Refractive index of concentrated colloidal dispersions

Abstract

This paper describes the measurement and theoretical interpretation of the refractive index, n′, of a range of disordered aqueous colloidal dispersions. Concentrations up to 50% by volume and particle sizes in the range 0.1–1.5 µm were used. All dispersions were highly turbid and required special refractometric methods. These methods gave concordant results, experimental error generally increasing with increasing particle size. The results for all dispersions show that n′ is linearly related to the volume fractions of the continuous and dispersed phases. For most dispersions n′ also appears to be the volume average refractive index of constituent phases, the exception being a dispersion containing inhomogeneous clay particles. Our results, for disordered dispersions, are entirely concordant with refractive index measurements made by previous workers using ordered, lattice-forming dispersions. Two theories of n′ in concentrated dispersions are discussed, one based on light scattering and the other on ray optics. Both accurately describe our experimental results.