Interactions between quaternary ammonium surfactants and polyethylenimine at high pH in film forming systems.

Hypothesis: Film formation and film structure in films formed on solutions of cationic surfactants with polyethylenimine is influenced by the surfactant structure, including both the tail length and the nature of the headgroup, which alter the micelle properties.

Experiments: A series of cationic surfactants were synthesized and conductivity measurements were used to compare the critical micelle concentrations for these surfactants and their behaviour with and without polyethylenimine at high pH. Small angle neutron scattering measurements were used to characterise the size and shape of the micelles in the presence and absence of the polymer. Film formation between polyethylenimine and the various surfactants was trialled, and the interfacial film structures measured using neutron reflectivity.

Findings: Film formation is shown to depend on surfactant tail length, with thicker films forming for surfactants with longer hydrophobic tails. The surfactant headgroup structure affects counterion binding to the micelles, and in the case of the aromatic headgroups, the headgroup affects the extent of micellar growth when polymer is added. Films with the greatest mesostructural ordering were grown using hexadecylpyridinium bromide surfactants.