Low Temperature Phase Transitions of the Ionic Liquid 1-Ethyl-3-methylimidazolium Dicyanamide

TL;DR

This study investigates the low-temperature phase transitions and polymorphism of the ionic liquid [C2C1im][N(CN)2], revealing two crystalline phases and explaining its glass-forming ability through combined experimental and computational approaches.

Contribution

It provides new insights into the polymorphic behavior and structural origins of glass formation in [C2C1im][N(CN)2] using Raman spectroscopy, X-ray diffraction, calorimetry, quantum chemistry, and molecular dynamics.

Findings

Identification of two crystalline phases at different temperatures.

Polymorphism related to conformational differences of the cation.

Delocalization of anion positions hampers crystallization, enabling glass formation.

Abstract

Several calorimetric measurements have shown that 1-ethyl-3-methylimidazolium dicyanamide, [C2C1im][N(CN)2], is a glass-forming liquid, even though it is a low-viscous liquid at room temperature. Here we found slow crystallization during cooling of [C2C1im][N(CN)2] along Raman spectroscopy measurements. The low-frequency range of the Raman spectrum shows that the same crystalline phase is obtained at 210 K either by cooling or by reheating the glass (cold-crystallization). Another crystalline phase is formed at ca. 260 K just prior the melting at 270 K. X-ray diffraction and calorimetric measurements confirm that there are two crystalline phases of [C2C1im][N(CN)2]. The Raman spectra indicate that polymorphism is related to [C2C1im]+ with the ethyl chain on the plane of the imidazolium ring (the low-temperature crystal) or non-planar (the high-temperature crystal). The structural reason for the glass-forming ability of [C2C1im][N(CN)2], despite of the relatively simple molecular structures of the ions, was pursued by quantum chemistry calculations and molecular dynamics (MD) simulations. Density functional theory (DFT) calculations were performed for ionic pairs in order to draw free energy surfaces of the anion around the cation. The MD simulations using a polarizable model provided maps of occurrence of anions around cations. Both the quantum and classical calculations suggest that the delocalization of preferred positions of the anion around the cation, which adopts different conformations of the ethyl chain, is on the origin of the crystallization being hampered during cooling and the resulting glass-forming ability of [C2C1im][N(CN)2].