structures percolate to an oil-continuous structure as the volume fraction of oil in the microemulsions increases. Percolation thresholds measured as a function of alkane chain length are interpreted in terms of the phase behavior of the microemulsionand the strength of droplet interactions.

Introduction Microemulsions are thermodynamically stable dispersions of water, oil, and surfactant. One property of a microemulsion is that surfactant sheets separate water and oil domains to form a microstructured solution. In general, oil-swollen micelles form when water is the majority component and inverted water-swollen micelles form when water is the minority component. Either one of these closed droplet structures percolate to a "bicontinuous" structure as the water and oil concentrations are made comparable. The bicontinuous structure is characterized by intertwined, sample-spanning pathways of both components separated by surfactant sheets. Many experimental techniques have been used to probe microemulsion microstructure.' Thermodynamically stable, microstructured solutions also form when surfactant is added to oil and a polar solvent other than water.2 Polar organics such as formamide and alkanediols are suitable replacements for water in many microemulsion recipes, but little is known about the microstructure ofthese mixtures. In formamide, CTAB, and hydrocarbon mixture^,^^^ electrical conductivity and small angle X-ray scattering results are consistent with the presence of a structured solution, and dynamic light scattering measurements from AOT, glycerol, and heptane microemulsions have been interpreted in terms of a dispersion of monodisperse spherical micelle^.^ Earlier, we used SAXS to show glycerol, propylene glycol, oil, and CiE, mixtures were microstructured, and we investigated the interfacial properties of the surfactant sheets.6 Surfactant aggregation in polar organic solvents is reduced relative to that occurring in water. Critical micelle concentrations are higher in polar organics than

* To whom correspondence should be addressed. t Sandia National Laboratories.

University of Delaware. Abstract published in Advance ACS Abstracts, February 1, 1995. (1)Langevin, D. ACC.Chem. Res. 1988,21, 255. (2)Friberg, S. E.; Liang, Y. C. Microemulsions: Structure and D.ynamics; Friberg, S. E., Bothorel, P., Eds.; CRC: Boca Raton, FL, 1987;Chapter 3. (3)Rico, I.; Lattes, A. J . Colloid Interface Sci. 1984,102,285. (4)Auvray, X.;Peptipas, C.; Anthore, R.; Rico, I.; Lattes, A.;AhmahZadeh Samii, A.; de Savignac, C. Colloid Polym. Sci. 1987,265,925. (5)Fletcher, P.D. I.; Galal, M. F.; Robinson, B. H. J . Chem. Soc., Faraday Trans. 1 1984,80,3307. (6)Martino, A.;Kaler, E. W. J . Phys. Chem. 1990,94,1627. f

