Processes taken place at the solid/aqueous solution interface have a strong impact on the evolution of materials in the fields of construction, environment, geochemistry, membranes, catalysis, and nuclear wastes. Since many materials in these application fields are completely or partially nanoporous (cementitious materials, biominerals, clay, secondary minerals…) and filled with aqueous solutions, the processes and chemical reactions occurring in this nanoconfinement has a major impact on the materials evolution on a macroscopic scale. The study of such processes and chemical reactions occurring in nanometer-sized porosities in contact with aqueous solutions (ions sorption, electrolyte diffusion, phase precipitation, pore wall dissolution and the recondensation of dissolved species) is important since it differs from the ones occurring in dense materials. Since water takes part in most of these processes and chemical reactions, the understanding of water molecules behavior in such media is essential to be able to predict the behavior of such materials.
Generally, the macroscopic evolution of materials in solution is described or predicted using modelling. Thermodynamic models and rate laws developed and integrated in computer programs (PHREEQC, JChess…) generally take into account some data arising from measurements performed in diluted media (concentration of dissolved ions, pH, kinetics and thermodynamic constants…). For nanoporous materials, the validity of these models and rate laws are still not proven yet, thus prediction can be defective. Indeed, under confinement and in presence of ions, the water behavior is modified by the strong interactions between the pore surfaces and the ions structuring water, and slowing down its dynamics/transport from nanoscale to macro-scale. Such effects modify the electrostatic interactions in the system and thus the ions free energy landscape. Since chemical reactions such as silica hydrolysis/recondensation processes are controlled by the solution-silica interfacial layer, the study of the modification of this interfacial layer in nanoconfinement is of paramount importance to describe the evolution of nanoporous materials in aqueous solution. In-fine, this will allow the improvement of the thermodynamic models and rate laws of processes occurring in nanoconfinement.
In this study, we investigated the water properties (structure and dynamics) in the presence of ions in silica nanoconfinement and relate it to the evolution of silica mesoporous materials in aqueous solutions. We used an original approach, consisting in the use of electrolyte solutions having ions with various kosmotropic property XCl2 (X = Ba, Ca, Mg) confined in model systems such as two parallel and plane silica surfaces spaced of 3 and 5 nm (nanochannels) and highly ordered mesoporous silica materials represented by SBA-15 (6 nm pore size and microporous pore wall) and MCM-41 (3 nm pore size and dense pore wall).
Diane Rébiscoula, Markus Bauma, Cyrielle Reya, Fanni Juranyib, Francois Rieutordc
a- ICSM – UMR 5257 CEA-CNRS-UM-ENSCM, 30207 Bagnols-sur-Cèze Cedex, France
b- Paul-Scherrer-Institute, 5232 Villigen, Switzerland
c- Univ. Grenoble Alpes, INAC-MEM, F-38000 Grenoble, France
UMR 5257 CEA-CNRS-UM-ENSCM
Laboratoire des Nanomatériaux pour l'Energie et le Recyclage
Site de Marcoule, Bâtiment 426, BP 17171
F-30207 Bagnols sur Cèze Cedex