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

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 T 1 and T 2 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 T NMR at which dispersed hydrates are involved in isotropic motion represents the melting point of the hydrates located in the silica pores. The decrease of the melting point for the dispersed hydrates is in good accordance with the Gibbs−Thompson effect for dispersed materials. In dispersed hydrates, water molecules reorient isotropically 1 order of magnitude faster in the temperature range 230−490 K; that is, water is more mobile in the dispersed hydrates. The slow isotropic reorientation of water molecules is influenced by both the quantity of water in the hydrate and the dispersibility of the hydrates. In the case of the hydrate with n = 4, the activation energy of this motion decreases by ca. 3 times when the hydrate becomes dispersed in the silica pores.