The system poly(ethylene imine)/cetyltrimethylammonium bromide (PEI/CTAB) is an unusual aqueous polymer/surfactant mixture, as the components are strongly associating without electrostatic attraction being the interaction driving force. This system has provoked interest over the last few years because thick, mesostructured films form spontaneously at the air/water interface. We have carried out a study to understand the self-assembly mechanism at the surface using three reflectometry techniques. From ellipsometry, we show that PEI with a high molecular weight (750 kDa) forms two types of films with CTAB. Below the critical micelle concentration (cmc) of the surfactant, the films are stable, around 20-40 nm thick, and exhibit lateral inhomogeneity on the micron-scale. Yet above the cmc, a striking growth-collapse sequence is observed (Figure 1, left) where the maximum thickness and timing of the collapse are both clearly linked to the amount of polymer in the bulk phase. Complementary neutron reflectivity measurements have shown that we can relate the mesostructured nature of the film to the observed changes in surface excess (Figure 1, right). We have also recorded Brewster angle microscopy measurements on the different film states to assess the lateral film structure. Recently measurements were recorded under different ambient gases to examine effects on the film growth of humidity and carbon dioxide absorption. The underlying reasons for this newly-discovered interfacial mechanism are presented, along with comparisons to different strongly interacting polymer/surfactant systems.

Reasons for a growth-collapse mechanism for thick, mesostructured PEI/CTAB films

246th National Meeting (2013)

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