Effect of Concentration on the Interfacial and Bulk Structure of Ionic Liquids in Aqueous Solution

Abstract

Bio and aqueous applications of ionic liquids (IL) such as catalysis in micelles formed in aqueous IL solutions or extraction of chemicals from biologic materials rely on surface-active and self-assembly properties of ILs. Here, we discuss qualitative relations of the interfacial and bulk structuring of a water-soluble surface-active IL ([CMIm][Cl]) on chemically controlled surfaces over a wide range of water concentrations using both force probe and X-ray scattering experiments. Our data indicate that IL structuring evolves from surfactant-like surface adsorption at low IL concentrations, to micellar bulk structure adsorption above the critical micelle concentration, to planar bilayer formation in ILs with <1 wt % of water and at high charging of the surface. Interfacial structuring is controlled by mesoscopic bulk structuring at high water concentrations. Surface chemistry and surface charges decisively steer interfacial ordering of ions if the water concentration is low and/or the surface charge is high. We also demonstrate that controlling the interfacial forces by using self-assembled monolayer chemistry allows tuning of interfacial structures. Both the ratio of the head group size to the hydrophobic tail volume as well as the surface charging trigger the bulk structure and offer a tool for predicting interfacial structures. Based on the applied techniques and analyses, a qualitative prediction of molecular layering of ILs in aqueous systems is possible.

Real-time visualization of metastable charge regulation pathways in molecularly confined slit geometries

“…Our results show that molecular layers measured in sSFA displayed an oscillatory profile of 2.2 nm, which matches the AFM phase-distance characteristics, revealing the multilayered IL structures with complementary techniques 35 . Comparably weaker but still clearly oscillatory sSFA F-D profiles (less load required to expel a given molecular layer) were resolved in the IL-gold system.…”

Proteins in Ionic Liquids: Reactions, Applications, and Futures

“…Ions with high charge density (most anions studied) are preferentially dispersed in bulk water at low concentrations (Haberler and Steinhauser, 2011; Haberler et al, 2012), and are those that strip the non-localized water molecules. Chloride ions in aqueous solution require six water molecules to form a first hydration shell; in a study of the behavior of [C 8 C 1 Im][Cl] at both the interface and in bulk solution, the calculated water to ion pair molar fraction was found to be 3.6 at ~20 wt%, meaning that the chloride ion was preferentially surrounded by one cation and 3-4 water molecules (Cheng et al, 2018). This structuring, and rearrangement of anionic coordination sphere could also have implications for the nature of the interaction of ions with a protein surface at different water contents.…”

Atomic Force Spectroscopy on Ionic Liquids

“…Depending on the molecular configuration of the ionic liquids, this transition can also be related to the formation of micelles. Reverting the roles of ionic liquid and water, Cheng et al [113] investigated the formation of the mica/liquid interface when an ionic liquid, here [C 8 MIm][Cl], was subsequently added to water. As is shown in Figure 15, force-separation histograms were constructed for different concentrations of the ionic liquid in the mixture.…”