Adsorbed monolayers of mixed surfactant solutions of sodium dodecylsulfate and cetylpyridinium chloride studied by infrared external reflection spectroscopy

Infrared external reflection (IER) spectra of monolayer adsorbed on mixed surfactant solutions of sodium dodecylsulfate (SDS) and cetylpyridinium chloride were measured as a function of mole fraction of SDS (X-SDS). The alkyl chains of the surfactants in the adsorbed monolayer (Gibbs monolayer) at the air-solution interface were in all-trans conformations in the solid state, regardless Of XSDS. The adsorbed monolayers at any X-SDS had a single layer structure, except for X-SDS approximate to 0.6-0.7, which had multilayer structures. Furthermore, the time dependency of the IER spectra revealed that the formation of the multilayer structures occurs in two processes: a faster single layer formation followed by subsequent slower multilayer formation.