Orientation and motion of interlamellar water: an infrared and NMR investigation of water in the galleries of layered Cd_0.75PS₃K_0.5(H₂O)_y

Abstract

The motion of water in the galleries of layered Cd_0.75PS₃K_0.5(H₂O)_1.2, prepared by the ion-exchange intercalation of hydrated potassium ions into CdPS₃, have been investigated using a combination of orientation dependent NMR and infrared (IR) spectroscopies as a function of temperature. Within the galleries of the intercalated compound the potassium ions are displaced toward the layers and are immobile, while the water molecules form monolayer-thick two-dimensional islands. Two types of interlamellar water are distinguishable by both NMR and IR spectroscopy. A loosely bound isotropically tumbling water which is easily lost on mild evacuation and a more tightly bound water with restricted degrees of rotational freedom. The angular dependence of the NMR and IR spectral features of the latter indicate that their C→₂ symmetry axis is at a fixed orientation with respect to the interlamellar normal and rotate rapidly about it. In analogy with bulk aqueous solutions these two types of interlamellar water may be considered as two-dimensional solventlike water and water coordinated to the potassium ion forming part of its hydration shell. A remarkable feature of the water in the galleries of Cd_0.75PS₃K_0.5(H₂O)_1.2 is the extremely slow rate of exchange (τ » 10⁻⁵ s) between the two-dimensional solventlike water and the coordinated water, in direct contrast to the rapid exchange observed for K ions in bulk aqueous solutions. The present experimental results provide the first observation of the effect of confinement on solvation shell exchange. It is suggested that the slowing down could be the effect of confinement in two dimension which would strongly inhibit any mechanism involving transition or intermediate states which require an expansion of the coordination shell around the potassium ion.