A Molecular-Based Framework for Studying the Thermodynamic Properties of Ionic Liquids

CBET-0829062 McCabeIonic Liques(ILS)是完全由熔点在室温或低于室温的离子组成的液体，区别于高温熔盐。与传统有机溶剂相比，ILS在使用替代和更环保的溶剂方面走在了前列，因为它们的挥发性可以忽略不计，从而将大气污染的风险降至最低，并减少了相关的健康问题。离子液体在催化、生物催化、分离和光化学电池等领域显示出广阔的应用前景。对离子液体的兴趣源于其独特的物理性质，这些性质超出了普通分子溶剂的范围，包括(除了无挥发性或低挥发性外)热稳定性、液体的广泛温度范围以及溶解包括大分子在内的各种有机和无机材料的能力。ILS通常由大的有机阳离子和弱配位的无机或有机阴离子组成，这阻碍了堆积并降低了熔点。阳离子和阴离子各自赋予IL不同的物理化学性质，并且可以被取代以获得所需的性质，并且如果需要，进行官能化以提供进一步的控制。虽然最初对离子液体的兴趣主要集中在它们的溶剂性质上，但最近利用它们新颖的、可调的物理和化学性质来设计广泛应用的新功能材料的潜力已经激发了人们对离子液体的兴趣。因此，到目前为止，IL设计已经取得了很大的进展；然而，这些进展中的很大一部分是通过对属性修改的经验方法实现的。大量可能的阳离子-阴离子组合使得能够根据它们的组成合理地预测IL的性质变得至关重要。因此，有相当大的动力来开发一种准确的工具，以基于对ILS内的结构和相互作用的物理理解来设计特定于任务的ILS。我们工作的目标是通过发展一个基于分子的理论框架来研究ILS的热力学性质，以满足这一重要需求。ILS中存在的相互作用的复杂性质使得他们使用计算工具进行研究从根本上是一个具有挑战性的课题。总的目标是开发一个框架，用来准确地模拟IL系统及其与其他分子物种的混合物，并能够预测它们的热力学性质。在这项工作中，我们将重点发展一个纯ILS的基于分子的模型，这将为未来ILS和分子溶质/溶剂混合物的建模和特定任务ILS的预测设计奠定基础。开发的方法将根据实验数据和从头计算进行优化，并将能够根据化学成分预测IL性质，从而消除确定给定离子组合的性质以及最终确定它们与分子物种的混合行为的猜测工作。具体地说，项目e将创建一个理论工具来描述建立在SAFT框架上的ILS，用于模拟流体相行为。与文献中提供的各种基于工程的状态方程相比，SAFT提供了一种基于分子的方法，并且在将分子形状、大小和相互作用的影响捕捉到分析状态方程中的能力方面确实是无与伦比的。为了用上述分子模型描述离子液体的热力学性质，PI将为离子液体体系开发一个理论框架，该框架将积分方程式理论(精确描述偶极和离子流体的结构)的解析解与SAFT-VR方法的分子框架相结合。拟议方法的核心将是由PI开发的异段SAFT模型，12该模型与基于同段方法的SAFT模型形成对比(即模型链中的每一段具有相同的相互作用参数的大小和能量)。拟议研究的智力价值-拟议的工作将导致新的方法，将从量子化学计算和分子模拟获得的性质映射到基于统计力学的模型所需的参数上，并导致液态理论的新发展。这项工作将通过提供基于对IL结构和相互作用的深入物理理解的预测性IL设计方法，为分子热力学的重大进步奠定基础。拟议研究的广泛影响-拟议研究与PI教育活动的整合将确保Vanderbilt的学生接触并参与分子建模和分子热力学的前沿。将通过范德比尔特的NSF TLSAMP4计划寻求少数族裔的参与，并通过PIS参与工程学院的RET现场计划寻求高中教师的参与。总体计划-目标1：纯ILS的基于分子的模型AIM 2：ILS和分子溶剂/溶液混合物的分子模型AIM 3：特定任务ILS的预测设计PI将把这一修订的探索性项目的范围限制为AIM One，一个基于分子的纯ILS模型，因为它代表了工作中最具挑战性和新颖性的部分。这项工作将集中在(1)描述离子液体中静电相互作用、电荷离域和极性的综合效应的液态理论的新发展；(2)发展将量子化学计算和分子模拟结果映射到基本理论模型中的参数的新方法；以及(3)将这些新发展应用于实验离子液体系统。

CBET-0829062McCabeIonic liquids (ILs) are liquids comprised entirely of ions that have melting points at or below room temperature, distinguishing them from high temperature molten salts. ILs are at the forefront in the use of alternative and greener solvents due to their negligible volatility, which minimizes the risk of atmospheric contamination and reduces associated health concerns in comparison with conventional organic solvents. ILs show promise in a wide range of applications from catalysis and biocatalysis, to separations, and photochemical cells. The interest in ILs stems from their unique physical properties, that extend beyond the range of normal molecular solvents, and can include (in addition to no or low volatility) thermal stability, a wide range of temperature over which they are liquid, and the ability to solubilize a wide variety of organic and inorganic materials, including macromolecules. ILs generally consist of a large, organic cation with a weakly coordinating inorganic or organic anion, which frustrates packing and lowers the melting point. The cations and anions each impart different physiochemical properties to the IL and can be substituted to obtain the properties desired, and if needed, functionalized to provide further control. While initial interest in ILs focused primarily on their solvent properties, more recently the potential to use their novel, tunable, physical and chemical properties in the design of new functional materials for a wide range of applications has fueled the interest in ILs. As a result, there has been a great deal of progress to date in IL design; however, much of that progress has been achieved through an empirical approach to property modification. The vast number of possible cation-anion combinations makes it vitally important to be able to rationally predict IL properties based on their constituents. Thus, there is considerable incentive to develop an accurate tool for designing task-specific ILs based upon a physical understanding of the structure and interactions within ILs. The goal of our work is to