Influence of Pore Surface Chemistry on the Rotational Dynamics of Nanoconfined Water
Benjamin Malfait, Aîcha Jani, J. Benedikt Mietner, Ronan Lefort, Patrick Huber, Michael Fröba, Denis Morineau
Abstract
We have investigated the dynamics of water confined in mesostructured porous silicas (SBA-15, MCM-41) and four periodic mesoporous organosilicas (PMOs) by dielectric relaxation spectroscopy. The influence of water–surface interaction has been controlled by the carefully designed surface chemistry of PMOs that involved organic bridges connecting silica moieties with different repetition lengths, hydrophilicity, and H-bonding capability. Relaxation processes attributed to the rotational motions of nonfreezable water located in the vicinity of the pore surface were studied in the temperature range from 140 to 225 K. Two distinct situations were achieved depending on the hydration level: at low relative humidity (33% RH), water formed a nonfreezable layer adsorbed on the pore surface. At 75% RH, water formed an interfacial liquid layer sandwiched between the pore surface and the ice crystallized in the pore center. In these two cases, the study revealed different water dynamics and different dependence on the surface chemistry. We infer that these findings illustrate the respective importance of water–water and water–surface interactions in determining the dynamics of the interfacial liquid-like water and the adsorbed water molecules as well as the nature of the different H-bonding sites present on the pore surface.