Anisotropic diffusion of water molecules in hydroxyapatite nanopores

Abstract

New insights into the dynamical properties of water in hydroxyapatite (HAP) nanopores, a model system for the fluid flow within nanosize spaces inside the collagen-apatite structure of bone, were obtained from molecular dynamics simulations of liquid water confined between two parallel HAP surfaces of different sizes (20 Å ≤ H ≤ 240 Å). Calculations were conducted using a coreshell interatomic potential for HAP together with the extended simple point charge model for water. This force field gives an activation energy for water diffusion within HAP nanopores that is in excellent agreement with available experimental data. The dynamical properties of water within the HAP nanopores were quantified in terms of the second-order water diffusion tensor. Results indicate that water diffuses anisotropically within the HAP nanopores, with the solvent molecules moving parallel to the surface twice as fast as the perpendicular direction. This unusual dynamic behaviour is linked to the strong polarizing effect of calcium ions, and the synergic interactions between the water molecules in the first hydration layer of HAP with the calcium, hydroxyl, and phosphate ions, which facilitates the flow of water molecules in the directions parallel to the HAP surface.

Self-diffusion coefficient of bulk and confined water: a critical review of classical molecular simulation studies

“…Prakash et al [232] characterized water transport properties by MD simulations applying different water potentials and found that the SPC/E [39] water together with the coreshell potential for hydroxyapatite is the most accurate combination for predicting diffusion properties. With these models, Prakash et al [231] calculated the anisotropic self-diffusion coefficient of the second-order diffusion tensor and found that the perpendicular component is significantly lower than the parallel ones for all the studied widths. The calculation of transport properties showed a dependency on the size of the nanopore, confirming the work of Pham et al [233], which showed this behaviour for water confined in hydroxyapatite pores from 2.0 to 6.0 nm wide at different temperatures.…”

Transport properties of water molecules confined between hydroxyapaptite surfaces: A Molecular dynamics simulation approach

“…In the first model hereafter named CS-water, the interactions between particles were represented by the core shell interatomic potential developed by de Leeuw and Parker for HAP and water systems [18,37], which includes electronic polarizability via the shell model of Dick and Overhauser. This model was used in our previous studies [11,12]. In this CS-water model the phosphate, hydroxyl group, and oxygen-hydrogen (Hw ) bonds are described as the sum of a Morse and a Coulombic potential, the phosphate and water bond angles by a harmonic potential, and non-bonded interactions by Buckingham potentials.…”

“…This clearly states that two different kind of diffusion mechanism are possible in struc-tures presenting a HAPwater interface. This may be an evolution from a quasi 1D diffusion process in the narrow pores to anisotropic diffusion of water molecules for lower confinement [12].…”

“…During biomin-eralization, bone-water interface plays an important role in the mechanism of bone reorganization [10]. Thus, the investigation of HAPwater interface materials received widespread attention to understand the chemical, physical and mechanical properties of these materials considering the confinement effect of water near the HAP surface [5,[11][12][13].…”

Nanoscale kinetics of amorphous calcium carbonate precipitation in H2O and D2O

“…As methods for preparing nanopipettes of smaller dimensions improve, this technique may be able to probe effects related to confinement, where mass transport properties can change dramatically. [52][53][54]…”