Interfacial structure and double layer capacitance of ionic liquids
Monchai Jitvisate is a PhD-student in the Department of Biomechanical Engineering. His supervisor is Professor Serge Lemay from the Faculty of Science and Technology.
Ionic liquids are organic salts that are in liquid phase at room temperature. Their wide liquidus range, particularly at room temperature, results from the liquids’ large and asymmetric molecular geometry. This leads to a collection of unique properties, such as, high ionic strength, extremely low vapor pressure, wide electrochemical window, and high thermal and chemical stabilities. These properties make ionic liquids a promising material in many applications, for example, electrochemical energy storage devices, electrically tunable lubrication, high-temperature/vacuum material synthesis, and novel self assembly media. However, ionic liquids do not seem to obey the well established classical electrical double layer models for electrolytes, leading to the necessity of both theoretical and experimental studies.
This thesis is devoted to experimental investigations of the interfacial behavior of ionic liquids, in both pure and dilute forms at various physical and electrical conditions, aiming to reveal their true interfacial nature. Atomic force microscopy (AFM) is used to directly probe the double layer force while electrochemical techniques can be used to measure the response to electric field of the liquids in terms of a double layer capacitance. The observations from these two experimental techniques will lead to a molecular scale picture of the double layer and its charging mechanism, respectively.
As a result, we observed discrete near-wall structure of the ionic liquids formed on flat solid substrates, and their properties vary with the influences of, for example, surface materials and concentration of the liquids. In addition to this near-wall measurement, the long-range electrostatic force is also measurable in dilute ionic liquids by using colloidal probe microscopy, where partial ion dissociation can be inferred from the measured screening length. Double layer capacitance is measured using electrochemical techniques, showing capacitance curves that well fit with a recently proposed theoretical model for ionic liquids, leading to an understanding of the charging process of the ions. The results explained in this thesis are relevant for many applications ranging from tunable lubrication and electrochemical energy storage devices.
