Classical molecular dynamics (MD) simulations have been employed to study the interaction of the saccharides glucuronic acid (GlcA) and N-acetylgalactosamine (GalNAc) with the (0001) and (011̄0) surfaces of the mineral hydroxyapatite (HAP). GlcA and GalNAc are the two constituent monosaccharides of the glycosaminoglycan chondroitin sulfate, which is commonly found in bone and cartilage and has been implicated in the modulation of the hydroxyapatite biomineralization process. MD simulations of the mineral surfaces and the saccharides in the presence of solvent water allowed the calculation of the adsorption energies of the saccharides on the HAP surfaces. The calculations show that GalNAc interacts with HAP principally through the sulfate and the carbonyl of acetyl amine groups, whereas the GlcA interacts primarily through the carboxylate functional groups. The mode and strength of the interaction depends on the orientation of the saccharide with respect to the surface and the level of disruption of the layer of water competing with the saccharide for adsorption sites on the HAP surface, suggesting that chondroitin 4-sulfate binds to the layer of solvent water rather than to HAP.

We have investigated the thermodynamics of mixing between aragonite (orthorhombic CaCO 3 ) and strontianite (SrCO 3 ). In agreement with experiment, our simulations predict that there is a miscibility gap between the two solids at ambient conditions. All Sr x Ca 1Àx CO 3 solids with compositions 0.12 < x < 0.87 are metastable with respect to separation into a Ca-rich and a Sr-rich phase. The concentration of Sr in coral aragonites (x $ 0.01) lies in the miscibility region of the phase diagram, and therefore formation of separated Sr-rich phases in coral aragonites is not thermodynamically favorable. The miscibility gap disappears at around 380 K. The enthalpy of mixing, which is positive and nearly symmetric with respect to x = 0.5, is the dominant contribution to the excess free energy, while the vibrational and configurational entropic contributions are small and of opposite sign. We provide a detailed comparison of our simulation results with available experimental data.

The dehydration of cations is generally accepted as the rate-limiting step in many processes. Molecular dynamics (MD) can be used to investigate the dynamics of water molecules around cations, and two different methods exist to obtain trajectory-based water dehydration frequencies. Here, these two different post-processing methods (direct method versus survival function) have been implemented to obtain calcium dehydration frequencies from a series of trajectories obtained using a range of accepted force fields. None of the method combinations reproduced the commonly accepted experimental water exchange frequency of 10–8.2 s–1. Instead, our results suggest much faster water dynamics, comparable with more accurate ab initio MD simulations and with experimental values obtained using neutron scattering techniques. We obtained the best agreement using the survival function method to characterize the water dynamics, and we show that different method combinations significantly affect the outcome. Our work strongly suggests that the fast water exchange kinetics around the calcium ions is not rate-limiting for reactions involving dissolved/solvated calcium. Our results further suggest that, for alkali and most of the earth alkali metals, mechanistic rate laws for growth, dissolution, and adsorption, which are based on the principle of rate-limiting cation dehydration, need careful reconsideration.

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The effect of stoichiometry on the new formation and subsequent growth of CaCO 3 was investigated over a large range of solution stoichiometries (10 –4 < r aq < 10 4 , where r aq = {Ca 2+ }:{CO 3 2– }) at various, initially constant degrees of supersaturation (30 < Ω cal < 200, where Ω cal = {Ca 2+ }{CO 3 2– }/ K sp ), pH of 10.5 ± 0.27, and ambient temperature and pressure. At r aq = 1 and Ω cal < 150, dynamic light scattering (DLS) showed that ion adsorption onto nuclei (1–10 nm) was the dominant mechanism. At higher supersaturation levels, no continuum of particle sizes is observed with time, suggesting aggregation of prenucleation clusters into larger particles as the dominant growth mechanism. At r aq ≠ 1 (Ω cal = 100), prenucleation particles remained smaller than 10 nm for up to 15 h. Cross-polarized light in optical light microscopy was used to measure the time needed for new particle formation and growth to at least 20 μm. This precipitation time depends strongly and asymmetrically on r aq . Complementary molecular dynamics (MD) simulations confirm that r aq affects CaCO 3 nanoparticle formation substantially. At r aq = 1 and Ω cal ≫ 1000, the largest nanoparticle in the system had a 21–68% larger gyration radius after 20 ns of simulation time than in nonstoichiometric systems. Our results imply that, besides Ω cal , stoichiometry affects particle size, persistence, growth time, and ripening time toward micrometer-sized crystals. Our results may help us to improve the understanding, prediction, and formation of CaCO 3 in geological, industrial, and geo-engineering settings.

