Selective adsorption of octadecylamine hydrochloride on the surface of KCl crystal: adsorption energy based on density functional theory

Flotation has become a significant technique for the KCl separation from NaCl-containing complex systems, but the true nature of adsorption selectivity of collector on the crystal surface of KCl is still unclear. In this study, using density functional theory calculations, we investigated the adsorption behaviors of a commonly used collector, octadecylamine hydrochloride (ODA), on the crystal surfaces of KCl and NaCl, respectively. The results demonstrate that the structure of built crystals for calculation is validity and the crystal surfaces containing two-layer atoms are most stable. All adsorption energies of ODA⁺ cation and H₂O molecule on the crystal surfaces of KCl and NaCl are negative with the decreased absolute value according to the sequence of KCl-ODA⁺>NaCl-H₂O > KCl-H₂O > NaCl-ODA⁺, suggesting that the interaction between ODA and KCl is stronger than that in the other considered systems. The differences of interaction energies demonstrate the adsorption selectivity of ODA on KCl, resulting in the selectively flotation. Therefore, we construct accurate models to simulate the actual crystal surface structures of KCl and NaCl, and we provide adsorption energies as complementary evidences to interpret the floatability of KCl from the NaCl mixture, which can help to design new collectors for the flotation of soluble salts.