INTERACTIONS IN COLLOIDAL MIXTURES - PARTIAL STRUCTURE FACTORS IN MIXTURES OF COLLOIDAL SILICA AND AN ANIONIC OIL-IN-WATER MICROEMULSION

We have investigated the colloidal stability and interactions in a mix ture of colloidal silica (Ludox(TM)) and an oil-in-water microemulsion (AOT/decane/brine). Stable silica-microemulsion mixtures containing l arge fractions of dispersed phase (phi(oil) + phi(Ludox) > 0.36) can b e prepared. For tired silica and AOT concentrations, the addition of d ecane increases the size of the microemulsion droplets. Since the surf actant concentration remains constant, this droplet growth is balanced by a decrease in the number density of droplets. Partial structure fa ctors for the mixture are extracted from sets of small-angle neutron s cattering (SANS) data using a solvent contrast variation technique. Th e extracted partial structure factors are compared qualitatively to mo del calculations for a multicomponent sticky hard sphere (SHS) fluid. A shallow contact potential (approximately 2.6 kT) between the Ludox p articles is necessary to obtain agreement between model and experiment in the low-q region of the partial structure factors. The magnitude o f this contact potential does not change with decane concentration, in agreement with the macroscopic observation that there is no observabl e change in the stability of these silica-microemulsion mixtures. (C) 1998 Academic Press.