Small-angle scattering from colloidal dispersions

Abstract

A theory of the small-q behaviour of the static structure factor S(q) for monodisperse colloidal dispersions is presented in which the long-range forces between the colloid particles are treated in the random phase approximation. By using a hard-sphere reference fluid, explicit results for the coefficients to (q⁴) in a small-q expansion of c(q), the Fourier transform of the direct correlation function, are obtained. The theory gives an expression for the osmotic compressibility of a dispersion that is equivalent to an empirical relation used to interpret light-scattering experiments on microemulsions. When applied to charge-stabilised dispersions the theory predicts the existence of a phase diagram closely analogous to one normally observed in molecular systems.