Terminal crystalline solid solutions, solubility enhancements and T–X phase diagram of salicylic acid – 4-hydroxybenzoic acid

Abstract

The binary T–X phase diagram of salicylic acid (SA) and 4-hydroxybenzoic acid (4HBA) has been constructed from 20 °C to melting, revealing a partially miscible system with an eutectic composition of 27.3 mol% 4HBA in SA. Terminal crystalline solid solutions were obtained at the extremes of the phase diagram with solid-state miscibility limits below 0.4% at 20 °C. The limited phase boundaries could be captured experimentally by both DSC analyses at around melting temperature and solid–liquid equilibria studies at 20 °C in two solvent systems. The NRTL model was applied to regress phase boundaries and generate the final binary T–X phase diagram. The NRTL model was also used to regress solubility data, and reproduce the ternary SA/4HBA/solvent phase diagram at 20 °C and 1 atm. 4HBA was obtained as two crystal forms, viz. anhydrate and monohydrate. It is shown how the monohydrate of 4HBA is less miscible with SA in the solid state than the anhydrous form of 4HBA. As compared to pure SA and 4HBA, the crystalline solid solutions exhibited significant changes in physical properties that are relevant for organic and pharmaceutical materials in the context of impurity effects. A lattice incorporation of just 0.2 mol% 4HBA in SA caused a 10% reduction in melting enthalpy and a 66% solubility increase in 40 wt% MeOH in H₂O. The reasons for this thermodynamic effect are discussed.