Growth-collapse mechanism of PEI-CTAB films at the air-water interface

We reveal a growth-collapse mechanism for self-assembled films of polyethyleneimine (PEI) and cetyltrimethylammonium bromide (CTAB) at the air-water interface. The work involved the quantification of film thickness on the micron scale through tracking oscillations in the amplitude and phase angles using single-angle, single-wavelength ellipsometry. Our results allow us to propose the following interfacial mechanism: film growth resulting from the association between trimethylammonium groups on surfactant micelles with lone pairs of electrons on the primary amine groups of long polymer chains, then arrest of the growth resulting from CO2 adsorption which increases the cationic charge density of the polyelectrolyte, followed by total film collapse resulting from its drying out and breakup into solid particles. We relate stages of the film development to the evolution of a Bragg peak using neutron reflectometry, where the highest degree of micellar ordering occurs just after the rapid growth stage and the loss of definition of the mesostructure is well correlated with the final collapse of the film. We also link the development to characteristic changes in the lateral film morphology using Brewster angle microscopy, including wrinkling during film growth and circular defects during collapse.