HEAT CAPACITY OF INTRA-CRYSTALLINE WATER MOLECULES IN ZEOLITES

Intra-crystalline (zeolitic) water is an intrinsic chemical and structural component of zeolites and other crystalline hydrates. Knowledge of the heat capacity of these water molecules is necessary for prediction of the hydration state of these materials as a function of temperature, pressure, and the chemical potential of water. In the present study, simultaneous differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) was employed to obtain continuous heat capacity functions from –130 to > 200 ºC for homologous hydrated and dehydrated analcime, wairakite, and natrolite. These minerals were chosen for study because they contain nominally only one crystallographically and energetically distinct water site. Simultaneous acquisition of a TGA signal eliminates ambiguities associated with uncertain hydration states at elevated temperatures in these materials; some experiments were conducted under humid atmospheres to extend the temperature region over which the minerals remained hydrated. The heat capacity of intra-crystalline water was taken as the difference in heat capacity between the homologous hydrated and dehydrated forms, normalized to one mole of water molecules. In the case of analcime, the heat capacity of zeolitic water is relatively insensitive to temperature above 25 ºC in accordance with statistical mechanical models of gas absorption in zeolites. Water molecules in natrolite and wairakite undergo phase transitions leading to a dramatic temperature dependence of the heat capacity on temperature. In wairakite, zeolitic water exhibits a distinct lambda-type phase transition at ~150 ºC, which corresponds to a lambda feature seen in the hydrated, but not the dehydrated form of this mineral. Above this transition, the heat capacity per mol of water is essentially temperature invariant at a value similar to that of gaseous water. In natrolite, the heat capacity of zeolitic water steadily increases with temperature to ~ 75 ºC, at which point it becomes temperature invariant at a value approximately 2*R greater than that of gaseous water. The marked temperature dependence of the heat capacity of zeolitic water in natrolite and wairakite suggests that caution should be exercised in assuming that the heat capacity of dehydration reactions in zeolites is independent of temperature.