Molecular Control of Reactive Gas Uptake "on Water"

Abstract

Reactive gas uptake on environmentally realistic aqueous surfaces is expected to be affected by a combination of multiple interactions. This issue is herein explored in experiments where the formation of Me_3NH^+ on neat and doped water microjets exposed to Me_3N(g) is monitored within <1 ms by online electrospray ionization mass spectrometry as a function of pH of the bulk liquid (pH_(BLK)). Notably, M_e3N(g) is protonated on the surface of neat water microjets below pH_(BLK)~4, rather than at pH_(BLK) pKA ≾ (Me_3NH^+) = 9.8 as in bulk water. Me_3N(g) uptake is significantly enhanced by anionic surfactants and fulvic acid (a surrogate of complex natural organic matter) above pH_(BLK)~4, uniformly depressed by cationics (which otherwise counteract FA effects), and unaffected by n-octanol. The direct hydrogen isotope effects associated with enhanced uptake of Me_3N(g) on H_2O/D_2O microjets implicate a process controlled by proton transfer from interfacial donors whose coverage is electrostatically modulated by ionic headgroups. The finding that the combined effect of fulvic acid and tetrabutylammonium bromide closely matches the geometric mean of their separate effects on TMA uptake is evidence of strong dopant interactions.

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