Gas Adsorption at Structured Ionic Liquid Surfaces

Lead Research Organisation: University of Nottingham
Department Name: Sch of Chemistry


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?

Planned Impact

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) ( 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.)


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Description We have applied Line of Sight Mass Spectrometry (LOSMS) to studying the interaction of vapours with bulk ionic liquid surfaces. Water, methanol, acetone, 1,3,5 and 1,2,3 Triflurobenzene on [C2C1Im][EtSO4] and acetone on [C2C1Im][Tf2N].
Deyko and Jones Farday Discuss 154, 2012, 265

We have developed a low temperature co-adsorption method to quantitatively measure the strength of interaction between ILs and small molecules using temperature programmed desorption. Acetone in [C2C1Im][Tf2N], and acetone, SO2 and water in [C8C1Im][BF4]. The small molecules are heavily stabilised by the ionic liquid.
Hessey and Jones Chem. Sci. 4 2013 2519

We have applied X-ray photoelectron spectroscopy, ultra-violet photoelectron spectroscopy and LEED to Ionic liquid adsorption on gold and copper surfaces, and coadsorption of gases in ionic liquids. [C2C1Im][Tf2N] and [C8C1Im][Tf2N] on Au(110). [C2C1Im][Tf2N] and [C8C1Im][Tf2N] and on Cu(111). Acetone in [C8C1Im][Tf2N] on Cu(111).
Foulston et al Phys. Chem. Chem. Phys. 14 2012 6054

Project Studentship associated with this grant (M. Buckley) PhD thesis passed. Covered the following work:
Interaction energy of [OMIM][BF4] and acetone
Interaction energy of [OMIM][BF4] and SO2
Interaction energy of [OMIM][BF4] and H2O
Structure of [EMIM][Tf2N] on Au(110) using NEXAFS and NIXSW

We have developed and implemented low temperature Line of Sight Mass Spectrometry for studying the interaction of carbon dioxide with ionic liquids. To apply LOSMS to study the interaction of CO2 with an ionic liquid by coadsorption, a lower temperature of ca 40 K is required. We have constructed a low temperature (40 K) cryopumped line of sight mass spectrometer which successfully operates with CO2. It has been used to quantitatively measure the interaction between CO2 and [C8C1Im][BF4] and [C8C1Im][Tf2N]. Both ionic liquid stabilise the CO2.
Gibson et al papers in preparation

We have studied the structure of bulk ionic liquid surfaces and of the ionic liquid /solid substrate surface using five runs at the Diamond synchrotron. Two runs were on I07 using X-ray reflectivity to determine the structures of IL/vacuum surfaces. Three runs were on I09 to determine the structures of monolayers of IL on Au(110) using near edge X-ray absorption fine structure (NEXAFS) and normal incidence X-ray standing wave (NIXSW) analysis. The structures of multilayers of IL and acetone in IL, on Au(110) and Si(001) were studied using the completely new technique of variable period X-ray standing wave (VPXSW) analysis. Analysis of the NIXSW data has necessitated the development of a new methodology to deal with the large amount of data from the large IL molecules. Software to analyse the VPXSW data has been written from scratch.
Gibson, J. S.; Syres, K. L.; Buckley, M.; Lee, T.-L.; Thakur, P. K.; Jones, R. G. In depth analysis of thin films using variable period X-ray standing waves. Nature (submitted) 2016.

In a collaboration we have used helium atom scattering to probe the liquid behaviour of [C2C1Im][Tf2N] on Au(111).
McIntosh et al Chem. Sci. 5, 2014, 667

We have developed techniques to form thin layers (sub-monolayer to 100's Å) of ionic liquid surfaces by evaporative techniques at atmosphere (on gold), and cleavage techniques under solution (on mica). Analysis was by XPS.
Exploitation Route We have studied the structure of the ionic liquid/vacuum and the ionic liquid/solid interfaces. We have quantitatively measured the interactions between small molecules notably water, acetone, SO2 and CO2 with ionic liquids. We have measured rates of dissolution of small moleclues across bulk ionic liquid surfaces and the interactions energies of these molecules with ionic liquids. Academically, this develops the area of ionic liquid surface science, and allows us to understand why particular molecules dissolve in particular ionic liquids and what the interactions are between the two, as a function of structure. It also allows us to understand the structuring of liquids at both the liquid solid interface and the liquid/gas interface. Non-academically, our work should impact on: carbon capture using ionic liquids; gas capture generally (e.g. SO2) using ionic liquids; surface ionic liquid phase (SILP) catalysis where high surface area thin layers of ionic liquid combine the benefits of heterogeneous and homogeneous catalysis; electrochemical reduction of CO2 and other molecules where the interactions between the solute and the ionic liquid at the electrode are paramount in determining the reduction potential; tribology and lubrication where an understanding of the structuring of liquids at solid surfaces is required. Longer term the structured ionic liquid/gas and ionic liquid/solid interfaces could become environments where fabrication of nanostructures is greatly facilitated over current solid/vacuum techniques.
Sectors Chemicals,Energy,Environment,Manufacturing, including Industrial Biotechology

