PUB - Publikationen an der Universität Bielefeld

Using quasielastic neutron scattering, we have investigated the water dynamics of powders of the compound with the stoichiometry of [Mo72Fe30O252(CH3COO)(12)[Mo2O7(H2O)](2)[H-2 Mo2O8(H2O)](H2O)(91)]center dot approximate to 150H(2)O. It contains about 150 crystal water molecules in the voids between the spherical {Mo72Fe30} nanocapsules, which are considered unique polytropic inorganic acids with well-defined hydrophilic surfaces due to the presence of H2O and O atoms. It has been proposed that {Mo72Fe30} can be used as a structurally well-constrained experimental model of oxide mineral surfaces for earth scientists.(I) In this respect, it is of fundamental importance to understand the dynamics of the water molecules at the surface of the nanoclusters. Our measurements show that the dynamics of these water molecules is as expected profoundly different from that of bulk water at the same temperature, especially because of the strong hydrogen bonding between the crystal and cluster surface water molecules. In fact, our data show a non-Debye relaxation behavior. The momentum transfer dependence of the dynamics is close to that expected for a purely diffusive motion. This suggests that the nonexponentiality of the dynamics originates from a distribution of relaxation times, probably related to the different local environments experienced by the water molecules. The dynamics of the crystal water in the voids between the well-defined and arrayed nanocages is significantly slower than that of bulk water at the same temperature that has often been reported for interfacial water. In the investigated range, the temperature dependence of the relaxation time can be described in terms of an Arrhenius law, indicating that the dynamics is triggered by the breaking of the bonds connecting the crystal water molecules with the hydrophilic nanocage surfaces.