Thermodynamics of molecular solids in organic solvents

The thermodynamics of solid-solution solubility are examined using data for two different compounds: butyl paraben and benzoic acid. Solubility data in different solvents are used to estimate melting properties of the pure solid solutes, and are used to estimate the solid state Gibbs free energy relative to the super cooled melt. The relation to melting data experimentally determined is analysed, and solution activity coefficients are calculated. The work shows that there is a strong relation between solid-liquid solubility data and thermodynamic data of the pure solute. For these compounds, the melting temperature of the pure solute can be fairly accurately estimated by extrapolation of solid-liquid solubility data up to mole fraction equal unity. The estimation of the melting enthalpy is less successful showing deviations in the order of 20% to 30% from the experimental values determined by differential scanning calorimetry. For butyl paraben and benzoic acid, the best estimation of the solid state Gibbs free energy is obtained if DSC determination of melting properties is combined with an estimation of the melt-solid heat capacity difference versus temperature relation by correlation to solubility data