Oliver HAMMOND1,2, Guillaume BOUSREZ1,2, Manishkumar SHIMPI2,3, Oleg ANTZUTKIN3, Sergei GLAVATSKIH4, Mark RUTLAND4, Anja-Verena MUDRING1,2
1Aarhus University, Aarhus, Denmark
2Stockholm University, Stockholm, Sweden
3Luleå University of Technology, Luleå, Sweden
4KTH Royal Institute of Technology, Stockholm, Sweden
Ionic liquids (ILs) are interesting as potential lubricants for mechanical contacts. However, many common ILs corrode interfaces, either directly due to halide content, or indirectly through decomposition, such as hydrolysis of anions such as [PF6]- and [BF4]-. Orthoborate anions, such as bis(oxalato)borate and bis(mandelato)borate, are an attractive potential replacement for these anions, since they open up avenues for interfacial boron chemistry: On degradation, a sacrificial tribofilm is formed comprising B2O3, BN, and various borides, which is autolubricating and protects against further wear. This can be complemented by a tribofilm formed by the cation, for example, from phosphonium or imidazolium cations. As such, these systems have been explored and shown to self-assemble into nanostructures at the interface, where they exert a strong reduction in interfacial friction, which can be electrochemically controlled.
Despite studies into their properties and structure at the interface, orthoborate IL systems remain completely unstudied in terms of their bulk structure, with no direct experimental evidence reported to date. The bulk structure of ILs has been a topic of high interest for a long time, due to the potential to form anomalous spongelike L3 nanostructured phases. Here, we therefore explored the effect of molecular architecture, through various modifications, such as methylation, altering the alkyl chain length, and varying cation/anion type, to synthesise a large family of orthoborate ILs. The bulk structure of the ILs was measured using small-angle neutron scattering (SANS). Analysis of the SANS data shows how these various molecular modifications can be used to alter the bulk nanostructure, to favour an L1 or L3 phase, and in some cases, we observe how the bulk aggregates are destabilized through modified packing and topological effects.