Structures of Ionic Liquids Having Both Anionic and Cationic Octyl Tails: Lamellar Vacuum Interface vs Sponge-Like Bulk Order

Abstract

Numerous experimental and computational studies have shown that the structure of ionic liquids is significantly influenced by confinement and by interactions with interfaces. The nature of the interface can affect the immediate ordering of cations and anions, changing important rheological characteristics relevant to lubrication. Most studies suggest that such changes are local or short-ranged and that bulk properties are reestablished on a length scale of a few nanometers. The current study focuses on the 1-methyl-3-octylimidazolium octylsulfate ionic liquid for which both the cation and anion have moderate length linear alkyl tails. For this system, we find that the bulk phase is dominated by the very common sponge-like morphology characteristic of many ionic liquids. However, at the vacuum interface, a lamellar structure is observed that is not restricted to the vicinity of the surface but instead extends across the full 9 nm slab of our simulation. We suspect that in reality it could extend significantly beyond this.

Fast multipole method for three‐dimensional systems with periodic boundary condition in two directions

“…In recent years, a number of MD calculations of three‐dimensional (3D) systems with two‐dimensional periodic boundary condition (slab geometry) have been performed for the interfaces of electrodes and substances with layer structures . To evaluate the electrostatic interactions for these systems, a calculation method based on the Ewald sum (Ewald2D) has been proposed .…”

Surface-Induced Changes in the Thermochromic Transformation of an Ionic Liquid Cobalt Thiocyanate Complex

“…One possibility could be related to the intrinsic potential of the IL/vacuum interface for inducing order in the near-surface region, as was witnessed, for example, by the preferential orientation of free [SCN] − anions at the surface observed by ARXPS (see above) or vertical layering effects found by other groups. 19,20 If this ordering is reduced by the formation of the octahedral complex plus its extended solvation shell (as observed by us by the loss of free [SCN] − orientation), and thus, if the reaction entropy in the near-surface region is slightly higher than that in the bulk, where those ordering effects are absent, then a shift of the complex equilibrium (Scheme 1) is expected toward the right, which leads to the observed increase in transition temperature in the near-surface region. It is important to note that such surface-induced phenomena could be of high relevance for all concepts where high-surface-area materials are applied.…”

The study of ionic liquid behavior at solid-liquid interfaces

“…59 The long-range ordering in ILs as shown by two of our previous publications were supported by recent experimental and computational studies on various ILs using different techniques. 112,[114][115][116] The reported increase in density and van der Waal's energy in this interfacial region implies solid-like behavior of the film. 127 All these above described studies characterize interfacial layers extending up to few tens of angstroms.…”

“…10 nm x 10 nm) and predict that the ordering phenomenon might extend beyond these dimensions. 114 A recent study of supported ionic liquid membranes showed that the IL dynamics under confinement are slower than the bulk liquid which is attributed to the change in the IL's calculated viscosity under confinement (4 times that of the bulk viscosity) and is predicted to be a result of long-ranged influence (ca. 100 nm) of the interfaces.…”

“…113 Margulis et al showed introduction of vacuum interfaces changed the sponge-like morphology observed in bulk ILs to lamellar structures that extends across the full simulation slab (10 nm) and are predicted to extend beyond these dimensions. 114 The study of supported ionic liquid membranes (SILMs) showed that the orientational relaxation of molecules like SeCNand CO2 are slower in SILMs (pore size of 100 nm to 350 nm) than in the bulk ILs, which implies that the pore walls can influence structure at distances of more than a hundred nanometers. 134 An interfacial study by Siria et al showed formation of a solid-like IL film under nanoconfinement with the thickness of this film depending on the material of nanoconfinement.…”