The effect of functional groups on the glass transition temperature of atmospheric organic compounds: a molecular dynamics study

Organic compounds constitute a substantial part of atmospheric particulate matter not only in terms of mass concentration but also in terms of distinct functional groups. The glass transition temperature provides an indirect way to investigate the phase state of the organic compounds, playing a crucial role in understanding their behavior and influence on aerosol processes. Molecular dynamics (MD) simulations were implemented here to predict the glass transition temperature (T-g) of atmospherically relevant organic compounds as well as the influence of their functional groups and length of their carbon chain. The cooling step used in the simulations was chosen to be neither too low (to supress crystallization) nor too high (to avoid T-g overprediction). According to the MD simulations, the predicted T-g is sensitive to the functional groups as follows: carboxylic acid (-COOH) > hydroxyl (-OH) and (-COOH) > carbonyls (-C=O). Increasing the number of carbon atoms leads to higher T-g for the linearly structured compounds. Linear compounds with lower molecular weight were found to exhibit a lower T-g. No clear correlation between O : C and T-g was observed. The architecture of the carbon chain (linear, or branched, or ring) was also found to impact the glass transition temperature. Compounds containing a non-aromatic carbon ring are characterized by a higher T-g compared to linear and branched ones with the same number of carbon atoms.