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

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.