water - Have "molecular clusters" for azeotropes been identified?

A different question about azeotropes got me thinking about this point again. Azeotropes have a very specific composition so it seems that the azeotrope ought to have some sort of physical structure. It seems to be a "molecular cluster" of some sort.

The azetrope for water and ethanol is about 95.5% ethanol by weight. A little fiddling and it seems that the ratio is 8 ethanol molecules to 1 water molecule. Is there some particular physical configuration of molecules to which this would correspond?

Answer

In positive azeotropes for which the boiling point is less than the boiling points of any of the constituents the intermolecular interaction of the different molecules is less than in the pure liquid phase. Therefore it is not very likely that clusters involving the different molecules will be formed.

In negative azeotropes that have a higher boiling point than the constituents cluster structures in the liquid phase are more likely. E.g. for concentrated hydrochloric acid the existence of clusters has been suggested¹.

              

^{1 Agmon, N., Structure of Concentrated HCl Solutions, J. Phys. Chem. A 1998, 102, 192-199}