Dynamics of Ultra-Confined Water in Hemimorphite

T.R. Prisk¹, C. Hoffmann², A.I. Kolesnikov², E. Mamontov², A.A. Podlesnyak³, X. Wang², and L.M. Anovitz¹

¹Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge TN, 37831
²Chemical and Engineering Materials Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831
³Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831

Porous media constitute a broad class of natural and synthetic materials which play keys roles in geological processes and the modern chemical industry. The structure, properties, and phase behavior of fluids may be strongly modified from their bulk counterparts when they are adsorbed to solid surfaces or confined within porous media. Water confined within microporous minerals presents an extreme example of molecular confinement, where the water molecule is trapped within cages or channels which are not much larger than the water molecule itself. In these materials, individual water molecules or small clusters of water molecules may interact with a highly restricting, anisotropic crystal framework by means of hydrogen bonding. For instance, hemimorphite is a hydrous microporous mineral which ultra-confined water molecules form a two-dimensional network of hydrogen bonds within the pore channels. In this presentation, we report neutron scattering investigations into the structure and dynamics of water confined within hemimorphite. The total diffusive motion of the water molecule is a superposition of two processes, one being a slow jumping from one hydrogen-bonded location to another and the other being rapid, localized motions at each hydrogen-bonded location. The first motion is an analog to the rotation of water molecules in the bulk fluid, while the second motion has no known analog in either the bulk or interfacial fluid. As shown by this example, water molecules may exhibit qualitatively new behavior in extreme cases of confinement, where the character of their dynamics depends critically upon the chemical architecture of their surrounding environment.

This research was sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy. This research at Oak Ridge National Laboratory's Spallation Neutron Source was also sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

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