Antonio Tilocca

Local ordering and electronic signatures of submonolayer water on anatase TiO2(101)

The interaction of water with metal oxide surfaces is of fundamental importance to various fields of science, ranging from geophysics to catalysis and biochemistry. In particular, the discovery that TiO(2) photocatalyses the dissociation of water has triggered broad interest and intensive studies of water adsorption on TiO(2) over decades. So far, these studies have mostly focused on the (110) surface of the most stable polymorph of TiO(2), rutile, whereas it is the metastable anatase form that is generally considered photocatalytically more efficient. The present combined experimental (scanning tunnelling microscopy) and theoretical (density functional theory and first-principles molecular dynamics) study gives atomic-scale insights into the adsorption of water on anatase (101), the most frequently exposed surface of this TiO(2) polymorph. Water adsorbs as an intact monomer with a computed binding energy of 730 meV. The charge rearrangement at the molecule-anatase interface affects the adsorption of further water molecules, resulting in short-range repulsive and attractive interactions along the [010] and directions, respectively, and a locally ordered (2x2) superstructure of molecular water.

Structural models of bioactive glasses from molecular dynamics simulations

The bioactive mechanism, by which living tissues attach to and integrate with an artificial implant through stable chemical bonds, is at the core of many current medical applications of biomaterials, as well as of novel promising applications in tissue engineering. Having been employed in these applications for almost 40 years, soda-lime phosphosilicate glasses such as 45S5 represent today the paradigm of bioactive materials. Despite their strategical importance in the field, the relationship between the structure and the activity of a glass composition in a biological environment has not been studied in detail. This fundamental gap negatively affects further progress, for instance, to improve the chemical durability and tailor the biodegradability of these materials for specific applications. This paper reviews recent advances in computer modelling of bioactive glasses based on molecular dynamics simulations, which are starting to unveil key structural features of these materials, thus contributing to improve our fundamental understanding of how bioactive materials work.

On the unusual stability of Maya blue paint: molecular dynamics simulations

Abstract Classical molecular dynamics simulations of the indigo-containing clay palygorskite (Maya blue) have been carried out at different temperatures in order to get insight into the high stability of this paint. After several tens of picoseconds the molecules set into stable sites along the channels of the clay, where they remain for relatively long times even at high temperature (573 K). The hydrogen bonds between indigo carbonyl and structural water are seemingly the most important guest–host interactions, however, there is some evidence that they are not essential in keeping the organic dye trapped in the stable sites. Therefore, van der Waals interactions are expected to be crucial in stabilizing Maya blue, allowing to fit the bulky indigo molecules into the palygorskite channels. Furthermore, we successfully tested the possibility that a direct interaction between indigo and clay octahedral cations (not mediated by structural water) could also play an important role in trapping the indigo molecules.

Reaction pathway and free energy barrier for defect-induced water dissociation on the (101) surface of TiO2-anatase

The adsorption of a water molecule on a partially reduced TiO2 anatase (101) surface has been studied by first-principles molecular-dynamics simulations. At variance with the stoichiometric surface, dissociation of water close to the oxygen vacancy is energetically favored compared to molecular adsorption. However, no spontaneous dissociation was observed in a simulation of several picoseconds, indicating the presence of an energy barrier between the molecular and dissociated states. The free energy profile along a possible dissociation path has been determined through constrained molecular dynamics runs, from which a free energy barrier for dissociation of ∼0.1 eV is estimated. On the basis of these results, a mechanism for the dissociation of water at low coverage is proposed.

Modeling the water-bioglass interface by ab initio molecular dynamics simulations.

The hydration of the surface of a highly bioactive silicate glass was modeled using ab initio (Car-Parrinello) molecular dynamics (CPMD) simulations, focusing on the structural and chemical modifications taking place at the glass-water interface immediately after contact and on the way in which they can affect the bioactivity of these materials. The adsorption of a water dimer and trimer on the dry surface was studied first, followed by the extended interface between the glass and liquid water. The CPMD trajectories provide atomistic insight into the initial stages relevant to the biological activity of these materials: following contact of the glass with an aqueous (physiological) medium, the initial enrichment of the surface region in Na+ cations establishes dominant Na+-water interactions at the surface, which allow water molecules to penetrate into the open glass network and start its partial dissolution. The model of a Na/H-exchanged interface shows that Ca2+-water interactions are mainly established after the dominant fraction of Na is leached into the solution. Another critical role of modifier cations was highlighted: they provide the Lewis acidity necessary to neutralize OH(-) produced by water dissociation and protonation of nonbridging oxygen (NBO) surface sites. The CPMD simulations also highlighted an alternative, proton-hopping mechanism by which the same process can take place in the liquid water film. The main features of the bioactive glass surface immediately after contact with an aqueous medium, as emerged from the simulations, are (a) silanol groups formed by either water dissociation at undercoordinated Si sites or direct protonation of NBOs, (b) OH(-) groups generally stabilized by modifier cations and coupled with the protonated NBOs, and (c) small rings, relatively stable and unopened even after exposure to liquid water. The possible role and effect of these sites in the bioactive process are discussed.

