Osmotic properties of a cation exchange membrane in contact with an aqueous solution of a non-electrolyte: influence of preferential solvation of counterions

A system in which a membrane separates two aqueous solutions of a non-electrolyte of different but constant compositions is used to demonstrate a continuous transition from positive to negative osmosis and from positive to negative retention coefficients in filtration experiments. The membrane is a cation exchange membrane loaded with two deliberately chosen counterion species which compensate the electrical fixed charges of the membrane matrix. The parameter of the experiments is the mole fraction of one of the mobile counterion species in the pore fluid of the membrane. The counterion species which is preferential solvated by the non-electrolyte acts as a carrier for that component across the membrane. Increasing the mole fraction of the carrier ion species in the pore fluid causes two effects: it increases the transport rate of the non-electrolyte across the membrane as well as its coupling with the osmotic volume flow across the membrane. This leads to the stated changes of the transport properties of the membrane.