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Microheterogeneity in Aqueous Acetonitrile Solution Probed by Soft X-ray Absorption Spectroscopy
Masanari Nagasaka1,2, Hayato Yuzawa1, Nobuhiro Kosugi1,2
1Institute for Molecular Science , Myodaiji, Okazaki 444-8585 , Japan.
Abstract:
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.
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