The Nature of Molecular Association and Solvation of Aliphatic and Perfluorinated Aliphatic Carboxylic Acids

Yoonkook Park Tuskegee大学脂肪族和全氟脂肪族羧酸的分子缔合和溶剂化的性质羧酸在工业中具有重要意义，因为许多用途作为其他化学品的原料，作为溶剂，以及在药物，染料，纺织和食品工业中。 此外，羧酸在生物应用中发挥重要作用。例如，短链脂肪酸（SCFA）如乙酸、丙酸、异丁酸和正丁酸以及异戊酸和正戊酸用作细菌活性的指示剂。 因此，SCFAs的分析在肠道健康和疾病的研究中具有重要意义。 对这些酸的基本分子认识有助于合理设计具有所需性质的工艺和产品。 鉴于需要改善涉及这些物种的工艺的绿色工程，本提案将侧重于了解羧酸在超临界流体中的分子行为。甲酸和SCFAs代表了一系列有趣的分子，它们提供了研究和关联分子结构与这些分子自缔合倾向的机会。 然而，溶剂化可以对羧酸自缔合产生巨大影响，即使在具有适度不同烷基的羧酸的情况下。 一些研究小组已经研究了溶剂对某些羧酸二聚反应的影响。 在早期的工作中，PI和co-PI使用傅里叶变换红外（FTIR）光谱和改进的晶格流体氢键（MLFHB）模型来研究溶剂密度和溶剂化在超临界（SC）CO2和C2 H6中甲酸缔合的影响。结果表明，在SC CO2溶剂中，甲酸的缔合作用随溶剂密度的增加而线性下降。 与乙烷作为溶剂时相比，CO2（溶剂）与甲酸单体（溶质）的相互作用对二聚体的形成具有显著的影响。 由于乙烷中的溶剂-溶质相互作用较弱，甲酸在C2 H6中的缔合常数因此比在CO2中大一个数量级。 CO2和羧酸单体的强相互作用对平衡行为有很大影响。 此外，这种行为已经成功地使用MLFHB模型建模。 特定目的：SC CO2可以与某些分子，特别是具有供电子官能团的分子发生强烈相互作用。 在这项研究中，PI将研究羧酸的分子缔合和溶剂化的性质。这项研究包括三个主要领域。1)将使用FTIR光谱法测定各种溶剂（例如SCF CO2和C2 H6）中各种密度下羧酸的缔合平衡常数（KA）。帕克博士和他的塔斯基吉大学本科生将与罗伯茨博士和他的全职奥本博士合作。学生在罗伯茨的实验室收集实验数据。 2)二氧化碳和脂肪族（和氟化脂肪族）羧酸在“惰性”溶剂（如戊烷）中的特定相互作用也将在罗伯茨实验室和塔斯基吉实验室进行研究。 Park博士和他的学生将从理论上确定CO2和脂肪族（和氟化脂肪族）羧酸之间最可能的相互作用位点。 将实验获得的结合能与模型势得到的结合能进行比较。 3)这些系统的分子建模将在Gupta博士的实验室进行，以提高我们对实验结果的基本理解并指导实验研究。从头算和状态方程的方法将被利用。 更广泛的影响：这些研究将是重要的，以提高我们的理解溶剂对羧酸的相互作用在SCF。 了解CO2的基本分子属性在合理设计廉价和基于绿色CO2的工艺中起着重要作用。 关于二氧化碳和脂肪族（和氟化脂肪族）羧酸之间的特定相互作用的信息将提供一个指南，应考虑哪些因素，以提高整体亲二氧化碳性。 拟议的研究计划提供了多种机会，培养学生在分子热力学和绿色化学和工程领域。这种发展的机制包括博士学位。学生和本科生的培训，以及我们的发现和方面，这一重要领域的绿色化学在我们的分子热力学课程在塔斯基吉大学和奥本大学。

Yoonkook ParkTuskegee UniversityThe Nature of Molecular Association and Solvation of Aliphatic and Perfluorinated Aliphatic Carboxylic Acids Carboxylic acids are of significant importance in industry due to numerous uses as raw materials for other chemicals, as solvents, and in the drug, dye, textile and foodstuffs industries. In addition, carboxylic acids play an important role in biological applications. For example, short-chain fatty acids (SCFAs) such as acetic, propionic, iso- and n-butyric and iso- and n-valeric acids, serve as an indicator of bacterial activity. Hence, analysis of SCFAs is of importance in studies of health and disease in the intestinal tract. Fundamental molecular understanding of these acids can help in rational design of processes and products with desired properties. In light of the need to improve the green engineering of processes involving these species, this proposal will focus on understanding the molecular behavior of carboxylic acids in supercritical fluids.Formic acid and SCFAs represent an interesting series of molecules that provide the opportunity to study and correlate molecular structure with the tendency for these molecules to self-associate. However, solvation can have a tremendous impact on carboxylic acid self-association even in the case of carboxylic acids with modestly different alkyl groups. The influence of solvents on dimerization of some carboxylic acids has been investigated by several research groups. In earlier work, the PI and co-PIs used Fourier transform infrared (FTIR) spectroscopy and a modified lattice-fluid hydrogen-bonding (MLFHB) model to examine the effect of solvent density