MEMBRANE-WATER STRUCTURAL INTERACTIONS IN REVERSE OSMOSIS TRANSPORT

Reverse-osmotic water permeabilities, equilibrium water sorption levels, and rates of approach to sorption equilibrium were measured for a series of polymers, including hydroxyethyl methacrylate (HEMA), copolymers of HEMA and ethyl methacrylate (EMA), cellulose acetate, cellulose nitrate, and poly(urethans). Pronounced equilibrium solvent clustering behavior was observed for these systems as vapor saturation was approached in sorption experiments. However, clustering tendency was not found to be a function of total membrane water content at saturation but rather appears to be a function of the chemical nature of the polymer in question. Moreover, clustering of water molecules in (relatively) hydrophobic membranes resulted in low effective diffusivities (reverse osmotic permeability divided by equilibrium water content) whereas clustering in hydrophilic membranes led to higher effective water diffusivities. Clustering tendency was not as strong in the case of the weakly interacting membranes (i.e., the cellulose acetates). These conclusions were supported by theoretical diffusivity calculations. Predictions were based on analyses of transient sorption data, employing a dual-mode sorption model, and considering ordinary Fickian diffusion with simultaneous first-order reversible penetrant localization at water-binding sites in the polymer matrices. Means were found for correcting these diffusivity predictions to those values obtained experimentally under reverse osmosis conditions by accounting for the nonideality of the water flux under the latter conditions. © 1969, Taylor & Francis Group, LLC. All rights reserved.