Microscopic Viscosity of Aqueous Solution of Saccharides: A Study by Ultrafast Pump-Probe Spectroscopy

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  • NAGASAWA Yutaka
    Department of Chemistry, Graduate School of Engineering Science and Research Center for Materials Science at Extreme Conditions, Osaka University
  • NAKAGAWA Yukako
    Department of Chemistry, Graduate School of Engineering Science and Research Center for Materials Science at Extreme Conditions, Osaka University
  • KENMOCHI Junya
    Department of Chemistry, Graduate School of Engineering Science and Research Center for Materials Science at Extreme Conditions, Osaka University
  • OKADA Tadashi
    Department of Chemistry, Graduate School of Engineering Science and Research Center for Materials Science at Extreme Conditions, Osaka University

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  • 1. Microscopic Viscosity of Aqueous Solution of Saccharides : A Study by Ultrafast Pump-Probe Spectroscopy

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Ultrafast pump-probe (PP) spectroscopy was applied to the measurements of microscopic viscosity of the aqueous solutions of saccharides. A dye molecule, malachite green (MG), the electronic excited state lifetime of which depends on solvent viscosity, was used as a probe molecule. The viscosity of the solution measured by a viscometer increased nonlinearly with saccharide concentration and this increase was the largest for the trehalose solution. The decay of the PP signal of MG was fitted with a multi-exponential function and it showed that the molecular dynamics occurring in the ultrafast time region, <200 fs, was not affected by the viscosity. The dynamics slower than 1 ps showed clear viscosity dependence and at high concentration of disaccharide, the picosecond decay became bi-exponential. This indicates that heterogeneity of the solvent has increased, i.e. the excited state lifetime of MG molecules trapped in the vicinity of the aggregated sugar molecules becomes longer because they sense higher microscopic viscosity. The time constants for the picosecond decay were plotted against the viscosity. Although trehalose showed the largest nonlinear increase of the viscosity with the concentration, the viscosity dependence of the picosecond time constants was the weakest among the measured saccharides. This result indicates that aggregated hydrogen-bonding network of trehalose efficiently interrupts macroscopic translational motion of the solution, although its effect on microscopic molecular motion, such as the rotation of the phenyl group, is small.

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