All ETDs from UAB

Advisory Committee Chair

Sergey Vyazovkin

Advisory Committee Members

Renato P Camata

Derrick R Dean

Tracy P Hamilton

Eugenia Kharlampieva

Document Type

Dissertation

Date of Award

2015

Degree Name by School

Doctor of Philosophy (PhD) College of Arts and Sciences

Abstract

The objective of this dissertation was to study the effects of nanoconfinement on the thermodynamic and kinetic parameters of phase transitions in selected compounds. The compounds were loaded into silica gel nanopores and filled them partially. A model was developed for estimating the fullness of the nanopores as well as the height of the compound layer deposited on the pore walls. A study of the phase transition in NH4Cl confined to the native silica gel nanopores revealed that the heat of transition increases with the increasing the layer height. Also the transition temperature measured on heating and cooling respectively increases and decreases by decreasing the layer height. The temperature dependencies of the effective activation energy derived from isoconversional kinetic analysis of Differential Scanning Calorimetry (DSC) data were parameterized in terms of the Turnbull-Fisher model. It was found that the transition in the pores encounters a larger free energy barrier to nucleation. Since the nanoconfinement effect in the native pores is a combination of both size and surface interaction effects, the separation of the effects was attempted. For this, NH4NO3, NH4ClO4 and NaNO2 were studied in native and organically modified silica gel nanopores. A DSC analysis of the solid-solid transitions in these compounds revealed that, in organically modified pores, the transition temperature shifts to the lower temperatures in comparison than the bulk; whereas, in the native pores, the direction of the shift is generally unpredictable. Kinetic analysis of the transitions in terms of a nucleation model indicated that the shifts in temperature can be explained by changes in pre-exponential factor and free energy barrier of nucleation parameters. As an extension of the study, the coil-to-globule transition of poly(N-isopropoylacrylamide) in aqueous solution was investigated. It was found that nanoconfinement and dilution cause an increase in the transition temperature. Kinetics analysis of the respective DSC data in terms of a nucleation model demonstrated that the temperature rise due to nanoconfinement was associated with a decrease in the preexponential factor. However, the rise due to dilution was because of an increase in the free energy barrier.

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