J. Phys. Chem. 1984, 88, 4586-4589
4586
is similar to that employed by Russell et al.I4 and Bewick.I5 In these other investigations spectra were taken in situ by pressing the electrode up against an infrared transparent window. We have taken the different approach of isolating the electrode from the supporting electrolyte for spectroscopic examination. This eliminated materials compatibility problems that have been reported in these other studies and thus eliminated problems of contamination. In the work presented here the irreversible nature of the adsorption permitted the electrode to be removed from the elec(15) Bewick, A. J. Electroanal. Chem. Interfacial Electrochem. 1983,150, 481. (16) Pons, S.J. Electroanal. Chem. Interfacial Electrochem. 1983, 150, 495.
trolyte. Clearly this is a limitation of our approach; however, the influence of contaminants, such as halide ions, on the adsorption process mitigated against any other approach. The results presented here do show conclusively a structural transformation of adsorbed quinoid compounds with solute concentration. This transformation is consistent with the orintational transition model proposed by Soriaga and Hubbard.’ The results also suggest that the edge-bonded H Q and BQ are structurally similar, and its is suggested they both adsorb as 2,3-q2 bonded diphenols. Lastly, these results indicate the feasibility of spectroscopic examination ouf adsorbed layers by separating the electrode and supporting electrolyte. Registry No. Pt, 7440-06-4; HQ, 123-31-9; BQ, 106-51-4.
Evidence for Zero Mean Curvature Microemulsions Ldc Auvray,*t$ Jean-Pierre Cotton,$ Raymond Ober,? and Christiane Taupint Laboratoire de Physique de la Matiere Condensee, College de France, 75231 Paris Cedex 05, France, and Laboratoire Leon Brillouin, C.E.A.-C.E.N.Saclay, 91 191 Gif-sur-Yvette Cedex, France (Received: May 23, 1984)
We studied Winsor type microemulsions containing equal brine and toluene volumes by X-ray and neutron scattering. The contrast factors between the microemulsion constituents are chosen to separate the oil, water, and surfactant contributions to the scattering. The spectra were recorded in different angular ranges. At large angle, the interfacial surfactant film between oil and water is directly evidenced by the asymptotic intensity behavior. At zero angle, the contrast variation method shows that the concentration fluctuations of water and surfactant are not correlated; we deduce from this result that the mean curvature of the interfacial film is zero on average. The oil and water spectra and film spectra suggest that, when the film curvature fluctuates, the water and oil domain size remains well-defined.
In the Winsor phases’ of brine, hydrocarbon (“oil”), ionic surfactant, and alcohol mixtures, as the water ionic strength increases, one observes the following successively: (i) a two-phase equilibrium: an oil-in water microemulsion coexists with an oil excess; (ii) a three-phase equilibrium: the middle-phase microemulsion contains comparable oil and brine volumes and separates the oil and brine phases; (iii) another two-phase equilibrium: a water-in-oil microemulsion coexists with a brine excess. This sequence has raised much interest1” but is not yet fully understood. To interpret it partially, it has been proposed that salt addition changes the c~rvature’g”~ of the surfactant interfacial film between oil and brine, by screening the ionic surfactant polar head repulsion. In the twQ-phase equilibria, the microemulsion is “classical”, Le., made of oil-in-water or water-in-oil droplets; in the middle phase, as the water and oil volumes are very close and the microemulsion is not d i l ~ t a b l ethe , ~ existence of well-defined droplets is very unlikely and bicontinuous