Water mimicry enables the formation of a solid solution of caffeine and theophylline sulphate and a new type of non-stoichiometric hydrate

A solid solution of caffeine and theophylline is realized as sulphate salt hydrate. The molecules enter the structure with one and two equivalents of water, respectively, creating a novel type of non-stoichiometric hydrate. The solid solution is more thermally stable and enables an increased dissolution rate in water for caffeine and theophylline than the respective hydrated sulphate salts. In spite of the increase in solubility, the solid solution shows reduced permeability of caffeine through a synthetic skin membrane when tested against a physical mixture of the parent salts.