Polymerization and Surface Behavior of Alkyl-Substituted Aniline Surfactants at the Air−Aqueous Interface:  A Kinetic Study

The surface behavior of 2-, 3-, and 4-pentadecylaniline (3a, 3b, and 3c, respectively) was studied at the air−aqueous interface of an acidic subphase. Surface dipole moments (μn) were calculated from the surface potentials (ΔV) of 3a−c and were related to the orientation of the anilinium ion at the interface. Further, 3a and 3b were polymerized in a Langmuir film, and analysis of the absorption spectra showed that the resulting polymers were in different oxidative states upon completion of the polymerization. A method that monitors the change in the mean molecular area (A) of the surfactant at a constant applied surface pressure and predicts the concentrations of monomer and polymer and/or oligomer at the interface in order to determine the formation rate (R p) of poly(alkylaniline) is presented. The R p values of 3a and 3b are expressed in terms of the concentrations of the monomer and polymer at the interface. It was determined that the R p of 3a was dependent on the applied surface pressure while 3b showed little dependence on surface pressure. Differences in R p can be explained by the conformation of the anilinium group and the steric hindrance of the alkyl group at the interface. A characteristic “autoacceleration effect” was observed for both 3a and 3b but was more significant in the case of 3a.