Structure of the Potassium Dihydrogen Phosphate (KDP) Solid–Solution Interfaces

Crystallization from solution is a fundamental process in both natural systems and industrial applications, yet the molecular-level mechanisms governing crystal growth at solid–liquid interfaces remain poorly understood. In this work, we employ molecular dynamics simulations to investigate the interfacial structure and dynamics of potassium dihydrogen phosphate crystals in aqueous solution, with a particular focus on the prismatic (100) and pyramidal (101) surfaces. Our results reveal distinct differences in interfacial behaviour depending on crystallographic orientation. A smooth (100) surface exhibits pronounced kinetic stabilisation, characterised by minimal dissolution; the introduction of defects leads over timescales of several µs to the formation of a structured, layered interface consisting of a distorted crystalline layer, an intermediate ordered layer, and a solution-like region. This structure is accompanied by notable charge separation between K+ and H2PO4- ions, giving rise to a pronounced electric double-layer that may impact interfacial dynamics, or attract cationic impurities. In contrast, the (101) surface undergoes spontaneous dissolution without defect introduction, and appears able to achieve an equilibrium solubility, although still on a µs timescale. The interface is more diffuse and dynamically evolving, featuring a much-reduced electric double-layer. The layered structure within both interfaces is examined and compared with experiments. These findings suggest a molecular-level explanation for the observed anisotropic growth behaviour of KDP crystals and highlight the critical role of interfacial structure in governing crystallization from solution. More broadly, this work demonstrates that the solid–liquid interface can, in systems such as KDP, be a spatially extended region, whose structure and properties must be explicitly considered in predictive models of crystal growth.