Strong Lewis acid-Lewis base interactions between supercritical carbon dioxide and carboxylic acids: Effects on self-association

The equilibrium constants for dimerization of three different carboxylic acids in near-critical and supercritical carbon dioxide and ethane at various temperatures and densities are presented. Fourier transform infrared spectroscopy was used to determine the equilibrium constants by examining the C=O stretching frequencies of the monomer and dimer of the acids in solution. The equilibrium constants for formic acid, propionic acid, and trifluoroacetic acid decrease with temperature for both ethane and CO2 solvents and strongly depend on CO2 solvent density. The modified lattice fluid hydrogen-bonding (MLFHB) model was used to interpret this density dependence of the equilibrium constants. The density 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 examination 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.