Solute transport in solvent-resistant nanofiltration membranes for non-aqueous systems: Experimental results and the role of solute-solvent coupling

Application of reverse osmosis (RO) and nanofiltration (NF) processes to non-aqueous systems can lead to several applications in the food and pharmaceutical industries. Transport studies for such system has been limited to pure solvent permeation with a few literature results for solute transport. This study is directed towards understanding solute transport aspects by using specific marker molecules in non-aqueous medium through polymeric membranes. Solute transport studies were performed using organic dyes and triglycerides in polar and non-polar solvents. Choice of these solutes was limited due to solubility constraints. Literature and our experimental observations point out the important role of solvent and membrane material for solute separation. For example, the rejection of Sudan IV (384MW organic dye) in n-hexane medium is about 25% at 15bar and that in methanol is about -10% at about 20bar for a hydrophobic (PDMS-based) membrane. However, for a hydrophilic polyamide-based NF membrane, the direction of separation is reversed (86% in methanol and 43% in n-hexane). Six different membranes (hydrophilic and hydrophobic) were compared for rejection characteristics. It was concluded from this comparison that there is a distinct analogy between aqueous and non-aqueous systems with respect to the ratio of the molar volumes of the minor and the major permeating species. However, several other factors like the solute-membrane interactions and the solute charge/conformation in different solvents must also be addressed. From our experimental data with two types of membranes it is clear that coupling of the solute and solvent fluxes cannot be neglected. Two traditional transport theories that consider coupling were evaluated with literature and our experimental solute permeation data. The Spiegler-Kedem model was used to obtain the convective and diffusive contributions, however, the model does not have specific parameters for solute-membrane interactions. Direct diffusive flux data was also incorporated in the model. The pore flow model, which considers convective coupling and interaction parameter, was also used.