SORPTION OF ORGANIC-SOLVENTS INTO DENSE SILICONE MEMBRANES .1. VALIDITY AND LIMITATIONS OF FLORY-HUGGINS AND RELATED THEORIES

Sorption isotherms of various organic solvents and aqueous mixtures into dense polydimethylsiloxane (PDMS) membranes have been analysed and the results are compared with Flory-Huggins theory, already reported to hold for solvent sorption into elastomeric materials. The original Flory-Huggins equation, as well as more complex versions derived later by different authors (variable interaction parameter, elastic contribution, modified entropic contribution) have been screened; it appears that although excellent agreement is offered by the Flory-Huggins simplest expression for good PDMS solvents (e.g. hydrocarbons and halogenated hydrocarbons), none of the modified Flory-Huggins theories leads to satisfactory data interpretation for poor solvents (ketones, alcohols), unless at least three adjustable parameters are used for the enthalpic contribution term. Cluster formation at high solvent activity, considered on the basis of Zimm-Lundberg clustering analysis, is supposed to explain the observed deviations for poor solvents. A semi-empirical power law (Freundlich type) gives the best fit in the latter case.