Small angle neutron scattering near Lifshitz lines: Transition from weakly structured mixtures to microemulsions

We have studied the phase behavior, wetting transitions, and small angle neutron scattering (SANS) of water, n-alkane, and n-alkyl polyglycol ether (C_iE_j) 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 well-structured mixtures (exhibiting nonwetting middle phases and well-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 smaller surfactant concentrations, and intersect there with the body of heterogeneous phases, depends on a number of factors that are discussed in some detail.