High Temperature Solid-Solid Transition in Ammonium Chloride Confined to Nanopores

This paper describes a calorimetric (DSC) study of the high temperature (similar to 190 degrees C) solid-solid phase transition in ammonium chloride in silica nanopores (4-30 nm) and in bulk The study focuses on the values of the transition heat and temperature as well as on the transition kinetics. Because ammonium chloride is loaded from a solution, the pores are only filled partially. Thermogravimetric analysis is employed to evaluate the pore fullness, which is further used to estimate the height of ammonium chloride layer deposited inside the pores. With increasing the layer height, the heat of transition increases toward the bulk value. Relative to the bulk value, the transition temperature measured on heating and on cooling respectively increases and decreases with decreasing the layer height. In larger pores (15 and 30 nm), the transition has revealed a second DSC peak that appears above 210 degrees C on heating and below 100 degrees C on cooling. The temperature dependencies of the effective activation energy derived from isoconversional kinetic analysis of DSC data have been parametrized in terms of the Turnbull-Fisher model. It is found that the transition in the pores encounters a larger free energy barrier to nucleation.