Rate Constants and Activation Energies for Gas-Phase Reactions of Three Cyclic Volatile Methyl Siloxanes with the Hydroxyl Radical.

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 (D), decamethylcyclopentasiloxane (D), and dodecamethylcyclohexasiloxane (D) 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 D and D with the OH radical are 1.9 × 10 (95% confidence interval (CI): (1.7-2.2) × 10) and 2.6 × 10 (CI: (2.3-2.9) × 10) cm molecule s, respectively, which are 1.9× and 1.7× faster than previous measurements. Our measured rate constant for D is 2.8 × 10 (CI: (2.5-3.2) × 10) cm molecule s and to our knowledge there are no comparable laboratory measurements in the literature. Reaction rates for D were 33% higher than for D (CI: 30-37%), whereas the rates for D were only 8% higher than for D (CI: 5-10%). The activation energies of the reactions of D, D, and D with OH were not statistically different and had a value of 4300 ± 2800 J/mol.