6 9 I H I G H L I G H T S 2 0 2 2
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)
 T. Welton, Biophys. Rev. 10, 691 (2018).  D. Pontoni et al., J. Mol. Liq. 338, 116587 (2021).  D. Pontoni et al., J. Mol. Liq. 300, 112280 (2020).  W. Helfrich, Z. Naturfor. C 28, 693 (1973).  E. Sloutskin et al., Phys. Rev. Lett. 89, 065501 (2002).  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 , 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 , 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 . 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 . 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.