Temperature Dependent Rate Coefficients for the Gas-Phase Reaction of the OH Radical with Linear (L 2 ,L 3 ) and Cyclic (D 3 ,D 4 ) Permethylsiloxanes Franc ̧ ois Bernard, †,‡ Dimitrios K. Papanastasiou, †,‡ Vassileios C. Papadimitriou, †,‡,§ and James B. Burkholder* ,† † Earth System Research Laboratory, Chemical Sciences Division, National Oceanic and Atmospheric Administration, Boulder, Colorado 80305, United States ‡ Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80309, United States * S Supporting Information ABSTRACT: 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 temper- ature 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. 1. INTRODUCTION Permethylsiloxanes are linear or cyclic molecules containing repetitive −Si(CH 3 ) 2 O− subunits that are commonly used in consumer products such as cosmetics, textiles, common health care, and household products. 1,2 Permethylsiloxanes have no known natural source and are emitted into the atmosphere during production, use, and disposal. Permethylsiloxanes are slightly water-soluble 3 and undergo only limited aqueous-phase degradation. 4,5 Permethylsiloxanes are expected to partition into the gas-phase with emissions expected to be greatest in highly populated regions, 6−9 while permethylsiloxanes have been observed in urban, 10,11 indoor, 12,13 and remote locations. 14,15 The bioaccumulation of permethylsiloxanes, which have octanol/water partition coefficients, log K OW , on the order of 7.0 to 8.9, in aquatic biota is also an area of active research. 16−19 The environmental chemistry of permethylsiloxanes has been the subject of several review articles. 20,21 It has been suggested, following the identification of silicon in atmospheric nanoparticles, 22,23 that the atmospheric photo- chemical processing of siloxanes may contribute to new particle 23 and secondary organic aerosol (SOA) formation. 24−26 In environmental chamber studies, the degradation of permethylsi- loxanes has been shown to lead to the formation of highly oxygenated low volatility compounds (e.g., silanols) 27,28 that may contribute to aerosol formation and growth with potential impacts on air quality. The predominant gas-phase removal process for permethylsiloxanes is expected to be via reaction with the OH radical, and permethylsiloxane atmospheric lifetimes are expected to be on the order of weeks. Knowledge of the rate coefficients for the OH radical reactions is therefore an essential element for the evaluation of their atmospheric lifetimes and initiation of their atmospheric degradation. There are several OH + permethylsiloxane kinetic studies in the current literature, 28−32 but there remains substantial Received: February 24, 2018 Revised: April 4, 2018 Published: April 6, 2018 Article pubs.acs.org/JPCA Cite This: J. Phys. Chem. A 2018, 122, 4252-4264 © 2018 American Chemical Society 4252 DOI: 10.1021/acs.jpca.8b01908 J. Phys. Chem. A 2018, 122, 4252−4264