Neutron scattering study on dynamics of water molecules in MCM-41

The dynamics of water molecules confined in MCM-41 was investigated by quas i-elastic neutron scattering. The measurement was performed for three water -filled MCM-41 samples with different pore sizes in the temperature range 2 00-300 K. The spectra were analyzed by using a model employed by Teixeria e t al. in a study for hulk water. This model is composed of two motions of w ater molecules: rotational and translational diffusions. For the translatio nal diffusion, water molecules in MCM-41 are, on the whole, less mobile tha n those in bulk water, and the mobility is decreased by narrowing of the po re size. The residence time of translational diffusion of the confined wate r molecules shows the Arrhenius type of temperature dependence, which is in contrast to a non-Arrhenius behavior for bulk water. This implies that a g rowth of the hydrogen-bond network of water is hindered in a confined space by surface field. Spectra of MCM-41 sample having monolayer water were als o measured and could be analyzed with a model in which only rotational diff usion is an allowed motion of the monolayer water molecules.