Professor John Ralston - Summit Speaker Series - September 2, 2016

Professor John Ralston                              

Emeritus Laureate Professor University of South Australia

September 2, 2016 2:00 pm

8-207 Donadeo Innovation Centre for Engineering

“Static and Dynamic Boundary Conditions for Confined Ionic Liquids”

Room temperature ionic liquids [RTILs] structure at solid –liquid and solid gas interfaces, shown by electrochemical, X-Ray reflectance and spectroscopic evidence. This surface structuring impacts the motion of the three phase contact line during dynamic wetting and thin film spreading. We explore this behaviour through a variety of complementary techniques, revealing the subtle mechanisms involved.

The differential capacitance of the electrical double layer at glassy carbon, platinum and gold electrodes, immersed in various RTILs was measured using impedance spectroscopy and cyclic voltammetry. The differential capacitance/potential curves indicate specific adsorption of both cations and anions. Cations are adsorbed at the open-circuit potential and their surface excess concentration increases with negative polarization, whereas adsorption of anions occurs at positive potentials.

In complementary high-energy x-ray reflectivity studies, the temperature-dependent structures of RTILs in contact with a charged sapphire substrate were investigated at sub-molecular resolution. The RTILs show strong interfacial layering, commencing with an adsorbed cation layer at the solid substrate and decaying exponentially into the bulk liquid. The observed decay length and layering period together point to an interfacial ordering mechanism. This is similar to a charge inversion effect and may originate from strong correlations between the unscreened ions. This structure at the solid-liquid interface has a remarkable influence on solid-ionic liquid-solid interactions as well as interfacial friction.

The structure of the liquid-vacuum interface in RTILs was studied with angle-resolved X-ray photoelectron spectroscopy (ARXPS) and synchrotron X-ray photoelectron spectroscopy (SXPS). By varying the polar angle and comparing the results for the chosen ionic liquids, the presence of a surface layer, chemically different to the bulk, was detected. This layer is enriched by aliphatic carbon atoms from the saturated carbon chains of the anions and cations, and contains an unequal distribution of cations and anions in a direction normal to the surface, creating a potential gradient which extends from the surface into the liquid. This observed interfacial layering at solid -liquid and liquid- vapour interfaces appears to be a generic feature of RTILs at charged interfaces.

The spontaneous spreading of RTILs on a fluoropolymer surface in air was probed by high-speed video microscopy. The dependence of the dynamic contact angle on contact line velocity was interpreted with hydrodynamic and molecular-kinetic models. There is a good correspondence between the molecular dimensions of the liquids and the physical parameters of the molecular-kinetic model, correlating strongly with interfacial friction behaviour

Atomic Force Microscopy was used to investigate the spreading of molecularly thin precursor films emerging from drops of RTILs that partially wet smooth mica surfaces. The lateral extent of the film increased with time and reaches values as large as few millimeters within 12 hours. From the observations of the precursor film at several positions and times, its extent was estimated and used to determine bounds for the spreading coefficient D. The spreading rate and the film morphology (at micrometer scale) for three different RTILs of varying cation molecular structure were compared and correlated with surface analysis observations.