Microstructures and Dynamics of Tetraalkylphosphonium Chloride Ionic Liquids

TL;DR

This study uses atomistic simulations to explore how the length of aliphatic chains in tetraalkylphosphonium cations influences the microstructure, morphology, and dynamics of tetraalkylphosphonium chloride ionic liquids, revealing chain-length-dependent structural and dynamical heterogeneities.

Contribution

It provides detailed insights into the relationship between aliphatic chain length and the microstructural and dynamical properties of tetraalkylphosphonium chloride ionic liquids, which was previously not well understood.

Findings

Longer aliphatic chains lead to increased segregation of polar and apolar domains.

Microstructures exhibit sponge-like networks with chain-length-dependent morphology.

Dynamical heterogeneities increase with chain length, affecting ionic mobility.

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 properties 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 leads 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 intermediate q range exhibit complicated tendencies due to the complete cancellations 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 translational diffusion coefficients, van Hove correlation functions, and non-Gaussian parameters for ionic species in heterogeneous ionic environment. The increase of aliphatic chain length in tetraalkylphosphonium cations leads to concomitant shift of van Hove correlation functions and non-Gaussian parameters to larger radial distances and longer timescales, respectively, indicating the enhanced translational dynamical heterogeneities of tetraalkylphosphonium cations and the corresponding chloride anions.