Molecular Architecture Effects on Bulk Nanostructure in Bis(Orthoborate) Ionic Liquids.
Oliver S HammondGuillaume BousrezFilip MehlerSichao LiManishkumar R ShimpiJames DoutchLeide CavalcantiSergei GlavatskihOleg N AntzutkinMark W RutlandAnja-Verena MudringPublished in: Small (Weinheim an der Bergstrasse, Germany) (2023)
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 L 3 spongelike phases when the alkyl chains are longer than C 6 (hexyl). L 3 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.