Description Our finding are academic in nature and will feed into the general use of structured liquids and of ionic liquids, as outlined in "key findings". At present we cannot identify a demonstrable contribution from the chemistry we have explored. Several of the techniques we have invented in the course of this work should contribute to creative output. Variable Period X-ray Standing Wave (VPXSW) analysis. This technique gives a quantitative structural analysis with Å resolution of thin film structures from 10-500Å thickness. We have demonstrated it for the first time, and have developed the analysis software for it. It should find use wherever quantitative knowledge of thin film structures are required, from biology through to engineering. Analysis of NIXSW data. Until now this technique has only been applied to surface structural studies of small species. We have developed a methodology for fitting experimental data to surface models for large adsorbates such as ionic liquids. This will allow wider use of this technique to solve "real life" surface structural problems using synchrotron radiation sources such as Diamond. Development of a methodolog for forming ultra-thin liquid films on surfaces by a solvent evaporation process. This could have many applications from lubrication to gas sensing and catalysis. During the course of the project we developed helium cryopumping for the line of sight mass spectroscopy method and applied it to carbon dioxide studies (not previously possible). This technique is now fully realised and means that LOSMS is applicable to all gases except hydrogen and helium. The LOSMS technique can now be applied to systems from UHV to low vacuum conditions, with a consequent broadening of applications.
First Year Of Impact 2013
Sector Energy,Environment
Impact Types Economic

Description Probing liquid behaviour by helium atom scattering: surface structure and phase transitions of an ionic liquid on Au(111) 
Organisation University of Cambridge
Department Cavendish Laboratory
Country United Kingdom 
Sector Academic/University 
PI Contribution We supplied the ionic liquid and carried out analysis of LEED and helium diffraction data.
Collaborator Contribution They have the helium scattering equipment. They carried out the experimental runs.
Impact Publication. McIntosh et al Chem. Sci. 5, 2014, 667
Start Year 2012
Title MolXSWsim 
Description Program written in Igor code used to determine the adsorption geometry of molecular species on surfaces using normal incidence X-ray standing wave (NIXSW) data. Experimental coherent position/coherent fraction vectors for a range of Bragg reflections, for all the chemical species within an adsorbed molecule, are compared with calculated values for all orientations and positions of the molecule on the surface, to determine the best fit. 
Type Of Technology Software 
Year Produced 2015 
Impact Written to analyse data for the ionic liquid [EMIM][Tf2N] on Au(110) and produce a structural model. Reported in M. Buckley PhD thesis. 
Title NIXSW_2016 
Description Program written in Igor code to fit Normal Incidence X-ray Standing Wave (NIXSW) for a particular Bragg flection, producing the coherent fraction/coherent position structural data for the species under study. X-ray constants for the Bragg reflection are used plus photoelectron/Auger electron/background intensity vs X-ray energy data extracted from raw data using edcfitprog. 
Type Of Technology Software 
Year Produced 2016 
Impact Normal Incidence X-ray Stannding Wave Analsysis determination of surface structures. 
Title VPXSW_3layer_2016 
Description Program written in Igor code to simulate variable period X-ray standing wave data for comparison with experimental data. It calculates the variable period X-ray standing wave intensity produced over a range of angles for grazing reflection of an X-ray beam from a mirror surface, and then simulates the photoelectron/ Auger electron emissions expected, for a three layer system. The three layer system comprises vacuum, adsorbate, substrate for the purpose of X-ray reflection, while the composition of the adsorbate can be of arbitrary complexity for the purpose of simulating photoelectron/Auger electron emissions. The simulations are compared with intensity versus angle data obtained from fitting raw data using the edcfitprog program. The program is still operable, but still being developed to encompass heterogeneous surfaces. 
Type Of Technology Software 
Year Produced 2016 
Impact Being used to analyse the first variable period X-ray standing wave (VPXSW) data to be measured using photoelectron spectroscopy. VPXSW can be applied to thick adlayers (ca 30 nm) and gives chemical state information on the adsorbates. 
Title edcfitprog_2017 
Description Data reduction program written in Igor code. Used to extract photoelectron / Auger electron/ background intensities from sequences of energy distribution curves (EDCs) measured for the purposes of X-ray standing wave (XSW) / SEXAFS / NEXAFS experiments. The program fits peaks with Gaussian/Doniach-Sunjic/no-fit shapes and backgrounds with a flexible polynomial/structured shape. Intensities extracted can be peak heights or areas or raw-background-subtracted and provide input for XSW/SEXAFS/NEXAFS analysis. Program is continuously updated and extended to include data for new synchrotrons. Latest updates are for data taken at the Diamond Synchrotron for the normal incidence X-ray standing wave (NIXSW), Near Edge X-ray Fine Structure and Variable Period X-ray Standing Wave (VPXSW) techniques. 
Type Of Technology Software 
Year Produced 2017 
Impact Analysis of XSW/SEXAFS/NEXAFS experiments.