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.

No abstract

Differential scanning calorimetry (DSC) is used to study the kinetics of the coil-to-globule transition in aqueous solutions of poly( N -isopropoylacrylamide) (PNIPAM) prepared in the bulk (3 and 10 wt%) and nanoconfi ned (10 wt% inside 30 nm silica pores) forms. It is demonstrated that the kinetics can be described in terms of the classical nucleation model. The proposed treatment affords estimating the free-energy barrier and pre-exponential factor of the transition. The application of the nucleation model to the DSC data collected for the three systems studied provides physical insights into the effect of increasing the transition temperature due to dilution and nanoconfi nement. Dilution appears to raise the free-energy barrier, whereas nanoconfi nement causes a decrease in the pre-exponential factor. demixing of a two component mixture having the lower critical solution temperature. Per this treatment, raising temperature above the binodal (equilibrium) line brings the system into a metastable state that demixes (relaxes) via the process of nucleation, i.e., the formation of tiny droplets of a new phase. The process is relatively slow as it requires overcoming a thermodynamic free-energy barrier associated with the surface energy of the forming droplet.Notwithstanding a plethora of publications dealing with the coil-to-globule transition in PNIPAM, rather few of them [15][16][17][18][19][20][21][22][23][24] are devoted specifi cally to its kinetics. Surprisingly, we have not been able to fi nd any publications that would attempt to parameterize the kinetics of the coil-to-globule transition in the frameworks of a nucleation model. Such an attempt is made in this paper that combines the classical nucleation kinetics model [ 13,14 ] with isoconversional kinetic analysis [ 25,26 ] and applies them to calorimetric data on the coil-to-globule transition 305 306 307 0 10 20 30 40 50 10% 3% Silica

The solid–solid phase transitions in ammonium nitrate, ammonium perchlorate, and sodium nitrite confined to native and organically modified silica nanopores are analyzed by differential scanning calorimetry. The study reveals the effect of nanoconfinement on the transition temperature and links it to the kinetic parameters of the process. It is suggested that the effect of nanoconfinement is primarily the surface interaction effect in the native pores and the size effect in organically modified pores. It has been found that in organically modified pores the transition temperature is always depressed relative to the bulk, whereas in the native pores its behavior is generally unpredictable. Kinetic analysis of the transitions in terms of a nucleation model indicates that the experimentally observed shifts in temperature can be explained by a combination of changes in the free energy of nucleation and preexponential factor of the respective processes.