Solvation properties of silver ions in ionic liquids using attenuated total reflectance ultraviolet spectroscopy

•Solvation structures of ionic liquids (ILs) surrounding silver ions were experimentally and theoretically studied.•Combination of spectroscopy, multivariate analysis, and molecular dynamics simulations revealed the coordination number.•Opposite peak shift directions of ILs in the ultraviolet region upon additions of Li+ and Ag+ were detected.•The energy levels of the final states were different between Li+-coordinated and Ag+-coordinated ILs.•Electrical interaction between Ag+ and ILs was only within the first solvation layer, differing from the case of Li+. The solvation structures of N-butyl-N-methylpyrrolidinium dicyanamide ([BMP][DCA]) surrounding silver ions were investigated using attenuated total reflectance-ultraviolet (ATR-UV) spectroscopy. The addition of Ag+ to [BMP][DCA] red-shifted the [DCA]− absorption band. The shift direction was opposite to that caused by the addition of Li+, and these spectral shifts were qualitatively reproduced using quantum chemical calculations. Multivariate analysis (the multivariate curve resolution-alternating least squares; MCR-ALS) and molecular dynamics simulations were used to decompose the ATR-UV spectra into separate [DCA]− and Ag-solvated [DCA]− spectra. Both multivariate analysis and molecular dynamics simulation showed that the coordination number of [DCA]− around Ag+ was approximately-five. The proposed method using spectroscopic techniques, spectral decomposition, and simulations can be used for the quantitative investigation of various solvation structures, which are fundamental features in electrochemistry.