Association models for alcohol-water mixtures

Alcohol-water mixtures are often used as model systems for the study of hydrophobic interactions. In this respect, their thermodynamic properties are of particular interest since precise data are available over the whole miscibility range and they lend themselves readily to theoretical modelling. Three association models have been used to fit the volume data taken from the literature for aqueous methonol, ethanol and n-propanol at various temperatures. A simple micellization equilibrium explains the general trends but fails to fit the reduced excess volumes at both ends of the mole fraction scale. Better fits are obtained if interaction parameters are introduced, but these two parameters cannot easily be rationalized. A double association model (one for the alcohol and one for the water) gives the best fit and all the parameters have a physical significance. However, the parameters extracted, e.g. aggregation numbers, and their trends with temperature are not always realistic. Despite their limitations, these models illustrate well with the kind of avenues that can be explored to fit and interpret the experimental data of these complex systems.