The Use of Solution Theories for Predicting Water Vapor Absorption by Amorphous Pharmaceutical Solids: a Test of the Flory-Huggins and Vrentas Models

The limitations of traditional gas adsorption models for describing water vapor sorption by amorphous pharmaceutical solids are described and an alternative approach based on polymer solution theories is proposed. The approach is tested by comparing a priori predicted isotherms with literature data for the poly(vinylpyrrolidone)(PVP)-water system. The well-known Flory-Huggins model is able to describe the water vapor sorption isotherm only when the PVP-water mixture is in the rubbery state (i.e., above its glass transition temperature). However, a newer model developed by Vrentas and coworkers, which takes into account the plasticizing effect of water on the polymer, is able to describe the entire form of the isotherm. Consideration of the parameters in this model allows a number of critical variables to be identified and also enables the characteristic shape of the water vapor sorption isotherm to be explained.