@

in ~ a t e r , ~ - l and O liquid crystalline phase regions are smaller or absent in the presence of polar ~ r g a n i c s . l l - ~ ~ Calculations are also consistent with a reduced level of surfactant aggregation in nonaqueous microemulsions.15 Adjusting the amphiphilicity of a surfactant by mixing formamide and water allows for exploration of a variety of microemulsion properties.16 Some clues to the molecular-level reasons why surfac-. tant aggregation diminishes in polar organics are given by the study of liquid crystals. Some studies suggest that the lower interfacial tensions between the oil and nonaqueous solvents compared to that between oil and water reduces the attraction between surfadant head group^.'^-^^ Disorder of the surfactant tails due to enhanced penetration of polar organics into the interfacial region may also contribute to the diminished aggregation of liquid crystalline phases.20,21The reduction in aggregation may also be due to a decrease in the rigidity of the surfactant interface22or to the reduced number of hydrogen bonding sites per molecule in the polar organic solvents.23 Some organization of these observations in terms of the driving forces for self-assembly is given by Evans and Miller.24 In this report, we study quantitativeIy the microstructure of microemulsions made with glycerol, propylene (7) Magid, L. Solution Chemistry of Surfactants; Mittal, K. L., Eds.; Plenum Press: New York, 1979;Vol. 1, 427 pp. (8)Ray, A.Nature (London) 1971,231,313. (9)McDonald, C. J. Pharm. Pharmac. 1970,22,148. (10)Singh, H.N.; Saleem, S. M.; Singh, R. P.; Birdi, K. S. J . Phys. Chem. 1980,84,2191. (11)Friberg, S. E.; Liang, Y. C.; Lockwood, F. E. J . Dispersion Sci. Technol. 1987,8,407. (12)Friberg, S. E.; Liang, P. Colloid Polym. Sci. 1986,264,449. (13)Das, K. P.; Ceglie, A,; Lindman, B. J . Phys. Chem. 1987,91, 2938. (14)Rananavare, S. B.; Ward, A. J. I.; Osborne, D. W.; Friberg, S. E.; Kaiser, H.J . Phys. Chem. 1988,92,5181. (15) Martino, A.; Schick, M.; Kaler, E. W. J . Chem. Phys. 1990,93, 8228. (16)Schubert, K.-V.; Strey, R. J . Chem. Phys. 1991,95,8532. (17)Bergenstahl, B.; Jonsson, A.; Sjoblom,J.;Stenius, P.; Warnheim, T. Prog. Colloid Polym. Sci. 1987,74,108. (18)Backlund, S.; Bergenstahl, B.; Molander, 0.;Warnheim, T. J . Colloid Interface Sci. 1989,131,393. (19)Warnheim, T.;Jonsson, A. J . Colloid Interface Sci. 1988,125, 627. (20)Moucharafieh, N.;Friberg, S. E. Mol. Cryst. Liq. Cryst. 1979, 49,231. (21)Friberg, S.E.;Liang, P.; Liang, Y. C.; Greene, B.; Van Gilder, R. Colloids Surf. 1986,19,249. (22)Martino, A.;Kaler, E. W. To be submitted for publication. (23)Bergenstahl, B. A,; Stenius, P. J . Phys. Chem. 1987,91,5944. (24)Evans, D. F.; Miller, D. D. Organized Solutions; Friberg, S. E., Lindman, B., Dekker, M., Eds.; 1992;p 33.

0743-746319512411-0779$09.00/0 0 1995 American Chemical Society

Martino and Kaler

780 Langmuir, Vol. 11, No. 3, 1995

0‘4

2

0.2

sin(8/2),with A the wavelength of the radiation and 8 the scattering angle. In the absence of multiple scattering, the observed coherent differential scattering cross section per unit irradiated volume, a d s 2 (q),from a dispersion of droplets can be calculated in terms of the amplitude form factor F(q)and the structure factorS(q). The ensemble average over all particle orientations and positions ofF(q),(IF(q)I2), describesthe intraparticle scattering and can be calculated in closed form for particles of many shapes. S(q)accounts for interparticle scattering and depends on the interaction potential between the particles. For a monodisperse distribution of spherical droplets the scattering is given by

I c

I

0

20

40

wt. % C12E.5

Figure 1. Phase behavior of hydrogenated glycerol-propylene glycol, dodecane, and &E5 at constant temperature. y is the mass fraction of glycerol in the polar organic mixture. As glycerol content increases, surfactant beco_mesless soluble in the polar organic phase causing the 2-3-2 phase transitions. The ratio of glycerol plus propylene glycol to dodecane is 1.

glycol, straight-chain alkanes, and the nonionic surfactant pentaethylene glycol mono-n-dodecyl ether CUE^. We use N M R self-diffisionmeasurements to find a percolation threshold (the composition at which droplet structures give way to a bicontinuous structure) in microemulsions made with three oils: heptane (C,), dodecane (CIZ),and hexadecane ((216). Small angle neutron scattering(SANS) is used to determine the size and shape of the droplets in dilute microemulsions and to measure the interactions between the droplets as a function of alkane chain length. SANS has also been used to measure the length scales in the oil-continuous (and probably bicontinuous) microemulsions. To use SANS to probe such mixtures, either the oil or the polar material must be deuterated in order to provide contrast and reduce the amount of incoherent scattering. An earlier investigation extensively documented the phase behavior of these mixtures.6 Phase behavior at a fixed 1:l ratio of polar oganic and oil as a function ofy = (mass of glycerol)/(massof glycerol mass of propylene glycol)] and weight fraction of surfactant follows the phenomenological ideas describing water microemulsion 2-3-3 phase phase behavior outlined by K a h l ~ e i t .A~ ~ progression is observed with increasing y (the bar in this nomenclature represents the relative position of the surfactant rich microemulsion phase to the excess phases in a sample), and the phase diagram of polar organicl CiEJalkane mixtures at room temperature resembles a “fish”(Figure 1).We have investigated the microstructure in the microemulsions in the one phase region as the ratio of polar organic to oil is varied (i.e. as the mix point is moved off the 1:l plane in the tail of the fish).

