Evidence of supercoolable nanoscale water clusters in an amorphous ionic liquid matrix

Abstract

Nanoscale water clusters in an ionic liquid matrix, also called “water pockets,” were previously found in some mixtures of water with ionic liquids containing hydrophilic anions. However, in these systems, at least partial crystallization occurs upon supercooling. In this work, we show for mixtures of 1-butyl-3-methylimidazolium dicyanamide with water that none of the components crystallizes up to a water content of 72 mol. %. The dynamics of the ionic liquid matrix is monitored from above room temperature down to the glass transition by combining depolarized dynamic light scattering with broadband dielectric and nuclear magnetic resonance spectroscopy, revealing that the matrix behaves like a common glass former and stays amorphous in the whole temperature range. Moreover, we demonstrate by a combination of Raman spectroscopy, small angle neutron scattering, and molecular dynamics simulation that, indeed, nanoscale water clusters exist in this mixture.

Water in ionic liquids: Raman spectroscopic studies

“…[ 31 ] A combination of Raman spectroscopy, small‐angle neutron scattering and molecular dynamics simulation helped to establish the existence of nanoscale water clusters in [C 4 C 1 im][DCA], above a certain water content. [ 9 ] In a systematic study, recently, Balevicius et al . have demonstrated the existence of nanoscale water clusters (termed as ‘water pockets’) in an IL matrix using Raman spectroscopy.…”

“…have taken the help of several other techniques to prove that at a higher concentration of water, instead of a nanoscale water cluster, a percolating water network spreading across the whole sample exists. [ 9 ] Raman spectroscopic studies of the O‐H band and C‐N (from anion) clearly indicate the existence of a few water molecules interacting with anion below 60 mol. % water, whereas a percolating water network appears at higher water concentrations.…”

“…A green laser (532 nm, 10 mW, 5 cm −1 ) is used for excitation. The figure is reproduced from Pabst et al 9 . with permission from the publishers.…”

“…[ 8 ] Despite this knowledge, knowingly or unknowingly, water creeps into the IL system or remains in the ILs even after high‐vacuum in‐oven drying. [ 9 ]…”

“…The solid line represents the overall fit to the individual bands that appear for differently bonded water molecules. The concept of the figure is obtained from Pabst et al 9 …”