Mesoscopic Correlation Functions in Heterogeneous Ionic Liquids

Abstract

A common feature of ionic liquids composed of cations with long aliphatic side chains is structural heterogeneities on the nanometer length scale. This so-called microphase separation arises from the clustering of aliphatic moieties. The temperature dependence of the liquid bulk structure was studied by small-angle X-ray and neutron scattering for a set of methylimidazolium ([CCim], [CCim]) based ionic liquids with tris(pentafluoroethyl)trifluorophosphate ([FAP]), bis(trifluoromethylsulfonyl)imide ([NTf]), and bis(nonafluorobutylsulfonyl)imide ([NNf]) anions. The experimental data is quantitatively analyzed using a generalized Teubner-Strey model. Discussion of the resulting periodicity d and correlation length ξ shows that the structural heterogeneities are governed by the interplay between the alkyl chain length, the geometry of the anion, and entropic effects. Connections between the mesoscopic correlation functions, density, and entropy of fusion are discussed in comparison to alcohols. The observed dependencies allow predictions on the mesoscopic correlation functions based on macroscopic bulk quantities.

Surface structure evolution in a homologous series of ionic liquids

“…1A shows, the typical R/RF shape of n = 16 to 20 is recovered also for n = 22 at high temperatures. The new phase of n = 22 is tentatively identified as the surface equivalent of the bulk liquid-crystal phase reported recently for this compound (38,39). Indeed, the qz positions of the two peaks of n = 22 in Fig.…”

“…of the chains by introducing gauche defects, the interdigitation motif may largely survive in the RTILs' liquid phase as well, similar to that found in molten alcohols (63,64). The facts that the layer spacing d for all n ≥ 8 is intermediate between ln + 5Å and 2ln + 5Å, that, in the bulk, it increases with n by more than the projected C-C bond length, 1.27Å/CH2, but by less than 2 × 1.27Å/CH2 (6, 7), and that it decreases with increasing temperature despite the overall volume expansion (17,39) are taken, in this approach, as support for chain interdigitation.…”

“…The dξ/dn parallels therefore the expected increase in the vdW interaction due to the added methylenes, implying that the layering range is determined by that interaction, with negligible contribution from interactions between the polar headgroups' layers. This, in turn, suggests that the anomalously negative thermal expansion coefficients found for the surface d of [C18mim][FAP] (17), and for the bulk dI of n = 18 and n = 22 (39), originate in the alkyl layer, not in the interaction between the polar headgroups' layers. Finally, we also note that, since d increases with n with a much smaller slope than ξ, ξ/d still increases, reaching values of 1 for n ≥ 18.…”

Neutron scattering studies on short- and long-range layer structures and related dynamics in imidazolium-based ionic liquids

“…1). [8][9][10][11][12][13][14][15][16][17][18][19][20][21] These studies indicate that CnmimX have some higher-order structures in nanometer scale, which we call "nanostructure." The nanostructure consists of the aggregations of the charged parts of cations and anions and the neutral parts of alkylchains.…”

“…c) Electronic mail: yamamuro@issp.u-tokyo.ac.jp During the past decade, great progress has also been made in fundamental scientific studies. [4][5][6] The structural investigations for CnmimX with n < 10 have been performed by using x-ray and neutron diffraction (ND) techniques [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] and molecular dynamics (MD) simulations. 12,17,[22][23][24][25][26][27][28][29][30] In these studies, a characteristic peak indicative of Q low , which is called "low-Q peak" in this paper, appeared at Q low ≈ 0.3 Å −1 in addition to the peaks at Q ion ≈ 1 Å −1 corresponding to the correlation among ions and at Q alkyl ≈ 1.5 Å −1 corresponding mainly to the correlation among alkylchains (Fig.…”