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Slow dynamics of supercooled water confined in nanoporous silica materials

We review our incoherent quasielastic neutron scattering (QENS) studies of the dynamics of supercooled water confined in nanoporous silica materials. QENS data were analysed by using the relaxing cage model (RCM) previously developed by us. We first use molecular dynamics (MD) simulation of the extended simple point charge model (SPC/E) for bulk supercooled water to establish the validity of the RCM, which applies to both the translational and rotational motion of water molecules. We then assume that the dynamics of water molecules in the vicinity of a hydrophilic surface is similar to a bulk water at an equivalent lower supercooled temperature. This analogy was experimentally demonstrated in previous investigations of water in Vycor glasses and near hydrophilic protein surfaces. Studies were made of supercooled water in MCM-41-S (pore sizes 25, 18, and 14 Å) and MCM-48-S (pore size 22 Å) using three QENS spectrometers of respective energy resolutions 1, 30, and 60 µeV, covering the temperature range from 325 to 200 K. Five quantities are extracted from the analysis: they are β, the stretch exponent characterizing the α-relaxation; βγ, the exponent determining the power-law dependence of the relaxation time on Q; , the Q-independent pre-factor for the average translational relaxation time; , the relaxation time for the first-order rotational correlation function; and , the relaxation time for the second-order rotational correlation function. We discuss the temperature dependence of these parameters and note that, in particular, the dynamics is rapidly slowing down at temperature around 220 K, signalling the onset of a structural arrest transition of liquid water into an amorphous solid water.