Molecular Architecture Effects on Bulk Nanostructure in Bis(Orthoborate) Ionic Liquids

Abstract

A series of 19 ionic liquids (ILs) based on phosphonium and imidazolium cations of varying alkyl‐chain lengths with the orthoborate anions bis(oxalato)borate [BOB]−, bis(mandelato)borate, [BMB]− and bis(salicylato)borate, [BScB]−, are synthesized and studied using small‐angle neutron scattering (SANS). All measured systems display nanostructuring, with 1‐methyl‐3‐n‐alkyl imidazolium‐orthoborates forming clearly bicontinuous L3 spongelike phases when the alkyl chains are longer than C6 (hexyl). L3 phases are fitted using the Teubner and Strey model, and diffusely‐nanostructured systems are primarily fitted using the Ornstein‐Zernicke correlation length model. Strongly‐nanostructured systems have a strong dependence on the cation, with molecular architecture variation explored to determine the driving forces for self‐assembly. The ability to form well‐defined complex phases is effectively extinguished in several ways: methylation of the most acidic imidazolium ring proton, replacing the imidazolium 3‐methyl group with a longer hydrocarbon chain, substitution of [BOB]− by [BMB]−, or exchanging the imidazolium for phosphonium systems, irrespective of phosphonium architecture. The results suggest there is only a small window of opportunity, in terms of molecular amphiphilicity and cation:anion volume matching, for the formation of stable extensive bicontinuous domains in pure bulk orthoborate‐based ILs. Particularly important for self‐assembly processes appear to be the ability to form H‐bonding networks, which offer additional versatility in imidazolium systems.

Evidence for an L3 phase in ternary deep eutectics: composition-induced L3-to-Lα transition of AOT

“…The Teubner–Strey model is a shape-independent function used to fit microemulsion-like systems, and is parameterized in terms of periodicity ( d -spacing), correlation length ξ , and volume fraction ϕ . 53,54 The L 3 sponge phase swells with addition of solvent, and thus is characterised by a linear dependence between d -spacing and ϕ solvent , which we observed for samples with w ≤ 2, as is shown in Fig. 3a.…”

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

“…Such linear assemblies, driven by local attractive energy minima and stabilized by a common corona, have been observed in nanoparticle systems and are relatively uncommon for molecular self-assembly systems, in part due to the lower propensity for self-assembly in media with low values of dielectric constant and cohesive energy density, cf. the Gordon parameter. , These SANS results therefore offer a framework for understanding nanoscale structuring in these systems, which appears to be Coulombically dominated in the bulk, as has been suggested for other systems. , …”

“…71,72 These SANS results therefore offer a framework for understanding nanoscale structuring in these systems, which appears to be Coulombically dominated in the bulk, as has been suggested for other systems. 36,51 Neutron Reflectivity. 5% w/w [P 6,6,6,14 ][BxB] in 2-EHL.…”

“…In addition to NR, our constellation has conducted investigations of the bulk structure and interfacial electroresponsive behavior of various nonhalogenated ILs, both neat and as additives in base fluids (e.g., base oil, acetone, and propylene carbonate), using small-angle neutron scattering (SANS), quartz crystal microbalance (QCM), and sum frequency generation (SFG) spectroscopy, among other techniques. − Recent SANS measurements, in particular, have provided direct evidence of bulk nanostructure in neat bis­(orthoborate) ILs and how such structures and their intermolecular interactions are influenced by the specific choice of ion pairs and their molecular structures . Meanwhile, previous NR measurements have provided further information on the interfacial electroresponsive behavior and structural characteristics of nonhalogenated ILs and their mixtures with polar solvents at the liquid–solid interface under the influence of an external electric field. , Recently, in conjunction with NR measurements, the assessment of surface charge via QCM has revealed a voltage-induced interphase transition phenomenon occurring in a phosphonium orthoborate-based IL …”

“… 45 − 51 Recent SANS measurements, in particular, have provided direct evidence of bulk nanostructure in neat bis(orthoborate) ILs and how such structures and their intermolecular interactions are influenced by the specific choice of ion pairs and their molecular structures. 51 Meanwhile, previous NR measurements have provided further information on the interfacial electroresponsive behavior and structural characteristics of nonhalogenated ILs and their mixtures with polar solvents at the liquid–solid interface under the influence of an external electric field. 47 , 48 Recently, in conjunction with NR measurements, the assessment of surface charge via QCM has revealed a voltage-induced interphase transition phenomenon occurring in a phosphonium orthoborate-based IL.…”