lower fusion enthalpy modulated differential calorimetric mdsc data in fig3 shows that polymorph i has a melting endotherm onset t onset at 63 2 u00b0 c and a fusion enthalpy u03b4h f of 65 6 j g as compared to the corresponding properties for polymorph ii 62 2 u00b0 c and 81 5 j g respectively the differences in free energy between the polymorphs were measured as a function of temperature in order to determine the transition temperature and relative stability versus temperature isothermal competitive co slurries of polymorph i and ii 1 1 ratio at 5 20 and 35 u00b0 c showed that polymorph i converts to polymorph ii at all three temperatures indicating that polymorph ii is the more stable form over the temperature range of 5 35 u00b0 c fig4 shows the thermodynamic equilibrium solubility of the anhydrous polymorphs in anisole at 6 u00b0 c and 25 u00b0 c at both temperatures polymorph ii has the lower solubility indicating that polymorph ii is the lower energy form at temperature 6 25 u00b0 c anisole was selected as solvent based on good stability and measurable solubility of bimatoprost acid polymorphs in the studied temperature range solubility studies could not be performed at temperatures above 40 u00b0 c in order to determine the enantiotropic polymorph transition temperature because bimatoprost acid transforms to an oil in anisole another solvent nitromethane was selected to detect the solubility of bimatoprost acid at 9 and 22 u00b0 c at 9 u00b0 c the solubility of polymorph i 4 1 u00b1 0 3 mg ml is significantly higher than the solubility of polymorph ii 3 3 u00b1 0 2 mg ml which suggests the polymorph ii is more stable than.