Abstract
The solubilites of phenylbutazone polymorphs, pure α, β and δ, were measured over a temperature range of 35°C, from 50°C to 15°C. Both the Van't Hoff plots of lnX2 against 1/T and the Hildebrand plots of lnX2 against lnT are nonlinear, where X2 is the mole fraction solubility at an abslute temperature, T. The data were treated by multiple regression analysis according to the equation: ln(solubility)=(-Q/R)(1/T)+(b/R)lnT+c, where a, b and c are constant and R is the gas constant. From a, b and c, the heat of solution ΔH*2(soln) may be calculated e. g. ΔH*2(soln)=a+bT, and ΔCp=b, where ΔCpis the difference between the heat capacities of the liquid and the solid forms of the solute. Also, the heat of dissolution, ΔH2(diss), and dissolution activation energy, Ea, for these polymorphs were obtained by using the initial disolution rate at various temperatures. Their ideal mole fraction solubilities (α2) were calculated from the heats of fusion and the melting points of these polymorphs. The activity coefficient, γ2, the partial molal heat of mixing, ΔH2(mix), and the differential heat of solution, ΔH2(soln) may also be calculated from the heat of fusion, the activity coefficient, and the mole fraction solubility. The relationships between these ΔH*2(soln), ΔH2(diss) and Ea were discussed.
