Physical and electrochemical properties of low-viscosity phosphonium ionic liquids as potential electrolytes

Understanding the impact of the central atom on the ionic liquid behavior: Phosphonium vs ammonium cations

“…Although it could be expected that the introduction of a heavier atom would increase the densities and viscosities, as commonly observed, or that the large cation's alkyl chains used in this study would shield the central atom and therefore smooth or even remove its influence on the interactions, and ultimately on the properties of the IL, the viscosity and density results show that even surrounded by large alkyl chains, the cation's central atom still has an important impact on these basic thermophysical properties. The two pairs here reported present the same behavior previously observed by Tsunashima et al 8 and Shirota et al 10 Tsunashima et al 8 reported a 1 H NMR chemical shift towards a high magnetic field, of the ammonium CH 2 groups adjacent to the cationic center compared to the corresponding phosphonium. The authors suggested that the shifts observed are attributed to relatively high electron density of the protons in the phosphonium cations and that this electron density increase leads to lower acidity and lower electrostatic interactions of the phosphonium ILs and therefore, to a density decrease.…”

“…In contrast to the ion substitution studies, the atom substitution, of the constituent ions, has been object of a limited number of studies. [8][9][10][11][12][13] Shirota and Castner 11 reported that the substitution of carbon by silicon in the neopentyl group at the imidazolium cation (1-methyl-3-neopentylimidazolium ([Cmim] + ) vs 1-methyl-3-trimethylsilylmethylimidazolium ([Simim] + )) leads to a substantial reduction of viscosity. On the anionic side, the heavy atom substitution in a series of [XF 6 ] − anions (X = P, As, or Sb) in 1-butyl-3-methylimidazolium ([C 4 mim] + )-based ILs also affects the viscosity: 290 cP for [C 4 6 ].…”

“…12 In addition to studies with aromatic cationbased ILs, reductions in the viscosities of ILs with heavier atoms in the same position were also reported for nonaromatic cation-based ILs. 8,10,13,14 Tsunashima and co-workers 8,9 were the first to compare the behavior of pairs of [NTf 2 ] − anion-based ammonium and phosphonium ionic liquids investigating the differences upon viscosities, conductivities, and thermal stability. Contrary to most neutral molecular liquids, where the substitution of a constituent atom by a heavier atom leads to a viscosity increase, the authors 8 reported lower viscosities and higher conductivities for the phosphonium, when compared to those of corresponding ammonium ILs.…”

“…8,10,13,14 Tsunashima and co-workers 8,9 were the first to compare the behavior of pairs of [NTf 2 ] − anion-based ammonium and phosphonium ionic liquids investigating the differences upon viscosities, conductivities, and thermal stability. Contrary to most neutral molecular liquids, where the substitution of a constituent atom by a heavier atom leads to a viscosity increase, the authors 8 reported lower viscosities and higher conductivities for the phosphonium, when compared to those of corresponding ammonium ILs. Furthermore, in a later study 9 the authors reported higher thermal stability for benzylsubstituted phosphonium, compared to the corresponding benzyl-substituted ammonium compounds.…”

Tribo-Chemistry of Phosphonium-Derived Ionic Liquids

“…Tetraalkylphosphonium (R 4 P)-derived RT-ILs, which have a structure analogous to tetraammonium cations, are known to exhibit better thermal stability than alkylimidazolium-derived ones [17][18][19]. To date, there are brief reports on the tribological properties of R 4 P-derived RTILs [13,[20][21][22][23][24][25].…”