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

脂肪族和全氟脂肪族羧酸的分子结合和溶剂化的性质羧酸在工业中具有重要意义，因为它作为其他化学品的原料，作为溶剂，以及在药物，染料，纺织和食品工业中有许多用途。此外，羧酸在生物领域也有重要的应用。例如，短链脂肪酸（SCFAs），如乙酸、丙酸、异丁酸和正丁酸以及异戊酸和正戊酸，可以作为细菌活性的指标。因此，分析短链脂肪酸在肠道健康和疾病研究中具有重要意义。对这些酸的基本分子理解有助于合理设计具有所需性能的工艺和产品。鉴于需要改进涉及这些物种的绿色工程过程，本建议将重点放在理解超临界流体中羧酸的分子行为上。甲酸和scfa代表了一系列有趣的分子，为研究分子结构和这些分子自我结合的倾向提供了机会。然而，溶剂化可以对羧酸的自缔合产生巨大的影响，即使在羧酸具有适度不同的烷基的情况下。一些研究小组研究了溶剂对某些羧酸二聚化反应的影响。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对超临界（SC） CO2和C2H6中甲酸缔合的影响。结果表明，在SC - CO2溶剂存在下，甲酸的结合力随着溶剂密度的增加而线性降低。与乙烷为溶剂相比，CO2（溶剂）与甲酸单体（溶质）的相互作用对二聚体的形成有显著的影响。由于乙烷中溶质-溶质相互作用较弱，甲酸在C2H6中的缔合常数比在CO2中的缔合常数大一个数量级。二氧化碳与羧酸单体的强相互作用对平衡行为有很大影响。此外，使用MLFHB模型已经成功地对这种行为进行了建模。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的相互作用。在这项研究中，pi将研究羧酸的分子结合和溶剂化的性质。本研究由三个主要领域组成。1)用FTIR光谱测定不同密度下羧酸在不同溶剂（如SCF CO2和C2H6）中的缔合平衡常数（KA）。朴博士和他在塔斯基吉大学的本科生将与罗伯茨博士和他在奥本大学的全日制博士生一起在罗伯茨的实验室收集实验数据。2)在“惰性”溶剂（如戊烷）中，二氧化碳与脂肪族（和氟化脂肪族）羧酸之间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二氧化碳与脂肪族（和氟化脂肪族）羧酸之间最可能的相互作用位点。将实验得到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。从头算和状态方程两种方法都将被使用。更广泛的影响：这些研究对于提高我们对SCFs中羧酸相互作用的溶剂效应的理解将是重要的。了解二氧化碳的基本分子属性对合理设计廉价和绿色的二氧化碳工艺起着重要作用。关于二氧化碳与脂肪族（和氟化脂肪族）羧酸之间具体相互作用的信息将为应考虑哪些因素以改善总体的二氧化碳亲和性提供指导。拟议的研究计划为学生在分子热力学和绿色化学与工程领域的发展提供了多种机会。这一发展的机制包括博士生和本科生的培训，以及在塔斯基吉大学和奥本大学的分子热力学课程中纳入我们的发现和绿色化学这一重要领域的各个方面。

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.