Self-assembly and phase behaviour of PEI : cationic surfactant aqueous mixtures forming mesostructured films at the air/solution interface

Self-assembly processes and corresponding phase boundaries depend on the structure of interacting molecules. We have studied the effect of surfactant head-group structure and counterion on the self-assembly processes occurring in aqueous mixtures of the cationic polymer polyethylenimine (PEI) and the cationic surfactants cetyltrimethylammonium bromide (CTAB), cetylpyridinium bromide (CPB) and cetylpyridinium chloride (CPC). Surface tension and conductometric methods were employed, as well as X-ray scattering and reflectometry techniques. The results indicate that the phase behaviour of PEI : cationic surfactant mixtures under dilute conditions can be tuned by altering head-group and counterion. The surfactant head-group exerts a greater influence overall, especially over micellization, since the counterion effect is screened at high polymer concentrations. Also, the self-assembly processes occurring at lower surfactant concentrations are not significantly affected by the counterion effect. The variation of surface tension with surfactant concentration shows a particular ‘well-like’ profile with a distinct break-point that can be directly related to the onset of film formation. Phase diagrams constructed using surface tension, conductivity and Brewster angle microscopy (BAM) data show three different regions where initial interactions, film formation and micellization take place, respectively, characterized by the two main self-assembly boundaries. Based on these results, we propose a molecular interpretation of the aggregation and film formation processes occurring in PEI : cationic surfactant mixtures under increasing surfactant concentrations within the dilute regime.