Temperature-induced pH changes govern hydrate transformation during cooling crystallization of potassium acid phthalate

Abstract Achieving greater control on crystallization to obtain desired crystal structure and morphology is a significant challenge in material synthesis. The highly stochastic nature of crystallization makes it necessary to conduct thorough experimental and computational investigations of crystalline materials. Polymorphic transformation, including hydrate formation during crystallization of potassium acid phthalate (KAP) is one such example. The hydrated form of crystals can appear during crystallization when water interacts strongly with functional groups of solute molecules, leading to its inclusion into the crystal lattice. Here, we show the temperature-dependent transformation of anhydrous KAP (form-I) to monohydrate KAP (form-II) crystal. Such polymorphic and associated morphological changes as a function of temperature and supersaturation are studied using in-situ hot-stage optical microscopy and validated with morphology domain analysis. While the measured growth rates and morphologies do not vary significantly with supersaturation, they show a stronger temperature dependence due to the formation of hydrated forms. The computational results uncover the previously unknown effect of temperature-dependent pH increase, which causes deprotonation of the carboxylic group leading to destabilization of [110] facets. The appearance of hydrated form seems to be due to temperature-induced pH changes. These results provide fundamental insights into the mechanism of hydrate transformations during cooling crystallization.