Towards compartmentalized micelles: Mixed perfluorinated and hydrogenated ionic surfactants in aqueous solution

Hypothesis: Aqueous solutions of mixtures of hydrogenated and perfluorinated ionic surfactants are known to display anomalous aggregation behavior due to the mutual phobicity between hydrogenated and perfluorinated chains. Despite all efforts, different experimental limitations prevented so far a definite interpretation of the existing experimental results: both intermicellar and intramicellar segregation remain acceptable possibilities. Method: The potential for segregation of mixtures of fluorinated and hydrogenated ionic surfactants in water was assessed using atomistic molecular dynamics simulations. Aqueous mixtures of hydrogenated and perfluorinated ionic surfactants were studied: mixtures of anionic surfactants (sodium dodecylsulfate (SDS) + sodium perfluorooctanoate (SPFO)) and catanionic surfactants (decyltrimethylammonium bromide (DeTAB) + SPFO) were simulated. Findings: The mixture of anionic surfactants, SDS + SPFO, exhibits clear intramicellar segregation between fluorinated and hydrogenated chains, displaying hydrogenous-rich and fluorous-rich regions. Compartmentalized micelles are thus clearly formed. The simulation results also suggest the possibility of intermicellar segregation. Conversely, catanionic mixtures of DeTAB and SPFO in water solution assemble into a large oblate structure, containing all available molecules in the simulation box, resembling a double layer membrane or a vesicle wall. In this case mixing between fluorinated and hydrogenated surfactants is dictated by charge alternation.