THE DEVELOPMENT OF A PREDICTIVE METHOD FOR THE ESTIMATION OF FLUX THROUGH POLYDIMETHYLSILOXANE MEMBRANES .1. IDENTIFICATION OF CRITICAL VARIABLES FOR A SERIES OF SUBSTITUTED BENZENES

The steady-state flux of 31 monosubstituted benzene derivatives was determined through polydimethylsiloxane membranes using isopropyl alcohol as the solvent. These diffusants constituted a diverse group of compounds possessing a wide range of hydrophobic, steric and electronic characteristics. Various parameters representing these physico-chemical properties were employed to develop an empirical model capable of relating the flux to these characteristics of the substituent on the benzene ring. Molecular volume and hydrophobicity greatly influenced the flux, while electronic properties of the substituent had a relatively small, but significant, effect on the flux. Higher diffusion rates were observed for compounds possessing lipophilic, exiguous, electron-donating substituents. Molar refractivity was the best descriptor of molecular volume while cyclohexane-water fragmental constants were better predictors of hydrophobicity.