Study of the Formation of Precursor Clusters in an Aqueous Solution of KH₂PO₄ by Small-Angle X-ray Scattering and Molecular Dynamics

Abstract

The structure of an aqueous solution of potassium dihydrogen phosphate (KH2PO4, KDP) was studied by small-angle X-ray scattering (SAXS) and molecular dynamics (MD). According to SAXS data, the octameric species (KH2PO4)8 are formed in solution in addition to K+, (H2PO4)−, and KH2PO4, while the presence of other types of oligomers is not observed. When the temperature drops below the saturation temperature (~60 °C), the volume fraction of octamers increases sharply, reaching 50% at 4 °C. The results of MD calculations of the temporal stability relationships of dimers (KH2PO4)2, tetramers (KH2PO4)4, and octamers (KH2PO4)8 show that the dimers and tetramers disintegrate rapidly (50–100 ps), while the octamers remain stable. A comparative analysis of the bonds between the octamers and the KDP crystal lattice was carried out when the octamer was inserted during crystal growth in the directions [001] and [100]. The possible relationship of the obtained results with the changes in the anisotropy of growth rates (habitus) of KDP crystals at different degrees of supersaturation is discussed.

Keywords

• three-stage crystallization model
• nonclassical crystallization
• precursor clusters
• water solution
• small-angle X-ray scattering
• KDP