Water vapor absorption into amorphous sucrose-poly(vinyl pyrrolidone) and trehalose-poly(vinyl pyrrolidone) mixtures

Previous studies from this laboratory suggested that a solution model (Flor y-Huggins equation) modified by a free volume model (Vrentas equation) coul d satisfactorily describe water absorption into an amorphous solid composed of a sugar or a polymer, This paper has extended the studies of single sol utes to binary mixtures of trehalose-and sucrose-poly(vinyl pyrrolidone) (t rehalose-PVP and sucrose-PVP, respectively) either co-lyophilized or indivi dually lyophilized and then physically mixed. Water vapor absorption isothe rms of the binary mixtures were determined at 30 degreesC. Co-lyophilized P VP-sugar mixtures take up essentially the same amount of water as predicted by the weight average of individual isotherms, whereas sugar crystallizati on is significant retarded in the molecular dispersions. The sugar-PVP inte raction, as reflected in the Flory-Huggins chi interaction parameter, was e stimated by fitting the high relative pressure (p/p(0)) region of the isoth erm, at which the system is in a liquid state, with a three-component Flory -Huggins-type model. The estimated sugar-water PVP-water, and sugar-PVP int eraction parameters suggest that the solute-water interactions are not sign ificantly affected by the sugar-PVP interaction; that is, the solute-water interaction parameters in a binary solute system are similar to those in th e corresponding single solute systems. Based on these interaction parameter s, the sucrose-PVP interaction appears to be stronger than that of trehalos e-PVP. Manipulation of the interaction parameters suggest that the water va por absorption isotherm is not a sensitive indicator of possible sugar-PVP interactions. Density, glass transition temperature, T-g, and the heat capa city change, DeltaC(p), at T-g were determined to estimate the excess water absorption energy due to the plasticizing effect of water using the struct ural relaxation model, as described by Vrentas. Results suggest that PVT is a better antiplasticizer for sucrose than for trehalose. Consequently, the excess free energy arising from structural relaxation was disproportionall y reduced by the presence of PVP in these molecular dispersions. Finally, t he entire isotherms of co-lyophilized sugar-PVP mixtures are reasonably des cribed with an extended three-component Flory-Huggins model and Vrentas gla ss structural relaxation model. (C) 2001 Wiley-Liss, Inc. and the American Pharmaceutical Association.