Sugar-based gemini surfactants with pH-dependent aggregation behavior: Vesicle-to-micelle transition, critical micelle concentration, and vesicle surface charge reversal

In a recent report, we presented data on the rich and unusual pH-dependent aggregation behavior of a sugar-based (reduced glucose) gemini surfactant (Johnsson et al. J. Am. Chem. Soc. 2003, 125, 757). In the present study, we extend the previous investigation by introducing a different sugar headgroup (reduced mannose), by varying the spacer between the two main surfactant parts, and by introducing, in one of the surfactants, an amide linkage (instead of an amine linkage) between the headgroup and the unsaturated (C 18: 1) hydrocarbon tails. The aggregation behavior of these four gemini surfactants has been studied and compared by means of light scattering, cryo-transmission electron microscopy, electrophoretic mobility, and fluorescence measurements. We find that all four surfactants form vesicles near neutral or high pH. However, the vesicles made from the amine-containing geminis are transformed into cylindrical or wormlike micelles at lower pH values (pH < similar to5.5). The micellization is driven mainly by an increased electrostatic repulsion, caused by the protonation of the tertiary amino groups, and we find that the nature of the sugar or spacer has little influence on this process. At low pH (pH 2), solely small globular micelles are found, and the critical micelle concentration at this pH is about 0.005-0.010 mM for the different amine-containing surfactants. As was expected, the gemini surfactant with the amide instead of the amine functional groups in the headgroup does not undergo the vesicle-to-micelle transition but displays only vesicle formation within the investigated pH range. The electrophoretic mobility measurements on the vesicular samples formed from the amine-containing geminis show that the vesicles are cationic below pH similar to7-7.5; however, the vesicles acquire a substantial negative charge at a higher pH. The most probable explanation for this charge reversal is a strong adsorption (or binding) of hydroxide ions onto the vesicle surface. In accordance with this hypothesis, we find that the vesicles made from the amide-containing gemini are anionic (no protonation) even at a low pH (pH < 5). Using a simple Poisson-Boltzmann model, we are able to describe the obtained zeta-potential profiles reasonably well and derive a hydroxide-ion binding constant (K-OH) for the respective systems. We find that the nature of the sugar does have a small influence on KOH. The colloidal stability of all four types of the gemini vesicles seems to be well-described by the classical Derjaguin-Landau-Verwey-Overbeek theory, and the vesicles aggregate/flocculate rapidly in the limit of low surface potential. However, the flocculated vesicles can be easily redispersed by, for example, raising the pH of the solution, and this flocculation/redispersal process is completely reversible.