where n is the droplet number density. If the droplets are approximated as hard spheres of diameter u (i.e., only excluded volume interactions are important), the interaction potential is UHS= m for r u and UHS= 0 for r > u,where r is the center-to-center spacing. In this case, an analytical expression for S(q) is obtained in the Percus-Yevick approximation.26 An analytical expression is also available if the droplets are assumed to act through an attractive square well potential given as USW= m for r < u,-e/kBT for u < r < Au,and 0 for r > Au,where dA - 1)is the width of the well, e the depth of the well, kg the Boltzmann constant, and T the temperat~re.~~ The scattering from polydisperse distributions of spheres or particles of nonspherical shapes can be described approximatelyusing “decoupling”assumptions. An exact expression is available only for polydisperse hard spheres.% The decoupling assumptions lead to modifications of the averages of the particle form factors and the nature of the structure factor so thatz9

+

Small Angle Scattering Analysis SANS from microemulsions results from neutron scattering length differences that occur between the colloidal sized oleic and polar domains. The experimental intensity curves are compared to the scattering predicted by statistical mechanical models developed for dispersions of droplets of different sizes and shapes interacting in various ways. A time-averaged picture of the dispersion is obtained on a size scale inversely proportional to the magnitude of the scattering vector, q, where q = (4dA) ~

~~

~

~~

~

~~

(25) Kahlweit, M.; Strey, R.; Hasse, D.; Firman, P. Langmuir 1988, 4, 785.

(3) For a polydispersedistribution of spheres, the droplet sizes and droplet separations are assumed to be uncorrelated and the ensemble averages in eqs 2 and 3 are over particle sizes. For nonspherical droplets, separations and orientations are assumed to be uncorrelated and the ensemble averages are over orientation^.^^ The above treatment is appropriate for microemulsions dilute in either water or oil when other measurements show that droplets exist, but a more general analysis is needed to describe bicontinuous microemulsions. Such an analysis can be developed in terms of the correlation function, y(r), of the fluctuations in the density of the ~ ( r ) ) l ( ~ ( r )A~ ) . ~ l scattering centers V(r), i.e., y(r) = (~(0) Landau free energy expansion suggests that (26) Wertheim, M. S. Phys. Rev. Lett. 1963, 10, 321. (27) Sharma, P. V.; Sharma, K. C. Ph.ysica (Utrechti 1977,89A, 213. (28) Vrij, A. J . Chem. Phys. 1979, 71, 3267. (29) Kotlarchyk, M.; Chen, S. H. J . Chem. Phys. 1983, 79, 2461. (30) Kaler, E. W. Modern Aspects of Small-Angle Scattering; Brum-

berger, H., Ed.; Kluwer Academic Publishers, in press. (31) Vonk, C. G. Small Angle X-ray Scattering; Glatter, O., Kratky, O., Eds.; Academic Press: New York, 1982.

Microstructure of Nonaqueous Microemulsions

y(r)=

d

exp(-r/c) sin(2dd)

Langmuir, Vol. 11, No. 3, 1995 781 (4)

would be a good description of a microem~lsion.~~ Here, d is the characteristic domain size (periodicity) and is the correlation length, or a measure of the dispersion of d. The scattering cross section corresponding to this correlation function is the Teubner-Strey (T-S) formula

