Theoretical insights into nanoparticle interfaces as dual carrier-reactors: Catalytic amplification of methylsiloxane oxidation and secondary organic aerosol formation.

The environmental fate of volatile methylsiloxanes (VMS), pervasive pollutants, is critically governed by their interactions with nanoscale interfaces, yet these processes remain poorly understood. This study reveals how aqueous and mineral nanoparticles alter the oxidation processes of linear (L2) and cyclic (D3, D4) VMS by atmospheric oxidants through comparing the VMS reaction in the air-phase, bulk water, air-water, and air-dust interface by combining quantum chemical calculations with molecular dynamics simulation methods. Results show that VMS exhibit a surface preference at the air-water interface with retention rates from 64 % to 92 %, and exhibit higher interaction energy (> 10 kcal mol-1) on the mineral dust surface, with a stronger surface affinity at the air-dust interface than at the air-water interface. The presence of nanoparticles changes the reactivity from •Cl > •OH > NO3• to •Cl > NO3• > •OH. The aqueous nanoparticle interface catalyzes the reaction of D3 with •Cl/NO3• but inhibits the reaction of D3 with •OH, and the mineral nanoparticle promotes all the reactions with rate constants increased by 1 ∼ 6 orders of magnitude. These catalysis processes aggravate the formation burden of formaldehyde, and the coincidence of mineral nanoparticle and high-O/Si-ratio products increases the exposure risk of secondary organic aerosols. Although the •OH-initiated air-phase reaction has the smallest half-life (t1/2), the t1/2 value recognizes the dominant contribution of •Cl and NO3• to VMS attenuation in the presence of aqueous and mineral nanoparticles respectively. Our findings provide essential mechanistic insights for predicting VMS environmental impact and designing targeted mitigation strategies for nanoparticle-mediated pollution.