Solid–Liquid Equilibrium of Theophylline in Solvent Mixtures

In this work, the thermodynamics of theophylline anhydrate and monohydrate were investigated. The solubilities of theophylline anhydrate and monohydrate in methanol–water and isopropanol–water mixtures were measured from 278.15 K to 303.15 K. Then the solubility data were correlated with the combined nearly ideal binary solvent/Redlich–Kister (CNIBS/R–K) model and the modified Jouyban–Acree model, respectively. The water activity aT that corresponds to the transformation between hydrate and anhydrate forms of theophylline was investigated. The water activity was accurately calculated by the Wilson equation and correlated by the van’t Hoff equation. The transformation enthalpy and entropy were calculated as well. The ternary phase diagrams of the theophylline–methanol–water system and theophylline–isopropanol–water system thus were obtained.