Gas Adsorption at Structured Ionic Liquid Surfaces

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The absorption, or capture, of gases into liquids has been studied since the early 1800's. However, the study of liquid surfaces has been restricted to techniques that can operate at or near to atmospheric pressure because common liquids have high vapour pressures (>10-6 mbar), due to the relatively weak van der Waals interactions that hold them together as liquids. This meant that liquid surfaces could not be studied (with some exceptions) on the molecular scale because such study requires the use of powerful, vacuum based, surface science techniques developed for solid surfaces over the past half century. However, ionic liquids, consisting of relatively large, low symmetry, organic cations, matched with inorganic or organic anions, have ultra low vapour pressures at room temperature (<10-10 mbar), due to the strongly cohesive Coulomb potential between the ions, making them vacuum compatible. Therefore, the liquid surfaces of ILs can be studied with molecular detail using vacuum based surface science techniques, opening up a new field of liquid surface science. Our goal is to quantitatively determine the surface structure of ionic liquids, and relate that structure to how gases adsorb onto the surface layers and then pass by absorption into the bulk of the ionic liquid. There are good academic and industrial reasons for such work. Academically, because ionic liquids are composed of large complex species, they have the potential for a level of surface self-organisation that is completely beyond anything simple solvents can attain. The most obvious examples of this self organisation are surface freezing, where long alkyl chains align themselves at the surface into a semicrystalline layer (thus providing an oleaginous barrier to adsorbing gas), and the formation of an ionic underlayer where the charge carriers of the anion and cation form a charged double layer which can act as a surface trap for adsorbed species. By understanding how such self-organisation depends on the nature of the IL, and how the self-organised structure then affects the adsorption of gases, we open up a new area of task specific liquid surface science. Crucially, the ultra-low volatility of ionic liquids is also the property that gives them their huge industrial potential, because the IL does not contaminate gas phase reactants and products with its vapour. Of particular relevance to this application is the SILP (surface ionic liquid phase) process, which combines the advantages of homogeneous and heterogeneous catalysis, and the possibilities of using ILs as capture agents for CO2 in carbon capture and storage (CCS). In this application we use the low volatility of ILs to apply ultra-high vacuum surface science techniques to their surfaces. Surface structures will be determined using angle resolved XPS and X-ray reflectivity, while surface kinetics will be determined using line of sight mass spectroscopy to measure absolute sticking probabilities and temperature programmed desorption. Our work will be the first coherent study of adsorption of any type on well defined liquid surfaces, and will be seminal in the development liquid surface science, comparable to the advances in understanding of solids surfaces when ultra-high vacuum adsorption studies were first carried out in the late '60s and early '70s.Longer term (15-30 years) we will start to answer more complex questions such as; how can this highly structured, anisotropic, but mobile, surface environment be deliberately modified to facilitate the formation of nanostructures using material from both sides of the surface; can we design new types of liquid surfaces which will facilitate directed self assembly of nanoparticles and nanomachines; can we begin to engineer liquid membranes with embedded moieties similar to those in living cells?