44. Bell, Philip W.; Thote, Amol J.; Park, Yoonkook; Gupta, Ram B.; Roberts, Christopher B. Strong Lewis Acid-Lewis Base Interactions between Supercritical Carbon Dioxide and Carboxylic Acids: Effects on Self-association. Industrial & Engineering Chemistry Research (2003), 42(25), 6280-6289.
The equil. consts. for dimerization of three different carboxylic acids in near-crit. and supercrit. carbon dioxide and ethane at various temps. and densities are presented. Fourier transform IR spectroscopy was used to det. the equil. consts. by examg. the C:O stretching frequencies of the monomer and dimer of the acids in soln. The equil. consts. for formic acid, propionic acid, and trifluoroacetic acid decrease with temp. for both ethane and CO2 solvents and strongly depend on CO2 solvent d. The modified lattice fluid hydrogen-bonding (MLFHB) model was used to interpret this d. dependence of the equil. consts. The d. dependence provides evidence for specific solvent-solute interactions between CO2 and the carboxylic acid functional group, which is attributed to strong Lewis acid-Lewis base interactions. Since carboxylic acids contain both a Lewis acid and a Lewis base moiety, the examn. of a strong acid such as trifluoroacetic acid provides insight as to whether CO2 acts as a Lewis acid or as a Lewis base in these specific interactions. Ab initio computations are also presented and indicate that CO2 interacts strongly with the carboxylic acids.