title: Rate Constants and Activation Energies for Gas-Phase Reactions of Three Cyclic Volatile Methyl Siloxanes with the Hydroxyl Radical.
abstract: Reaction with hydroxyl radicals (OH) is the major pathway for removal of cyclic volatile methyl siloxanes (cVMS) from air. We present new measurements of second-order rate constants for reactions of the cVMS octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) with OH determined at temperatures between 313 and 353 K. Our measurements were made using the method of relative rates with cyclohexane as a reference substance and were conducted in a 140-mL gas-phase reaction chamber with online mass spectrometry analysis. When extrapolated to 298 K, our measured reaction rate constants of D4and D5with the OH radical are 1.9 × 10-12(95% confidence interval (CI): (1.7-2.2) × 10-12) and 2.6 × 10-12(CI: (2.3-2.9) × 10-12) cm3molecule-1s-1, respectively, which are 1.9× and 1.7× faster than previous measurements. Our measured rate constant for D6is 2.8 × 10-12(CI: (2.5-3.2) × 10-12) cm3molecule-1s-1and to our knowledge there are no comparable laboratory measurements in the literature. Reaction rates for D5were 33% higher than for D4(CI: 30-37%), whereas the rates for D6were only 8% higher than for D5(CI: 5-10%). The activation energies of the reactions of D4, D5, and D6with OH were not statistically different and had a value of 4300 ± 2800 J/mol.
authors.affiliation: Department of Environmental Science and Analytical Chemistry (ACES) Stockholm University SE-10691 Stockholm Sweden.
authors.forename: Andreas
authors.initials: A
authors.lastname: Safron
authors.affiliation: Department of Environmental Science and Analytical Chemistry (ACES) Stockholm University SE-10691 Stockholm Sweden.
authors.forename: Michael
authors.initials: M
authors.lastname: Strandell
authors.affiliation: Department of Environmental Science and Analytical Chemistry (ACES) Stockholm University SE-10691 Stockholm Sweden.
authors.forename: Amelie
authors.initials: A
authors.lastname: Kierkegaard
authors.affiliation: Department of Environmental Science and Analytical Chemistry (ACES) Stockholm University SE-10691 Stockholm Sweden.
authors.forename: Matthew
authors.initials: M
authors.lastname: Macleod