The use of static and dynamic physical property measurements to infer structural properties of associated liquids: Acetic acid–water

Abstract

The anomalies which occur in most static and dynamic physical property measurements on solutions of water in acetic acid can be used with critical analysis to infer associative molecular structures. The anomalies indicate significant structural changes. Proton magnetic resonance chemical shifts for acetic acid–water solutions show a significant structural change occurring in the region of equal molar concentration. Literature values of carbon-13 spin–lattice relaxation times, density, molar enthalpy of mixing, index of refraction, and relative viscosity for acetic acid–water solutions also show anomalies in the region of equal molar concentration. Careful measurement of carbon–13 spin–lattice relaxation time for 20 wt. % water in acetic acid was compared with literature values. A critical analysis of these anomalous physical properties infer a preponderance of the dihydrated dimeric acetic acid structure. Microwave absorption at 10.525 GHz for acetic acid–water solutions was found to have a significant structural change occurring in the region of 70 wt. % water. The deviation appeared in the exponential response of microwave adsorption for polar liquids. It was found that literature values of carbon-13 spin–lattice relaxation times, molar enthalpy of mixing, and carbon-13 chemical shifts for acetic acid–water solutions also show anomalies in the region of 70 wt. % water. Critical analysis of these measurements and their anomalies imply a dihydrated monomeric acetic acid structure predominates from approximately 70 wt. % water in acetic acid to infinite acetic acid dilution.