Models of structure, dynamics and reactivity of bioglasses: a review

Due to their high biocompatibility and osteoconductivity, bioactive silicate glasses are the core components of biomaterials used to repair, restore and regenerate bone and tissues in the human body. One of the key features, which control their bioactivity, is the fast surface dissolution in a biological medium, with the release of critical amounts of ions in the surrounding environment. Being able to understand these inorganic processes at the atomistic level is essential if a more rational approach to the use of these materials is sought. Over the past five years, atomistic simulations of bioglasses have revealed details of bulk structural features which affect the glass dissolution and thus its bioactivity, such as the connectivity of the silicate network and the tendency to form chains, rings and clusters. Further simulations have started to focus directly on the details of the glass surface and of its reactivity in an aqueous environment. This article reviews recent computational approaches used to investigate the properties of bioglasses crucial for their bioactive behaviour.

Short- and medium-range structure of multicomponent bioactive glasses and melts: An assessment of the performances of shell-model and rigid-ion potentials.

Classical and ab initio molecular dynamics (MD) simulations have been carried out to investigate the effect of a different treatment of interatomic forces in modeling the structural properties of multicomponent glasses and melts. The simulated system is a soda-lime phosphosilicate composition with bioactive properties. Because the bioactivity of these materials depends on their medium-range structural features, such as the network connectivity and the Q(n) distribution (where Q(n) is a tetrahedral species bonded to n bridging oxygens) of silicon and phosphorus network formers, it is essential to assess whether, and up to what extent, classical potentials can reproduce these properties. The results indicate that the inclusion of the oxide ion polarization through a shell-model (SM) approach provides a more accurate representation of the medium-range structure compared to rigid-ion (RI) potentials. Insight into the causes of these improvements has been obtained by comparing the melt-and-quench transformation of a small sample of the same system, modeled using Car-Parrinello MD (CPMD), to the classical MD runs with SM and RI potentials. Both classical potentials show some limitations in reproducing the highly distorted structure of the melt denoted by the CPMD runs; however, the inclusion of polarization in the SM potential results in a better and qualitatively correct dynamical balance between the interconversion of Q(n) species during the cooling of the melt. This effect seems to reflect the slower decay of the fraction of structural defects during the cooling with the SM potential. Because these transient defects have a central role in mediating the Q(n) transformations, as previously proposed and confirmed by the current simulations, their presence in the melt is essential to produce an accurate final distribution of Q(n) species in the glass.

About the influence of lattice vibrations on the diffusion of methane in a cation-free LTA zeolite

Abstract The influence of lattice vibrations on the diffusion of methane in a cation-free zeolite of structure Type LTA is examined. It is shown that contrary to earlier published results the self-diffusion coefficients obtained with flexible and with rigid lattices are practically the same. This finding is true over a wide range of temperatures and for different interaction parameters. The reason why earlier papers did not state this independence of D on the lattice vibrations is explained.

Surface signatures of bioactivity: MD simulations of 45S and 65S silicate glasses.

The surface of a bioactive (45S) and a bioinactive (65S) glass composition has been modeled using shell-model classical molecular dynamics simulations. Direct comparison of the two structures allowed us to identify the potential role of specific surface features in the processes leading to integration of a bioglass implant with the host tissues, focusing in particular on the initial dissolution of the glass network. The simulations highlight the critical role of network fragmentation and sodium enrichment of the surface in determining the rapid hydrolysis and release of silica fragments in solution, characteristic of highly bioactive compositions. On the other hand, no correlation has been found between the surface density of small (two- and three-membered) rings and bioactivity, thus suggesting that additional factors need to be taken into account to fully understand the role of these sites in the mechanism leading to calcium phosphate deposition on the glass surface.

Sodium migration pathways in multicomponent silicate glasses: Car-Parrinello molecular dynamics simulations

The mechanism of sodium migration in low-silica alkali-alkaline earth silicate glasses is investigated through Car-Parrinello molecular dynamics (MD) simulations. The transport of sodium to the glass surface and its subsequent release is critical for the use of these glasses in biomedical applications. The analysis of the MD trajectory, mainly through a combination of space and time correlation functions, reveals a complex mechanism, with some common features to the migration in mixed-alkali silicate glasses and several important differences. The low site selectivity of Na cations in this glass allows them to use both Na and Ca sites in the migration process. The high fragmentation and the corresponding flexibility of the silicate network enable an additional mechanism for ion migration, not favorable in the more rigid network of common higher-silica glasses, involving the creation of empty transient sites through the correlated forward-backward motion of an Na or a Ca cation. We also show that because sodium migration must involve an undercoordinated intermediate, sharing of oxygen atoms in the initial and final coordination shells is a way to reduce the energetic cost of losing favorable Na-O interactions and Na migration proceeds between corner-sharing NaO(x) polyhedra, where x=5-7. For these low-silica compositions, the present simulations suggest that due to the participation of calcium in the Na migration, the latter will not be significantly hampered by extensive mixing with less mobile Ca ions, or, in any event, the effect will be less marked than for higher-silica glasses.