Two‐dimensional Raman spectroscopy study of ionogel phase formation in long‐chain ionic liquid/water systems

Abstract

The phase behavior of the imidazolium‐based room temperature ionic liquids (RTILs) 1‐decyl‐3‐methyl‐imidazolium bromide and chloride ([C10mim][Br], [C10mim][Cl]) were studied by 2D Raman correlation spectroscopy and principal component analysis. The hydrogen‐bonded network changes and the liquid crystalline ionogel phase formation were observed when water content was continuously increased in the system, and the Raman shift of C–H stretching modes in spectra was monitored. The extent of liquid crystalline ionogel phase was determined from the discontinuities in the concentration dependencies. The focus to the Raman bands originated from the imidazolium ring C–H stretching mode region (3000–3200 cm−1) when water content in the IL was increased allowing to determine the lower border of ionogel phase formation in RTIL/water systems. The upper border of the ionogel phase was determined when main attention was carried out to the Raman bands originated from the O–H stretching vibration region (3200–3800 cm−1). The hypothesis of weaker hydrogen bonds involving the C(4)‐H and C(5)‐H groups and stronger hydrogen bonds of the C(2)‐H groups was supported by Raman results. Copyright © 2016 John Wiley & Sons, Ltd.