Influence of atmospheric oxygen on the structure and electronic properties of bitumen components – A DFT study
Szymon Malinowski, Lidia Bandura, Agnieszka Woszuk
Abstract
Extensive research on oxidative ageing has been carried out in recent years, mainly by means of experimental methods. In spite of continuous development of theoretical methods and their wider application in road construction, in literature there is no description of electronic properties of individual bitumen components in the aspect of their sensitivity to atmospheric O2 and changes due to the oxidation reaction. For this purpose, quantum chemical modelling by using the DFT method can be successfully applied. The results of theoretical analyses presented in this paper demonstrate that asphaltenes are the most exposed to atmospheric O2. Among the analysed bitumen components, asphaltenes are characterised by the highest chemical reactivity as well as the ability to donate and accept electrons; therefore, they undergo the most intensive oxidative ageing processes, both by radical reactions and formation of the intermediate product, i.e. –COOH. The quantum-chemical calculations carried out at DFT/B3LYP/6–311 g-dp theory level showed that during asphaltene exposure to O2, the mechanism of their ageing changes. Prolonged exposure to atmospheric O2 reduces the participation of radical reactions in favour of oxidation by the formation of –COOH. This effect is seen for all the bitumen fractions, except saturates. They do not undergo oxidative ageing due to their low electron donating and accepting capacity as well as high chemical stability.