α-Hydroxyalkyl-hydroperoxides (α-HHs), from the addition of water to Criegee intermediates in the ozonolysis of olefins, are reactive components of organic aerosols. Assessing the fate of α-HHs in such media requires information on the rates and products of their reactions in aqueous organic matrixes. This information, however, is unavailable due to the lack of analytical techniques for the detection and identification of labile α-HHs. Here, we report the mass spectrometric detection (as Cl– adducts) of the α-HH produced in the ozonolysis of a C15 diolefin in water (W):acetonitrile (AN) mixtures of variable composition containing inert NaCl. α-HH decays into a gem-diol + H2O2 within τ1/e ≈ 52 min in 50% (v:v) water, but persists longer than a day in ≤10% water mixtures. The strong nonlinear dependence of τ1/e on solvent composition reveals that water content is a major factor controlling the fate of α-HHs in atmospheric particles. It also suggests that α-HH decomposes while embedded in WnANm clusters rather than randomly dissolved in molecularly homogeneous W:AN mixtures.

Multiple pathways in crystal nucleation are now known to be more common than previously predicted; it is, therefore, crucial to understand the early stages of crystallization. Even in relatively simple vapor-phase homogeneous nucleation, the process has significant potential diversity. Here, we experimentally show crystalline Al2O3 nanoparticles forming via precisely two steps in the nucleation process from supersaturated vapor with a moderate cooling rate. In situ FT-IR measurement of nucleation allowed us to observe the formation of Al2O3 nanoparticles. Liquid-like particles first nucleated from the vapor before crystallizing. The crystalline phase was preserved by quenching without further transformation into the most stable α-Al2O3 polymorph. The precipitated phase changed from δ-Al2O3 for pure Al2O3 to γ-Al2O3 or θ-Al2O3 by adding Sb or Cr, respectively. We demonstrate that a two-step process occurs in homogeneous nucleation of refractory materials from supersaturated vapor, which may facilitate polymorphic control in industry and improve understanding of cosmic dust formation in space.

Many chemical reactions in Nature, the laboratory, and chemical industry occur in solvent mixtures that bring together species of dissimilar solubilities. Solvent mixtures are visually homogeneous, but are not randomly mixed at the molecular scale. In the allimportant binary water-hydrotrope mixtures, small-angle neutron and dynamic light scattering experiments reveal the existence of short-lived (<50 ps), short-ranged (∼1 nm) concentration fluctuations. The presence of hydrophobic solutes stabilizes and extends such fluctuations into persistent, mesoscopic (10-100 nm) inhomogeneities. While the existence of inhomogeneities is well established, their impacts on reactivity are not fully understood. Here, we search for chemical signatures of inhomogeneities on the surfaces of W:X mixtures (W = water; X = acetonitrile, tetrahydrofuran, or 1,4-dioxane) by studying the reactions of Criegee intermediates (CIs) generated in situ from O 3 (g) addition to a hydrophobic olefin (OL) solute. Once formed, CIs isomerize to functionalized carboxylic acids (FC) or add water to produce α-hydroxy-hydroperoxides (HH), as detected by surface-specific, online pneumatic ionization mass spectrometry. Since only the formation of HH requires the presence of water, the dependence of the R = HH/FC ratio on water molar fraction x w expresses the accessibility of water to CIs on the surfaces of mixtures. The finding that R increases quasi-exponentially with x w in all solvent mixtures is consistent with CIs being preferentially produced (from their OL hydrophobic precursor) in X-rich, long-lived OL:X m W n interfacial clusters, rather than randomly dispersed on W:X surfaces. R vs x w dependences therefore reflect the average m, n composition of OL:X m W n interfacial clusters, as weighted by cluster reorganization dynamics. Water in large, rigid clusters could be less accessible to CIs than in smaller but more flexible clusters of lower water content. Since mesoscale inhomogeneities are intrinsic to most solvent mixtures, these phenomena should be quite general.Published under license by AIP Publishing. https://doi.org/10.1063/1.5055684The addition of solutes to binary mixtures generates the larger (in the 10-100 nm mesoscale), more persistent (from ARTICLE scitation.org/journal/jcpIn our experiments, (β-C + NaCl) solutions in W:X mixtures are injected (at 100 µl min −1 ) as liquid microjets (v y = 21 cm/s, see Fig. S1) into the spraying chamber of the mass spectrometer (ES-MS, Agilent 6130 Quadrupole LC/MS Electrospray System at NIES, Japan) through an electrically grounded stainless-steel syringe needle (100 µm bore). The ARTICLE scitation.org/journal/jcp (2017).

Biogenic monoterpenes are major sources of Criegee intermediates (CIs) in the troposphere. Recent studies underscored the importance of their heterogeneous chemistry. The study of monoterpene CI reactions on liquid surfaces, however, is challenging due to the lack of suitable probes. Here, we report the first mass spectrometric detection of the intermediates and products, which include labile hydroperoxides, from reactions of CIs of representative monoterpenes (α-terpinene, γ-terpinene, terpinolene, d-limonene, α-pinene) with water, cis-pinonic acid (CPA) and octanoic acid (OA) on the surface of liquid microjets. Significantly, the relative yields of α-hydroxy-hydroperoxides production from CIs hydration at the gas-liquid interface-α-terpinene (1.00) ≫ d-limonene (0.18) > γ-terpinene (0.11) ∼ terpinolene (0.10) ≫ α-pinene (0.01)-do not track the rate constants of their gas-phase ozonolyses. Notably, in contrast with the inertness of the other CIs, the CIs derived from α-terpinene ozonolysis readily react with CPA and OA to produce C and C ester hydroperoxides, respectively. Present results reveal hitherto unknown structural effects on the reactivities of CIs at aqueous interfaces.

Recent studies have shown that atmospheric particles are sufficiently acidic to enhance the uptake of unsaturated volatile organic compounds (VOCs) by triggering acid-catalyzed oligomerization. Controlling factors of oligomerization at the aqueous surfaces, however, remain to be elucidated. Herein, isoprene (2-methyl-1,3-butadiene, ISO), 1,3-butadiene (1,3-b), 1,4-pentadiene (1,4-p), 1-pentene (1-p), and 2-pentene (2-p) vapors are exposed to an acidic water microjet (1 ≤ pH ≤ 5), where cationic products are generated on its surface within ∼10 μs and directly detected using surface-sensitive mass spectrometry. We found that carbocations form at the air-water interface in all the cases, whereas the extent of oligomerization largely depends on the structure in the following order: ISO ≫ 1,3-b > 1,4-p ≫ 1-p ≈ 2-p. Importantly, the cationic oligomerization of ISO yields a protonated decamer ((ISO)10H+, a C50 species of m/z 681.6), while the pentenes 1-p/2-p remain as protonated monomers. We suggest that ISO oligomerization is uniquely facilitated by (1) the resonance stabilization of (ISO)H+ through the formation of a tertiary carbocation with a conjugated C[double bond, length as m-dash]C bond pair, and (2) π-electron enrichment induced by the neighboring methyl group. Experiments in D2O and D2O : H2O mixtures revealed that ISO oligomerization on the acidic water surface proceeds via two competitive mechanisms: chain-propagation and proton-exchange reactions. Furthermore, we found that ISO carbocations undergo addition to relatively inert 1-p, generating hitherto uncharacterized co-oligomers.