MULTINUCLEAR MAGNETIC-RESONANCE STUDY OF SAPONITE HYDRATION AND OF ACETONITRILE-WATER COMPETITION

Saponite is a trioctahedral 2:1 clay with strongly held water layers i n between the platelets. Homoionic Li-saponite is suspended in mixture s of acetonitrile-d(3) and heavy water. Reporter quadrupolar nuclei mo nitor the occurrence of local order. They are H-2 and O-17 for water; H-2 and N-14 for acetonitrile; and Li-7 for the counterions. The resid ual quadrupolar splittings in the resonances for these nuclei are inte rnally consistent within the molecules. As the acetonitrile content of the binary solvent mixtures is raised, more and more lithium ions are extracted from the Stern into the Gouy-Chapman diffuse layer. Stacks of saponite flakes segregate layers of CD3CN and D2O molecules from on e another. At low acetonitrile content (<20%), some CD3CN molecules or der themselves together with the lithium ions at the clay interface. T hey follow the outside migration of the lithium ions. They orient them selves, on the average, at the magic angle with respect to the clay su rface, at 20 vol % CD3CN. Up to 50%, the average orientation of interf acial acetonitrile molecules changes continuously with composition of the binary solvent mixture. In the entire 0-50% acetonitrile concentra tion range, water orientation at the clay surface remains conversely i nvariant.