Bulk structure of mixed ionic liquid systems
Naomi ELSTONE1, John SLATTERY1, Duncan BRUCE1, Sarah RODGERS2, Najet MAHMOUDI2, Bruno DEMÉ3, Emily SHAW1, Karina SHIMIZU4, Jose Nuno CANONGIA-LOPES4
1University of York, York, United Kingdom
2ISIS, Didcot, United Kingdom
3Institut Laue-Langevin, Grenoble, France
4Universidade de Lisboa, Lisboa, Portugal
Traditionally, ILs have been tuned by varying the nature of the cation or anion, but the huge number of potential structures means that making and testing vast numbers of ILs to find the perfect liquid for the desired application is extremely time consuming and costly. To address this, research into IL mixtures is being carried out. Here the advantage is that a small number of ILs can be made and then used to quickly and simply generate a large range of mixtures in which the physical properties and the interfacial and bulk structures may be tuned by varying the composition.These mixed systems have the potential to allow us to optimise the performance of ILs in a range of applications. We are particularly interested in supported ionic liquid phases, SILPs, in which a thin IL film containing a homogeneous catalyst is deposited on a porous support to produce a pseudo-heterogeneous catalyst. Using IL mixtures should allow modification of both the gas-liquid interfacial structure of SILPs, tuning substrate passage into the bulk across the interface, and their bulk structure, which modifies the microenvironment experienced by the catalyst dissolved in the IL.
We have investigated mixtures of imidazolium-based ILs with different chain lengths and with different degrees of fluorination on the chain. Bulk structure has been determined using a combination of small-angle X-ray and neutron scattering techniques, SAXS and SANS and in some cases molecular dynamics simulations. The bulk structure of the ILs varies with the composition of the mixtures. Isolated aggregates of long chain components are seen at some compositions, percolated domains are observed for others, depending upon the nature of the ILs used. We also observe variations in the interfacial structure as a function of bulk composition.