On The Structural Compatibilities of Membrane Filters

Abstract

Compatibility relationships between solid polymerics, such as membranes, and the liquids they encounter reflect the similarity of their chemical bondings as defined by their cohesive energy densities. Thus, two liquids, each sufficiently dissimilar from the filter in cohesive energy density as to provide compatibility may yield an incompatible fluid on mixing. Small quantities of components may, therefore, produce unsuspected and subtle, but significant, incompatibilities. For this reason, the customary gross compatibility charts may be insufficient guides. An examination was made of three different membrane types (selected from one manufacturer in order to avoid invidious comparison) in each of several different liquids, e.g., sesame seed oil, tri-t-butyl phosphate, and aqueous benzyl alcohol solutions of varying strengths. Equilibration times were experimentally defined. Contact periods of 48 hr were generally found to be more than ample. The compatibility results are described in terms of tensile and elongation properties, bubble point, flow, and dimensional changes in the filler. Scanning electron micrograph photos show the changes undergone in filter morphology. None of these, unfortunately, serves as a convenient index of incompatibility. Organism retention alone suffices. The property changes usually involved decreasing flows—a consequence of plasticization—and decreasing bubble point values, a result of molecular chain stress relaxation. The findings argue that compatibility complexities and subtleties require very specific assessments involving the particular filter type and the precise solutions it is to encounter. This can be done in terms meaningful to organism retention by the user within the framework of filter validations.