Pressure-dependent morphology of trihexyl(tetradecyl)phosphonium ionic liquids: A molecular dynamics study

Abstract

In the present molecular dynamics study, we investigate the effects of increasing pressure on the structural morphology of trihexyl(tetradecyl)phosphonium bromide (P/Br) and trihexyl(tetradecyl)phosphonium dicyanamide (P/DCA) ionic liquids (ILs). Special attention was paid to how charge and polarity orderings, which are present in the microscopic structure of these ILs at ambient conditions, respond to very high external pressure. The simulated X-ray scattering structure functions, S(q)s, of the two systems reveal that both the characteristic orderings show appreciable responsiveness towards the applied pressure change. At a given pressure, a slight difference between the polarity ordering (PO), charge ordering (CO), and adjacency correlations (AC) for both the systems points towards different microscopic structure of the two ILs due to change in anion. Beyond a certain pressure, we observe emergence of a new low-q peak in the S(q)s of both the systems. The new peak is associated with formation of crystalline order in these systems at higher pressures and the real space length-scale corresponding to the crystalline order lies in between those of polarity- and charge-ordering. Beyond the transition pressure, the crystallinity of both the systems increases with increasing pressure and the corresponding length-scale shifts towards smaller values upon increasing pressure. We also observe that the extent of the usual polarity ordering decreases upon increasing pressure for both the P/Br and P/DCA systems. We demonstrate that the disappearance of the usual polarity peak is due to decreased polar-polar and apolar-apolar correlations and enhanced correlations between the charged and uncharged groups of the ions. This scenario is completely reversed for the components corresponding to the crystalline order, the polar-polar and apolar-apolar correlations are enhanced and polar-apolar correlations are diminished at higher pressure. In addition, the charge ordering peak, which is not so obvious from the total S(q) but from ionic and sub-ionic partial components of it, shifts towards lower q values for P/Br. Instead, for the P/DCA, at the highest pressure studied the CO peak occurs at a q-value higher than that at the ambient pressure.

The Effect of Phenyl Substitutions on Microstructures and Dynamics of Tetraalkylphosphonium Bis(trifluoro‐ methylsulfonyl)imide Ionic Liquids

“…This observation is correlated with the short carbon‐carbon bond length in phenyl groups in comparison with that in hexyl chains, and additionally, with the effective mediation of [NTF 2 ] anions between tetraalkylphosphonium cations. The latter is consistent with computational results for ILs consisting of tetraalkylphosphonium cations coupled with bromide and dicyanamide anions …”

“…The peaks at intermediate q range around 7 nm −1 (0.89 nm in real space) are an indicative of mesoscopic liquid organization characterized by positive‐negative charge alternations in IL matrices . This charge ordering behavior is the need to maintain a lattice‐like arrangement of cations and anions to maximize cation‐anion Coulombic attractions and minimize anion‐anion and cation‐cation Coulombic repulsions in IL matrices, and thus is associated with a length scale between ions of the same charge separated by ions of opposite charge . For some ILs, this charge alternation peak is present as a weak shoulder because of an almost complete cancellation of peaks and anti‐peaks that offset this ordering phenomena at intermediate length scale .…”

On the volumetric properties of 2-hydroxy ethylammonium formate ionic liquid under high-pressures: Measurement and molecular dynamics

“…Zhao et al [26] reported all-atoms MD simulations of 1-butyl-3methylimidazolium hexafluorophosphate IL showing conformational changes in the cation upon pressure increase resulting in changes in density upon compression and large effects of pressure on ion mobility resulting in viscosity increase. Sharma et al [27] showed the appearance of solid-like ionic arrangements for trihexyl(tetradecyl)phosphonium-based ILs with a large reorganization of molecular functionalities upon compression. Dhungana et al [28] showed remarkable conformational changes upon compressions leading to the changes in physical properties and fluids' reorganization.…”

Molecular Dynamics Investigation of Efficient SO2 Absorption by Anion-Functionalized Ionic Liquids

“…It is evident that the structure of these ILs remain unaltered even in the presence of the solute. 56,57,60,85 3.2b SO 2 -ion g(r): Several experiments 39,41,42,86 and molecular simulation 15,16,60 studies exhibited that the nature of anions has a profound effect on the interaction between SO 2 and ILs. Figure S3 (in SI) shows the RDFs between the center of mass (COM) of the ions and that of SO 2 .…”