We consider two charged semipermeable membranes which bound bulk electrolyte solutions and are separated by a thin film of salt-free liquid. Small ions permeate into the gap, which leads to a steric charge separation in the system. To quantify the problem, we define an effective surface charge density of an imaginary impermeable surface, which mimics an actual semipermeable membrane and greatly simplifies the analysis. The effective charge depends on separation, generally differs from the real one, and could even be of the opposite sign. From the exact and asymptotic solutions of the nonlinear Poisson-Boltzmann equation, we obtain the distribution of the potential and of ions in the system. We then derive explicit formulae for the disjoining pressure in the gap and electro-osmotic velocity and show that both are controlled by the effective surface charge.

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