A first study of the behavior of amino functions in mesoporous hybrid thin films M 1-x (Si-(CH 2 ) 3 NH 2 ) x O 2-x/2 (M ) Si, Ti, Zr; 0.05 e x e 0.2) with accessible Im3 j m-or Fm3 j m-derived pore mesostructures is presented. An XPS study of surface nitrogen species shows two different sites corresponding to amino and ammonium groups. The ratio of these species changes with pH and is related to the nature of M, suggesting that the interaction between the organic functions and the surface M-OH groups can be tailored to tune the surface acid-base behavior. Density functional theory (DFT) calculations were used to rationalize the XPS observations showing that -NH 3 + functions irreversibly transfer a proton to neighboring M-Osurface groups. The acid-base surface properties can be further modified by adding a phosphonate "capping" on the M surface sites. Our findings have a series of interesting implications in surface functionalization: attachment of biomolecules to surfaces, design of perm-selective or philicity-selective membranes, or design of catalysts that show a well-defined organic reactive function near surface hydroxyl groups.

Continuum solvent models have become a standard technique in the context of electronic structure calculations, yet no implementations have been reported capable to perform molecular dynamics at solid-liquid interfaces. We propose here such a continuum approach in a density functional theory framework using plane-wave basis sets and periodic boundary conditions. Our work stems from a recent model designed for Car-Parrinello simulations of quantum solutes in a dielectric medium [D. A. Scherlis et al., J. Chem. Phys. 124, 074103 (2006)], for which the permittivity of the solvent is defined as a function of the electronic density of the solute. This strategy turns out to be inadequate for systems extended in two dimensions: the dependence of the dielectric function on the electronic density introduces a new term in the Kohn-Sham potential, which becomes unphysically large at the interfacial region, seriously affecting the convergence of the self-consistent calculations. If the dielectric medium is properly redefined as a function of the atomic coordinates, a good convergence is obtained and the constant of motion is conserved during the molecular dynamics simulations. The Poisson problem is solved using a multigrid method, and in this way Car-Parrinello molecular dynamics simulations of solid-liquid interfaces can be performed at a very moderate computational cost. This scheme is employed to investigate the acid-base equilibrium at the TiO(2)-water interface. The aqueous behavior of titania surfaces has stimulated a large amount of experimental research, but many open questions remain concerning the molecular mechanisms determining the chemistry of the interface. Here we make an attempt to answer some of them, putting to the test our continuum model.

We study water confined in calcite (104) slit pores from 6 to 1 nm by molecular dynamics. By determining NMR parameters combined with hydrogen bond network analysis, we provide an important contribution to the understanding of the dynamics of water confined. The water dynamics was found uncorrelated upon confinement within calcite, with the translational dynamics highly dependent on the local density variations and the rotational dynamics varying with local hydrogen bond connectivity. A water layered structuring is observed, and the layer by layer analysis reveals that translational dynamics are the main contribution to spin relaxation of near surface water molecules. The T 2 relaxation time for water molecules directly hydrogen bonded to the surface is short and pore size independent; however, a bulk-like spin relaxation is observed at the center of pores larger than 3 nm. The hydrogen bond network of confined water has a more continuous path topology that results in the slightly longer rotational correlation time for water located up to 2 nm from the surface. Moreover, the number of tetrahedral geometric patterns which are associated with bulk water is reduced upon confinement. The confinement effects are enhanced mainly in the 1 nm pore due to overlap of surface effects.