On negative retentions in organic solvent nanofiltration

Some recent literature reports negative retentions in organic solvent nanofiltration. However, this phenomenon is little understood and requires systematic evaluation. The aim of this work is to extend the systematic investigations of solute transport through a dense PDMS-based polymeric OSN membrane. Experiments with four different solute classes, namely polyethylene glycol (194–590 Da), linear carboxylic acid (228–340 Da) polystyrene (MW=741 Da) and n-alkanes (142–339 Da) were performed. These solutes are dissolved in n-hexane, toluene, isopropanol or methanol. Depending on the solvent and the solute, there exist considerable differences in retentive behavior. Above mentioned negative retentions could be observed for linear carboxylic acids or n-alkane as solutes dissolved in isopropanol or methanol. The generally observed positive retention was measured for polyethylene glycol and polystyrene as solute. This systematic investigation proofs that strong solute–solvent–membrane interactions can cause a solubility determined solute transport resulting in up-concentration of the larger solute into the permeate. The surprising observation of larger transport rates for the larger solute molecules correlates with the growing contribution of the increasing partial molar volume to the chemical potential.