Fundamental origin of flux non-linearity in organic solvent nanofiltration: Formulation of a thermodynamic/diffusion framework

Abstract

In this paper we critically discuss the hypothesis of membrane compaction under pressure, which has been invoked to explain solvent flux vs. Δp non-linearity in organic solvent nanofiltration (OSN) experiments with dense polymer membranes. Although physically sound, this hypothesis does not have experimental support. To demonstrate that the molecular origin of flux non-linearity is purely thermodynamic, we propose a thermodynamic-diffusion framework which describes solvent transport in OSN membranes in terms of the concentration gradient produced by the applied pressure across the membrane. Solvent diffusion coefficient in the membrane increases with increasing Δp, which further confirms that flux decline is not related to membrane compaction. The developed framework allows to quantify both frame of reference and non-ideal thermodynamic effects on solvent diffusion coefficients in OSN membranes. This study demonstrates that the solution-diffusion model, if properly corrected for frame of reference (i.e., convection) and non-ideal effects, provides a satisfactory description of small molecule transport in OSN membranes, without the need to resort to more complicated transport models.