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Published January 14, 2019 | Supplemental Material + Published
Journal Article Open

Chemical signatures of surface microheterogeneity on liquid mixtures


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 all-important 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 xwxwexpresses the accessibility of water to CIs on the surfaces of mixtures. The finding that Rincreases quasi-exponentially with xwxw in all solvent mixtures is consistent with CIs being preferentially produced (from their OL hydrophobic precursor) in X-rich, long-lived OL:X_mW_n interfacial clusters, rather than randomly dispersed on W:X surfaces. R vs xwxwdependences therefore reflect the average ⟨m, n⟩ composition of OL:X_mW_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.

Additional Information

© 2019 Published under license by AIP Publishing. Submitted: 9 September 2018; Accepted: 10 December 2018; Published Online: 8 January 2019. We are grateful to Dr. Satoshi Inomata, Dr. Akihiro Fushimi, Dr. Kei Sato, and Dr. Yu Morino of NIES and Dr. Lijie Li and Dr. Michael Hoffmann of Caltech for valuable discussions. This work was supported in part by the JSPS KAKENHI (Grant Nos. 15H05328 and 15K12188). A.J.C. acknowledges support from the National Science Foundation, USA (Grant No. AGS-1744353).

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