A small angle x-ray scattering study of the droplet-cylinder transition inoil-rich sodium bis(2-ethylhexyl) sulfosuccinate microemulsions

A method for nonlinear fitting of x-ray scattering data from polydisperse m ixtures was developed. It was applied to the analysis of the structural cha nges in the droplet phase of oil-rich water-in-oil (w/o) sodium bis(2-ethyl hexyl) sulfosuccinate (AOT) microemulsions with increasing temperature or u pon addition of salt. Data were collected at different temperatures (15 to 60 degrees C) and salt concentrations (up to 0.6% NaCl) within the one-phas e region of the L-2 phase (w/o microemulsion) for different droplet sizes ( water/AOT molar ratio w(o) = 25 to 56) and concentrations (droplet weight f raction c(w) = 2% to 20%). This allowed us to distinguish between contribut ions from individual scattering particles, e.g., droplets and cylinders to the total scattering intensity. The complete data set containing over 500 s cattering curves could be interpreted by fitting the scattering of weighted sums of AOT covered water droplets, long cylinders, and inverse AOT micell es containing bound water only, to the experimental scattering curves. The polydispersity of the droplets and cylinders is described by Schulz distrib utions and the interactions between the droplets are calculated using a sti cky hard-sphere potential in the Percus-Yevick approximation. The volume fr actions of the components, their average sizes and polydispersity, and the stickiness of the water/AOT droplets are determined by a nonlinear fit to t he experimental data. (C) 2000 American Institute of Physics. [S0021-9606(0 0)50528-5].