Behaviour of simple ions in the system cellulose–anionic dye–simple electrolyte. Part 2.—Diffusion coefficients

Abstract

The permeability of water-swollen cellulose membranes (undyed or dyed with various amounts of Direct Blue 1) to Na⁺ and Cl^– was measured by means of suitable tracers. Care was taken to minimize, and correct for, the effect of stagnant layers in the electrolyte bath adjacent to the membrane surfaces. The behaviour of each ion was studied as a function of the concentration of electrolyte in the bath and of fixed anionic groups in the membrane, in conjunction with the corresponding equilibrium partition behaviour. The results obtained were used to throw further light on the alternative theoretical treatments examined in Part 1. The inadequacy of the simple equipotential volume model (EVM) was further demonstrated. It was also difficult to reconcile satisfactorily the partition and diffusion data with the EVM augmented with the assumption of “cation binding” by the fixed anionic groups. On the other hand, a semiquantitative treatment based on the non-equipotential volume model which was reasonably successful with the partition data, could reproduce the salient features of the diffusion behaviour. This required an additional assumption about the fine structure of the membrane, but there is some precedent for it. Conversely, significant information about important structural features of the membrane can be obtained in this way.