We have prepared and analyzed thin film structures formed by polyethylenimine and alkyltrimethylammonium bromide (C(n)TAB) surfactants at the air/water interface, using both surface and bulk sensitive techniques. In initial experiments it was observed that hexagonal arrays of rodlike micelles surrounded by the polymer were formed at the solution surface, with the principal axis of the micelles running parallel to the surface. In the studies reported here, the formation of these ordered mesostructured films was identified as being kinetically but not thermodynamically favored, with some of the systems examined showing a loss of structure from their neutron reflectometry profiles with time. The polymer was used in both an as-diluted state (with a small net positive charge) and a neutral state, through the addition of sodium hydroxide to the solution. The primary interaction in these systems was found to be that of a neutral polymer with a cationic surfactant; however, by modifying the charge on the polymer it is possible to alter the distance between micelles by up to 6 angstrom without destroying the structure of the films. Analysis of the bulk solution with small-angle neutron scattering showed that the micelles in solution are elliptical rather than rod-shaped, and so the assembly of the hexagonal mesostructure occurs at the air/water interface rather than adsorbing to the interface from the bulk.
O'Driscoll, B. M. D., Milsom, E., Fernandez-Martin, C., White, L., Roser, S. J., & Edler, K. J. (2005). Thin films of polyethylenimine and alkyltrimethylammonium bromides at the air/water interface. Macromolecules, 38(21), 8785-8794. https://doi.org/10.1021/ma050469k