MEMBRANE PORE CHARACTERIZATION - COMPARISON BETWEEN SINGLE AND MULTICOMPONENT SOLUTE PROBE TECHNIQUES

Two methods of membrane characterization are compared. Both methods are based on the permeation of dilute solutions containing probe solutes through a membrane. The first method characterizes the membrane by the successive permeation of dilute solutions containing a single solute of varying molecular weight. The second method uses one solution containing a mixture of these solutes to obtain a molecular weight cut-off for the membrane. The use of mixtures tends to under-estimate the molecular weight cut-off and pore size of the membrane. Membrane pore sizes obtained from mixtures are relative and depend on the composition of the feed mixture used in the characterization tests. Larger components in the mixture were found to have the greatest influence on membrane performance. Differences between characterization methods were quantified by calculating membrane pore sizes based on a radially averaged restricted transport model. For a 10,000 molecular weight cut-off membrane, pore sizes of 35.2 angstrom for single solutes and 28.0 angstrom for a five component mixture were obtained. Based on the Stokes-Einstein radius of polyethylene glycols (PEG), the molecular weight equivalent for absolute retention (100% separation) was 17,700 daltons for single solutes and 11,200 daltons for a five component mixture. This difference in pore sizes could not be attributed to experimental error, nor to differences in the analysis method. These results indicate that solute-solute interferences occur in the permeation of mixed solutes through membrane pores. The discrepancies are explained by different lag times for solutes translating through the membrane. Larger solutes hinder the movement of smaller ones resulting in higher separations. If mixtures are to be used in characterizing membranes, molecular weight cut-off measurements and restricted transport models should be altered to account for solute-solute interferences. The single solute method combined with automation techniques is a preferred choice for characterizing membranes.