Molecular dynamics simulations of the interaction of citric acid with the hydroxyapatite (0001) and (011ˉ0) surfaces in an aqueous environment

Abstract

Molecular dynamics simulations are employed to investigate the adsorption of citric acid molecules to the (0001) and (010) surfaces of hydroxyapatite. An aqueous environment is added through the explicit introduction of water molecules in the simulation cell. The citric acid is found to attach to the surfaces, rather than remain in the bulk water and the calculated average adsorption energies are +291.4 kJ mol⁻¹ and −17.4 kJ mol⁻¹ for the (0001) and (010) surfaces respectively. Citric acid thus adsorbs to the (010) surface and hence would inhibit growth of this surface more effectively than growth of the (0001) plane with which it does not interact strongly in an aqueous environment where the water competes with the citric acid for adsorption. The implication is that the hydroxyapatite crystal would grow more rapidly in the [0001] direction than in the [010] direction in the presence of citric acid, leading to elongation in the c-direction and more pronounced expression of the (010) surface in the hydroxyapatite morphology.