Computing the melting point and thermodynamic stability of the orthorhombic and monoclinic crystalline polymorphs of the ionic liquid 1-n-butyl-3-methylimidazolium chloride

Abstract

The melting point, enthalpy of fusion, and thermodynamic stability of two crystal polymorphs of the ionic liquid 1-n-butyl-3-methylimidazolium chloride are calculated using a thermodynamic integration-based atomistic simulation method. The computed melting point of the orthorhombic phase ranges from 365 to 369 K, depending on the classical force field used. This compares reasonably well with the experimental values, which range from 337 to 339 K. The computed enthalpy of fusion ranges from 19 to 29 kJ/mol, compared to the experimental values of 18.5-21.5 kJ/mol. Only one of the two force fields evaluated in this work yielded a stable monoclinic phase, despite the fact that both give accurate liquid state densities. The computed melting point of the monoclinic polymorph was found to be 373 K, which is somewhat higher than the experimental range of 318-340 K. The computed enthalpy of fusion was 23 kJ/mol, which is also higher than the experimental value of 9.3-14.5 kJ/mol. The simulations predict that the monoclinic form is more stable than the orthorhombic form at low temperature, in agreement with one set of experiments but in conflict with another. The difference in free energy between the two polymorphs is very small, due to the fact that a single trans-gauche conformational difference in an alkyl sidechain distinguishes the two structures. As a result, it is very difficult to construct simple classical force fields that are accurate enough to definitively predict which polymorph is most stable. A liquid phase analysis of the probability distribution of the dihedral angles in the alkyl chain indicates that less than half of the dihedral angles are in the gauche-trans configuration that is adopted in the orthorhombic crystal. The low melting point and glass forming tendency of this ionic liquid is likely due to the energy barrier for conversion of the remaining dihedral angles into the gauche-trans state. The simulation procedure used to perform the melting point calculations is an extension of the so-called pseudo-supercritical path sampling procedure. This study demonstrates that the method can be effectively applied to quite complex systems such as ionic liquids and that the appropriate choice of tethering potentials for a key step in the thermodynamic path can enable first order phase transitions to be avoided.

Dispersion interactions in room-temperature ionic liquids: Results from a non-empirical density functional

“…In contrast with both, LDA/GGA and force field calculations, results obtained with the DRSLL and κ = 1 functionals produce much smaller energy differences between the two experimentally observed polymorphs, i.e., smaller than 0.01 eV per ionic pair. These results are consistent with the observation of both the monoclinic and orthorhombic structures and with the results of Jayaraman and Maginn, 46 and they do not rule out the possibility of a temperature-driven phase transition between them.…”

“…44 but, due to the small energy difference, the possibility of a phase transition between the two structures upon increasing temperature is re-instated, in agreement with the classical force field results of Jayaraman and Maginn. 46 Electronic energy gaps at the -point of the Brillouin zone are similar for the two structures. LDA, GGA-PBE, and VDW energy gaps are larger in the monoclinic structure, consistently with a higher stability, but the difference is only about 0.2 eV.…”

“…9 A contrasting classical MD study using a different force field produced the intriguing result that the free energy difference between the two polymorphs was vary small, and thus it was not possible to establish which of the two structures was more stable. 46 In order to shed more light on this controversy, we computed the energy difference between the two structures using the DRSLL functional. In addition, we have re-computed the energy difference for LDA and PBE using the improved basis set.…”

Molten salt eutectics from atomistic simulations

“…The dependence of μ AB on the choice of particle was found to be within statistical uncertainties, which were computed using the error estimation procedure described in Ref. [30].…”

New quantum chemistry-based descriptors for better prediction of melting point and viscosity of ionic liquids

“…In general, these models are either Quantitative Structure Property Relationship (QSPR) correlations [6][7][8] , group contribution (GC) type [9][10][11][12][13][14][15][16] , or molecular dynamics (MD) simulations. [17][18][19][20][21][22][23] The main drawback of the QSPR and GC models is the narrow restrictions in choice of applicable compounds while the main limitation of MD simulations is the high computational demands and the need for starting geometry. 24 In this paper, we present two empirical correlations to predict melting point and viscosity of ILs in a way that does not need experimental input or tedious simulations, but rely on inputs from simple quantum chemical calculations.…”