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H. Cheng, H. Weiss, P. Stock, Y. Chen, C. Reinecke, J. Dienemann, M. Mezger, M. Valtiner:
"Effect of Concentration on the Interfacial and Bulk Structure of Ionic Liquids in Aqueous Solution";
Langmuir, 34 (2018), 8; 2637 - 2646.

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 ([C8MIm][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 selfassembled
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.