Nanostructural Organization in Ionic Liquids

Abstract

Nanometer-scale structuring in room-temperature ionic liquids is observed using molecular simulation. The ionic liquids studied belong to the 1-alkyl-3-methylimidazolium family with hexafluorophosphate or with bis(trifluoromethanesulfonyl)amide as the anions, [C(n)mim][PF(6)] or [C(n)mim][(CF(3)SO(2))(2)N], respectively. They were represented, for the first time in a simulation study focusing on long-range structures, by an all-atom force field of the AMBER/OPLS_AA family containing parameters developed specifically for these compounds. For ionic liquids with alkyl side chains longer than or equal to C(4), aggregation of the alkyl chains in nonpolar domains is observed. These domains permeate a tridimensional network of ionic channels formed by anions and by the imidazolium rings of the cations. The nanostructures can be visualized in a conspicuous way simply by color coding the two types of domains (in this work, we chose red = polar and green = nonpolar). As the length of the alkyl chain increases, the nonpolar domains become larger and more connected and cause swelling of the ionic network, in a manner analogous to systems exhibiting microphase separation. The consequences of these nanostructural features on the properties of the ionic liquids are analyzed.

Dynamics of RTILs: A comparative dielectric and OKE study

“…This points to a breathing mode of mesoscopic aggregates essentially normal to the average dipole orientation of the cluster. Aggregates of π-stacked cations and their surrounding anions are reasonable candidates compatible with MD simulations [40,41,42] and scattering experiments [43]. It should be noted that the lifetime of the caging effects observed in MD simulations of RTIL diffusion, see e.g.…”

Communication: Anion-specific response of mesoscopic organization in ionic liquids upon pressurization

“…As a matter of fact, their structure that was originally considered to be characterised by an onion-like alternation of opposite sign ions (similarly to the organisation in conventional high temperature molten salt, e.g., NaCl) has been discovered to show an enhanced level of organization, stemming from the alternation not only of charged and uncharged moieties, but also of polar and apolar domains. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] These alternations are now known to be fingerprinted by well-defined X-ray and/or neutron scattering 13,[21][22][23][24] features in the low momentum transfer (Q) range. The interest toward this feature is witnessed by the wealth of studies addressing its nature and dependence upon chemical nature of the ions, [25][26][27][28][29][30] temperature, 13,15,31 mixture composition, [32][33][34][35][36][37][38][39][40][41] and, only recently, pressure.…”

Intermolecular vibrations and fast relaxations in supercooled ionic liquids

“…(1)]. It turns out that typical correlation length ξ is of the It is interesting to compare the length ξ with the characteristic size of heterogeneities that has been obtained from the wave-vector of the first-sharp diffraction peak, k FSDP , observed in neutron or X-ray spectra, 26,[66][67][68][69] and also molecular dynamics (MD) simulations, [69][70][71][72][73] 69 The FSDP in the static structure factor, S(k), indicates that an intermediate-range order (IRO) develops in the liquid, but the nature of such IRO in glass-forming liquids is another lively debated issue because different explanations have been proposed. In SiO 2 , the network of connected SiO 4 2− tetrahedral produces the correlations of the IRO.…”

“…[70][71][72][73] On the other hand, the FSDP in ionic liquids has been recently assigned to correlation of the second shell of cations, with no need of a picture of nanoscale polar/non-polar heterogeneity. 68 However, it is worth stressing that the physical picture of structural heterogeneity in ionic liquids has been proposed by MD simulation not only on the basis of the occurrence of a FSDP in the calculated S(k) but also on the basis of anions depletion in regions occupied by the alkyl chains.…”

“…68 However, it is worth stressing that the physical picture of structural heterogeneity in ionic liquids has been proposed by MD simulation not only on the basis of the occurrence of a FSDP in the calculated S(k) but also on the basis of anions depletion in regions occupied by the alkyl chains. [70][71][72][73] Several inorganic glass-formers present correlation between the characteristic length ξ obtained from the boson peak and from the FSDP. 61,62 This correlation between ξ −1…”