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

Abstract

This paper describes a calorimetric (DSC) study of the high temperature (∼190 °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 °C on heating and below 100 °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.

Isoconversional Methodology

“…This expands the application area of the methods to such processes as crystallization [48,49], gelation [50], as well as some other phase transitions [51] that can be initiated by decreasing temperature. If a flexible method is based on integration with respect to temperature (e.g., Eq.…”

Nucleation‐Driven Kinetics of Poly(ethylene terephthalate) Melting

“…The present study is undertaken with a purpose to learn whether the nucleation model proposed by Toda et al can predict the temperature dependence of the effective activation energy of polymer melting as estimated experimentally by an isoconversional kinetic analysis and whether the experimentally derived dependence can be used for evaluating parameters of the model. Note that isoconversional analysis has been effective in estimating parameters of kinetic models related to other nucleation‐driven processes such as crystallization of polymer melts and glasses, gelation of polymer solutions, and solid–solid phase transition …”

Gelation of Poly(Vinylidene Fluoride) Solutions in Native and Organically Modified Silica Nanopores

“…In principle, this equation can be used to evaluate the effect of nanoconfinement on the kinetics of a nucleation driven process. It has been employed to evaluate such an effect on the kinetics of the morphological solid-solid transition [ 18 , 51 ], and coil-to-globule transition [ 52 ]. This present study appears to be the first attempt using the Turnbull-Fisher model for parameterizing the effect of nanoconfinement on the process of gelation.…”