Computational studies of supercritical fluids

超临界二氧化碳是化学反应的一种令人兴奋的溶剂。它价格便宜，无毒，不易燃，无污染，其物理性质受相对较小的温度和压力变化的影响很大，使反应结果得以控制。然而，与水等溶剂相比，二氧化碳中可溶的化学物质较少，这限制了超临界二氧化碳在化学过程中的使用。为了增加有机分子在二氧化碳中的溶解度，可以用氟原子取代碳氢链上的氢原子。或者，氟化烃分子(HFC)可以用作表面活性剂的基础，使二氧化碳溶剂中存在小水滴。然后，即使所涉及的化学物质不能溶解于周围的二氧化碳中，水滴内部也可以发生化学反应。氟化分子对环境的影响很小，一些小型氢氟碳化物已经取代了消耗臭氧的氯氟化碳作为制冷剂。不幸的是，生产较大氟化分子的高昂成本限制了HFC/二氧化碳混合物的商业潜力。此外，氟化分子在二氧化碳中的溶解度增加的原因尚不清楚，这使得设计出更具成本效益的HFC替代品变得困难。拟议的研究将解决与HFC/二氧化碳混合物相关的这些理论和实践问题。这项工作将在诺丁汉大学进行，是在分子相互作用(Wheatley)、模拟(Hirst)和超临界二氧化碳实验测量(Poliakoff和Ke)领域拥有相当专业知识的现有研究小组之间的新合作。我们将开始开发新的计算方法，基于PI已经成功使用的方法，以比以前可能的更准确和更详细的方式研究氢氟碳化合物与二氧化碳分子之间的相互作用。使用实验COIS开发的最先进的设备对混合物的相平衡、临界现象和热力学性质进行实验测量，将有助于验证和改进这些计算方法，适用于含有小型氢氟碳化物的混合物，以及目前可用的少数大型氢氟碳化物。计算的分子间相互作用将用于氢氟碳化物/二氧化碳混合物的结构和物理性质的原子模拟。赫斯特在模拟大有机分子方面的经验将确保克服这项工作的技术挑战，而模拟对计算资源的需求将由该大学新的1024节点万亿次浮点计算集群来满足。这些模拟得到的结果将使我们能够探索氟化提高溶解度的原因，并为一系列新的HFC分子产生新的溶解度数据--目前无法从实验中获得。从长远来看，这项研究将使我们能够预测使分子在超临界二氧化碳中溶解的特征，从而有助于设计新的、高效的、环境友好的化学溶剂。

Supercritical carbon dioxide is an exciting solvent for chemical reactions. It is cheap, non-toxic, non-flammable and non-polluting, and its physical properties are greatly influenced by relatively small variations in temperature and pressure, which enable the reaction outcomes to be controlled. However, fewer chemicals are soluble in carbon dioxide than in solvents such as water, which restricts the use of supercritical carbon dioxide in chemical processes.To increase the solubility of organic molecules in carbon dioxide, hydrogen atoms in hydrocarbon chains can be replaced by fluorine atoms. Alternatively, fluorinated hydrocarbon molecules (HFCs) can be used as the basis of surfactants, which enable small droplets of water to exist within the carbon dioxide solvent. Chemical reactions can then take place inside the water droplets, even when the chemicals involved are not soluble in the surrounding carbon dioxide.Fluorinated molecules can have a low environmental impact, and a number of small HFCs have replaced ozone-depleting CFCs as refrigerants. Unfortunately, the high cost of producing larger fluorinated molecules has limited the commercial potential of HFC / carbon dioxide mixtures. Furthermore, the reason for the enhanced solubility of fluorinated molecules in carbon dioxide is not known, which makes it difficult to design more cost-effective alternatives to HFCs.The proposed research will tackle these theoretical and practical problems associated with HFC / carbon dioxide mixtures. The work will be carried out at the University of Nottingham, and is a new collaboration between established research groups with considerable expertise in the areas of molecular interactions (Wheatley), simulations (Hirst) and experimental measurements on supercritical carbon dioxide (Poliakoff and Ke).We shall begin by developing new computational methods, based on those already used successfully by the PI, to investigate the interactions between HFCs and carbon dioxide molecules with more accuracy and in more detail than previously possible. Experimental measurements of the phase equilibria, critical phenomena and thermodynamic properties of the mixtures, using state-of-the-art equipment developed by the experimental CoIs, will allow validation and improvement of these computational methods for mixtures involving small HFCs, and for the few large HFCs that are currently available.The calculated intermolecular interactions will be used in atomic simulations of the structures and physical properties of HFC / carbon dioxide mixtures. Hirst's experience of simulations of large organic molecules will ensure that the technical challenges of this work are overcome, and the demand that the simulations will place on computing resources will be met by the new 1024-node teraflop computing cluster at the University. The results obtained from these simulations will allow us to explore the reason for the enhancement of solubility by fluorination, and to produce new solubility data - which cannot currently be obtained from experiment - for a range of new HFC molecules. In the longer term, the research will allow us to predict the characteristics that make molecules soluble in supercritical carbon dioxide, and will therefore assist in the design of new, efficient, environmentally-friendly chemical solvents.