NMR study of adsorbed water. II. Molecular motions in the monolayer hydrate of halloysite

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Abstract
In the electrically neutral interlayer space of endellite, the water monolayer does not exhibit a preferential orientation such as that observed in phyllosilicates with water hydrating the interlayer cations (see, for example, the first paper in this series). The influence of the surface is to provoke an anisotropy of the molecular motions which can be separated into a rapid 180° flipping motion about the C2 axis and a slow tumbling about an axis perpendicular to C2. The activation energy of the former motion is 4.6 kcal mole−1, whereas that of the latter is 3.6 kcal mole−1. The proton and deuteron spin–spin and spin–lattice relaxation times can be accounted for by this simple model. Two other types of experimental results complement this model. The observed infrared OH frequencies suggest weak hydrogen bonds between water molecules and surface lattice hydroxyls, whereas strong hydrogen bonds lock water molecules to each other. The molar heat capacity of adsorbed water parallels that of ice up to the temperature (130 °K) where a torsional motion about the C2 axis is transformed into the 180° flipping motion.