Water Dynamics in Bulk and Dispersed in Silica CaCl2 Hydrates Studied by 2H NMR

Abstract

The mobility of water in deuterated analogues of CaCl2·nH2O (n = 2, 4, 6) hydrates has been studied by solid-state 2H NMR spectroscopy. Dynamics of water molecules in hydrates dispersed in the mesopores of silica are compared with those in the bulk state. Analysis of the 2H NMR line shape and T1 and T2 relaxation times allowed us to characterize the water mobility in a wide temperature range (120−493 K). In both crystalline and melted hydrates, the mobility of water molecules is governed by O−D···Cl hydrogen bonding. Both bulk and dispersed hydrates have been found to exhibit three types of molecular motion. Two of these represent fast internal and local motions performed on a time scale of 10−10−10−11 s. The third, slow isotropic reorientation occurs on a time of 10−6−10−7 s. Dispersed hydrates become involved in the slow isotropic motion at temperatures 50−130 K lower than the corresponding bulk hydrates. The temperature TNMR at which dispersed hydrates are involved in isotropic motion represents the me...