Water confined in mesoporous TiO2 aerosols: insights from NMR experiments and Molecular Dynamics simulations

SCHERLIS, DAMIAN; VELASCO, MANUEL I.; SOLER ILLIA, GALO J. A. A.; GONZALEZ SOLVEYRA, ESTEFANIA; FRANCESCHINI, ESTEBAN A.; FRANZONI, M. BELEN

Congreso; 16th NMR Users Meeting; 2017

AUREMN

Porousmaterials are of great interest because of their ability to interact with avariety of entities like atoms, ions, molecules, and nanoparticles; not only atthe surfaces level but also in the bulk of the materials. Due to their tunablesurface area, porosity, pore volume, pore shape, and framework compositions,mesoporous materials are currently under intense research. Some of the mainareas where these materials are applied include catalysis, adsorption or drugdelivery. It is well known that mesopore size affects the thermodynamics,structural properties and dynamics of liquids confined within them. Inrestricted geometries, water molecules interact with surfaces throughhydrophobic, hydrophilic or hydrogen bonds; where a competition betweensurface?liquid and liquid?liquid interactions is present. This leads tomolecular structuring not observed in bulk water, which gives rise to partialordering of water molecules in the vicinity of the confining surface.Among thewide variety of materials, mesoporous TiO2 is of particular interestdue to its outstanding features such as low cost, environmental benignity,plentiful polymorphs, excellent chemical and thermal stability,biocompatibility and tunable electronic and optical properties. In consequence,TiO2 mesoporous systems are of paramount importance in biomaterials,photocatalysis, solar cells or solar fuel productionIn this workthe adsorption of water in nanoporous TiO2 was studied by NMR andmulti-scale molecular dynamics simulations. Mesoporous TiO2 aerosolswere synthesized and further characterized by N2 sorption, renderinga pore diameter of 3.5 nm. Water dynamics in systems with different fillingdegree were studied by 1H NMR spectroscopy and double quantum NMR,where three populations with different mobilities are clearly identified: ahighly structured adsorbed layer, a second less structured layer, and a mobilewater component which increases with the filling degree. Comparison withmultiscale molecular dynamic (MD) simulations enables the identification ofthe  capillary condensation onset, from which a pore diameter of around4.5 nm is determined. The difference in the results obtained from N2sorption isotherms and from the combination of NMR and MD is ascribed to the presenceof a residual first layer of structured water, which is always present due tothe high hidrophilicity of the surface. The combination of theoretical andexperimental studies is a very powerful tool for a complete analysis of theproperties of water confined in the mesoporous material, which is critical inapplications, such as selective membranes, photocatalysts and biomaterials.