Ionic Liquid Mixture at the Vacuum Interface and the Peaks and Antipeaks Analysis of X-ray Reflectivity

Abstract

This article presents work on the structure of the ionic liquid mixture between 1-methyl-3-octylimidazolium octylsulfate and 1-ethyl-3-methylimidazolium ethylsulfate at the vacuum interface. In particular, we focus on the relative concentration of components as a function of distance to the interface and the formation of a nanoscopic apolar blocking layer that encloses a second thicker and much more polar interfacial layer. We present a thorough analysis of the time scale for the convergence of structural features as they dissipate into the bulk and provide a clear estimation of the interface persistence lengthscale. We also show how the peaks and antipeaks analysis that our group has developed to better understand polar–apolar and positive–negative alternations in the bulk structure factor S(q) can be recast to provide novel and insightful information about interfacial reflectivity. The analysis dissects the nearly featureless overall normalized reflectivity into a rich collection of peaks and antipeaks that provides significant insight into the location and width of apolar and charged layers. The peaks and antipeaks analysis captures fine structural details within layersincluding the pattern of charge alternation that is non-lateralassociated with the preferential arrangement of charged species within a polar layer.

Temperature‐Dependent Surface Enrichment Effects in Binary Mixtures of Fluorinated and Non‐Fluorinated Ionic Liquids

“…Therefore, investigations of the topmost layers of IL mixtures are getting more into the focus of research. A variety of studies has been performed, using reactive‐atom scattering with laser‐induced fluorescence detection (RAS‐LIF), neutron scattering, small‐angle X‐ray scattering and X‐ray reflectivity, time of flight secondary ion mass spectrometry (TOF‐SIMS), Rutherford backscattering spectroscopy (RBS), low‐energy ion scattering (LEIS), X‐ray photoelectron spectroscopy (XPS) and molecular dynamics (MD) simulations …”

Understanding the Photoexcitation of Room Temperature Ionic Liquids

“…At very early times, i. e. before the time scale of solvent reorganization, a so‐called “dry” electron (hole) can occur [33] . In more general, this is the topic of excess electron solvation in an ionic liquid [33,70] such as 1,3‐dimethyl‐imidazolium [34] or in 1‐methylpyridinium chlorides [35] …”

Local and Long-Range Organization in Room Temperature Ionic Liquids

“…Long-range organization is seen in RTILs by a number of methods, both experimental and theoretical. Such organization is not consistent with models that are appropriate for dilute solutions, despite data that suggests very limited dissociation in RTILs. , Another body of data that is not consistent with the treatment of RTILs as “normal” liquid phase solvents is evidence, both experimental and theoretical, for nanoscale heterogeneities in RTILs. ,,, The rationale for the existence of such domains stems from the amphiphilic nature of the cationic RTIL constituents when nonpolar moieties, typically on the cation, are sufficiently large to form nanodomains. Such compositional heterogeneity has been suggested as the basis for the ability of RTILs to solubilize nonpolar species …”

“…16,17 Another body of data that is not consistent with the treatment of RTILs as "normal" liquid phase solvents is evidence, both experimental and theoretical, for nanoscale heterogeneities in RTILs. 14,15,45,46 The rationale for the existence of such domains stems from the amphiphilic nature of the cationic RTIL constituents when nonpolar moieties, typically on the cation, are sufficiently large to form nanodomains. Such compositional heterogeneity has been suggested as the basis for the ability of RTILs to solubilize nonpolar species.…”