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 x_A 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 x_A, 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 x_A ≤ ∼0.4, and water clusters eventually predominate in the mixtures at x_A ≤ ∼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 x_A decreases, and hydrogen bonds among water molecules are significantly formed in the mixtures at x_A ≤ ∼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.