Molecular features contributing to the lower viscosity of phosphonium ionic liquids compared to their ammonium analogues

Abstract

Molecular features contributing to the lower viscosity of phosphonium based ionic liquids (ILs) compared to ammonium based ILs are investigated by static quantum chemistry calculations and classical molecular dynamics simulations. The larger bond distance and the higher flexibility of bond angles and dihedral angles in the phosphonium compounds tend to reduce their viscosity compared to ammonium analogues, while the strongly localized charge at the central atom has the opposite effect. Fast translational ion dynamics is also found to be related to a short counter-ion association lifetime in the investigated compounds. Furthermore, a weak structuring between the center of charges also seems to increase mobility. Interestingly, the order of ion pair interaction energies in the gas phase is reversed compared to the order of counter-ion association lifetimes in the liquid, which highlights the important role of solvation in ILs. Overall, the higher flexibility of the bond and dihedral angles of the phosphonium compounds appears to be the most important factor in producing the lower viscosity of these ILs compared to their ammonium analogues.

Experimental validation of calculated atomic charges in ionic liquids

“…[P6, 6,6,14][NO3] is an exception as only D3 was used, due to difficulties with low negative frequencies when using D3(BJ) for these structures. 67,68 The self consistent field convergence criteria were set to 10 7 on the energy matrix and 10 9 on the density matrix.…”

“…9 Multiple other MD studies have found that varying charge distribution can lead to 3 significant changes in predicted structural, dynamic or energetic properties. [10][11][12][13][14][15][16][17] Therefore, understanding the charge distribution within ILs is key to linking IL composition with macroscopic physical properties.…”

“…[9][10][11][12][13][14][15][16][17] Varying the charge distribution within each ion, while keeping a ±1 e charge on each ion, can significantly affect dynamic properties. For example, Li and Kobrak compared MD simulations using octahedral ions with either a symmetric (charge spread equally over the ion) or non-symmetric (charge localised on one half of the ion) charge distribution.…”

Towards Phosphorus Free Ionic Liquid Anti-Wear Lubricant Additives

“…There are subtle differences between the ammonium and phosphonium cation that may influence the behavior of the ILs in terms of miscibility. Phosphonium is a more flexible cation and also has a more centralized local charge as compared to a similar ammonium cation [17]. This increased flexibility may result in the phosphonium cation being better able to match the structure of the base oil, while the more central charge could be well screened from the oil by the attached alkyl chains on the cation.…”

Hydrophosphination of Styrene and Polymerization of Vinylpyridine: A Computational Investigation of Calcium-Catalyzed Reactions and the Role of Fluxional Noncovalent Interactions

“…[1][2][3]4 Moreover, organophosphines are also of significant interest as derivatives to phosphonium-based ionic liquids (ILs). Phosphonium ILs exhibit higher thermal stabilities and conductivities when compared to their corresponding ammonium IL counterparts, [5][6][7][8][9][10] are an attractive alternative class of solvents to more traditional volatile organic compounds, 11,12 can be used for gas capture 13 and demonstrate antimicrobial properties. 14,15 Experimentally, there are a number of Ca-based complexes that have been shown to catalyse the hydrophosphination reaction.…”