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

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 nonplanar (the high-temperature crystal). The structural reason for the glass-forming ability of [C2C1im]­[N­(CN)2], despite the relatively simple molecular structures of the ions, was pursued by quantum chemistry calculations and molecular dynamics (MD) simulations. Density functional theory 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].

Themis: A Software to Assess Association Free Energies via Direct Estimative of Partition Functions

“…Hamiltonian 23 as implemented in the MOPAC software. 24 More recently, it was shown that free energy surfaces of first coordination shell of ion pairs of ionic liquids obtained at DFT level produces results in excellent agreement with the ones obtained from liquid phase MD simulations, 25,26 demonstrating that even in some complex cases, Themis is able to probe the structure of condensed phases. This enables the comparison of forcefields with higher theory methods such as DFT, which are too costly to be applied even for small clusters.…”

Role of density and electrostatic interactions in the viscosity and non-newtonian behavior of ionic liquids – a molecular dynamics study

“…The dispersion energy per ion pair is unaffected by the shear in the newtonian regime, but for very high shear rates it starts to increase. This behavior can also be described by the Carreau equation, which can be used to describe the dependence of several properties with g 0 , as will be shown in the following, by generalizing it to different properties A as in eqn (3), where A 0 is the zero shear value of A, and the other parameters hold the same meaning as discussed before with subscript ''A'' added to indicate that the values of fitting parameters may be different for different properties.…”

“…Most ionic compounds are crystalline solids at ambient temperature, with very high melting temperatures, due to strong electrostatic interactions between oppositely charged ions. However, the melting point can be lowered by the presence of bulkier, flexible and low symmetry ions, [1][2][3][4] such as salts with melting point smaller than 100 1C called ionic liquids (ILs). The ability to combine different pairs of cations and anions, as well as variations in their molecular structure, earned the ILs the title of ''designer solvents'', meaning that the physical properties may be tuned by changing the molecular structure if the correlation between them is known.…”

Effect of alkyl-group flexibility on the melting point of imidazolium-based ionic liquids

“…The IL 1-ethyl-3-methyl-imidazolium dicyanamide, [C2C1IM][DCA], has two solid phases that differ from each other due to the in-plane or out-of-plane conformation of the cation ethyl group, as showed by Raman spectroscopy. 10 Computer simulations demonstrate that the conformation of the cation significantly changes the local spatial distribution of the anion. 8 The IL 1-butyl-3-methyl-imidazolium chloride, [C4C1IM] [Cl], has a monoclinic crystalline form with a gauche dihedral between the first and the second carbon of the butyl group and an orthorhombic form with the same dihedral in the trans conformation.…”