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Organophilic silica dispersions, containing spherical monodisperse particles up to concentrations of 80% (w/v) of silica, were studied with the (static) light-scattering method. Cyclohexane and chloroform were chosen as solvents in order to minimize the scattering intensity. The results were interpreted with fluid-state theories. In a large range of silica concentrations the osmotic compressibility as well as the scattering at finite angles could be described rather well with hardsphere interactions. The hard-sphere radius was rather close to the hydrodynamic radius of the particle. At the highest particle concentrations the scattering intensity sharply increases for small scattering angles. This is attributed to formation of large dense clusters of primary particles.

The contrast variation method, well known from small-angle neutron and X-ray experiments, is applied to a light-scattering study of colloidal dispersions of spherical particles. The theory is rederived in terms of light-scattering parameters and extended to cases of preferential adsorption of one of the solvent components at the particle surface. Experiments were carried out with spherical monodisperse lyophilic silica dispersions in (binary) mixtures of cyclohexane and transdecaline. It is found that the silica particles have a low mean refractive index and that significant optical density variations occur in the particle. Cyclohexane is preferentially adsorbed. For the system under consideration the light-scattering contrast variation method appears to be a valuable technique for particle characterization.