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Abstract

Using an asymmetric cellulose acetate (CA) membrane annealed at 82.5°C, the membrane potential has been determined as a function of the NaCl concentration, c′_s in hyperfiltration experiments at pressure differences, ΔP, of up to 10 atm. These membrane potential determinations have been repeated under dialysis conditions (c′_s = c″_s). Both the membrane potential in dialysis, Δ_Ψm, as well as in hyperfiltration, Δ_ϕm, are strongly dependent upon the feed concentration, c′_s, and even change their sign at c′_s ∼- 0.07 mole/liter. The concentration dependence of the hyperfiltration membrane potential, Δ_ϕm, is briefly discussed in terms of the finely porous membrane model applied to both layers of an asymmetric membrane. Furthermore, employing the same model, the difference of membrane potentials, Δ_Ψm − Δ_ϕm, is related to different concentration profiles within the porous sublayer of an asymmetric cellulose acetate membrane under the different boundary conditions. The small potential contributions of the porous sublayer to the entire membrane potential are due to larger diffusion coefficients and a smaller fixed charge concentration within the porous sublayer. Employing the integral of the Nernst-Planck equations and the Donnan potential relation, the membrane potential is calculated as a function of brine concentration in hyperfiltration at 10 atm as well as a function of the hydrostatic pressure difference, ΔP, for different brine concentrations.