Solvation of Lauric Acid Studied by Vibrational Spectroscopies

Abstract

Solvation of lauric acid [CH3(CH2)10COOH] has been investigated in different solvents (apolar, polar-aprotic, and protic solvents) by infrared absorption and Raman scattering in a large molar ratio range in order to investigate the intermolecular interactions between fatty acids and molecules of different types of solvents. Temperature effects on these mixtures have also been investigated between 24 and 95°C. We have especially concentrated on the carbonyl stretching region (between 1600 and 1800 cm−1), the hydroxyl stretching region (between 2500 and 3600 cm−1), and the C-H stretching region (2800–3000 cm−1). Chloroform behaves, with respect to lauric acid, almost as an apolar solvent. In acetone and in DMSO, a dynamical equilibrium between both cyclic dimers and H-bonded lauric acid molecules with solvent molecules seems to occur, the proportion of the two species depending on the lauric acid concentration. Cyclic dimers appear for a lauric acid: DMSO molar ratio x > 0.5 ( x > 0.2 in acetone). In alcohols, cyclic dimers are present from x &thkap, 0.2. For x < 0.2, two different complexes exist, in contrast with results from recent literature. These may be monosolvate and disolvate complexes. When the alcohol is deuterated, exchange between hydroxyl groups of both components is observed. Finally, Raman scattering appears as a powerful tool to clarify IR spectra in the case of solvated carboxylic acids.