Interaction kinetics of anionic surfactants with cationic vesicles

The interaction mechanism of cationic diethylester dimethylammonium chloride (DEEDMAC) vesicles and the anionic surfactant sodium dodecyl sulphate (SDS) was investigated. Electrophoretic mobility measurements showed that the outer monolayer of the DEEDMAC vesicles became negatively charged immediately after addition due to sorption of an excess SDS. The subsequent increase of the vesicle surface charge density reflected the entering of SDS into the complete vesicle. Turbidity measurements showed that the flocculation behaviour was more pronounced than expected based only on the electrostatic interactions. The dynamic surface tension obtained immediately after mixing an SDS solution and a DEEDMAC dispersion was lower than the value of an SDS solution of the same concentration without any DEEDMAC, although DEEDMAC alone did not lower the surface tension. Based on the turbidity and surface tension observations, it is postulated that sorption of SDS leads not only to neutralisation of surface charges but also to a more hydrophobic vesicle surface due to the non-immediate penetration of the SDS dodecyl-chains into the DEEDMAC alkyl chain region. After waiting for a longer time, the surface tension of the DEEDMAC–SDS mixtures reached the value of pure water, indicating that all SDS is incorporated within the vesicles.