Thermodynamic equilibrium of hydrocortisone form I in different solvent systems

The solubility of hydrocortisone form I in seven pure solvents and one kind of binary solvent system from (278.15-323.15) K was experimentally determined by using a dynamic method under atmospheric pressure. The results show that the solubility of hydrocortisone form I monotonously increases with increasing of temperature and reaches the maximum at ethanol molar fraction of 0.60 in binary solvent mixture of ethyl acetate and ethanol at constant temperature. The experimental solubility data in pure solvents were correlated by the modified Apelblat equation, the λh equation and the van't Hoff equation while the experimental solubility data in binary solvent systems were correlated by the modified Apelblat equation and the CNIBS/R-K model. In addition, the apparent thermodynamic properties (enthalpy, entropy, and Gibbs energy) of mixing processes and dissolution processes were also calculated based on the NRTL model. The calculated results indicate that the dissolution process of hydrocortisone form I in tested solvents is spontaneous and endothermic.