MOBILITY OF ANION SPIN PROBES IN HECTORITE GELS - VISCOSITY OF SURFICIAL WATER

The mobility of water adjacent to layer silicate clay surfaces and the spatial range of influence of these surfaces on the physical properti es of water have been subjects of some controversy. In an attempt to f urther characterize the surficial water of clays, electron spin resona nce (ESR) spectroscopy was used to determine the rotational mobility o f an anionic nitroxide spin probe (3-carboxy-2,2,5,5-tetramethyl-1-pyr rolidinyloxy) 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 w ater within 100 angstrom of the surface. The spin probe results indica te that, at low water contents, the addition or removal of water chang es the rigidity of all the water, so that the viscosity of the interla mellar water cannot be estimated by averaging that of the adsorbed and added free water. The results place the maximum range of silicate sur face influence on dynamic water properties at about five molecular dia meters, with up to two monolayers of water at each surface being one t o two orders of magnitude more rigid than free water.