Anomalous Dependence of Translational Diffusion on the Water Mole Fraction for Solute Molecules Dissolved in a 1-Butyl-3-methylimidazolium Tetrafluoroborate/Water Mixture

Yoshifumi Kimura; Rie Akai; Kohei Shimada; Mizuki Hirata; Kaori Fujii; Takatsugu Endo

doi:10.1021/acs.jpcb.3c01978

Anomalous Dependence of Translational Diffusion on the Water Mole Fraction for Solute Molecules Dissolved in a 1-Butyl-3-methylimidazolium Tetrafluoroborate/Water Mixture

J Phys Chem B. 2023 Jun 29;127(25):5684-5692. doi: 10.1021/acs.jpcb.3c01978. Epub 2023 Jun 13.

Authors

Yoshifumi Kimura^{1

2}, Rie Akai¹, Kohei Shimada², Mizuki Hirata¹, Kaori Fujii¹, Takatsugu Endo¹

Affiliations

¹ Division of Molecular Chemistry and Biochemistry, Faculty of Science and Engineering, Doshisha University, 2-1, Tatara Miyakodani, Kyotanabe 610-0321, Kyoto, Japan.
² Department of Applied Chemistry, Graduate School of Science and Engineering, Doshisha University, 2-1, Tatara Miyakodani, Kyotanabe-City 610-0321, Kyoto, Japan.

PMID: 37310854
DOI: 10.1021/acs.jpcb.3c01978

Abstract

Translational diffusion coefficients of carbon monoxide (CO), diphenylacetylene (DPA), and diphenylcyclopropenone (DPCP) were determined in mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate ([C₄mim]BF₄) and water using transient grating spectroscopy at different mole fractions of water (x_w). While DPA exhibited a larger diffusion coefficient than DPCP at low water mole fractions (x_w < 0.7), as observed for conventional liquids and ionic liquids (ILs), it was smaller at high mole fractions (x_w > 0.9). The apparent molecular radius of DPA determined using the Stokes-Einstein equation at x_w > 0.9 is close to the radius of an IL cluster in a water pool as determined from small-angle neutron scattering experiments (J. Bowers et al., Langmuir, 2004, 20, 2192-2198), suggesting that the DPA molecules are trapped in IL clusters in the water pool and move together. The solvation state of DPCP in the mixture was studied using Raman spectroscopy. Dramatically strong water/DPCP hydrogen bonding was observed at higher water mole fractions, suggesting that DPCP is located near the cluster interfaces. The large diffusion coefficient of DPCP suggests that hopping of DPCP between IL clusters occurs through hydrogen bonding with water.