C O M P L E X S Y S T E M S A N D B I O M E D I C A L S C I E N C E S

S C I E N T I F I C H I G H L I G H T S

6 8 H I G H L I G H T S 2 0 2 2 I

Fig. 58: a,b) Measured C12,n, n=1-22, scattering curves. c) RTILs layered nanostructure [3]. d) Layer spacing (symbols, dash-line fits) at temperatures K.

e) Layering decay lengths xI/dI (symbols). Dash-dot lines denote pure Cn.

Mixed, stirred or shaken: tuning the nanoscale structure of ionic liquids by mixing Although of great scientific and applicative interest, the nanostructure of ionic liquid mixtures is still mostly unexplored. Using X-ray scattering, an intriguing cation-chain-length-driven trimodal structure evolution from a solution of lipid-like bilayers to alternating layers of polar headgroups and apolar interdigitated chains was found. These insights should facilitate the design of mixtures for specific applications.

Room-temperature ionic liquids (RTILs) are a novel class of organic salts exhibiting intriguing complex-fluid nanostructures and properties. Their bulky and irregular- shaped ions inhibit solidification to ≤100°C, unlike the ≥700°C of conventional salts. RTILs are intensively studied for their unusual combination of competing interactions, intricate hierarchical nanostructure and many applications for batteries, supercapacitors, lubrication, metal recovery from industrial wastes, pharmaceutics synthesis, targeted in-vivo drug delivery etc. [1]. Mixing RTILs is a cheap and easy method to fine- tune properties to specific applications. However, detailed understanding of the relation between the properties of the mixture and of its constituents is scarce [2].

With the support of the ESRF s Partnership for Soft Condensed Matter (PSCM), small-angle X-ray scattering (SAXS) was employed at beamline ID15A to study the nanostructure of [C12mim]0.5[Cnmim]0.5[NTf2] (denoted

C12,n), n=1=22, binary mixtures of a previously studied [3] model RTIL family, [Cnmim]+[NTf2] (denoted Cn), comprising cations with alkyl tails, H(CH2)n, and polar methylimidazole headgroups and (smaller) fluorinated anions. In Cn, n≥3, the apolar tails segregate from the polar moieties, self-organising locally into a layered polar-apolar superstructure, with partly overlapping, layer-normal, interdigitated chains [3].

The SAXS curves (Figure 58a) exhibit the same three- peak signature (I, II, III) of pure RTILs [3], corresponding, respectively, to the layer spacing dI, the lateral headgroup spacing dII, and the chain-chain and other adjacencies spacing dIII (Figure 58c). Peak I exhibits strong non-monotonic position and width variations with n (Figure 58b). Teubner-Strey-like model fits [3] reveal the structural evolution of the mixtures over broad n=1-22 and T=293 K-373 K ranges, hitherto unavailable for any RTIL family. The components identical headgroups and chain widths render minute the mixtures lateral spacing (dII and dIII) variation over the full n range: 4% and 0.5%, respectively. dI, however, varies by 25%-30% (Figure 58d), identifying layer contraction/expansion to be the main structural effect of mixing.

dI exhibits a non-monotonic trimodal n-variation (Figure 58d). For n≥12, all C12,n s dI exceed pure C12 s dI , albeit following its n-increase, demonstrating the longer component s domination of the mixture layering. The mixtures slightly lower slope and values, relative to pure Cn, are due to the presence of the shorter, C12, component. The longer component also dominates xI/dI (Figure 58e), which follows, albeit slightly below,