Diffusion- and convection-induced transport of nonelectrolytes in aqueous solution across a cation-exchange membrane

Abstract

Results of experiments are reported in which the influence of convection on the diffusive transport rate of four nonelectrolytes in aqueous potassium chloride solution across a −SO 3 − cation-exchange membrane is studied. The nonelectrolytes have different molar masses (60 g mol −1< M n <595 g mol −1). The convective volume flow is generated by passing an electric current across the membrane (electroosmosis). An osmotic difference between the bulk phases originating from a difference in concentration of the nonelectrolyte and that of potassium chloride contributes to the convection of the pore fluid. The concentration difference of the electrolyte is built up by electrical transference. The influence of the convection on the diffusive transport increases strongly with increasing molar mass of the nonelectrolytes, as theoretically expected. A quantitative comparison of the experimental data with that of the theoretical prediction shows that the influence of the volume flow density on the transport rate of the nonelectrolytes is smaller than expected and it strongly depends on the molar mass of the nonelectrolyte. It is hypothesized that this is a consequence of the inhomogeneous structure of the membrane. If one bulk phase contains two nonelectrolytes of different molar masses (e.g. M n(1)=60 g mol −1; M n(2)=595 g mol −1), a sorting effect is observed if the diffusive and the convective flows have opposite directions ( j n (1) j n (2) ≈100 ; j n(1) and j n(2) are the molar flow densities of the nonelectrolytes 1 and 2, respectively). If j n(1) and j n(2), and the volume flow density have the same direction, the ratio of the molar flow densities of the nonelectrolytes approaches the value 1.