Thomas M. Koller

This work summarizes the fundamentals of the description of molecular diffusion in liquid and gaseous non-electrolyte and electrolyte systems at constant temperature and pressure and in the absence of external fields. On the basis of Fick’s law and the theory of Maxwell and Stefan, the description of diffusive fluxes in commonly used reference frames and a recommendation for the terminology of the associated diffusion coefficients are given. For non-electrolyte systems, the corresponding equations for the diffusive fluxes are outlined for a general n-component mixture and are explicitly stated for binary and ternary mixtures. Additionally, the transformation of the component order is discussed for a general n-component mixture. For electrolyte mixtures, explicit equations for the diffusive fluxes are stated for binary mixtures consisting of a molecular solvent and a dissolved electrolyte component and binary mixtures consisting of two electrolyte components sharing a common ion. Furthermore, explicit equations for the diffusive fluxes are given for ternary systems consisting of one electrolyte component dissolved in two non-electrolyte solvents. For all the aforementioned systems, transformations of the diffusion coefficients between commonly used reference frames are included.

We investigated the surface tension and surface composition of binary ionic liquid (IL) mixtures of ILs sharing the same cation. As model system, binary mixtures of 1-ethyl-3-methylimidazolium acetate ([C₂C₁Im][OAc], molar volume: 256.2 cm³·mol^-1 at 293 K) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₂C₁Im][Tf₂N], molar volume: 154.4 cm³·mol^-1 at 293 K), with very different surface tensions, and with anions of very different chemical structure and size were studied over the whole composition range. The surface tension was obtained by pendant-drop (PD) measurements in the presence of 0.1 MPa argon between 294 and 323 K, and the surface composition was determined by angle-resolved photoelectron spectroscopy (ARXPS) in ultrahigh vacuum at 293 K. The ARXPS results reveal a strong preferential enrichment of [C₂C₁Im][Tf₂N] at the vacuum-liquid interface of the binary mixtures, which is more pronounced at lower [C₂C₁Im][Tf₂N] bulk contents. This microscopic behavior is reflected in the macroscopic surface tensions, which are significantly lower than calculated assuming a linear mixing behavior based on the bulk composition. A previously developed prediction model to correlate the surface tension with the molar surface composition yields deviations of more than 5% from the measured values, which we attribute to the strongly different sizes of the anions. By accounting for the surface areas occupied by the ILs, we present an improved new model which describes the experimental data very well within 1.4%.

In der homogenen Katalyse geht man üblicherweise davon aus, dass sich die Konzentrationen der Übergangsmetallkomplexe an der Gas‐Flüssig‐Grenzfläche und im Volumen der Lösung nicht unterscheiden. Tatsächlich zeigen aber umfangreiche Untersuchungen an gelösten Metallkomplexen in ionischen Flüssigkeiten (Englisch: Ionic Liquids, ILs) ausgeprägte Oberflächenanreicherungs‐ und Segregationseffekte, die auch für praktische Anwendungen von großer Bedeutung sein können. Diese Untersuchungen basieren auf Methoden der Oberflächenforschung, die aufgrund der vernachlässigbaren IL‐Dampfdrücke unter wohldefinierten Vakuumbedingungen anwendbar sind. Dieser Übersichtsartikel bietet einen Überblick über die jüngsten Fortschritte bei der Steuerung der Grenzflächenzusammensetzung von katalytischen Systemen auf der Basis ionischer Flüssigkeiten. Ein besonderer Schwerpunkt liegt auf der gezielten Oberflächenanreicherung. Insbesondere werden eine Vielzahl von Parametern vorgestellt, welche die lokale Komplexkonzentration an der Oberfläche beeinflussen wie etwa die Wahl der Liganden, die Konzentration im Volumen, die Temperatur und die Natur des IL‐Lösungsmittels. Als experimentelle Methoden wurden vor allem winkelaufgelöste Röntgenphotoelektronenspektroskopie (Englisch: Angle‐Resolved X‐Ray‐Photoelectron Spectroscopy, ARXPS) und Oberflächenspannungsmessungen mit der Methode des hängenden Tropfens (Englisch: Pendant Drop Method) unter ultrareinen Vakuumbedingungen eingesetzt. Die hier zusammengefassten Ergebnisse bilden die Grundlage für die Weiterentwicklung von maßgeschneiderten katalytischen Anwendungen mit großer innerer Oberfläche, z.B. in der Katalyse mit geträgerten ionischen Flüssigphasen (Englisch: Supported Ionic Liquid Phase, SILP); bei solchen Systemen hat das Design der Grenzfläche einen erheblichen Anteil an der katalytischen Effizienz.

When talking about homogeneous catalyst systems, it has long been assumed that the system at hand consists of a transition metal complex in solution with the liquid interface representing the composition of the bulk solution. Now, in light of considerable developments in the study of metal complexes dissolved in ionic liquids with their negligible vapor pressures, more detailed studies of the composition at the liquid/gas interface became possible. These investigations revealed pronounced surface enrichment and segregation effects of high relevance for practical applications. This article reviews recent advancements in tailoring the interfacial composition of ionic liquid‐based catalytic systems. A particular focus is dedicated to surface enrichment phenomena, and a variety of parameters are presented for deliberate control of the local concentration of the complexes at the surface, that is, the nature of the ligands, the bulk concentration, the temperature, and the nature of the IL solvent. As experimental methods, angle‐resolved X‐ray photoelectron spectroscopy (ARXPS) and vacuum‐based pendant‐drop surface tension measurements were applied. The reviewed results are intended to provide the basis for the advancement of catalytic systems with high surface areas, such as in supported ionic liquid phase (SILP) catalysis, where the interface design is directly interconnected with catalytic performance.