structures, where the surfactant interfacial film has no preferred curvature, have been imagined. They are either ordered, generated by minimal surfaces (lamellar,8 cubic9, or random: ’Ovl’ the microemulsion volume is divided into cells, randomly filled by oil and water, and the surfactant is distributed at the oil-water interface. In the original Talmon-Prager model (ref lo), the cells are generated by a Voronoi tessellation and the cell size distribution is large. In ref 11, the interfacial film is flexible and fluctuates, but the curvature fluctuations are important only at scales larger than Ek, the “persistence length” of the film; this fixes the basic size (- 100 %.)of the cells, which are taken to be identical and cubic. In both models, the oil and water domain size 5 (& in ref 11) is related to the chemical composition by the geometrical constraint
t College de France
* Laboratoire LBon Brillouin 0022-3654/84/2088-4586$01 S O / O
($o and +w, oil and water volume fractions; Cs,surfactant concentration; 2, area per surfactant molecule in the interfacial film, 2 = 60 A2). This relation is at variance from the equation giving the radius R of, say, water-in-oil spheres: R = 34W/(CSZ) (2) Phase diagrams of the random models have been constructed in have calculated the X-ray ref 10-12 and Kaler and Prager l 3 (W) scattering by the oil and water parts of the microemulsion using the Voronoi model. They predict that the scattered intensity decreases monotonously as the scattering vector q increases. Recently, the authors of ref 14 and the present authorsI5 have studied the middle-phase structure by small-angle X-ray scattering. We observed that the X-rays were mainly scattered by the in(1) P. A. Winsor, “Solvent Properties of Amphiphilic Compounds”, Butterworths, London, 1954. (2) K. Mittal, Ed., “Micellization, Solubilization, and Microemulsions“, Plenum Press, New York, 1977. (3) A. M. Cazabat, D. Langevin, J. Meunier, and A. Pouchelon, Adu. Colloid Interface Sci., 16, 175 (1982). (4) S. Friberg, I. Lapczynska, and G. Gillberg, J. Colloid Interface Sci., 56, 19 (1976). (5) M. L. Robbins in ref 2, Vol. 2, p 713. (6) R. Hwan, C. A. Miller, And T. Fort, Jr., J. Colloid Interface Sci., 68, 221 (1979). (7) P. G. de Gennes and C. Taupin, J. Phys. Chem., 86, 2294 (1982). (8) Chun Huh, J. Colloid Interface Sci., 71, 408 (1979); 0. Parodi, Communication at the Workshop “Colloidal Crystals”, Les Houches, Feb
__19RA..
(9) L. E. Scriven in ref 2, Vol. 2, p 877. (10) Y. Talmon and S. Prager, J. Chem. Phys., 69, 2984 (1978). (1 1) P. G. de Gennes, J. Jouffroy, and P. Levinson, J. Phys. (Orsay, Fr.), 43, 1241 (1982). (12) B. Widom, J. Chem. Phys. 81, 1030 (1984). (13) E. W. Kaler and S. Prager, J. Colloid Interface Sci., 86, 359 (1982). (14) E. W. Kaler, K. E. Bennett, H.T. Davis, and L. E. Scriven, J. Chem. Phys., 79, 5673 (1983); E. W. Kaler, H. T. Davis, and L. E. Scriven, J. Chem. Phys., 79, 5685 (1983). (15) L. Auvray, J. P. Cotton, R. Ober, and C. Taupin, J. Phys. (Orsay, Fr.), 45, 913 (1984).
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984 4587
Letters
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4 ' Figure 2. Asymptotic behavior of a film spectrum. q2i(q)as a function of 42.
Figure 1. Variations of the scattering length density (sld) through the interfacial film: (a) Film type contrast no = n, = 5.36 X 10" cm-2. Cm8 = 0.06, 0.07, 0.08, 0.09, 0.1 g/mL. (b) Two-phase contrast (n, = 6.2 x 1010 no = 9.1 X lo9 cm-2); we neglect the SDS polar head contribution and the contrast between SDS aliphatic tails and toluene (dashed lines). Cms= 0.07, 0.08, 0.09, 0.1 g/mL (in the X-ray experiments, we also studied the corresponding fully hydrogenated samples plus the samples Cml = 0.05, 0.055, 0.06, 0.065, 0.075, and 0.11 g/mL).
