Scattering of light by charged colloidal particles in salt solutions

Abstract

In the interpretation of light scattering by colloidal electrolytes in salt solutions the interaction between the colloidal particles and the low molecular weight ions has to be taken into account. When fluctuation theory is applied for the derivation of a light-scattering equation, nonelectroneutral fluctuations may be neglected in most cases. The total light scattering can be split into three contributions, one due to density fluctuations, one due to concentration fluctuations in the low molecular weight components, and one due to the colloidal particles. In the last-named contribution the (usually negative) adsorption of the low molecular weight salts by the colloid is included. This can be taken into account in good approximation by using in the light-scattering equations the refractive index increment at constant chemical potential and not at constant concentrations of the other components of the system. This quantity can be measured directly in membrane equilibria or it can be calculated from concentration differences in a membrane equilibrium combined with the more usual refractive index increments at constant concentrations. The theoretical treatment is confirmed by measurements of light scattering and membrane equilibria with half-neutralized polymethacrylic acid in 0.1 M sodium halide solutions and in a few other salts. The correction on the molecular weight varies from 10% in NaF to 25% in NaI and amounts even to 45% in 0.01 M (NH 4) 6Mo 7O 24.