This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication.Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available.You can find more information about Accepted Manuscripts in the author guidelines.Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal's standard Terms & Conditions and the ethical guidelines, outlined in our author and reviewer resource centre, still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. ture at the surface, and how this changes from that of the bulk.In the past few years, our groups and others have extensively used state-of-the-art highperformance computing methods, usually molecular dynamics (MD) simulations, to characterise the structure and properties of PBGs. In molecular dynamics, the individual atoms move due to the interatomic forces they experience from other atoms. Because the full three-dimensional structure of the material is available at all times, an MD trajectory contains a great deal of information from which the material's properties may be deduced. Our work has involved new methodological developments in the form of a new model of the phosphate interatomic forces 9 , and MD simulations have given new insight into the structural motifs which affect the dissolution which are not accessible to experimental methods 10 . Recently, the different properties of PBG at the surface of the glass, from where the dissolution takes place have been shown, which will also be important in unravelling the full connections between structure and dissolution 11 . Molecular dynamics methods for preparation of glassesA full review of molecular dynamics methods is beyond the scope of this paper; the reader is invited to turn to relevant books 12,13 . Broadly, there are two types of molecular dynamics used to prepare realistic glass models: classical and first-principles. In classical MD, the interatomic forces are represented by an empirical expression with a small number of parameters which are chosen to reproduce existing data on the material. This method has the advantage of being relatively easy to implement and computationally inexpensive, allowing for large models (typically ∼ 10 3 − 10 4 atoms) to be simulated over long timescales (typically ns). The disadvantage is that the accuracy of the simulation is limited by the accuracy of the approximations of the interatomic potential model. First-principles MD, by contrast, uses a quantum-mechanical expression for th...

The incorporation of cobalt in mixed metal carbonates is a possible route to the immobilization of this toxic element in the environment. However, the thermodynamics of (Ca,Co)CO3 solid solutions are still unclear due to conflicting data from experiment and from the observation of natural ocurrences. We report here the results of a computer simulation study of the mixing of calcite (CaCO3) and spherocobaltite (CoCO3), using density functional theory calculations. Our simulations suggest that previously proposed thermodynamic models, based only on observed compositions, significantly overestimate the solubility between the two solids and therefore underestimate the extension of the miscibility gap under ambient conditions. The enthalpy of mixing of the disordered solid solution is strongly positive and moderately asymmetric: calcium incorporation in spherocobaltite is more endothermic than cobalt incorporation in calcite. Ordering of the impurities in (0001) layers is energetically favourable with respect to the disordered solid solution at low temperatures and intermediate compositions, but the ordered phase is still unstable to demixing. We calculate the solvus and spinodal lines in the phase diagram using a sub-regular solution model, and conclude that many Ca1-xCoxCO3 mineral solid solutions (with observed compositions of up to x=0.027, and above x=0.93) are metastable with respect to phase separation. We also calculate solid/aqueous distribution coefficients to evaluate the effect of the strong non-ideality of mixing on the equilibrium with aqueous solution, showing that the thermodynamically-driven incorporation of cobalt in calcite (and of calcium in spherocobaltite) is always very low, regardless of the Co/Ca ratio of the aqueous environment.

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