Diffusion of Ions in Phosphonium Orthoborate Ionic Liquids Studied by 1H and 11B Pulsed Field Gradient NMR

Abstract

Non-halogenated boron-based ionic liquids (ILs) composed of phosphonium cations and chelated orthoborate anions have high hydrolytic stability, low melting point and exceptional properties for various applications. This study is focused on ILs with the same type of cation, trihexyltetradecylphosphonium ([P 6,6,6,14 ] +), and two orthoborate anions, such as bis(salicylato)borate ([BScB] −) and bis(oxalato)borate ([BOB] −). We compare the results of this study with our previous studies on ILs with bis(mandelato)borate ([BMB] −) and a variety of different cations (tetraalkylphosphonium, dialkylpyrrolidinium and dialkylimidazolium). The ion dynamics and phase behavior of these ILs is studied using 1 H and 11 B pulsed-field-gradient (PFG) NMR. PFG NMR is demonstrated to be a useful tool to elucidate the dynamics of ions in this class of phosphonium orthoborate ILs. In particular, the applicability of 11 B PFG NMR for studying anions without 1 H, such as [BOB] − , and the limitations of this technique to measure self-diffusion of ions in ILs are demonstrated and discussed in detail for the first time.

Aromatic heterocyclic anion based ionic liquids and electrolytes

“…1 V ( vs. Li/Li + ), which is due the presence of acidic alpha methylene protons in the (P 4444 ) + cation. 51,57,58…”

“…In agreement with the conductivity data above, the diffusivities of both cations and anions reduce VFT fits of the diffusivity data result in parameters comparable for the cations and anions of the ILs, but with marked difference for the electrolytes (Figure 4 and Table S2) The former can be understood as diffusion in a homogeneous matrix, primarily determined by the free volume available. 57 The latter is a consequence of increased ion-ion interactions, here manifested as a slightly changed apparent activation energy, E D , as well as in D 0 . As a sidenote, the T 0 from the ionic conductivity data are lower than the T 0 from the diffusivity data; the latter is an average of the diffusion coefficients of isolated, paired, and clustered ions, regardless of charge, whereas the former only has contributions from charged species.…”

Anion Architecture Controls Structure and Electroresponsivity of Anhalogenous Ionic Liquids in a Sustainable Fluid

“…47,48 In addition, 11 B NMR measurements reported that the [BMB] − anion exhibits a lower diffusion coefficient than [BOB] − and [BScB] − anions. 36 At −1 V, a small but unambiguous change in the innermost layer thickness indicates a consolidation of [P 6,6,6,14 ] + cations at the gold interface to compensate the larger surface charge. Compared to the low electroresponse of the innermost layer, the outer solvent-rich layer exhibits a relatively pronounced response under different bias potentials, despite it consisting predominantly of solvent (cf.…”

“…To address these issues, a series of nonhalogenated ILs, primarily containing phosphorus- and boron-based ions, has been synthesized and developed. ,− In addition to the absence of corrosive halides, boron-based ILs are well-regarded for their ability to reduce wear and friction through the formation of sacrificial tribofilms or lubricating boundary layers, while phosphonium-based ILs show enhanced thermal stability. ,, These nonhalogenated ILs demonstrate promising antiwear and friction-reduction properties, both as neat lubricants − and as additives. , Prior efforts to characterize the properties and tribological performance of these nonhalogenated ILs have employed a range of experimental techniques, including atomic force microscopy (AFM), − nuclear magnetic resonance (NMR), , Fourier transform infrared (FTIR), , scanning electron microscopy (SEM), time-of-flight-secondary ion mass spectrometry (ToF-SIMS), , and macrotribology tests, , as well as molecular dynamics (MD) simulations. , For readers interested in the bulk properties of the studied ILs, we refer to a comprehensive thesis by Rohlmann . Recently, a macroscale tribological test system was developed that allowed studying the tribotronic control of phosphonium orthoborate ILs dispersed in 2-ethylhexyl laurate: 2-EHL, a biodegradable oil, revealing a systematic variation in lubricant film thickness when controlled by an applied electric field.…”

Ionic Liquids and Electrolytes with Flexible Aromatic Anions

“…The large differences in the VFT fits between ILs and electrolytes (Figure 6 and Table S2) can be understood by the free volume available, [54] decreasing upon Li‐salt doping, which also increases the ion‐ion interactions, manifested as slightly changed apparent activation energies, E D , as well as in D 0 . As a sidenote, the T 0 obtained from the ionic conductivity data are lower than the T 0 obtained from the diffusivity data; this as the latter is an average of the diffusion coefficients of isolated, paired, and clustered ions, regardless of charge, whereas the former only has contributions from charged species.…”