Explaining characteristic azeotropic behaviour

An explanation of the formation of minimum-boiling, maximum-boiling and saddle-type azeotropes is given (with illustrative examples) in terms of the molecular interactions in the solutions and in the pure liquids. “Interaction weakening” and “interaction strengthening” effects are defined which tend to be associated with the formation of minimum-boiling and maximum-boiling azeotropes respectively. As defined these effects correspond respectively to positive and to negative values of the interchange energy in regular solution theory, though the concept is considered to be of more general application than is this simple model. When more than two components are present it is possible for both effects to occur leading to the possibility of saddle-type azeotropy.By considering the nature of the intermolecular fones it is possible to predict the kind of azeotope which will be formed, if an azeotrope is formed at all. Although present knowledge of intermolecular fones is far from complete, enough is known to enable this prediction to be made in many cases. The more exacting question of whether or not an azeotope will be formed cannot yet be answered with the same certainty partly because of the lack of a completely satisfactory model of the liquid state.