Langmuir monolayers from substituted aromatic carboxylic acids

Abstract

A series of 5′-phenyl-m-terphenyl carboxylic acid derivatives with methyl, phenyl, chloro, p- chlorophenyl and fluoro substituents have been characterised as Langmuir monolayers at the air/water interface by measuring the surface pressure and the electric surface potential upon monolayer compression. The three-layer capacitor model proposed by Demchak and Fort [(1974) J Colloid Interface Sci 46:191] is employed to relate the experimental surface potentials of the monolayers investigated to the molecular dipole moment calculated using semiempirical quantum methods. The local dielectric permittivity in the vicinity of hydrophobic groups was calculated to be 4.8. By adopting a dielectric constant of 7.6 in the vicinity of the hydrophilic groups, the contribution from the water reorientation was found to be 0.15 D, very close to that estimated for small aromatic molecules forming Gibbs monolayers at the air/ water interface.