Gas Adsorption at Structured Ionic Liquid Surfaces

气体吸收或捕获到液体中的研究始于19世纪初的S。然而，对液体表面的研究一直局限于在大气压或接近大气压下工作的技术，因为普通液体具有较高的蒸气压力(&gt；10-6mbar)，这是因为相对较弱的范德华相互作用将它们作为液体保持在一起。这意味着不能在分子尺度上研究液体表面(除了一些例外)，因为这种研究需要使用过去半个世纪为固体表面开发的强大的、基于真空的表面科学技术。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，由于离子之间强烈的库仑结合势，在室温下(&lt；10-10mbar)具有超低的蒸汽压，使它们与真空兼容。因此，利用基于真空的表面科学技术可以对离子液体表面进行详细的分子研究，开辟了液体表面科学的新领域。我们的目标是定量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面层上，然后通过吸收进入离子液体的主体相关联。这样的工作有很好的学术和工业原因。在学术上，由于离子液体是由大型复杂物种组成的，它们具有表面自组织的潜力，这是简单溶剂完全无法达到的。这种自组织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了油性屏障)，以及离子底层的形成，其中阴离子和阳离子的电荷载流子形成了带电的双层，可以作为吸附物种的表面陷阱。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，我们开辟了特定任务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其蒸气污染气相反应物和产品。与这一应用特别相关的是表面离子液体(SILP)工艺，它结合了均相和多相催化的优点，以及在碳捕获和储存(CCS)中使用离子液体作为二氧化碳捕捉剂的可能性。在这一应用中，我们利用离子液体的低挥发性将超高真空表面科学技术应用于它们的表面。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学来测量绝对粘着概率和程序升温脱附。我们的工作将是对任何类型的吸附在定义明确的液体表面上的第一次连贯研究，并将对液体表面科学的发展产生开创性的影响，可与60年代末和70年代初首次开展超高真空吸附研究时对固体表面的理解进步相媲美。从更长的时间来看(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改变，以促进使用表面两侧的材料形成纳米结构；我们能否设计出新类型的液体表面，以促进纳米颗粒和纳米机器的定向自组装；我们能开始设计嵌入部分类似于活细胞的液膜吗？

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?