  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 non-freezable water located in the
vicinity of the pore surface were studied in the temperature range from 140 K
to 225 K. Two distinct situations were achieved depending on the hydration
level: at low relative humidity (33% RH), water formed a non-freezable 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 the 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.
