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

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 −CH2OH 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 (CH3(CH2)29OH) and triacontanoic acid (CH3(CH2)28COOH) 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.

@article{tikhonov2006tail,
  title   = {Tail ordering due to headgroup hydrogen bonding interactions in surfactant monolayers at the water-oil interface},
  author  = {Tikhonov, Aleksey M and Patel, Harshit and Garde, Shekhar and Schlossman, Mark L},
  journal = {The Journal of Physical Chemistry B},
  volume  = {110},
  number  = {39},
  pages   = {19093--19096},
  year    = {2006},
  doi     = {10.1021/jp064120q}
}