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Microheterogeneity in Aqueous Acetonitrile Solution Probed by Soft X‑ray Absorption Spectroscopy

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journal contribution
posted on 10.02.2020, 20:13 by Masanari Nagasaka, Hayato Yuzawa, Nobuhiro Kosugi
Chemical processes in solution are influenced by microheterogeneity (MH), where two liquids seem to be mixed in a macroscopic scale but are microscopically inhomogeneous. We have investigated one of the simplest MH systems, aqueous acetonitrile solution, using soft X-ray absorption spectroscopy (XAS). Molecular interactions of acetonitrile were revealed by C and N K-edge XAS at different concentrations, and those of solvent water were separately revealed by O K-edge XAS. The energy shift of the CN π* peak at the C K-edge shows three characteristic concentration regions and a phase-transition-like behavior between them. By comparing the energy shifts in XAS spectra with ab initio quantum chemical inner-shell calculations, we have determined local structures of acetonitrile–water mixtures in three concentration regions and found that the dipole interaction between acetonitrile and water is the key structure to emerge the MH state in the middle concentration region.