DIFFUSION-INDUCED AND CONVECTION-INDUCED TRANSPORT OF NONELECTROLYTESIN AQUEOUS-SOLUTION ACROSS A CATION-EXCHANGE MEMBRANE

Results of experiments are reported in which the influence of convecti on on the diffusive transport rate of four nonelectrolytes in aqueous potassium chloride solution across a -SO3- 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 passi ng an electric current across the membrane (electroosmosis). An osmoti c difference between the bulk phases originating from a difference in concentration of the nonelectrolyte and that of potassium chloride con tributes to the convection of the pore fluid. The concentration differ ence of the electrolyte is built up by electrical transference. The in fluence of the convection on the diffusive transport increases strongl y 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 t han expected and it strongly depends on the molar mass of the nonelect rolyte. It is hypothesized that this is a consequence of the inhomogen eous structure of the membrane. If one bulk phase contains two nonelec trolytes of different molar masses (e.g. M-n(1)=60 g mol(-1); M-n(2)=5 95 g mol(-1)), a sorting effect is observed if the diffusive and the c onvective flows have opposite directions (j(n)(1)/j(n)(2)approximate t o 100; j(n)(1) and j(n)(2) are the molar flow densities of the nonelec trolytes 1 and 2, respectively). If j(n)(1) and j(n)(2), and the volum e flow density have the same direction, the ratio of the molar flow de nsities of the nonelectrolytes approaches the value 1.