Small angle neutron scattering studies of critical phenomena in a three-component microemulsion

Abstract

Critical density fluctuations of a “water-in-oil” microemulsion consisting of water, benzene, and BHDC (benzyldimethyl-n-hexadecyl ammonium chloride) have been observed near the phase boundary by a small-angle neutron scattering (SANS). The observed profiles were well described by the product of a form factor of spherical droplets and a structure factor, consisting of a term describing the inter-droplet correlations and also an Ornstein-Zernike component describing the droplet density fluctuations. Allowance was also made for the droplet polydispersity, though the width of the distribution turned out to be very small (1–2%). The observed temperature dependence of the osmotic compressibility was fitted using the crossover function proposed by Belyakov et al., and the Ginzburg numbers were obtained of the order 10⁻². These results indicate that longrange inter-droplet forces are not significant in this system, which displays upper critical solution temperature (UCST) behavior. In contrast, previous studies of systems displaying with lower critical solution temperature (LCST) behavior [e.g. water, n-decane and AOT (dioctyl sulfosuccinate sodium salt)] indicate that long-range interactions appear to dominate the phase separation behavior.