Nano-segregation in ionic liquids: scorpions and vanishing chains

Abstract

The present study analyses the large structural differences, first observed using X-ray diffraction, between 1-alkyl-3-methylimidazolium-based ionic liquids, [Cnmim][Ntf2] (n = 3, 6, 9), and their counterparts with ether-substituted alkyl side chains, [(C1OC1)(n/3)mim][Ntf2] (n = 3, 6, 9). The MD simulations-obtained using a non-polarizable atomistic force-field to model the ionic liquids under discussion-demonstrate that the suppression of the nanostructured nature in the ionic liquids with ether chains is persistent along the entire series and it is not due to any modification of the polar network of the ionic liquid but rather due to the different morphologies of the non-polar regions that surround it. The modification of the non-polar regions-shift from bulky segregated domains in [Cnmim][Ntf2] to thin enveloping ones in [(C1OC1)(n/3)mim][Ntf2]-are caused by the inability of the oxygen-substituted alkyl side chains to pack effectively side by side, the existence of kinks along the chain that lead eventually to intra-molecular, scorpion-like interactions between the chains and the imidazolium ring, and by their stronger interactions with the cations of the polar network via the lone electron pairs of the ether oxygen atoms.

Connecting Structural and Transport Properties of Ionic Liquids with Cationic Oligoether Chains

“…X-ray scattering results.-Transport properties of ionic liquids are intimately linked to their nanoscale structure, and previous structural and molecular dynamics studies of imidazolium, ammonium and phosphonium ILs with pendant ether groups 1,2,28,[39][40][41] have shown that their organization on the molecular scale is different than their normal alkyl chain congeners. While alkyl chains tend to exist in extended conformations, ether chains have a large fraction of gauche conformations, particularly across C-C bonds connected to ether oxygens.…”

“…1 H and 13 C NMR spectra were recorded on a Bruker 400 MHz NMR spectrometer. The water content measurements were performed using a Mettler Toledo DL39 Karl Fischer Coulometer.…”

“…Researchers are delving deeper into the nanostructural interactions between the ions to determine the topographical landscape of the ions within the liquids that influence IL properties. [1][2][3] Such information is valuable when tuning the properties of ILs for particular applications. The ability to tune the properties of ionic liquids for specific applications is a major factor in their allure as remarkable solvents that make it possible to do extreme chemistry without extreme conditions.…”

“…28 More recently, MD simulations and X-ray diffraction studies have shown that in cations with ether tails or side chains, the tails are curled toward the positive head of the cation or wrap the cation if long enough (Figure 2), 1,2,12,28 effectively reducing the coulombic interactions between the cations and anions. The anions adjoin both the polar ether sites as well as the positive site of the cation.…”

Exploring the Use of Ionic Liquid Mixtures to Enhance the Performance of Dicationic Ionic Liquids

“…More recently MD simulations and X-ray diffraction data have shown that this decrease in viscosity can be attributed to the attraction between the polar ether chain and the cation resulting in the ether chain curling inwards. 43,44 This in turn disrupts and suppresses the nanosegregation typically observed in ILs with alkyl side chains, resulting in heterogeneity and disorder. The anions adjoin both the positive site of the cation as well as the polar ether chains and there is no polar-apolar segregation.…”