Model thin film composite membranes for forward osmosis: Demonstrating the inaccuracy of existing structural parameter models

•Forward osmosis (FO) membranes have intrinsic and effective structural parameters, S.•A model FO membrane whose intrinsic S (Sint) value is known a-priori is fabricated.•The membrane is tested in FO to yield effective S (Seff) values.•Seff values are greater than Sint and they also vary widely with test conditions.•Modifications to the existing approach of characterizing S are needed.

Structural parameter, or S, is a widely used metric for assessing a membrane׳s suitability for use in osmotic processes. Currently, S is only calculated by an indirect approach using models derived from the governing flux equation. Our prior work has shown that this method is fraught with inaccuracy and that the true, or intrinsic, S value can differ substantially from the effective S value determined by fitted parameter methods. In this work, we prove that hypothesis definitively by using a membrane with a structural parameter that is known a-priori. We synthesized a thin film composite membrane using a well-characterized porous support membrane. The material we chose as the support is a polycarbonate track-etched membrane, which has an easy-to-characterize thickness, porosity, and tortuosity. These membranes have an intrinsic structural parameter of 133 μm but an effective structural parameter that ranged widely from 159 μm to 1950 μm when quantified using conventional methods. This finding should cause concern regarding the validity of existing mass transfer models. New approaches are necessary to fairly compare new membranes designed for osmotic processes.