A determination of the activity of water in water–alcohol mixtures using mobile order thermodynamics

The non-ergodic theory of mobile order thermodynamics (MOT) is employed to study how the structure of ethylene glycol and several common alcohols influences the activity of water in binary mixtures. Results suggest that alcohol molecules with a smaller alkyl group or a greater number of hydroxyl groups are more effective at lowering the activity of water. The influence of alcohol branching among isomers is examined as well. Calculations are in good agreement with widely used algorithms such as the UNIQAC equation. With more complex alcohols like diethylene and triethylene glycols, the MOT framework requires additional development. Alcohols are known to be thermodynamic inhibitors that hinder the formation of gas hydrates. Since hydrate stability is determined by an equilibrium condition of equal chemical potentials for water in the hydrate and liquid phases, the present study may help clarify the inhibition mechanism for alcohols.