Crystallization of mixtures of hydrophilic ionic liquids and water: Evidence of microscopic inhomogeneities

O Palumbo; F Trequattrini; J-B Brubach; P Roy; A Paolone

doi:10.1016/j.jcis.2019.05.034

Crystallization of mixtures of hydrophilic ionic liquids and water: Evidence of microscopic inhomogeneities

J Colloid Interface Sci. 2019 Sep 15:552:43-50. doi: 10.1016/j.jcis.2019.05.034. Epub 2019 May 10.

Authors

O Palumbo¹, F Trequattrini², J-B Brubach³, P Roy³, A Paolone⁴

Affiliations

¹ CNR-ISC, UOS La Sapienza, Piazzale A. Moro 2, 00185 Roma, Italy. Electronic address: oriele.palumbo@roma1.infn.it.
² CNR-ISC, UOS La Sapienza, Piazzale A. Moro 2, 00185 Roma, Italy; Dep. of Physics, Sapienza University of Rome, Piazzale A. Moro 2, 00185 Roma, Italy.
³ Synchrotron SOLEIL, 91192 Gif Sur Yvette, France.
⁴ CNR-ISC, UOS La Sapienza, Piazzale A. Moro 2, 00185 Roma, Italy.

PMID: 31100689
DOI: 10.1016/j.jcis.2019.05.034

Abstract

Hypothesis: We compare the effects of water, either intentionally added or due to absorption from the air, on the phase diagram of the hydrophilic 1-butyl-3-methylimidazolium dicyanamide ionic liquid, extending previous investigations to lower temperatures (down to 140 K), with a special attention to the changes of the environment of water molecules and the interface between water and ionic liquid as a function of temperature.

Experiments: Combined infrared spectroscopy and ab-initio calculations provide information about the phase transitions and the intermolecular changes occurring in the liquid.

Findings: The temperature dependence of the mid-infrared spectrum in the temperature range between 140 and 330 K indicates that in both cases the liquid undergoes a glass transition, but, when the water content is only due to absorption from air, a cold crystallization takes place on heating between ≈240 and ≈265 K, while it is suppressed when water is intentionally added in a greater amount. The analysis of the OH stretching bands indicates the existence of two different "liquid like" water environments. When cold crystallization takes places the water molecules, which seem less coordinated to the other H₂O molecules and more related to the anions, appear to be part of the crystallized sample. In both cases, it seems that at microscopic level the sample is not homogeneous, but more likely it is composed of separated clusters or regions of bulk water confined in the ionic liquid.

Keywords: Infrared spectroscopy; Interactions; Ionic liquids; Water.