THE USE OF SOLUTION THEORIES FOR PREDICTING WATER-VAPOR ABSORPTION BYAMORPHOUS PHARMACEUTICAL SOLIDS - A TEST OF THE FLORY-HUGGINS AND VRENTAS MODELS

The limitations of traditional gas adsorption models for describing wa ter vapor sorption by amorphous pharmaceutical solids are described an d an alternative approach based on polymer solution theories is propos ed. The approach is tested by comparing a priori predicted isotherms w ith 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 s tate (i.e., above its glass transition temperature). However, a newer model developed by Vrentas and co-workers, which takes into account th e 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.