Developing Distinct Chemical Environments in Ionic Liquid Films

Abstract

We report the reversible transitions from two distinct chemical environments (interfacial and bulk) to a single, globally ordered dominant environment that extends to 800 nm, within two triflate-based ionic liquid (IL) films. Vibrational spectra for supported IL films exhibit multiple peaks for the same vibrational mode, indicating the presence of multiple chemical environments (interfacial and bulk) in the film. After a quiescent maturation time, the vibrational spectra show much simpler absorption profiles indicating coalescence of the IL molecules into a global preferred phase, which resembles the interfacial environment that propagates throughout the film as a function of time. Data analysis suggests significant reorientation of the triflate anion with small changes of the cation, indicating a weakly interacting cation−anion pair. The distal extent of the self-organization is much thicker than that generally reported for solid−fluid interfaces. The ordering is reversible on replenishing the film with fresh fluid. This report, describing propagation of interfacial molecular orientation to form extended ordered structures, is a motivation for future studies to apply this phenomenon toward the thoughtful design of new IL systems for use in materials and devices.

The effect of dilution on induced free charge density gradients in room temperature ionic liquids

“…7,11–13 The Fayer group, in a series of elegant experiments using small molecules as probes, has identified organization with a persistence length on the order of tens to hundreds of nm in RTILs, 14–19 and the Shaw group has identified the evolution of structural order on the micrometer length scale in thin RTIL films. 20–23 Very recently, the Welton group has identified spatial variation in n , the refractive index, over distances of tens to hundreds of nm using Raman scattering. 24 It is clear from all these studies that RTILs exhibit structural order over length scales vastly in excess of what is typical for liquid phase solvents.…”

Local and Long-Range Organization in Room Temperature Ionic Liquids

“…As noted above, there is abundant evidence for structural organization and for the influence of charged surfaces on dynamics in RTILs. Most of these reports place such organization up to multiple tens of nanometers, ,,− ,− and in some instances the organization is limited by the thickness of the RTIL films examined. − Our group has found evidence for induced charge density gradients (ρ f ) in RTILs with a spatial extent of tens of micrometers. − We discuss our findings and their implications below, with an eye toward understanding how the free charge density gradients we find may be influenced by structural organization in RTILs. One important consideration is that the structural order reported for thin RTIL films may exhibit longer-range effects in thicker samples, bringing structural organization and free charge density gradients into closer agreement.…”

“…The Shaw group has found compelling evidence for long-range structural organization in ionic liquids. They formed thin RTIL films using a dynamic wetting technique and acquired FTIR, optical ellipsometry, and second harmonic generation measurements. − The Shaw group monitored the (polarized) IRRAS spectra of BMIM + TFSI – thin films as a function of time (minutes to days) and observed the gradual changes of band positions and intensities for several bands in the 1000–1400 cm –1 region. While several of the bands did not change over time, there were four bands that were seen to exhibit changes in intensity and/or frequency.…”

“…For these ionic liquids, a liquid crystalline mesophase has not been reported, but it is possible that subtle organization may exist in these systems. There are a number of reports that indicate thin films of imidazolium-based ionic liquids exhibit ordering that is induced by the presence of an interface. − In addition, a recent article by the Brand group demonstrates the ability to alter the organization of an ionic liquid monolayer deposited on Au by controlling the potential of the gold surface . They found that the orientation of the monolayer imidazolium aliphatic chains exhibited a surface-potential-dependent tilt angle.…”

Optical Spectroscopy of Surfaces, Interfaces, and Thin Films: A Status Report

“…These researchers demonstrated that ionic liquids form long-rage ordered structures out to ∼2 μm from the solid surface over a time period up to 2 h after dewetting. In a following study, these same researchers proved the propagation of ordering at the solid–ionic liquid interface to form ordered extended structures and the role of shear as a disruptive force in such ordering …”