Determining Preexponential Factor in Model-Free Kinetic Methods: How and Why?

Sergey Vyazovkin

doi:10.3390/molecules26113077

Sergey Vyazovkin

Open Access

https://doi.org/10.3390/molecules26113077

Copy DOI

Abstract

The kinetics of thermally stimulated processes in the condensed phase is commonly analyzed by model-free techniques such as isoconversional methods. Oftentimes, this type of analysis is unjustifiably limited to probing the activation energy alone, whereas the preexponential factor remains unexplored. This article calls attention to the importance of determining the preexponential factor as an integral part of model-free kinetic analysis. The use of the compensation effect provides an efficient way of evaluating the preexponential factor for both single- and multi-step kinetics. Many effects observed experimentally as the reaction temperature shifts usually involve changes in both activation energy and preexponential factor and, thus, are better understood by combining both parameters into the rate constant. A technique for establishing the temperature dependence of the rate constant by utilizing the isoconversional values of the activation energy and preexponential factor is explained. It is stressed that that the experimental effects that involve changes in the preexponential factor can be traced to the activation entropy changes that may help in obtaining deeper insights into the process kinetics. The arguments are illustrated by experimental examples.

Highlights

Kinetic studies of thermally stimulated processes provide a key to understanding the thermal behavior of materials
This article has highlighted the importance of determining the preexponential factor as a part of model-free kinetic analysis
The emphasis has been on using a model-free way of estimating the preexponential factor because it is suitable for both single- and multi- step kinetics, i.e., for cases when the isoconversional activation energy does not practically vary with conversion and when it varies with conversion significantly

Summary

Introduction

Kinetic studies of thermally stimulated processes provide a key to understanding the thermal behavior of materials. Proper kinetic analysis can be performed via model-fitting and modelfree approaches as long as the calculations are based on simultaneous use of data obtained at multiple temperature programs. Most commonly, it means the usage of data collected at multiple heating (or cooling) rates. The model-free approach includes primarily the Kissinger method [3] as well as a variety of isoconversional methods [4]. The latter have experienced dramatic growth in popularity over the past two decades [4,5]. An illustrative example is that out of 13 papers in the Special Issue “Thermal Analysis Kinetics for Understanding Materials Behavior”

Objectives

Methods

Conclusion