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

Tuskegee University, Tuskegee Institute AL

Investigators

Abstract

Yoonkook Park Tuskegee University The 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.