Tail Ordering Due to Headgroup Hydrogen Bonding Interactions in Surfactant Monolayers at the Water−Oil Interface (original) (raw)

Interactions between surfactants, and the resultant ordering of surfactant assemblies, can be tuned by the appropriate choice of head-and tailgroups. Detailed studies of the ordering of monolayers of long-chain n-alkanoic and n-alkanol monolayers at the water-vapor interface have demonstrated that rigid-rod all-trans ordering of the tailgroups is maintained upon replacing the alcohol with a carboxylic acid headgroup. In contrast, at the water-hexane liquid-liquid interface, we demonstrate that substitution of the-CH 2 OH with the-COOH headgroup produces a major conformational change of the tailgroup from disordered to ordered. This is demonstrated by the electron density profiles of triacontanol (CH 3 (CH 2) 29 OH) and triacontanoic acid (CH 3 (CH 2) 28 COOH) monolayers at the water-hexane interface, as determined by X-ray reflectivity measurements. Molecular dynamics simulations illustrate the presence of hydrogen bonding between the triacontanoic acid headgroups that is likely responsible for the tail ordering. A simple free energy illustrates the interplay between the attractive hydrogen bonding and the ordering of the tailgroup.