polar organic

Figure 2. Scattering length densities (SLD)for SANS samples

where the average squared contrast, ( A V ) ~is, the square made with deuterated polar organic compounds. of the difference in the scattering length density for neutrons between the oleic and polar domains, and the Gradients of u p to 100 G/cm were obtained by using a home-built parameters u2, c1, and c2 are related to the microemulsion coil assembly contained in the probe head. Gradient intensities The values of u2, c1, and c2 can also properties d and were controlled from the B-Z 18B Gradient unit, while pulse be used to define an amphilicity factor, AF = c1/(4a2~2)~/~. timing of both rf and gradient pulses was controlled from the CPX-200 console. Room temperature was maintained by blowing T h e o g 3 and experiment? show that AF changes from air over the probe. negative to positive as the amphiphile becomes weaker Small angle neutron scattering (SANS) experiments were and the microstructure becomes less defined. AI? equals performed at the 30-m camera at Oak Ridge National Laboratory. zero (c1= 0) defines a Lifshitz line, while AF = 1defines The neutron wavelength was 4.75 A,A M was 0.06, and the the disorder line where the correlation function becomes q-resolution was 0.008 A-l. The samples were held in quartz a simple exponential decay. cells with neutron path lengths of 1 or 2 mm. The scattering Finally, it is possible to extract model-independent spectra were corrected for background and sample cell scattering information about structure from SANS. The final slope and placed on an absolute scale using standards provided by the of the intensity spectra describes the nature of the Oak Ridge facility a s detailed elsewhere.35 surfactant interface.34 In the q range that corresponds to structures much smaller than the characteristic size of Results the microemulsion (L)and much larger than interfacial length scale ( I ) , the scattering follows Porod's law The phase behavior was determined in the tail of the fish (represented in Figure 1)for each of the three oils as the ratio of polar organic to oil was varied. These diagrams were determined using fully hydrogenated materials, and there is some isotope effect as discussed below. A onephase channel is found between the upper and lower twowhere S is the interfacial area per unit volume in the phase regions (Figure 3) on the propylene glycol/glycerol/ sample. Sample contrast is somewhat unique in the alkane Gibbs triangle with surfactant weight fraction microemulsions prepared with deuterated nonaqueous constant and equal to 0.19,0.26,and 0.30 for C7, C12, and compounds for SANS experiments (Figure 2), and the c16, respectively. The one-phase region appears a t surfactant head groups have a scattering length density progressively higher ratios of glycerol to propylene glycol approximately equal to that in the oleic domains. (higher y values) as the alkane chain length increases. The experiments were done on a line of constant y on the Experimental Section Gibbs phase diagrams, with the values ofy equal to 0.10, Propylene glycol and all hydrocarbons were purchased from 0.27, and 0.37 for C7, C12, and C16, respectively. NMR Aldrich (Gold Label, '99% pure). Anhydrous glycerol ('98% self-diffision measurements and SANS were used to probe pure) was purchased from J. T. Baker Chemical Co. Deuterated the structure of the microemulsions in each of the one propylene glycol (Ds) and glycerol (1,1,2,3,3-D~)were purchased phase regions to study the effect of varying the ratio of from Cambridge Isotope Laboratories and were 98% pure. polar organic to oil. Pentaethylene glycol mono n-dodecyl ether surfactant (C12E5) was purchased from Nikko Chemicals Co. and was greater than Self-diffusion coefficients for the oils in each of the 98% pure. All reagents were used as delivered except for microemulsion mixtures are reported with respect to the propylene glycol and glycerol. Molecular sieves were used to self-diffision coefficients ofbulk oil in the presence of Cl2E5 remove water from these components, and the water content as 4). The volume fraction of oil, &,,is taken as the (Figure determined by Karl Fischer titration was less than 1%by volume volume fraction of the alkane plus one-half the volume in the polar organic compounds and nonionic surfactants. fraction of the surfactant for all experiments. At low oil All samples were prepared on a weight basis. The phase concentrations, the relative self-diffusion coeficient in behavior is insensitive to temperature; thus, all experiments each microemulsion is low. As oil concentration increases, were done at room temperature (-21-24 "C). Samples were shaken by hand, and phase behavior was determined by visual a threshold value is reached and the self-diffusion coefinspection. ficient climbs. The threshold oil concentration decreases Pulsed gradient spin-echo Fourier transform NMR measurewith increasing alkane chain length and equals 0.38,0.32, ments of self-diffusion coefficients were made with a 200-MHz and 0.22 for C7, C12, and c16, respectively. Further Bruker CPX-200 spectrometer with a B-Z 18B gradient unit. A increases in oil volume fraction lead to a steady climb in standard high-resolution Bruker probe tuned to lH was used. the normalized diffusion coefficient until the oil volume fractions are high (-0.8). Eventually, the normalized (32) Teubner, M.; Strey, R. J . Chem. Phys. 1987, 87, 3195. diffusion coefficient remains constant and slightly under (33) Gompper, G.;Schick, M. SelfAssembling Amphiphilic Systems, Phase Transitions and Critical Phenomena; Domb, C., Lebowitz, J., 1. Eds., 1994; Vol. 16. (34) Porod, G. Small AngleX-rayScattering; Glatter, O., Kratky, O., Eds.; Academic Press: New York, 1982.

