Oxidation dynamics of e-liquids in electronic nicotine delivery systems with atomisers of various types
The processes of the thermal oxidative destruction of e-liquid components play a key role in the appearance of undesirable products in aerosols generated by electronic nicotine delivery systems (ENDS). The objective of the present studies is to obtain insight into understanding the key factors, that govern the oxidation processes in
e-liquids upon the ENDS operation. The basic methods we used pertained to the kinetic approaches of the oxidation chemistry, including measuring the chemiluminescence emission derived from interactions of reactive oxygen species (ROS) and peroxide reactions and monitoring the formation of oxidation products. The main results of the reported experiments are the following. We have shown for the first time that the temperature-dependent formation of carbonyl compounds in the ENDS aerosols proceeds through the free-radical chain mechanism with the intervention of hydroperoxides. However, the temperature, being the most important parameter, is not the only factor accounting for the occurrence of the pertinent oxidation products in the ENDS emissions, and materials of evaporators, contents of e-liquids and an aeration of atomizers influence significantly the oxidative status of the ENDS aerosols. In the present work, we examined the diversity of factors controlling the oxidative developments in e-liquids upon vaping with a particular emphasis on comparing the oxidation dynamics in cases of ceramic and metal atomizers. In this context, the advantages and disadvantages of each atomizer type are revealed. Besides, the direct microscopy of condensates of the ENDS aerosols enabled us to elucidate the structure of microparticles formed as a result of the thermal destruction of heating coils and fibrous materials of evaporators. The above-mentioned findings are of interest for improving the ENDS design and for optimizing their functioning and handling.