Is the science of thermal analysis kinetics based on solid foundations?: A literature appraisal

Abstract

A critical appraisal is made of the recent literature concerned with the kinetics and the mechanisms of thermal reactions, particularly research using the methods of thermal analysis (TA). It is concluded that many features of the theories and the practices in customary use are inadequate and/or unsatisfactory. The reaction models that are currently employed lack the necessary coherence required to introduce systematic and scientific order into the extensive information available. There are few reliable, general concepts and chemical principles which are capable of coordinating and classifying the experimental observations and which can contribute to the organic growth of an ordered discipline. Many, though not all, recent published TA studies appear as individual articles that remain largely unrelated to comparable investigations or to the general theoretical principles that are applicable throughout chemistry.This unacceptable situation is identified as having arisen during the emergence and establishment of thermoanalytical science as a distinct discipline, which effectively replaced the former interest in the thermal decompositions of solids, while also retaining concepts from this precursor topic. Overall this transformation was uncoordinated and perhaps has remained largely unrecognized. A consequence was that only some selected aspects of the heterogeneous and homogeneous kinetic theory and reaction models were incorporated into the developing (burgeoning) thermoanalytical subject area. However, during the initial rapid expansion of TA experimental methods, about three decades ago, shortcomings in the theories of thermal decomposition of solids were already becoming apparent, inhibiting advances. Consequently, some earlier optimistic hopes for progress in advancing solid state thermal chemistry were not fully realized. During the redirection of interest from thermal reactions of solids towards thermal methods, TA development proceeded without the appearance of new chemical principles or novel reaction models for general application. Progress in instrumentation predominantly focussed attention towards developing and applying automated calculation methods. This was essential to permit the analyses of larger number of accurate measurements that could be obtained, retained and processed by the more powerful computers that were becoming available to control experiments and to present processed results. The collection and collation of data was achieved more easily and efficiently than ever before, for ever larger number of data measurements, of ever greater precision, for an ever widening range of reactants and reaction conditions. The concurrent and continual extension of mathematical and computational techniques, for the analysis of thermal kinetic data, became, and remains, a principal preoccupation of TA research. Its early relationship with studies of thermal decompositions of solids has meant that thermal kinetic studies continue to apply, largely unchanged, aspects of the theory originally developed for application in thermal studies of solid state reactions.The adaptability and convenience of thermoanalytical methods for kinetic measurements has resulted in their application to a diversifying range of reactants, including thermal processes which sometimes proceed with melting and/or complex behaviours involving varied types of chemical changes. For these, the conventionally accepted range of kinetic models (predominantly solid state rate equations) may not be applicable, though the consideration of alternative possibilities is frequently ignored. The identification of chemical controls of reactivity and characterization of chemical reaction mechanisms are evidently now of much less importance in studies by TA methods than was usual formerly, when (originally) such interest was specifically concerned with the decompositions of solids. In recent work much less attention is directed towards designing experiments suitable for complementing kinetic interpretations and to contribute towards the formulation of chemical reaction mechanisms. Relevant additional observations include microscopy, spectroscopy, X-ray crystallography, etc., intended to provide information of value in the elucidation of the sequence of chemical steps participating in any reaction being studied.This tendency to minimize the application of chemical concepts in the analysis of TA rate data is self-perpetuating (autocatalytic) and results in a lack of coherence (or absence of scientific order) within the large accumulation of individual articles that constitutes the recent TA literature concerned with reaction kinetics. Because there are no widely applicable theoretical concepts that unify the subject, the possible generality of important results in particular reports tends to be disregarded. Reasons for this unsatisfactory situation are identified, from the present survey, as including the following: shortcomings in establishing reaction stoichiometry, uncertainty in the significance of kinetic data (rate measurements are often accepted uncritically and may, therefore, be empirical, varying with experimental conditions, and unsuitable for fundamental interpretation of reaction chemistry, including reactivity controls or mechanisms), and ambiguities in the definitions of essential terms, including mechanism, rate constant, activation energy. These, and other problems evident in TA studies, are discussed, together with the tendency to consider kinetic behaviour exclusively through rate equations applicable to solid state reactions. This convention results in the effective exclusion from all consideration of the possible participation of melting, the intervention of intermediates, concurrent or consecutive rate processes, secondary rate controls and other alternative, but nevertheless possible, reaction mechanisms.It is concluded that an important part of the thermoanalytical literature, concerned with kinetic and mechanistic TA studies, is in urgent need of a general and radical reappraisal. The introduction of systematic order into the contents of the continually expanding set of publications is now urgent, so that the significance of each new set of observations reported can be appreciated in its widest possible context. Science represents systematic knowledge, including the expectation that inductive prediction will generalize theory, through expansion of established trends: neither of these reasonable aspirations (order and prediction) are discernable in much of the recent TA literature.Following the decline of interest in solid state chemistry, together with the tendency within current TA reports to focus primarily (sometimes only) on kinetic characteristics of the reactions studied, it is appropriate to examine critically the options now available. These include the following. For each individual rate process studied, reaction stoichiometry must be established and the possible effects of secondary controls determined. Removal of the conventional constraints (probably unintentional and unappreciated) on kinetic analysis of TA data by extending interpretations to include all possible reaction models and influences on rate behaviour. It is also necessary to provide support for conclusions by complementary experimental observations whenever possible. Theoretical models that are applicable to solid state thermal decompositions are discussed, together with allowance for the possible participation by alternative reaction mechanisms (i.e., melting, etc.). The overall, and optimistic, conclusion from this survey is that thermoanalytical kinetic studies is a subject of considerable interest and promise. However, its potential can only be realized through fundamental reassessment of the methods and theories that continue to be used in a topic that appears to have stagnated and in which the chemical foundations have become eroded, or even forgotten.