Chemical signatures of surface microheterogeneity on liquid mixtures

Abstract

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).

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

“…Although the flow of β-caryophyllene molecules injected into the spraying chamber in this case was much larger than in the first set of experiments, product signals remained within background noise ( Figure S2, Reference 26). 26 We inferred that most if not all of the FC and HH products we observed were produced on the surface of intact liquid microjets rather than on microdroplets or in the gas-phase. Similar experiments on the protonation of isoprene on acidic water, in which we tested the effect on product signals of the relative distance between the isoprene gas streams and the tip of the nebulizer, led to the same conclusion.…”

“…[31][32] More direct evidence was obtained from the dependence of the FC and HH (see entry 4, above) products signal intensities in the ozonolysis of β-caryophyllene as functions of the applied polarizing voltage. 26 It was found that both signal intensities strictly increased (with a correlation coefficient ρ 2 = 0.99) with the square root of the polarizing voltage V ½ (see Figure S3 of Reference 26). 26 We inferred the signature of a purely kinematic effect on the trajectories of charged microdroplets in the spraying chamber, rather than electric field effects on chemistry.…”

“…26 It was found that both signal intensities strictly increased (with a correlation coefficient ρ 2 = 0.99) with the square root of the polarizing voltage V ½ (see Figure S3 of Reference 26). 26 We inferred the signature of a purely kinematic effect on the trajectories of charged microdroplets in the spraying chamber, rather than electric field effects on chemistry.…”

Stability of Monoterpene-Derived α-Hydroxyalkyl-Hydroperoxides in Aqueous Organic Media: Relevance to the Fate of Hydroperoxides in Aerosol Particle Phases

“…Since microheterogeneity should be a general phenomenon in "internally mixed" aqueous organic mixtures, we suggest that the existence of inhomogeneities at the molecular level, as distinct from mesoscopic segregation, [63][64][65] could play unanticipated roles in atmospheric aqueous media. 56 The relatively short lifetimes of the -HHs derived from the ozonolysis of monoterpenes relative to the dramatic persistence of the -HH derived from -Tp (Table and Fig a  1/e = 1/k 1 catalyzed reaction. This is an important effect, considering that ambient cloud water and aqueous aerosols are more acidic than previously assumed.…”

Polarimetry as a method for studying the structure of aqueous carbohydrate solutions: correlation with other methods

“…This new type of "weak" (judging by the magnitude of the interaction energy, which does not exceed k B T [17]), but the extremely effective and spontaneous structuring of liquids has not attracted the attention of researchers for a long time. Only recently has its importance been identified for an adequate description of reactions and other chemical processes [8,9,13,14,[18][19][20][21][22][23][24][25][26][27][28][29].…”