(35) Wignall, G. D.; Bates, F. S. J . Appl. Crystallogr. 1987,20, 28.

Martino and Kaler

782 Langmuir, Vol. 11, No. 3, 1995 glycerol

6ol 0 0

4

8

12

16

20

c7

0, = 0.20)

q (x102 / AI) Figure 5. Comparison on absolute scale of SANS spectra for

glycerol

!w

dilute oil in polar organic microemulsions (points) with the scattering predicted by a model treating the microemulsions as dispersions of hard ellipsoids with an effective radius (solid lines). The parameter values are tabulated in Table 1. Table 1. Summary of the Hard Ellipsoid Droplet Parameters for Dilute Oil in Polar Organic Microemulsions As Measured by SANS" C7

glycol

C12 glycerol

C16

A /( \

90

@d

0.20 0.30 0.22 0.26 0.20 0.23

0.31 0.41 0.37 0.40 0.38 0.40

RA&

RAIRB

Rer(A)

Req(A)

129 234 133 209 143 229

11.6 11.9 8.8 11.5 11.5 17.0

33 49 35 42 33 45

38 70 45 64 44 60

a The measured droplet sizes include the surfactant head groups, because the contrast is between the deuterated polar organic solvent and the entire surfactant aggregate.

L422LAo Zphases

I phase

1

propylene 0

glycol

2 phases

1 C16

Figure 3. Phase behavior of hydrogenated propylene glycol,

glycerol with C7, C12, and c16 at constant C12E5 mass fraction of 0.19, 0.26, and 0.30, respectively. NMR and scattering samples are found on a line of constant glycerol content in the polar organic phase ((27, y = 0.10; C n , y = 0.27; Cx, y = 0.37). 0.4

0.3

-

0.2

-

0.1

-

0.0

Figure 4. Normalized alkane self-diffusioncoefficientsfor the three oils as a function of the oil volume fraction. Oil volume fraction is taken as the sum of the alkane volume fraction plus half the surfactant volume fraction. The self-diffusion coefficients increases at a threshold oil volume fraction.

The NMR data show that the microemulsions dilute in oil have droplet microstructures, so several models of interacting droplets were used in attempts to represent the SANS spectra for microemulsions ofthese compositions

using eqs 2 and 3. Models of the microemulsion as containing monodisperse or polydisperse spheres acting through either hard sphere or square well potentials all failed to match the data. We found the best fit of the spectra using a model of prolate ellipsoidal droplets interacting through an excluded volume potential (Figure 5). This excluded volume ellipsoidal model is described by four parameters (RA,RB,Rer, #d). The three varied fitting parameters are the semimajor axis (RA),the semiminor axis (RB),and an effective radius (Red. A vertical scaling factor is also used and is constrained to within 20%of unity for the fitting process (this reflects the estimated error in the absolute scale of the scattering spectra). The volume fraction of the dispersed phase, #d, is set equal to the volume fraction of the oil plus the volume fraction of surfactant. In the usual uses of the decoupling approximation (eqs 2 and 3) an equivalent spherical radius, Req, is used in S(q), and is calculated from RA and RB by equating the second virial coefficient of the dispersion of ellipsoids to that of a dispersion of spheres.36 In our case the aspect ratio of the ellipsoids is so high that Re,overestimates the excluded volume, so a third parameter Reais used as the effective hard sphere radius in the calculation of the structure factor. Results of the fitting show the large aspect ratios (Table l),and show that the effective hard radius is consistently lower than the equivalent spherical radius. Despite the upturn in intensities at low q for spectra from samples with larger oil volume fractions, a model of ellipsoids interacting via an attractive square well potential failed to fit the data, although it is difficult to separate the effects of substantial droplet growth and increasing attractive interactions. (36) Isihara, A. J. Chem. Phys. 1950, 18, 1446.

