Microstructures and dynamics of tetraalkylphosphonium chloride ionic liquids

Abstract

Atomistic simulations have been performed to investigate the effect of aliphatic chain length in tetraalkylphosphonium cations on liquid morphologies, microscopic ionic structures, and dynamical quantities of tetraalkylphosphonium chloride ionic liquids. The liquid morphologies are characterized by sponge-like interpenetrating polar and apolar networks in ionic liquids consisting of tetraalkylphosphonium cations with short aliphatic chains. The lengthening aliphatic chains in tetraalkylphosphonium cations lead to polar domains consisting of chloride anions and central polar groups in cations being partially or totally segregated in ionic liquid matrices due to a progressive expansion of apolar domains in between. Prominent polarity alternation peaks and adjacency correlation peaks are observed at low and high q range in total X-ray scattering structural functions, respectively, and their peak positions gradually shift to lower q values with lengthening aliphatic chains in tetraalkylphosphonium cations. The charge alternation peaks registered in the intermediate q range exhibit complicated tendencies due to a cancellation of peaks and anti-peaks in partial structural functions for ionic subcomponents. The particular microstructures and liquid morphologies in tetraalkylphosphonium chloride ionic liquids intrinsically contribute to distinct dynamics characterized by mean square displacements, van Hove correlation functions, and non-Gaussian parameters for ionic species in the heterogeneous ionic environment. Most tetraalkylphosphonium cations have higher translational mobilities than their partner anions due to strong coordination of chloride anions with central polar groups in tetraalkylphosphonium cations through strong Coulombic and hydrogen bonding interactions. The increase of aliphatic chain length in tetraalkylphosphonium cations leads to a concomitant shift of van Hove correlation functions and non-Gaussian parameters to larger radial distances and longer time scales, respectively, indicating the enhanced translational dynamical heterogeneities of tetraalkylphosphonium cations and the corresponding chloride anions.

How Molecular Chiralities of Bis(mandelato)borate Anions Affect Their Binding Structures With Alkali Metal Ions and Microstructural Properties in Tetraalkylphosphonium Ionic Liquids

“…BMB] anions leading to their less efficient packing in heterogeneous IL matrices[10,13]. In addition, a good agreement is observed between experimental data and computational results for [P4,4,4,8 ][BMB] and [P6,6,6,14 ][BMB] ILs at all studied temperatures.…”

“…W (r) is a Lorch window function defined as W (r) = sin(2πr/L) 2πr/L , which is used to minimize effect of finite truncation of r for the calculation of g ij (r). For a simulation system of moderate size, this window function does not hinder the physical meaning of peaks and anti-peaks in S(q) plots [11,13,57,58]. molecular chiralities.…”

“…It is shown in Fig. 13 one expects to find a same-charge ion there is a systematic absence of ions having opposite charge [11,13,53,55,[57][58][59][60]. In addition, all partial scattering structural functions present positive contributions to adjacency correlations between neighboring ion species at high q values around 1.51 nm −1 but with different portions of contribution.…”

The effect of structural heterogeneity upon the microviscosity of ionic liquids

“…The closest structures that are available are for [P 4444 ]Cl and [P 4448 ]Cl. 21 Both are described as having interpenetrating polar and non-polar networks, with [P 4448 ]Cl having a greater separation of ionic domains.…”

“…Phosphonium-based ionic liquids are less well studied, but it is generally understood that they also contain interpenetrating polar and non-polar domains that become more segregated as the alkyl chain length increases. 21 …”