Atmospheric Chemistry of Volatile Methyl Siloxanes – Kinetics and Oxidation Mechanism from Experimental and Theoretical Investigations

Mitchell Alton, ANYL PhD thesis defense,
Browne group

"Volatile methyl siloxanes (VMS) are solely anthropogenic chemicals that have come under recent scrutiny for their environmental persistence and tendency to bioaccumulate. Millions of tons of these chemicals are produced every year, with an estimated 30 kilotons of decamethylcyclopentasiloxane (D5) emitted into the environment every year, with additional contributions from other VMS. A large source of VMS to the environment is from use of personal care products containing these compounds. Due to environmental concerns about the impact of these compounds, the European Union placed restrictions on the quantity of VMS in wash-off personal care products in 2018 with recommendations to increase the restrictions to include some industrial processes in 2021. Although there is significant interest in these high-production chemicals, the understanding of the environmental fate of these compounds is incomplete. It is predicted that >90% of VMS released to the environment will partition into the atmosphere. Once these compounds are in the atmosphere, they can react with hydroxyl radicals (OH) or chlorine atoms (Cl) through hydrogen-abstraction reactions, though the previously reported rate constants for reactions with OH vary by more than a factor of 3 and only one measurement exists for the rate constant with Cl atoms. Additionally, previously published works reported inconsistent oxidation products of these compounds, likely due to unconstrained oxidation chemistry in their experiments. To better constrain the atmospheric chemistry and fate of these compounds, I measured the kinetics of seven VMS with OH radicals and Cl atoms in a 1 cubic meter FEP Teflon™ chamber I designed and built. Additionally, I measured the oxidation products of VMS in a variety of atmospheric conditions to better constrain the oxidation mechanism. Finally, I used quantum theoretical calculations to investigate the plausibility of the reactions proposed from our experimental results."