Langmuir, Vol. 11, No. 3, 1995 783

Microstructure of Nonaqueous Microemulsions 60 I

I -4-0

0

4

-E-

c1z

-A-

C16

12

8

16

20

q ( X l O 2 / A') Figure 6. Comparison of SANS for microemulsions of comparable polar organic and oil concentrations (points) with the scatteringpredicted by a model for bicontinuous microemulsions (lines). The resultingparametersare inTable 2. The surfactant concentrations in these samples are higher than in other samples due to the shift in phase behavior. The mass fraction of surfactant equals 0.25, 0.34, and 0.39 for Cv, Clz, and CIS microemulsions, respectively. Table 2. Summary of Bicontinuous Microemulsion Modeling Parameters c7 c12 C16

0.52 0.52 0.52

77.0 102.8 93.1

26.6 30.6 29.3

Table 3. Area per ClzEs Head Group Values, Ep, Measured by Porod's Law for SANS and, Ed, Calculated from Model Parameters c7 c12 C16

0.20 0.30 0.20 0.26 0.31 0.20 0.23

188 113 93 80 74 104 84

124 137 79 82 77 82

The SANS scattering curves for microemulsions with comparable amounts of polar organic and oil are well reproduced using the Teubner-Strey formula (Figure 6). The final surfactant mass fractions in these C7, CIZ,and CIS microemulsions are 0.25,0.34,and 0.39, respectively, and are higher than those used in the dilute microemulsions (0.19, 0.26, and 0.30,respectively). The higher concentrations required to make a single phase is the result of an isotope effect that moves the narrow one-phase channel around these compositions in Figure 3. Deuterated samples at the original surfactant concentrations contained two phases, so more surfactant was added to recover a single phase. The characteristic domain sizes, d , and the correlation lengths, 5, are given in Table 2. SANS spectra exhibit Porod law behavior at high q . The area per surfactant head group, Z, is calculated by multiplying the measured surface to volume ratio by the total sample volume and the inverse of the number of surfactant molecules per sample (Table 3). The values of & for CTmicroemulsions are appreciably higher than those of the Clz and C16 microemulsions. For all the oils, & decreases with increasing oil volume fraction. The SANS intensity spectra show slight positive deviations from Porod's law. Values ofthe area per surfactant head group calculated from the droplet model parameters, Ed, are included in Table 3. Generally, the E d values are lower than those found by using the Porod analysis.

Discussion The one-phase channel appears at higher y values on the Gibbs triangle of propylene glycol, glycerol, and oil at constant surfactant concentration as the oil becomes more hydrophobic. More surfactant is needed to solubilize all of the polar organic and oil as the molecular weight of the oil increases. These phase behavior results are consistent with the results of our earlier work.6 The phase behavior of these nonaqueous microemulsionsis analogous in many respects to the phase behavior of aqueous microemulsions made with the same nonionic ~ u r f a c t a n t . ~ ~ The NMR self-diffusion data show that oil droplets percolate into an oil-continuous,and likely bicontinuous, structure with increasing oil volume fraction. At low oil concentrations, the measured self-diffusion coefficient is consistent with the oil residing in droplets. The increase in the oil self-diffisioncoefficient at a threshold oil volume fractionindicates the formation of sample spanning paths, and the gradual climb in the rate of diffision is consistent with the obstructed diffusion of the oil due to a network of surfactant sheets in the bicontinuous structure. Fitting of the SANS spectra is consistent with the presence of disconnected oil-swollen ellipsoidal droplets at low oil concentrations. The model results show that for all three oils the droplets increase in size and axial ratio with increasing oil concentration, and the droplets are quite elongated before percolation. Although the SANS model is crude, the fitting on an absolute intensity scale makes it clear that the microstructure is not spherical in these solutions. More sophisticated models such as the random phase a p p ~ x i m a t i o nmight ~ ~ be more successful in fitting these spectra, but given the limited supply of deuterated materials a more extensive scattering survey was not practical. The SANS spectra for microemulsions with comparable amounts of polar organic and oil are successfully modeled with the T-S free energy expansion model developed to account for the bicontinuous nature of the microemulsions. As described above, fitting spectra with the T-S model allows the scattering to be put on a general amphiphilicity scale, and for these microemulsions AF is approximately -0.6. Thus these microemulsions have a well-defined microstructure, unlike other nonaqueous microemulsions made with formamide and weaker surfactants (i.e. C8E3) that do not have such well-defined microstructure. The repeat distances and correlation lengths are much smaller in these nonaqueous microemulsions than in microemulsions made with water>*due in part to the high surfactant concentrations used here. The measured & values are nearly twice those found in microemulsions made with water.38 Similarly larger values of the area per surfactant head group are reported by Schubert and Strey for microemulsions containing formamide.16 The larger areas per surfactant head group in nonaqueous liquid crystalline phases have been attributed to the low interfacial tensions found between polar organics and alkanes,17-19and a similar effect could explain the high values measured here. The disagreements between the Porod measured values of and the model calculated values of & have been observed before39and are consistent with positive deviations from Porod's law. Such positive deviations from Porod's law are caused by liquid-like scattering (due to density fluctuations on short length scales) that appears at higher values of q and tends to increase the measured (37) Long, M. A.; Kaler, E. W.; Lee, S. P.; Wignall, G. D. J.Phys. Chem. 1994,98,4402. (38)Billman, J. F.; Kaler, E. W. Langmuir 1991, 7, 1609. (39) Billman, J. F.; Kaler, E. W. Langmuir 1990, 6, 611.

