Vaporization and Layering of Alkanols at the Oil/Water Interface

TL;DR

This paper investigates how alkanols adsorb at the oil/water interface, revealing that long-chain alkanols form monolayers with phase transitions, while shorter chains form multilayers, with interfacial structure influenced by solvent chain length.

Contribution

It provides new insights into the chain-length dependence of phase transitions and layering behavior of alkanols at oil/water interfaces, highlighting the role of interfacial entropy.

Findings

Long-chain alkanols undergo thermodynamically favorable monolayer phase transitions.

Shorter alkanols tend to form multilayer structures instead of monolayers.

The critical chain length for phase transition is approximately six carbons longer than the solvent molecules.

Abstract

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.