Infrared Spectroscopy of Aqueous Carboxylic Acids: Comparison between Different Acids and Their Salts

The attenuated total reflection−infrared (ATR−IR) spectra in the 4800−700 cm-1 range of nine carboxylic acids and their sodium salts in aqueous solutions are obtained and analyzed. Overall, 22 species are studied. Six IR titrations are made with five different acids:  acetic acid, malic acid, betaine, glycine, and N,N-((butyloxy)propyl) amino diacetic acid (BOPA). From the spectra of these titrations, the spectra of four types of water (acidic, basic, saline, and pure water) are subtracted, giving spectra with flat baselines without any artificial adjustment. Factor analysis (FA) made on the water-subtracted spectra yield the spectra of the principal species, and their abundances. Titration curves obtained from these precisely fit the theoretical curves and the pKa values in the literature. The remaining water bands that are not subtracted are assigned to water solute close-bound situations. The hydration number varied from 5 to 1, with an average of almost 2 per carboxyl carbonyl group. The IR CO band po...