Solution thermodynamics of simvastatin in pure solvents and binary solvent mixtures

The solubility data of simvastatin in five pure solvents and binary solvent mixtures of isobutyl acetate and n-heptane were experimentally determined by a gravimetrical method under atmospheric pressure. It was found that the solubility of simvastatin increases with temperature rising in all investigated solvents. In binary solvent mixtures, the solubility of simvastatin increases with the decreasing of the mole fraction of n-heptane. The Solubility-Polarity model was modified and used to interpret the dissolution behaviors of simvastatin in binary solvent mixtures. The experimental solubility data in all investigated solvents were correlated by empirical models with the relative average deviation percentage between experimental and calculated solubility less than 5%. The thermodynamic properties of simvastatin, including the Gibbs energy change, the entropy and the enthalpy of dissolution process, were also calculated. The results indicate that the dissolution process of simvastatin is a spontaneous, endothermic and entropy-driving process in all solvent systems.