Orientation and motion of interlamellar water: An infrared and NMR investigation of water in the galleries of layered Cd0.75PS3K0.5(H2O)(y)

The motion of water in the galleries of layered Cd0.75PS3K0.5(H2O)(1.2), pr epared by the ion-exchange intercalation of hydrated potassium ions into Cd PS3, have been investigated using a combination of orientation dependent NM R and infrared (TR) spectroscopies asa function of temperature. Within the galleries of the intercalated compound the potassium ions are displaced tow ard the layers and are immobile, while the water molecules form monolayer-t hick two-dimensional islands. Two types of interlamellar water are distingu ishable by both NMR and IR spectroscopy. A loosely bound isotropically tumb ling water which is easily lost on mild evacuation and a more tightly bound water with restricted degrees of rotational freedom. The angular dependenc e of the NMR and IR spectral features of the latter indicate that their (C) over right arrow(2) symmetry axis is at a fixed orientation with respect t o the interlamellar normal and rotate rapidly about it. In analogy with bul k aqueous solutions these two types of interlamellar water may be considere d as two-dimensional solventlike water and water coordinated to the potassi um ion forming part of its hydration shell. A remarkable feature of the wat er in the galleries of Cd0.75PS3K0.5(H2O)(1.2) is the extremely slow rate o f exchange (tau >> 10(-5) s) between the two-dimensional solventlike water and the coordinated water, in direct contrast to the rapid exchange observe d for K ions in bulk aqueous solutions. The present experimental results pr ovide the first observation of the effect of confinement on solvation shell exchange. It is suggested that the slowing down could be the effect of con finement in two dimension which would strongly inhibit any mechanism involv ing transition or intermediate states which require an expansion of the coo rdination shell around the potassium ion.