Optical extinction and refraction of concentrated latex dispersions

Abstract

Experimental details are given of an apparatus which has been used to measure the optical extinction of colloidal dispersions with concentrations as high as 50% solids. For all dispersions examined the optical extinction showed a maximum as a function of concentration. Optical extinction, or turbidity, is interpreted as the imaginary part of the complex refractive index, ñ. An approximate theory of ñ for concentrated dispersions is presented. The theory takes into account the effect of neighbouring particles on the scattering cross-section of a particular scatterer by using the concept of a local radial distribution of refractive index. Computer calculations based on an idealised refractive-index profile are compared with experimental data for both real and imaginary parts of ñ.