  We examine theoretically porous membranes in aqueous mixtures. We show that
large membrane pores can be reversibly gated from `off' (co-solvent-rich, poor
conductor of ions and other solutes) to 'on' (water-rich, good conductor)
states by an external potential. The transition voltage or charge for switching
depends on the membrane hydrophilicity/hydrophobicity, the salt content, the
preferential solvation of the salt ions, and the temperature. These parameters
also determine whether the filling transition is abrupt or gradual.
