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Abstract

Using an asymmetric cellulose acetate (CA) membrane annealed at 82.5°C and the linear relationships of the thermodynamics of irreversible processes, the electrical and electroosmotic transport coefficients such as R_e, l_ep, and l_pe as well as the hydrodynamic permeability, l_p, have been determined as functions of the NaCl concentration, c_s. The electroosmotic coefficient, l_pe, and the so called “pressure-membrane potential” (pmp) coefficient, l_ep, have been found to be strongly dependent on the external salt concentration. At larger NaCl concentrations both coefficients even change their sign being positive for c_s < 0.07 mole/liter and negative for c_s > 0.07 mole/liter. The positive values of l_ep and l_pe at concentrations c_s < 0.07 mole/liter are in agreement with the cation exchange property of CA membranes. Furthermore, the hydrodynamic permeability, l_p, and the electrical resistance, R_e, of the asymmetric cellulose acetate membrane have been measured. The sign reversal of l_ep and l_pe is discussed by applying the finely porous membrane model to the two-layer membrane and taking into account the weak cation-exchange character of cellulose acetate membranes. The discussion indicates that both the weak cation-exchange character as well as the asymmetry of the cellulose acetate membrane are responsible for the significant sign reversal of the electrokinetic transport coefficients l_ep and l_pe at larger concentrations. Moreover, the value of the measured electrical resistance supports the idea of the two-layer model for the asymmetric cellulose acetate membrane.