Gas Adsorption at Structured Ionic Liquid Surfaces

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?

Liquids play a key role as solvents in chemistry, biology, industry and society in general, allowing molecular mobility of dissolved material such that chemical transformations can occur. In this application we use ionic liquids, which, with their ultra-low vapour pressures (unlike common solvents), allow us to study their surfaces in vacuum using surface science tools. We will study the most basic and wide ranging aspects of adsorption at liquid surfaces; the rate at which gases adsorb at, and are transmitted through, the liquid surface; and how those rates relate to the liquid surface structure. The project team is a unique combination of a highly experienced ultra-high vacuum scientist (RGJ) and an organic chemist/chemical engineer (PL) who initiated the field of liquid surface science by applying UHV techniques to the study of IL surfaces. Knowledge: Our work will open up the new field of liquid surface science using ionic liquids. Although one might think that ionic liquids are a small sub-set of all liquids, the possible number of ILs outnumber common solvents by at least 10000x, while still exhibiting all the diversity of common liquids. In the short term (5-10 years) the new frontier of liquid surface science will allow an unprecedented increase in our molecular level understanding of liquid surfaces. In particular the structure of the liquid surface and how it affects the adsorption of gases (this proposal). Slightly longer term (10-15 years) we will learn what types of new and novel chemical reactions can be initiated at liquid surfaces and long term (15-30 years) we design new types of liquid surfaces which facilitate directed self assembly of nanoparticles and nanomachines and engineer liquid membranes with embedded transmembrane moieties similar to those in living cells. Economic benefits. Ionic liquids are already finding widespread commercial and this trend is set to continue and increase. Central to all processes involving liquids and gases is the rate at which the gases pass into, and out of, the liquid surface. Our application addresses this by studying the behaviour of simple gases that are relevant to the SILP process and hydrogenation, hydroformylation and Fischer-Tropsch reactions, providing kinetic data that will be of direct use to the chemical engineer. The relation of that data to chemical structure (time scale 5-10 years) will allow the engineering of ILs to suit specific commercial applications. Societal benefits. Climate change, due to global warming caused by CO2 emissions, threatens society directly. One way of ameliorating the problem is to capture the CO2, but the process must be fast (emission rates are about 1/4 tonne of CO2 per second per power station). Ionic liquids show great promise in that they are effectively involatile, even at elevated temperatures, (much less volatile than the amine based agents currently being considered) and can be tuned to have the necessary solvent properties. Our application directly addresses the kinetics of carbon capture and will contribute directly to the technology needed to reduce CO2 emissions. Method for communication and engagement. The outcome of our research involves both advancement of knowledge and data that is of direct use to the utilization of ILs industrially and commercially. We shall therefore publish in high quality academic journals aimed at scientific understanding, and also in Journals that list factual data such as Industrial and Engineering Chemical Research. We will also deposit our data onto the Ionic Liquids Database- (ILThermo, NIST Standard Reference Database #147) (http://ilthermo.boulder.nist.gov/ILThermo/mainmenu.uix) We shall also attend international conferences on Surface Science (to promulgate our understanding of liquids surfaces), and on Ionic Liquids (where industry representatives attend to learn of the new thermodynamic and kinetic measurements being made with ILs.)