Abstract
The infrared absorbance spectrum of the ionic liquid 1-butyl-1-methylpyrrolidinium dicyanamide, mixed with water at two different concentrations, was measured between 160 and 300 K in the mid infrared range. Both mixtures do not crystallize on cooling; however, remarkably, the one with an ionic liquid (IL):water composition of 1:3 displays a cold crystallization process on heating in a restricted temperature range between 240 and 250 K. A portion of the water participates to the cold crystallization. On the contrary, with an IL:water composition of 1:6.6 no crystallization takes place. Upon water addition the vibration frequencies of the anion and of some lines of the cation are blue shifted, while the absorption lines of water are red shifted. These facts are interpreted as the evidence of the occurrence of the hydrogen bonding of water, as the hydrogen bonding acceptor with respect to the anion (anion∙∙∙O-H bonds develop) and as hydrogen donor for the cation (C-H∙∙∙O bonds can form). Microscopic inhomogeneities in the samples and their evolution with temperature are discussed.
Highlights
For many applications of ionic liquids (ILs), mixing them with solvents—in particular, water—is required
When cold crystallization took place, the water molecules, which seemed less coordinated with the other H2 O molecules and mostly linked to the anions, become part of the crystallized sample. It seems that at the microscopic level the samples were not homogeneous, but more likely they were composed of separated clusters or regions of bulk water confined in the ionic liquid. We extend this low temperature investigation of the infrared spectra of water-IL mixtures to the hydrophilic 1-butyl-1-methylpyrrolidinium dicyanamide ionic liquid at larger concentrations of water, in order to explore the water-rich region of the phase diagram
It must be noted that despite that the initial purity of the sample was very high and the initial water content was extremely low, during the charging of the cell, it absorbed a large quantity of water due to its hygroscopic nature, as it will be more deeply commented “Results”
Summary
For many applications of ionic liquids (ILs), mixing them with solvents—in particular, water—is required Among such possible utilizations one can cite the dissolution of cellulose [1], otherwise insoluble in pure water, specific treatments to recover organic pollutants [2], the storage and modulations of properties of biomolecules [3], the use as electrolytes in electrochemical devices [4]. Such as fuel cells [5], the production of microemulsions for synthesis, (bio-)catalysis, polymerization,.
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