Structure of mixed DTAB/DDAB adsorbed layers on quartz: A neutron reflectometry and atomic force microscopy study

The structure and composition of adsorbed layers formed by mixtures of dodecyltrimethylammonium bromide (DTAB) and didodecyldimethylammonium bromide (DDAB) on quartz have been determined using a combination of atomic force microscopy and neutron reflectometry. The mixed adsorbed layer was found to be substantially enriched in DDAB, stabilising the formation of a laterally unstructured bilayer even when there is only as little as 1 mol% present in the bulk mixed solute. At even higher DTAB content the adsorbed film morphology gradually transforms to globules over a very narrow composition range. DDAB was found to be significantly excluded from the mixed adsorbed film only after it was completely transformed into globular aggregates, the equilibrium adsorbed layer morphology of DTAB.