Habit Change of Monoclinic Hydroxyapatite Crystals Growing from Aqueous Solution in the Presence of Citrate Ions: The Role of 2D Epitaxy

Abstract

Calcium hydroxyapatite (HAp) has been crystallized from aqueous solutions in the presence of citrate ions, in two temperature intervals. At lower temperature, where citrate could form the stable 3D-ordered phase Ca-citrate-tetrahydrate (Ca-Cit-TH), only the monoclinic (P2 1 /c) HAp polymorph occurs and assumes the shape of fence-like aggregates, built by sharply [010] elongated lamellae dominated by the pinacoid {001}. This pronounced anisotropic growth habit is compared with the usually considered rod-like pseudo-hexagonal occurring in pure aqueous solution growth. The habit change is interpreted by assuming that 2D islands of Ca-citrate-tetrahydrate can be adsorbed as epimonolayers of thickness d 001 onto the different growth forms: {001}, {100}, 102 , {010}, and 101 of HAp. A comparison is made among the corresponding coincidence lattices, in order to explain on reticular basis the selective adsorption of citrate on the {001} HAp form. The role exerted by the 2D-epitaxially adsorbed Ca-Cit-TH as a "mortar" in the monoclinic HAp "brick" assembly is outlined as well.

Phosphoserine Functionalized Cements Preserve Metastable Phases, and Reprecipitate Octacalcium Phosphate, Hydroxyapatite, Dicalcium Phosphate, and Amorphous Calcium Phosphate, during Degradation, In Vitro

“…EDS analysis suggests that phosphoserine is incorporated directly into/onto these plate-like crystals. This modification could occur via multiple mechanisms, including non-specific binding (surface adsorption), preferential adsorption to crystal faces, or direct incorporation into the lattice [20,35,42].…”

“…These chelators improve the mechanical strength by reducing the crystal size, and extending the working/setting time. However, chelators produce a monomeric organic phase that binds to the mineral surface [16][17][18][19][20], rather than a molecularly intermixed composite [6,10] with macroscale interactions (i.e., polymeric or crosslinking) [21][22][23][24].…”

Pb2+ Uptake by Magnesite: The Competition between Thermodynamic Driving Force and Reaction Kinetics

“…After ageing, the nuclei of a secondary phase that cannot be filtered or centrifuged due to the size (Ø < 50 nm) are present in solution. These nuclei immediately deposit onto the {10.4} surfaces of MGS (Figure 7B) and they spread through a fast 2D island growth process (Figure 7A) [42,43]. The growth of this secondary phase (0.13 ± 0.04 nm/s) is more than one order of magnitude faster than the growth of MGS in the presence of Pb 2+ (0.010 ± 0.003 nm/s); thus, about two orders of magnitude faster than the growth of MGS in the absence of Pb 2+ (0.001 nm/s).…”

Impurity Effects on Habit Change and Polymorphic Transitions in the System: Aragonite–Calcite–Vaterite

“…Two solvents (water and formamide) interact with the growing NaCl-type crystals: (a) Bienfait et al , showed that water influences the {001} → {111} habit change, through the change of the growth mechanisms (spiral growth, 2D-nucleation) by the random adsorption of the water dipoles on the running step ledges; moreover, neither epilayers are formed by water on the {001} and {111} forms, nor do water molecules enter into the crystal bulk; (b) we proved − that as much as the formamide concentration increases, the octahedron {111} dominates on the cube {001}, due to the increasing coverage degree of 2D-epi highlands of the condensate 101 layers of formamide. Furthermore, these layers enter the {111} NaCl-type growth sectors, giving rise to anomalous mixed crystals alkali-halides/formamide, even if the growth solution is unsaturated with respect to the 3D-formamide crystal phase. Lastly, we worked on other systems where the 2D-epitaxy plays a fundamental role in the habit change: (a) KCl growing from aqueous solutions in the presence of Pb 2+ ions; (b) CaCO 3 polymorphs (calcite and aragonite) coexisting in the nacre structure through the epilayers of sericin; (c) hydroxyapatite crystals showing a rich variety of habits, when growing in the presence of citrate ions that generate epilayers of calcium citrate tetra-hydrate …”

“…Lastly, we worked on other systems where the 2D-epitaxy plays a fundamental role in the habit change: (a) KCl growing from aqueous solutions in the presence of Pb 2+ ions; (b) CaCO 3 polymorphs (calcite and aragonite) coexisting in the nacre structure through the epilayers of sericin; (c) hydroxyapatite crystals showing a rich variety of habits, when growing in the presence of citrate ions that generate epilayers of calcium citrate tetra-hydrate …”