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Mobility of Anion Spin Probes in Hectorite Gels: Viscosity of Surficial Water

Department of Soil, Crop and Atmospheric Sciences, Cornell Univ., Ithaca, NY 14853

* Corresponding author ( mbm7{at}cornell.edu).

ABSTRACT

The mobility of water adjacent to layer silicate clay surfaces and the spatial range of influence of these surfaces on the physical properties of water have been subjects of some controversy. In an attempt to further characterize the surficial water of clays, electron spin resonance (ESR) spectroscopy was used to determine the rotational mobility of an anionic nitroxide spin probe (3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxy) in hectorite gels of variable water content. There was a measurable decrease in average mobility of the anion probe only when the clay/water ratio exceeded 1 kg L^–1. Because "negative adsorption" tended to exclude the anion from the near-surface regions except at quite low water content, mobility of the anion probe was unaffected by adsorption processes and could be used to estimate the viscosity of water within 100 Å of the surface. The spin probe results indicate that, at low water contents, the addition or removal of water changes the rigidity of all the water, so that the viscosity of the interlamellar water cannot be estimated by averaging that of the adsorbed and added free water. The results place the maximum range of silicate surface influence on dynamic water properties at about five molecular diameters, with up to two monolayers of water at each surface being one to two orders of magnitude more rigid than free water.

Received for publication March 11, 1994.