Effect of molecular adsorption on water permeability of microporous membranes

The rate of water permeation through a microporous membrane is affected markedly by adsorption of hydrocarbon impurities and/or trace amounts of organic ions such as H(CH 2 17CO 2 - and H(CH 2) 16NR 3 +. Neutral and cationic hydrocarbon impurities are physisorbed primarily at the microcapillary outlets on the low-pressure side of an electropositive filter. Adsorption of the former at these critical sites causes flow through the affected capillaries to stop, and total flow to decrease accordingly, whereas adsorption of the latter at these sites causes a marked increase in total flow. Organic anions and molecules with electron donor substituents are chemisorbed on the high-pressure side of an electropositive filter. Those molecules that are held only by monodentate adsorption migrate through the microcapillaries to the low-pressure side of the filter as in ion-exchange chromatography, during which time water permeation is impeded significantly. Multidentate adsorbed molecules, such as gelatin or polyvinyl alcohol, are relatively immobile. Filters modified on one side only by such molecules exhibit marked anisotropic permeability, i.e., flow in one direction is much greater than in the other.