Abstract
Hydroxyapatite, the main mineral phase of mammalian tooth enamel and bone, grows within nanoconfined environments and in contact with aqueous solutions that are rich in ions. Hydroxyapatite nanopores of different pore sizes (20 Å ≤ H ≤ 110 Å, where H is the size of the nanopore) in contact with liquid water and aqueous electrolyte solutions (CaCl2 (aq) and CaF2 (aq)) were investigated using molecular dynamics simulations to quantify the effect of nanoconfinement and solvated ions on the surface reactivity and the structural and dynamical properties of water. The combined effect of solution composition and nanoconfinement significantly slows the self-diffusion coefficient of water molecules compared with bulk liquid. Analysis of the pair and angular distribution functions, distribution of hydrogen bonds, velocity autocorrelation functions, and power spectra of water shows that solution composition and nanoconfinement in particular enhance the rigidity of the water hydrogen bonding network. Calculation of the water exchange events in the coordination of calcium ions reveals that the dynamics of water molecules at the HAP–solution interface decreases substantially with the degree of confinement. Ions in solution also reduce the water dynamics at the surface calcium sites. Together, these changes in the properties of water impart an overall rigidifying effect on the solvent network and reduce the reactivity at the hydroxyapatite-solution interface. Since the process of surface-cation-dehydration governs the kinetics of the reactions occurring at mineral surfaces, such as adsorption and crystal growth, this work shows how nanoconfinement and solvation environment influence the molecular-level events surrounding the crystallization of hydroxyapatite.
Highlights
Crystallization is a phenomenon that is strongly dependent on the environment in which it occurs.it is well-recognized that many features of crystal growth are affected by confinement [1,2,3,4,5,6,7]and solution composition [2,8,9,10,11,12]
This study reports Molecular dynamics (MD) simulations of HAP nanopores of different pore sizes (20 Å ≤ H ≤ 110 Å)
Fluoride ions are in a more hydrated state than chloride ions and, on average, they are closer to the Molecular dynamics of hydroxyapatite nanopores of different
Summary
Crystallization is a phenomenon that is strongly dependent on the environment in which it occurs.it is well-recognized that many features of crystal growth are affected by confinement [1,2,3,4,5,6,7]and solution composition [2,8,9,10,11,12]. Crystallization is a phenomenon that is strongly dependent on the environment in which it occurs. It is well-recognized that many features of crystal growth are affected by confinement [1,2,3,4,5,6,7]. Modification of the solvation environment through the addition to the solution of organics [14], peptides [15], polymers [16], or simple ionic salts [17,18,19,20], can influence the kinetics and thermodynamics of crystal nucleation and growth. A perfect example of a crystallization process that occurs within a confined environment and in multicomponent aqueous solutions is biomineralization [2,21]. Hydroxyapatite (HAP), molecular unit formula (Ca10 (PO4 ) (OH)2 ), is arguably the most important biomineral as it represents the main constituent of bone [22]
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