Raman spectroscopy study of water confinement in ionic liquid 1-butyl-3-methylimidzolium nitrate

Clusterization process of water molecules called “water pockets” in the imidazolium-based room temperature ionic liquid (RTIL) 1‑butyl‑3‑methylimidazolium nitrate ([C4mim][NO3]) was investigated using Raman spectroscopy. Changes of Raman spectra were observed in four different spectral regions when the content of heavy water (D2O) in the RTIL-D2O system was continuously increased. It was found that different parameters of the Raman bands were sensitive to the added content of D2O. Discontinuous distribution in the concentration dependencies of those parameters was registered. The extent of “water pockets” formation process was determined from discontinuities in the concentration dependencies of monitored shifts and integral intensities of the Raman bands. The blue shift of symmetric NO3− stretching mode at 1041 cm−1 and the stretching mode at 2nd carbon position C(2)–H at 3100 cm−1 of imidazolium ring was noticed in the spectra even after adding of infinitesimal content of water to the mixture. The identical behavior analyzing the Raman band at 706 cm−1 assigned to NO3− bending mode was discovered. Moreover, the ratio of integral intensities of two Raman bands at 600 and 625 cm−1 assigned to GT and TT forms of butyl chain was also found to be dependent on the content of D2O in the mixture. The distribution among relative integral intensities of Raman bands assigned to vibrations of different water clusters was sensitive to the content of D2O in the mixture too. Overall, the lower boarder of “water pockets” formation process (X = 0.6 molar D2O ratio) in [C4mim][NO3] came up with the discontinuous spectral changes observed for NO3− stretching mode, and vibrational modes of different water clusters. The upper boarder of “water pockets” formation process (X = 0.86 M D2O ratio) was found out from discontinuities in concentration dependencies of imidazolium ring stretching mode, and vibrational modes of different water clusters. The conformational changes of [C4mim]+ cation at the same D2O concentration corresponding to the upper boarder of “water pockets” formation were observed.