address this important need through the development of a molecular-based theoretical framework with which to study the thermodynamic properties of ILs. The complex nature of the interactions present in ILs makes their study using computational tools a fundamentally challenging subject. The overall goal is to develop a framework with which to accurately model IL systems and their mixtures with other molecular species, and enable the prediction of their thermodynamic properties. In this work we will focus on developing a molecular-based model for pure ILs, which will lay the foundation for future work on modeling mixtures of ILs and molecular solutes/solvents and the predictive design of task-specific ILs. The approach developed will be optimized against experimental data and ab initio calculations and will enable the prediction of IL properties from the chemical composition, thus eliminating guess work in determining the properties of a given combination of ions and, ultimately, their mixture behavior with molecular species. Specifically, the project e will create a theoretical tool to describe ILs built on the SAFT framework for modeling fluid phase behavior. SAFT, in contrast to the various engineering based equations of state available in the literature, provides a molecular-based approach and is truly unparalleled in its ability to capture the effects of molecular shape, size and interactions into an analytical equation of state. In order to describe the thermodynamic properties of ILs with the molecular-based model outlined above, the PI will develop a theoretical framework for IL systems that combines analytical solutions from integral equation theory (that accurately describe the structure of dipolar and ionic fluids) with the molecular framework of the SAFT-VR approach. The centerpiece of the proposed approach will be the hetero-segmented SAFT model developed by the PI,12 which contrasts with SAFT models that are based on a homo-segmented approach(i.e., each segment in the model chain has the same size and energy of interactionparameters). Intellectual Merit of the Proposed Research - The proposed work will lead to new methods to map properties obtained from quantum chemistry calculations and molecular simulations onto the parameters needed in statistical-mechanics-based models and new developments in liquid state theory. The work will lay the foundation for a significant advance in molecular thermodynamics by providing a predictive IL design methodology based upon a deep physical understanding of IL structure and interactions.Broader Impact of the Proposed Research - The integration of the proposed researchwith the PIs educational activities will ensure that students at Vanderbilt are exposed to, and participate in, the frontiers of molecular modeling and molecular thermodynamics. Participation of minorities will be sought through the NSF TLSAMP4 program at Vanderbilt and high school teachers involved through the PIs participation in a School of Engineering RET site program.Overall plan - AIM 1: A molecular-based model for pure ILsAIM 2: A molecular-based model for mixtures of ILs and molecular solvents/solutesAIM 3: Predictive design of task-specific ILsThe PI will limit the scope of this revised exploratory project to aim one, a molecular-based model for pure ILs since it represents the most challenging and novel part of the work. The work will focused on (1) the derivation and application of new developments in liquid state theory to describe the combined effects of electrostatic interactions, charge delocalization, and polarity in ILs; (2) the development of new methods to map quantum chemistry calculations and molecular simulation results onto the parameters in the underlying theoretical models; and (3) application of these new developments to experimental IL systems.