Insights into the microscopic behaviour of nanoconfined water: host structure and thermal effects

Abstract

The hydration of nanoporous materials is relevant to many fundamental and industrial applications. In this context, zeolites are usually used, but metal–organic frameworks constitute an emerging class of useful materials. The singular properties of water ascribed to the molecular association lead to a variety of behaviors in the confining systems that are not well understood. There is little of both experimental and computational information available, and most of which are limited to room temperature. This work addresses the adsorption and structural properties of water in a series of porous materials for a fixed topology, in particular LTA, and at various temperatures in the range of 298–573 K. The targeted structures were the all-silica zeolite ITQ-29, the aluminosilicate form with charge-balancing sodium and calcium cations LTA-5A, and the Zn-based zeolitic imidazolate framework. The adsorption process was computed using Monte Carlo simulations in the grand canonical ensemble and comprehensively rationalized at the molecular level on the basis of energetic factors and radial distribution functions. The structure of confined water for the different hydration levels was characterized in detail by using a specific criterion of hydrogen bond formation. The influence of both host characteristics and temperature on the microscopic behaviour of adsorbed water was assessed. Overall, this work proves that water–water hydrogen bonding is enhanced by the hydrophobic character of the pore walls and the large confining spaces. The increase in temperature induces progressive destruction of hydrogen bonds, but the majority of water molecules remain associated even when saturation is not reached. Concerning the thermodynamics of water adsorption, it is mainly affected by the peculiarities of the pore structure.