Prigogine-Defay ratio for an ionic glass-former: molecular dynamics simulations
Mauro C C Ribeiro1, Tullio Scopigno, Giancarlo Ruocco
1Laboratório de Espectroscopia Molecular, Instituto de Química, Universidade de São Paulo, C.P. 26077, 05513-970 São Paulo, SP, Brazil.
The Journal of Physical Chemistry. B
|February 21, 2009
Summary
Molecular dynamics simulations reveal how pressure affects the glass-transition temperature (Tg) in CKN ionic glass. The Prigogine-Defay ratio deviates from predictions, increasing significantly with pressure.
Area of Science:
- Materials Science
- Condensed Matter Physics
- Computational Chemistry
Background:
- Understanding the pressure dependence of the glass-transition temperature (Tg) is crucial for characterizing the behavior of ionic glass-formers.
- The ionic glass-former 2Ca(NO3)2 x 3KNO3 (CKN) serves as a model system for studying pressure-induced changes in glass properties.
- Ehrenfest relations and the Prigogine-Defay ratio are key theoretical tools for analyzing the thermodynamic response of glass transitions.
Purpose of the Study:
- To investigate the pressure dependence of the glass-transition temperature, Tg(P), for the CKN ionic glass.
- To calculate the liquid-glass differences in thermal expansivity (deltaalpha), heat capacity (deltaCp), and isothermal compressibility (deltak) as a function of pressure.
- To evaluate the validity of the Ehrenfest relations and analyze the pressure dependence of the Prigogine-Defay ratio (pi).
Main Methods:
- Molecular dynamics (MD) simulations were employed to model the CKN glass system under varying pressures.
- Thermodynamic properties including thermal expansivity, heat capacity, and isothermal compressibility were computed.
- The pressure dependence of Tg was analyzed using two Ehrenfest relations and the Prigogine-Defay ratio.
Main Results:
- The first Ehrenfest relation (dTg/dP = TVdeltaalpha/deltaCp) accurately predicts the pressure dependence of Tg for CKN.
- The second Ehrenfest relation (dTg/dP = deltakappa/deltaalpha) was found to be less accurate for this system.
- The Prigogine-Defay ratio (pi) was approximately 1.2 at low pressures but increased by an order of magnitude at high pressures.
Conclusions:
- The study validates one Ehrenfest relation while highlighting limitations of another for describing Tg(P) in CKN.
- The significant pressure dependence of the Prigogine-Defay ratio suggests complex underlying mechanisms in the glass transition.
- These findings contribute to a deeper understanding of the thermodynamic anomalies and pressure effects in ionic glasses.