and solvation in formic acid association in both supercritical (SC) CO2 and C2H6. The results obtained indicated that in the presence of SC CO2 solvent, the association of formic acid decreased linearly with increases in solvent density. The interaction of CO2 (solvent) with formic acid monomer (solute) has a marked effect on the dimer formation compared to that when ethane was employed as the solvent. As a result of weaker solvent-solute interactions in ethane, the association constant of formic acid in C2H6 is thus an order of magnitude larger than that in CO2. The strong interactions of CO2 and carboxylic acid monomers has a tremendous effect on the equilibrium behavior. In addition, this behavior has been successfully modeled by using the MLFHB model. Specific Aims: SC CO2 can interact strongly with certain molecules, particularly, those with electron donating functional groups. In this study, the PIs will investigate the nature of molecular association and solvation of carboxylic acids. This study is comprised of three main areas. 1) The association equilibrium constant (KA) of carboxylic acids at various densities in a variety of solvents (e.g. SCF CO2 and C2H6) will be determined using FTIR spectroscopy. Dr. Park and his Tuskegee University undergraduate students will work with Dr. Roberts and his full-time Auburn Ph.D. student to collect the experimental data in the Roberts' lab. 2) The specific interaction between CO2 and aliphatic (and fluorinated aliphatic)carboxylic acids in an "inert" solvent, such as pentane, will also be investigated in the Roberts' lab as well as the Tuskegee laboratories. Dr. Park and his students will identify the most probable interaction site between CO2 and aliphatic (and fluorinated aliphatic) carboxylic acids theoretically. The experimentally obtained binding energies will be compared to those obtained from model potentials. 3) Molecular modeling of these systems will be carried out in Dr. Gupta's lab to improve our fundamental understanding of the experimental results and to guide the experimental studies. Both ab initio and equation of state approaches will be utilized. Broader Impact: These studies will be important in improving our understanding of solvent effects on carboxylic acid interactions in SCFs. Understanding the fundamental molecular attributes of CO2 plays a role in rational design of inexpensive and green CO2-based processes. Information about the specific interaction between CO2 and aliphatic (and fluorinated aliphatic) carboxylic acids will provide a guideline as to which factors should be considered to improve overall CO2-philicity. The proposed research program presents multiple opportunities to develop students in the areas of molecular thermodynamics and green chemistry and engineering. The mechanisms for this development include Ph.D. student and undergraduate student training, as well as incorporation of our findings and aspects of this ever important field of green chemistry within our molecular thermodynamics courses at both Tuskegee University and Auburn University.