Detecting Intermediates and Products of Fast Heterogeneous Reactions on Liquid Surfaces via Online Mass Spectrometry

Abstract

One of the research priorities in atmospheric chemistry is to advance our understanding of heterogeneous reactions and their effect on the composition of the troposphere. Chemistry on aqueous surfaces is particularly important in this regard because of their ubiquity and expanse. They range from the surfaces of oceans (360 million km2), cloud and aerosol drops (~10 trillion km2) to the fluids lining the human lung (~200 m2). Typically, ambient air contains reactive gases that may affect human health, influence climate and participate in biogeochemical cycles. Despite their importance, reactions between gases and solutes at air-aqueous interfaces are not well understood. New, surface-specific techniques are required that detect and identify the intermediates and products of such reactions as they happen on liquids. This is a tall order because genuine interfacial reactions are faster than mass diffusion into bulk liquids, and may produce novel species at low concentrations. Herein, we review evidence that validates online pneumatic ionization mass spectrometry of liquid microjets dosed by reactive gases as a technique meeting such requirements. Next, we call attention to results obtained by this approach on reactions of ozone, nitrogen dioxide and hydroxyl radicals with various solutes on aqueous surfaces. The overarching conclusion is that the outermost layers of aqueous solutions are unique media, where equilibria shift and reactions proceed faster than, in some cases along different pathways from the bulk liquids. The fact that the rates and mechanisms of reactions at air-aqueous interfaces may not be deduced from experiments in bulk liquids opens new conceptual frameworks and lines of research, and adds an overlooked dimension to atmospheric chemistry.

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

“…The similar values of χ n (χ 8 ≈ χ 10 ≈ χ 12 ≈ χ 14 ) observed at concentration ranging from 10 to 50 μM equimolar solution (Figure 4 and SI; Figure S3) implies the presence of similar surface number densities of the long-chain species (n = 8, 10,12,14) at the air−water interface. Notably, although the total signals for the ammonium cations (the sum of the signals at m/ z = 74.2, 116.2, 172.2, 200.3, 228.3, and 256.3; ΣQ i ) increased monotonically as a function of the logarithmic concentration of the alkylammonium salts (SI; Figure S6), each signal showed specific behaviors, in that those for the shorter-chain alkylammonium cations (n ≤ 4) decreased as the alkylammonium salt concentration increased, whereas those for the longer-chain alkylammonium cations (n = 10, 12, 14) increased at larger concentrations.…”

“…The corresponding mass spectra for 1, 50, and 500 μM equimolar solution are shown in the SI, Figure S4. Although the bulk concentrations were the same, the mass-signal intensities of the six alkylammonium cations Me-(CH 2 ) n−1 N + Me 3 (n = 1,4,8,10,12,14) increased with increasing chain length (n). 31 For example, the signal intensity of Me(CH 2 ) 13 N + Me 3 (n = 14) was ∼100 times larger than that of Me 4 N + (n = 1) obtained from microjets of a 500 μM equimolar solution (Figure 4; blue triangles).…”

“…Such a small ε is predicted for the outermost layer of the water−hydrophobe interface of <0.2 nm, according to molecular dynamics simulations. 36 Figure 5 shows plots for the mass signals of Me-(CH 2 ) n−1 N + Me 3 (n = 1,4,8,10,12,14), normalized by the mass signal in the absence of octan-1-ol, for 50 μM equimolar chloride solutions as a function of the concentration of octan-1-ol in the range 1−3000 μM. The sum of the Me-(CH 2 ) n−1 N + Me 3 signals decreased by 20% in the presence of 1000 μM octan-1-ol, implying hindrance by octan-1-ol of the ammonium cations.…”

“…11,12 The unique features of our method that are differentiated from conventional electrospray ionization mass spectrometry 13 are discussed in the Experimental Section and elsewhere. 14,15 Using this method, we have investigated several aspects of carbocation chemistry at the air−water interface. We found that a less hydrated hydronium species (H 3 O) + (H 2 O) x<6 that is formed in the topmost layers of a water surface of bulk pH < 4 can transfer a proton (H + ) to a gaseous unsaturated hydrocarbon of higher proton affinity (e.g., isoprene) to form a carbocation; we named this the proton-transfer (PT) reaction: 16,17…”

Reply to the ‘Comment on “The chemical reactions in electrosprays of water do not always correspond to those at the pristine air–water interface”’ by A. J. Colussi and S. Enami, Chem. Sci., 2019, 10, DOI: 10.1039/c9sc00991d

“…[2][3][4][5][6][7]23,[26][27][28] What should we infer from the wide range of molecules that interact with electrosprays of pH-adjusted water and exhibit similar titration curves due to proton transfers and/or protoncatalyzed reactions? 3,12,[30][31][32][33] Interestingly, the proton affinities of the gases in the gas-liquid collision experiments studied by Colussi & co-workers were always higher than that of water. [2][3][4] This observation suggests that they, in fact, investigated chemical reactions of those reactants with protonated water clusters, just like the one we unraveled in our own work.…”