Determination of diffusion coefficients and effective charge numbers of lignosulphonate. Influence of relative permittivity of the solvent

Abstract

The effect of relative permittivity on the diffusion coefficients and effective charge numbers of a polydisperse polyelectrolyte, lignosulphonate, has been studied. The measurements were carried out using a method which is based on a convective diffusion process across a porous membrane. The relative permittivity of the medium was lowered by means of mixed solvents, i.e. using different ethanol–water mixtures. A procedure to estimate the transport quantities for the base electrolyte and the internal standard from molar conductivities in infinitely dilute solutions and from transport numbers with the aid of Bjerrum's theory for ion association is presented.

The diffusion coefficients of lignosulphonate decreased and the radii of the molecules increased with decreasing relative permittivity. The effective charge numbers of lignosulphonate molecules fell to very low values but did not reach zero, i.e. molecules were not completely associated.