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An extensive small angle neutron scattering study is presented using contrast variation on the growth of nonionic microemulsion droplets as the temperature and thus spontaneous curvature is varied away from the emulsification failure boundary, EFB, toward zero spontaneous curvature. Two ternary systems are compared. They only differ by the chain length of the oil (decane and hexadecane, respectively). Droplets grow in size as one moves away from the EFB. SANS data from ten different contrasts were fitted simultaneously with a model of polydisperse prolate shaped droplets interacting with an effectively hard sphere potential, and the data set further included three different droplet concentrations and four different temperatures. The model of prolates provided a good description of the data. The prolate axial ratio, obtained from the fits, increased with increasing temperature but showed only a minor variation with the droplet concentration. The two systems with different oils behave quantitatively different. In both systems, however, the droplet growth is only minor, with maximum axial ratios of about 3-4, before a bicontinuous microemulsion is formed.

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The free energy of nonionic balanced microemulsions based on nonionic surfactants are analyzed using experimental data from (i) phase behavior, (ii) osmotic compressibility of the balanced microemulsion structure, which is obtained from small angle neutron scattering (SANS) experiments, and (iii) data on interfacial tensions obtained by T. Sottmann and R. Strey [J. Chem. Phys. 106 8606 (1997)]. The balanced microemulsion, where the spontaneous curvature vanishes at equal volumes of water and oil, has a finite swelling with the solvent with a minimum surfactant volume fraction, Phi*(S). At higher surfactant concentrations the balanced microemulsion phase having the surfactant volume fraction Phi(S1) coexists with a lamellar phase of volume fraction Phi(S2). Under the constraint of Phi(W) = Phi(O), where Phi(W) and Phi(O) are the water and oil volume fractions, respectively, the free energy density can be written as an expansion in the surfactant concentration. While the phase equilibria only depend on relative values of the expansion coefficients, absolute values can be obtained from compressibility and interfacial tension data. The osmotic compressibility of the surfactant film was measured by SANS through contrast matching water and oil. The phase behavior of nonionic surfactant-water-oil systems depends strongly on the chain length of the oil, when comparing a homologous series from octane to hexadecane using the same surfactant, here being pentaethylene oxide dodecyl ether (C(12)E(5)). The three concentrations Phi*(S), Phi(S1), and Phi(S2) increase markedly as the chain length of the oil is increased. However, from the analysis of the surface tension data it is concluded that there are no major changes in the bending rigidities as the oil is changed. The data are analyzed within the model free energy densities [formula--see text] and [formula--see text]. We find that within experimental accuracy, the first of these models provides a quantitatively consistent description of the data. For the second model there is a larger discrepancy between observed and calculated values.

Treatment of 2-(trimethylsilyl)ethyl 2,3-di-O-acetyl-4-O-(3,4,6-tri-O-acetyl-R-D-galactopyranosyl)β-D-galactopyranoside (12) and 2-(trimethylsilyl)ethyl 2,3-di-O-benzyl-β-D-glucopyranoside (15) with the novel reagent combination formaldehyde diphenylmercaptal-N-iodosuccinimide-trifluoromethanesulfonic acid [(PhS) 2 CH 2 /NIS/TfOH], gave the corresponding 6,2′-O-and 4,6-O-methylidene acetals 13 and 16 in 52% and 53% yield, respectively. Deacetylation of 13 gave 2-(trimethylsilyl)ethyl 4-O-R-D-galactopyranosyl-β-D-galactopyranoside (1). The conformation of 1 was similar to that of the non-acetalated parent glycoside. † Dedicated to Professor Hans Paulsen on the occasion of his 75th birthday.