Vaporization and layering of alkanols at the oil/water interface

This study of adsorption of normal alkanols at the oil/water interface with x-ray reflectivity and tensiometry demonstrates that the liquid to gas monolayer phase transition at the hexane/water interface is thermodynamically favourable only for long-chain alkanols. As the alkanol chain length is decreased, the change in excess interfacial entropy per area Delta S-a(sigma) decreases to zero. Systems with small values of Delta S-a(sigma) form multi-molecular layers at the interface instead of the monolayer formed by systems with much larger Delta S-a(sigma). Substitution of n-hexane by n-hexadecane significantly alters the interfacial structure for a given alkanol surfactant, but this substitution does not fundamentally change the phase transition behaviour of the monolayers. These data show that the critical alkanol carbon number, at which the change in excess interfacial entropy per area decreases to zero, is approximately six carbons larger than the number of carbons in the alkane solvent molecules.