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

Saponite is a trioctahedral 2:1 clay with strongly held water layers in between the platelets. Homoionic Li-saponite is suspended in mixtures of acetonitrile-d(3) and heavy water. Reporter quadrupolar nuclei monitor 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 residual quadrupolar splittings in the resonances for these nuclei are internally 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 one another. At low acetonitrile content (<20%), some CD3CN molecules order themselves together with the lithium ions at the clay interface. They follow the outside migration of the lithium ions. They orient themselves, on the average, at the magic angle with respect to the clay surface, at 20 vol % CD3CN. Up to 50%, the average orientation of interfacial acetonitrile molecules changes continuously with composition of the binary solvent mixture. In the entire 0-50% acetonitrile concentration range, water orientation at the clay surface remains conversely invariant.