Single particle dynamics of water confined in a nanopore is studied through
Computer Molecular Dynamics. The pore is modeled to represent the average
properties of a pore of Vycor glass. Dynamics is analyzed at different
hydration levels and upon supercooling. At all hydration levels and all
temperatures investigated a layering effect is observed due to the strong
hydrophilicity of the substrate. The time density correlators show, already at
ambient temperature, strong deviations from the Debye and the stretched
exponential behavior. Both on decreasing hydration level and upon supercooling
we find features that can be related to the cage effect typical of a
supercooled liquid undergoing a kinetic glass transition. Nonetheless the
behavior predicted by Mode Coupling Theory can be observed only by carrying out
a proper shell analysis of the density correlators. Water molecules within the
first two layers from the substrate are in a glassy state already at ambient
temperature (bound water). The remaining subset of molecules (free water)
undergoes a kinetic glass transition; the relaxation of the density correlators
agree with the main predictions of the theory. From our data we can predict the
temperature of structural arrest of free water.Comment: 14 pages, 15 figures inserted in the text, to be published in J.
Chem. Phys. (2000