784 Langmuir, Vol. 11,No.3, 1995 values of &. Of course, given the crude nature of the droplet model used here, even qualitative agreement in the comparison is encouraging. These observations about microstructure suggest why the observed percolation threshold depends on oil chain length. Three key experimental facts are: (1)regardless of the method used to determine head group areas, the values are substantially larger for C, microemulsions than for those with C12 or (216; (2) the percolation threshold for C7 is higher ( ~ 0 . 4 4than ) that of CIZ(0.35) or c16 (0.25); and (3)the microemulsions are made with a polar organic solvent withy [=G/(PG G ) ] of 0.1 for C,, 0.27 for (212, and 0.37 for CIS. Given that the droplets all grow substantially as oil is added, the difference in the observed percolation thresholds with different oils could be due to changes in attractive interactions between the droplets. These interactions in turn could arise from the change in the solubility of the head groups in mixtures of glycerol and propylene glycol. While nonionic surfactants are insoluble in glycerol, they are highly soluble in propylene glycol,6 probably due to hydrogen bonding between the surfactant and propylene glyc01.l~This strong difference in solubility could promote the formation of zones deficient in glycerol around the surfactant head groups. In complete analogy, it has been proposed that salt-deficient zones around oil-in-water microemulsions droplets yield an attractive interaction between such droplets and that this interaction accounts for the effect of alkane chain length on their percolation threshold^.^^^^^ In this view, the

+

(40) Florin, E.; Kjellander, R.; Eriksson, J. C. J.Chem.Soc., Faraday Trans. 1 1984, 80, 2889. (41)Florin, E. Macromolecules 1985,18, 360.

Martino and Kaler putative glycerol-deficient zones would be largest in the c16 microemulsions, where the glycerol concentration is highest, and this microemulsion would have the lowest percolation threshold, as observed.

Conclusions The microstructure of nonaqueous microemulsions made with propylene glycol, glycerol, C12E5, and three different alkanes has been studied using NMR selfdiffusionmeasurements and SANS. SANS models suggest ellipsoidal oil droplets grow and then percolate to form an oil-continuous structure as the oil volume fraction increases. The threshold percolation concentration increases with decreasing alkane chain length. Large surface areas per surfactant molecule indicate increased solubility of the surfactant head group in the polar solvent as compared to that in water. We suggest that regions deficient in glycerol may exist around the droplets, and attractive forces between these zones may account for the alkane chain length effect on the percolation thresholds. Acknowledgments.This work was supported by the National Science Foundation (PYIA-8351179) and with matching funds from the Exxon Educational Foundation. We acknowledge Oak Ridge National Laboratory for providing the facilities used in the neutron scattering experiments and George Wignall for his assistance. This research was supported in part by the Division of Materials Sciences, US.Department of Energy, under Contract No. DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. LA9405612