Gas Adsorption at Structured Ionic Liquid Surfaces

自19世纪初以来，人们一直在研究气体被液体吸收或俘获的问题。然而，对液体表面的研究仅限于可以在大气压或接近大气压下操作的技术，因为普通液体具有很高的蒸汽压（10-6毫巴），这是由于将它们作为液体保持在一起的相对较弱的范德华相互作用。这意味着液体表面不能在分子尺度上进行研究（有一些例外），因为这样的研究需要使用强大的、基于真空的表面科学技术，这些技术是在过去半个世纪里为固体表面发展起来的。然而，离子液体由相对较大的、低对称性的有机阳离子组成，与无机或有机阴离子相匹配，在室温下具有超低的蒸气压（<10-10毫巴），这是由于离子之间具有很强的粘性库仑势，使它们具有真空相容性。因此，利用真空表面科学技术对液体表面进行分子细节研究，开辟了液体表面科学的新领域。我们的目标是定量地确定离子液体的表面结构，并将这种结构与气体如何吸附到表层，然后通过吸收进入离子液体的主体有关。这样的工作有很好的学术和工业理由。在学术上，由于离子液体是由大型复杂物质组成的，因此它们具有表面自组织水平的潜力，这是任何简单溶剂都无法达到的。这种自组织的最明显的例子是表面冻结，长烷基链在表面排列成半结晶层（从而为吸附气体提供一个油性屏障），以及离子下层的形成，阴离子和阳离子的电荷载体形成一个带电的双层，可以作为吸附物质的表面陷阱。通过了解这种自组织如何依赖于IL的性质，以及自组织结构如何影响气体的吸附，我们开辟了一个任务特定液体表面科学的新领域。至关重要的是，离子液体的超低挥发性也赋予了它们巨大的工业潜力，因为离子液体不会用它的蒸汽污染气相反应物和产物。与此应用特别相关的是表面离子液相（SILP）工艺，它结合了均相和多相催化的优点，以及在碳捕集与封存（CCS）中使用离子液相作为CO2捕集剂的可能性。在这个应用中，我们利用il的低挥发性将超高真空表面科学技术应用于其表面。表面结构将使用角度分辨XPS和x射线反射率来确定，而表面动力学将使用视线质谱来测量绝对粘着概率和温度程序脱附。我们的工作将是第一个在明确定义的液体表面上进行任何类型吸附的连贯研究，并将在液体表面科学的发展中具有开创性意义，可与60年代末和70年代初首次进行超高真空吸附研究时对固体表面的理解的进展相媲美。长期（15-30年），我们将开始回答更复杂的问题，例如；这种高度结构化，各向异性，但可移动的表面环境如何被有意地修改，以促进使用表面两侧材料的纳米结构的形成？我们能否设计出新型的液体表面来促进纳米粒子和纳米机器的定向自组装？我们能否开始设计嵌入与活细胞相似的部分的液体膜？

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?