Salt Separation by Charged Gel-Filled Microporous Membranes

A series of membranes composed of a microporous polypropylene substrate filled with a polyelectrolyte gel of poly(N-benzyl-4-vinylpyridinium chloride) of different gel polymer volume fractions has been used to study a pressure-driven separation of mono-monovalent salts from aqueous solutions. The experimental results are compared with that obtained from theoretical calculations based on the Teorell−Meyer−Sievers model of charged membranes combined with the Manning−Oosawa counterion condensation theory being used to estimate the membranes effective charge density. For counterions of low specific binding to the polyelectrolyte such as chloride and fluoride, the theoretical predictions were found to be in a good agreement with the experiment over a wide range of the gel polymer volume fractions. For strong binding counterions such as iodide, the separation is substantially lower than that predicted and is accompanied by an almost 3-fold increase in flux suggesting a microphase separation in the gel. The implications of this study on the design of gel-filled membranes are discussed.