Vibrational Spectroscopic Studies on the Acid Salts of Capric Acid and Lauric Acid

Fatty acid and their aklali soaps were well known as biosurfactants inliving system , however , very few works had been published before on the fatty acid-soap complexes, which were revealed here possessing unusual structures and properties. The acid sodium salts and acid potassium salts in molar ratio of 1: 1 of both capric acid and lauric acid were investigated by FTIR and FT-Raman spectroscopies. Two dominant features were determined. One was that the very strong and broad ν(OH)modes of fatty acids was absent above 1 800 cm￣(-1) in the in-frared spectra ,and the other was the characteristic ν_(as) (COO￣-)vibrations of alkali soaps could not be observed in both infrared and Raman spectra. The spectral results showed that a very strong hydrogen bond formed between carboxylate groups;In the infrared spectra of acid salts, a diffuse weak absorption in the region of 1 500～800 cm￣(-1) was found and ascribed to the ν(OH) mode, In order to explain he unusual weakness of ν(OH ) vibrations ,the possibility of proton motions was discussed depending on the structural features of acid salts of fatty acids. The weak bands at 200 cm￣(-1) in the Raman spectra of acid salts other than the acids or salts showed that there may be some a defect conformation in the alkyl chains of the acid salts.