Acid-Catalyzed Oligomerization at the Air–Water Interface Modified by Competitive Adsorption of Surfactants

Abstract

Amphiphilic organic compounds that accumulate naturally on the outermost layers of aqueous aerosols totally change the mechanisms involved in the uptake and reactions of volatile organic compounds adsorbed on the surfaces. By means of mass spectrometry of aqueous microjets, we examined how quaternary alkylammonium cations Me(CH 2 ) n−1 N + Me 3 (n = 1, 4, 8, 10, 12, and 14) and nonionic octan-1-ol influence acid-catalyzed oligomerization of gaseous isoprene (ISO) at the air−water interface. The oligomerization is initiated by proton-transfer (PT) reaction from an interfacial hydronium ion to isoprene to form a (ISO)H + and is subsequently followed by chainpropagation (CP) reaction to form (ISO) m≥2 H + . We found that although the total mass spectral signals of (ISO) m H + products were suppressed by the presence of either the alkylammonium cations or octan-1-ol, the suppression by the former surfactants was much larger than that of the latter. The suppression was observed even at aqueous microjets of 5 μM equimolar solution of the alkylammonium cations at which the air−water interface is only sparsely occupied by the alkylammonium cations. We propose that electrostatic repulsions between the alkylammonium cation and the interfacial H 3 O + cause the hindrance of the PT reaction. In contrast, the subsequent CP reactions were not suppressed by the presence of either the alkylammonium cations or octan-1-ol. We suggest that the presence of long-chain organic surfactants hinders the initial PT reaction but hardly suppresses the subsequent CP reaction. Our study has revealed hitherto unrecognized interface-specific roles of surfactants that affect multiphase chemistry in the atmosphere.

Hydronium Ion Acidity Above and Below the Interface of Aqueous Microdroplets

“…Significantly, the gas-phase basicities of ISO and TMA are widely different, but their titration curves are identical within experimental error. Similar titration curves were obtained with other gas-phase olefins and terpenes. − We infer that the common equivalence point at pH 1/2 ∼ 3.6 is a property of OTL water rather than of the gases being protonated.…”

“…This was confirmed by the isotope effects, sometimes very large, observed in the protonation/deuteration of several gases on H 2 O/D 2 O microdroplets, ,, and by the sensitive dependence of the extent of protonation (and polymerization) of olefins on molecular structure. The relative rates of chain propagation, chain transfer, and hydride abstraction reactions of the carbocations resulting from the protonation of olefins proved to be quite sensitive to inductive and conjugative effects. − , …”

Interfacial Water Mediates Oligomerization Pathways of Monoterpene Carbocations

“…In previous studies, , we found that the uptake of gaseous unsaturated hydrocarbons and subsequent chain reactions on the surface of water are governed by chemistry that differs from that in the bulk solution. For example, whereas conjugated CC bonds suppress the CP reaction in bulk organic solution, at the air–water interface the CP reaction is promoted. ,, In the present study, partial hydration by interfacial water molecules is found to be a key controlling factor that makes the interfacial reaction unique.…”

“…For example, whereas conjugated CC bonds suppress the CP reaction in bulk organic solution, at the air−water interface the CP reaction is promoted. 19,35,48 In the present study, partial hydration by interfacial water molecules is found to be a key controlling factor that makes the interfacial reaction unique.…”

Aerosol Droplet Surface Measurement Methods

“…8 The small size of developing aerosol droplets makes it incredibly challenging to directly measure the surface tension, as well as the composition of individual micrometer sized droplets. [17][18][19] Current techniques rely on indirect measurements such as determining the composition of an accumulated aerosol droplet sample.…”