terfacial film at large angles and by the oil and the water at small angles, The characteristic microemulsion size 5, defined as a mean radius of curvature of the film in ref 15 or a "pseudoradius of gyration" in ref 14, is drawn from the spectra in the intermediate q range (q,$> 3); in both cases ,$ follows prediction 1. This suggests that these microemulsions have a bicontinuous random structure. However, we found that the Kp model is not sufficient to describe the long-range oil-water correlations. At very low q, in a q range unexplored in ref 14 for technical reasons, we observed an unpredicted peak in the spectra.I5 To understand this result, we focused our small-angle-scattering study on symmetric microemulsions, the most likely bicontinuous (q50 = q5, C, variable, salinity fixed at the optimal value defined in ref 15). In neutron scattering, a specific deuteration enables the scattering length densities of water, oil, and film n,, no, nf, respectively, to be varied. We used this possibility to study the interfacial film structure and the oil-water domain structure. We first present how the interfacial film is evidenced by neutron scattering. Applying the contrast-variation method16 (measurement of the zero-angle intensity when n, varies with the D20 fraction) we then show that the average mean curvature of the interfacial film is zero; finally, we present the oil-water spectra (also called "two-phase" spectra, no = nf # n,,,)and the film spectra (no = n, # nf). Samples. The microemulsions are made of brine (salinity 6.5%, weight percentage of brine), toluene, sodium dodecyl sulfate (SDS), and 1-butanol, with as much brice (volume V,) as toluene (volume Vo= V,, 9, = 4, = V,/(Vo + V,) = 0.5). For a given mass m, of SDS (CmS= m,/(Vo+ V,) (glml)) we adjusted the butanol volume to obtain at 23 "C a monophasic microemulsion. See the caption of Figure 1 for the isotopic and chemical compositions. In calculating the scattering length densities,17we took into account the butanol partition coefficient estimated from the titration curves (0.06 mL of butanol/mL of toluene, 0.02/mL of brine). We did not notice isotopic effects in most of the samples, either in butanol OH or OD titration or in X-ray scattering. However, we did not succeed in making, at 23 OC, the twophase-contrast sample Cml= 0.06 g/mL. Diffraction Experiments. The scattering vector q is defined by q = 4r(sin @/A, where 28 is the scattering angle and X is the (16) H.B. Stuhrmann, J . Appl. Crystallogr., 7,173 (1974). (17) The scattering lengths are calculated from L. Koester and W. B. Yelon, "Neutron Diffraction Newsletters", March 1983.
wavelength of the incident radiation. The X-ray experiments were done at the synchrotron radiation laboratory L.U.R.E. (Orsay) (X = 1.61 A, Point collimation, 0.2 > q > lo-' A-'). The neutron experiments were done at Laboratoire Lton Brillouin ("Orphte" reactor, Saclay) (spectrometer P.A.C.E. 4.5 X 1O'O cm-*, Figure 3 ) . is not directly measured. Applying formula 4, we found that the intensity minimum occurs x,,(O) gives the water concentration fluctuation in a macrofor the film contrast: hm= hf= no;plotting (i(n,,O) - i(n,f,0))1/2 scopic volume Vand is always positive. xwr(0)measures the cross as a function of n,, we found a straight line (Figure 3 ) ; from correlation between the water and film fluctuations; xwf(0)is formula 3 , we deduced that Xwf(0)= 0. positive if a water excess in Vleads to a surfactant excess. De(iii) Discussion. With 4o= 4, = 0.5, the radius of hypothetic veloping a remark of Widom,'z we want to show that xWf(O)is water-in-oil or oil-in-water spheres would be R = 120 8, (formula related to the curvature of the microemulsion interfacial film. 2, X = 60 A2); assuming d > 6 8, (as a lower bound, more The area per surfactant molecule in a microemulsion interfacial probably d = 10 A), nf = 0.4 X 1O'O cm-2 (hydrogenated SDS film does not fluctuate in first approximation; a surfactant conaliphatic tails), no = 3 X 1O'O cm-2, one expected i(n,,o) to vanish centration fluctuation is proportional to an area fluctuation of the (or to be a minimum if there i s polydispersity) at nwm> 3.5 X interfacial film. To accommodate a water fluctuation 64, in V, 1O'O cm-2 for water-in-oil spheres or at 4"'< 2.5 X 1O1O cm-2 for the interfacial film moves, and the resulting area variation in V oil-in-water spheres; both bounds are clearly outside the experidepends on the film mean curvature. mental determination: nwm= n,f. Case of "Classical" Microemulsions. For a microemulsion For the first time, we thus deduce unambiguously that mimade of water-in-oil droplets, a water excess in a volume Vmeans croemulsions with a zero average mean curvature do exist. It must a droplet excess, hence a surfactant excess: x,f(O) is positive. It be emphasized that this conclusion is true only on average; objects is the reverse for oil-in-water microemulsions. In the past, this with different curvature signs may locally and temporarily exist. distinction (formulated in another way) has been used to evidence Oil-Water and Film Spectra. The oil-water and film spectra microemulsion phase inversion.20 For spherical droplets (radius are shown in Figure 4. They are very different: the two-phase R , film thickness d), i(hm,o)vanishes. Plots of i(q,.,0)'I2determine X-ray and neutron spectra have a peak at q = q*. Experimentally if the microemulsion is made of oil-in-water spheres (x,f(O)/x,(O) q* = 122C, (C, in 8,"). The spectra are homothetic, i(q) = i(q*) = - 3 d / R , if d
