  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
favorable only for long-chain alkanols. As the alkanol chain length is
decreased, the change in excess interfacial entropy per area decreases to zero.
Systems with small values of excess interfacial entropy form multi-molecular
layers at the interface instead of the monolayer formed by systems with much
larger excess interfacial entropy. Substitution of n-hexane by n-hexadecane
significantly alters the interfacial structure for a given alkanol surfactant,
but this substitution does not change fundamentally the phase transition
behavior 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.
