Temperature Dependent Rate Coefficients for the Gas-Phase Reaction of the OH Radical with Linear (L₂, L₃) and Cyclic (D₃, D₄) Permethylsiloxanes

Permethylsiloxanes are emitted into the atmosphere during production and use as personal care products, lubricants, and cleaning agents. The predominate atmospheric loss process for permethylsiloxanes is expected to be via gas-phase reaction with the OH radical. In this study, rate coefficients, k (T), for the OH radical gas-phase reaction with the two simplest linear and cyclic permethylsiloxanes were measured using a pulsed laser photolysis–laser induced fluorescence technique over the temperature range of 240–370 K and a relative rate method at 294 K: hexamethyldisiloxane ((CH 3 ) 3 SiOSi(CH 3 ) 3, L 2 ), k 1; octamethyltrisiloxane ([(CH 3 ) 3 SiO] 2 Si(CH 3 ) 2, L 3 ), k 2; hexamethylcyclotrisiloxane ([-Si(CH 3 ) 2 O-] 3, D 3 ), k 3; and octamethylcyclotetrasiloxane ([-Si(CH 3 ) 2 O-] 4, D 4 ), k 4 . The obtained k (294 K) values and temperature-dependence expressions for the 240–370 K temperature range are (cm 3 molecule –1 s –1, 2σ absolute uncertainties): k 1 (294 K) = (1.28 ± 0.08) × 10 –12, k 1 ( T ) = (1.87 ± 0.18) × 10 –11 exp(−(791 ± 27)/ T ); k 2 (294 K) = (1.72 ± 0.10) × 10 –12, k 2 ( T ) = 1.96 × 10 –13 (T/298) 4.34 exp(657/ T ); k 3 (294 K) = (0.82 ± 0.05) × 10 –12, k 3 ( T ) = (1.29 ± 0.19) × 10 –11 exp(−(805 ± 43)/ T ); and k 4 (294 K) = (1.12 ± 0.10) × 10 –12, k 4 ( T ) = (1.80 ± 0.26) × 10 –11 exp(−(816 ± 43)/ T ). The cyclic molecules were found to be less reactive than the analogous linear molecule with the same number of −CH 3 groups, while the linear and cyclic permethylsiloxane reactivity both increase with the increasing number of CH 3 – groups. The present results are compared with previous rate coefficient determinations where available. The permethylsiloxanes included in this study are atmospherically short-lived compounds with estimated atmospheric lifetimes of 11, 8, 17, and 13 days, respectively.