This paper contends that counterion dehydration at the ion-exchange membrane/electrolyte solution interface is an instance of heterogeneous reaction whose kinetics is governed by a rate equation of the Butler-Volmer type. The finite rate of this reaction is hypothesized to constitute an important factor that affects counterion selectivity of the membrane. Simple theoretical examples show that this is what may lead to the relatively easier membrane transfer of the less mobile ions (e.g., ${\mathrm{Li}}^{+}$) compared to the more mobile ones (e.g., ${\mathrm{Na}}^{+}$).

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• Electric field effects
• Interface & surface thermodynamics
• Ionic transport
• Nonequilibrium & irreversible thermodynamics
• Surface & interfacial phenomena