Liquid structure of acetic acid-water and trifluoroacetic acid-water mixtures studied by large-angle X-ray scattering and NMR.

The structures of acetic acid (AA), trifluoroacetic acid (TFA), and their aqueous mixtures over the entire range of acid mole fraction xA have been investigated by using large-angle X-ray scattering (LAXS) and NMR techniques. The results from the LAXS experiments have shown that acetic acid molecules mainly form a chain structure via hydrogen bonding in the pure liquid. In acetic acid-water mixtures hydrogen bonds of acetic acid-water and water-water gradually increase with decreasing xA, while the chain structure of acetic acid molecules is moderately ruptured. Hydrogen bonds among water molecules are remarkably formed in acetic acid-water mixtures at xA<or=approximately 0.4, and water clusters eventually predominate in the mixtures at xA<or=approximately 0.18. The LAXS results have revealed that TFA molecules form not a chain structure but cyclic dimers through hydrogen bonding in the pure liquid. In TFA-water mixtures O...O hydrogen bonds among water molecules gradually increase when xA decreases, and hydrogen bonds among water molecules are significantly formed in the mixtures at xA<or=approximately 0.6. It has also been shown that TFA molecules are considerably dissociated to hydrogen ions and trifluoroacetate in the mixtures. 1H, 13C, and 19F NMR chemical shifts of acetic acid and TFA molecules for acetic acid-water and TFA-water mixtures have indicated strong relationships between a structural change of the mixtures and the acid mole fraction. On the basis of both LAXS and NMR results, the structural changes of acetic acid-water and TFA-water mixtures with decreasing acid mole fraction and the effects of fluorination of the methyl group on the structure are discussed at the molecular level.