Studies with inorganic precipitate membranes—XII: Consideration of membrane field strength and energetics of permeation

Electrolytic transfort processes occuring across parchment supported membranes have been described by Nernst Planck flux equation taking into account the membrane resistance R_m, membrane potential E_metc.E_m values for various electrolytes display very interesting phenomena. In the case of 1:1 electrolyte the E_m values are all positive, while in the case of (2:1) and (3:1) electrolytes surface charge reversal takes place. The diffusion rate sequence and selectivity of the membrane for different uni- bi- and trivalent cations was found to be primarily dependent on the difference in the hydration energies of counter ions in the external solution. On the basis of Eisenman-Sherry theory the diffusion rate sequence of alkali metal cations point towards the weak field strength of the fixed charge groups. Various thermodynamic parameters, ΔH^≠, ΔF^≠ and ΔS^≠ were evaluated by applying the theory of absolute reactions rates to the diffusion process through parchment supported membranes. The values of ΔS^≠ were found to be negative, indicating that diffusion takes place with partial immobilization in the membrane phase. The relative partial immobility was found to increase with increase in the valence of the ions constituting the electrolyte. A formal relation between ΔH_hydration, ΔF_hydration and ΔS_hydration of cations with the corresponding values of ΔH^≠, ΔF^≠ and ΔS^≠ for diffusion, was also found to exist for these membranes.