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 pK_a 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 positions (±16 cm^-1) are assigned to the different species:  1723 and 1257 cm^-1 for the un-ionized acid double and single bonds; 1579 cm^-1 for CO₂^- asymmetric stretch; 1406 cm^-1 for CO₂^- symmetric stretch; and 1094 cm^-1 for noncarboxylic ethoxy groups. The OH absorption covers the full region, from 3700 to 1700 cm^-1, in four bands that are ∼220 cm^-1 wide. The near-3400 cm^-1 band is assigned to solvated water, alcoholic OH, and NH groups, because these are hydrogen-bonded groups. The 3000 and 2600 cm^-1 bands are assigned to the carboxyl OH groups that are hydrogen-bonded to other carboxyl groups in the pure acrylic species or to water in the aqueous solutions cases. The 2100 cm^-1 band is assigned to a combination band that involves the far-IR absorption. The absorption from 3700 to 1700 cm^-1, which is sometimes called the “continuous absorption”, cannot be attributed to the hydronium ion (H₃O⁺), because the acids are not ionized; rather, it results from the strong hydrogen bonds between water and the carboxylic acids.