An aqueous ionic surfactant 1-dodecyl-4-dimethylaminopyridinium (DMP) bromide and the
corresponding zwitterion 2-(4-dimethylaminopyridinio)-dodecanoate (DPN) were explored
by means of molecular dynamics (MD) simulations and, for the ionic system, by infraredvisible
sum frequency generation (IR-VIS SFG). The molecular structure of the interfacial
layer was investigated for the ionic and zwitterionic systems as a function of surfactant
concentration, both in water and in salt (KF or KBr) solutions, by MD simulations in a slab
geometry. The build up of the surface monolayer and a sublayer was monitored, and density
and orientational profiles of the surfactants were evaluated. The difference between the ionic
and zwitterionic systems and the effect of the added salt were analyzed at a molecular level.
The results of MD simulations were compared to those of nonlinear optical spectroscopy
measurements. Infrared visible sum frequency generation (IR-VIS SFG) was employed to
study the DMP ionic surfactant in water and upon addition of simple salts. The influence of
added salts on the different molecular moieties at the interface was quantified in detail
experimentally.