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PRINCIPAL PUBLICATION AND AUTHORS

Binary mixtures of homologous room-temperature ionic liquids: Nanoscale structure evolution with alkyl lengths difference, D. Pontoni (a), M. DiMichiel (b), M. Deutsch (c), J. Mol. Liq. 355, 118874 (2022); https:/doi.org/10.1016/j.molliq.2022.118874 (a) Partnership for Soft Condensed Matter (PSCM), ESRF (b) ESRF (c) Physics Dept. & Nanotechnology and Advanced Materials Inst., Bar-Ilan University (Israel)

REFERENCES

[1] T. Welton, Biophys. Rev. 10, 691 (2018). [2] D. Pontoni et al., J. Mol. Liq. 338, 116587 (2021). [3] D. Pontoni et al., J. Mol. Liq. 300, 112280 (2020). [4] W. Helfrich, Z. Naturfor. C 28, 693 (1973). [5] E. Sloutskin et al., Phys. Rev. Lett. 89, 065501 (2002). [6] E. Sloutskin et al., Eur. Phys. J. E 13, 109 (2004).

Fig. 59: a) Deviations of RTIL mixtures dI from ideality for |Dn=12-n|. b) Linear deviations dependence on (Dn/ñ)2, ñ=(12+n)/2, implying nanostructure domination by the interchange energy w.

the n-increase of the pure Cn values, reasonably indicating that mixing reduces the layering order range.

n≤4 mixtures exhibit an antithetical behaviour. Here, C12 is the longer component. Yet, C12,1 s dI≈31 Å exceeds pure C12 s dI≈26.5 Å [3], indicating deviation from the interdigitated-chains layering structure of n>12. Rather, dI≈31 Å is within only ~1 Å from the dI≈32 Å calculated for the non-interdigitated bilayers of solvated lipids and bio-membranes [4], showing C12,1 to consist of non-interdigitated C12 bilayers solvated in the shorter component, C1, in line with the liquid-like xI/dI<<1 of C12,1 (Figure 58e). C12,n s fast dI decrease for n>1 (Figure 58d) indicates an n-evolution from a bilayer-solution structure towards the interdigitated-chains layering of n>12, however with constant liquid-like, short-range, decay lengths xI/dI≤0.3 (Figure 58e). 6≤n≤10 constitutes a transition range between the n≤4 and n>12 regimes. Here, the opposing dI tendencies cancel out, yielding near-constant di(n) values, although the increasing van der Waals (vdW) chain-chain interaction yields a xI/dI

layering range that increases towards the longer, C12 component s value (Figure 58e), likely driven by the vdW energy gain upon increasing chain-chain contact as their overlap l0 increases (Figure 58c).

The dI values conform to Vegard s modified mixing law of structured soft matter [5]. They deviate from the ideal liquid mixtures d0=[dI(12)+dI(n)]/2 by up to 35% for n<12, but only by ≤8% for n>12 (Figure 59a), showing the latter to be closer to ideality. The deviations exhibit a linear (Dn/ñ)2 dependence (Figure 59b) (Dn=12-n, ñ=(12+n)/2), found for w, the interchange energy due to chain length mismatch, dominating alkanes and alkanols mixtures [6]. Thus, Figure 59b may imply that the nanostructures of the RTIL mixtures are also dominated by the w of the cationic chains.

This foray into the scarcely explored realm of RTIL mixture nanostructures should provide insights for knowledge-guided design of RTIL mixtures with desired nanostructure and properties.