SMALL-ANGLE NEUTRON-SCATTERING NEAR LIFSHITZ LINES - TRANSITION FROM WEAKLY STRUCTURED MIXTURES TO MICROEMULSIONS

We have studied the phase behavior, wetting transitions, and small ang le neutron scattering (SANS) of water, n-alkane, and n-alkyl polyglyco l ether (C(i)E(j)) systems in order to locate the transition between w eakly structured mixtures and microemulsions, and to provide a measure for the transition. We first determined the wetting transition by mac roscopic measurements and then measured the location of the Lifshitz l ines by SANS. Starting with well-structured mixtures (exhibiting nonwe tting 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/wate r volume ratio, and then the Lifshitz line was crossed. Further, start ing with systems past the disorder line (weakly structured mixtures th at display wetting middle phases and no scattering peaks), local struc ture 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 qua ntitatively, allows determination of the amphiphilicity factor, which is a measure of the strength of the surfactant. The results suggest th ere is a sequence of roughly parallel surfaces within the three-dimens ional composition-temperature space. As the amphiphilicity factor incr eases, first a disorder surface is encountered, then a Lifshitz surfac e, and finally a wetting transition surface. How and to what extent th ese surfaces move in the one-phase region toward smaller surfactant co ncentrations, and intersect there with the body of heterogeneous phase s, depends on a number of factors that are discussed in some detail.