A structural description of liquid particle dispersions: Ultracentrifugation and small angle neutron scattering studies of microemulsionsWe have studied the phase behavior, wetting transitions, and small angle neutron scattering (SANS) of water, n-alkane, and n-alkyl polyglycol ether (C;E) systems in order to locate the transition between weakly structured mixtures and microemulsions, and to provide a measure for the transition. We first determined the wetting transition by macroscopic measurements and then measured the location of the Lifshitz lines by SANS. Starting with wel1-structured mixtures (exhibiting nonwetting middle phases and wel1-expressed scattering peaks, features that qualify them as microemulsions) the wetting transition was induced by increasing the chain length of the alkane or by changing the oil/water volume ratio, and then the Lifshitz line was crossed. Further, starting with systems past the disorder line (weakly structured mixtures that display wetting middle phases and no scattering peaks), local structure was induced by either increasing the surfactant concentration or decreasing the oil/water volume ratio or the temperature. In each case a Lifshitz line was crossed. Analyzing the scattering experiments quantitatively, allows determination of the amphiphilicity factor, which is a measure of the strength of the surfactant. The results suggest there is a sequence of roughly parallel surfaces within the three-dimensional composition-temperature space. As the amphiphilicity factor increases, first a disorder surface is encountered, then a Lifshitz surface, and finally a wetting transition surface. How and to what extent these surfaces move in the one-phase region toward smal1er surfactant concentrations, and intersect there with the body of heterogeneous phases, depends on a number of factors that are discussed in some detail. phase microemulsions become able to wet the water/oil interface for thermodynamic reasons. 24 Widom has suggested

A small-angle neutron scattering study of nonionic surfactant molecules at the water-oil interface: Area per molecule, microemulsion domain size, and rigidity Small angle neutron scattering near Lifshitz lines: Transition from weakly structured mixtures to microemulsions Small-angle neutron scattering measurements were performed on symmetric microemulsions containing equal volume fractions of water/formamide, n-octane, and a sufficient amount of nalkyl polyglycol ether (CiE j ). By changing the surfactant chain length in the order C 8 E 3 , . C 6 E 2 , and C 4 E 1 in pure water, as well as for given CgE3 and C 6 E2 by increasing the relative amount of formamide in the water/formamide mixture, the amphiphilic strength (the amphiphilicity) is reduced. We observe that the characteristic scattering peak becomes weaker and its position moves into q = 0 showing that the microstructure becomes disordered.However, a q -2 and a q -4 behavior for large q in film and bulk contrast, respectively, is still observed indicating the persistence of internal interfaces. As the microstructure is further weakened, a disorder line is passed at which the behavior of the real space correlation function changes from a damped oscillatory behavior to a monotonically decreasing one. We determine that this line has been passed from fits to the bulk scattering intensity. Simultaneously, the film scattering intensities show a transition from correlated to uncorrelated films as the disorder line is passed. Closely related to the weakening of the microstructure is the occurence of a n~nwetting ~ wetting transition in the vicinity of a Lifshitz line. Roughly speaking, mlcroemulslOns, that do show a scattering peak do not wet the water-oil interface. As predicted by Landau-Ginzburg theories, the wetting transition occurs on the microemulsion side of the disorder line. As the amphiphilicity is even further reduced, a tricritical point is reached with the interesting observation that substantial local structure still persists. 8532

Spherical, cylindrical, and lamellar microstructures were examined by ͑SANS͒ in the binary system water ͑D 2 O͒/pentaethylene glycol mono-perdeutero-n-dodecyl ether ͑C 12 D 25 E 5 ͒ and in the ternary system ͑D 2 O͒/pentaethylene glycol mono-n-dodecyl ether ͑C 12 E 5 ͒/perdeuterated n-octane ͑C 8 D 18 ͒. A model-independent picture of the structures emerges from Fourier transformation of the measured SANS spectra. The data analysis makes no a priori assumptions about the type of structure. Rather, the pair distribution functions obtained are so unique that an unambiguous assignment of the local geometry is possible. The different structures for a given mixture are obtained by changing temperature. For water-rich samples the sequence of spherical, cylindrical, and planar structures is obtained with increasing temperature. For oil-rich samples the same sequence occurs as a function of decreasing temperature. As an important additional result, the scattering length density profile normal to the interface is obtained. The interfacial profile is found to be rather diffuse, apparently because of solvent penetration.

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