Solubility enhancements through crystalline solid solutions, the non-linear Tammann diagram and the T–X phase diagram of salicylic acid–benzoic acid

Abstract

The binary T–X phase diagram of salicylic acid and benzoic acid has been experimentally determined from 25 °C to melting. It is found to be dominated by two crystalline solid solutions exhibiting solid-state miscibility limits of 10.9 mol% and 82.5 mol% benzoic acid in salicylic acid, respectively, at 25 °C. The eutectic composition was determined to be 60.0 mol% benzoic acid in salicylic acid, with a eutectic temperature of 112.69 °C. An example of non-linearity in a Tammann plot is demonstrated and the reasons for the deviations are shown to stem from the temperature dependence of the solvus and compositional non-uniformity in the material. The formation of the crystalline solid solutions resulted in significant thermodynamic changes relative to the pure components. For example, the solubility of salicylic acid in a solvent mixture composed of 40 w% methanol in water increased by 87% up to the solid-state miscibility limit. The implications of these significant thermodynamic alterations are discussed within the context of crystallization of organic compounds in the presence of structurally-similar impurities.