Solutes in solvent resistant and solvent tolerant nanofiltration: How molecular interactions impact membrane rejection

Solvent resistant nanofiltration (SRNF) and solvent tolerant nanofiltration (STNF) are energy-efficient membrane-based technologies to purify solvents or water-solvent mixtures, respectively, and/or to recover solutes from these streams. SRNF and STNF are rapidly gaining importance as they contribute to process intensification and to a circular economy. Whilst solute-solvent-membrane interactions determine the membrane performance, they are often not sufficiently taken into account, which possibly leads to misinterpretation of the obtained data. This review focusses specifically on the environment-dependent behavior and properties of commonly used solutes (dyes, oligomers, n-alkanes, esters, triglycerides, sugars and inorganic salts) through a theoretical physicochemical lens that focusses on molecular interactions. It reveals that the assumed fixed solute properties do not necessarily coincide with those in the chosen solvent medium. Indeed, solutes can adopt different configurations, charges or sizes in different (hydro-)organic media and this can significantly impact the membrane performance. By highlighting these complex solute-solvent interactions from a theoretical point of view, this review aims at more correctly interpreting, describing and representing membrane performance data, and to facilitate the selection of suitable solutes for a specific separation. Furthermore, the most adequate characterization techniques per solute class are discussed and some solutes that are less environment-dependent are proposed as more appropriate alternatives. In addition to achieving a better understanding of the fundamental solute-solute and solute-solvent interactions, the outlined insights aim at assisting membrane selection by end-users and at promoting improved data reliability.