Computer simulations of the adsorption of citric acid at hydroxyapatite surfaces

Computer modelling techniques are employed to investigate the adsorption of the citric acid molecule to a range of hydroxyapatite surfaces, and new interatomic potential parameters for the apatite/adsorbate interactions are presented. Citric acid coordinates mainly to the surfaces through interaction between its oxygen atoms to surface calcium ions, followed by hydrogen-bonded interactions to surface oxygen ions. Bridging between two or more surface calcium ions is the preferred mode of adsorption, when the geometry of the surface allows it, and multiple interactions between the surfaces and the functional groups of the citric acid molecule lead to the largest adsorption energies. The calculated adsorption energies range from 80 kJ mol−1 on the {0 0 0 1} surface to 280–330 kJ mol−1 on the {101¯0} and {11¯21} surfaces. Citric acid would thus more strongly inhibit growth of these latter surfaces than the {0 0 0 1} growth, leading to an elongated morphology in the c-direction of the apatite crystal.