Sodium Dodecylsulfate Bilayer Formation Under a Cationic Surfactant Langmuir Monolayer at the Air-Water Interface

An aqueous solution of sodium dodecylsulfate (SDS) was injected into a Langmuir monolayer of dioctadecyldimethylammonium chloride (DODAC) at the air-water interface. Infrared external spectra, atomic force microscopy (AFM), and quartz crystal microbalance (QCM) measurements revealed that the resulting surface monolayer was a multilayer structure consisting of a DODAC monolayer as the upper layer and an SDS bilayer as the underlayer. Furthermore, the SDS alkyl chain was in the all-trans conformation, and the molecular packing of the DODAC alkyl chains was hexagonal with the trans-zigzag plane of the methylene chain oriented randomly around the chain axis. We also discuss the structure of the multilayer obtained by injecting SDS into a mixed Langmuir monolayer of DODAC + SDS.