Thermodynamic analysis of moisture sorption isotherms.

This article illustrates how thermodynamic functions can be calculated from moisture sorption isotherms for water-solid systems. The materials evaluated include the drugs albuterol sulfate and indomethacin, the carbohydrates sucrose, raffinose, and trehalose, and the polymers poly(vinyl pyrrolidone) and microcrystalline cellulose. The results demonstrated that significant positive and negative deviations from ideality occurred for water and solid components, respectively, producing higher free energy materials than for the ideal mixtures. The calculations quantitatively discriminated major differences in thermodynamic activity and free energy for all of the solid phases studied, arising from the uniqueness in chemical interactions. This approach to analyzing moisture sorption data is invaluable in advancing our understanding of the physical chemistry of water-solid systems.