pH-dependent phase behavior of carbohydrate-based gemini surfactants. The effects of carbohydrate stereochemistry, head group hydrophilicity, and nature of the spacer.

The pH-dependent phase behavior and hydroxide-ion adsorption ability of a series of (reduced) carbohydrate-based gemini surfactants were studied between pH 2 and 12. Static and dynamic light scattering were employed to address transitions in the aggregate morphologies and cryo-electron microscopy was used to provide further evidence for the morphologies present in solution. Changes in aggregate structure as a result of a change in solution pH and an accompanying change in protonation state or a change in molecular structure can be rationalized in terms of the variations in the packing parameter. In this paper we have focused our attention on the size of the carbohydrate moiety, the carbohydrate stereochemistry and the nature of the spacer (hydrophobic vs hydrophilic). At near neutral pH, most of the gemini surfactants form vesicles. Upon lowering of the pH, the vesicles undergo a transition toward wormlike micelles followed by a transition to spherical micelles. Upon increasing the solution pH, flocculation occurs due to charge neutralization followed at still higher pH by redispersion and charge reversal of the vesicles through the specific adsorption of hydroxide ions to the vesicular surface. Upon decreasing head group size at constant, but low, degrees of protonation, the packing parameter has a tendency to become larger than one resulting in the formation of inverted phases. Upon further decrease in the head group size, oil droplets are observed. In case of a hydrophobic spacer, the carbohydrate stereochemistry affects the pH of the transitions, but not the type of the transitions. By contrast, for a hydrophilic spacer, the pH of the transitions remains unaffected. Adsorption of hydroxide ions at basic pH follows similar trends, but was only found for vesicles and oil droplets. The large range of structural variations that we have examined allows a better understanding of the requirements for the phase transitions for carbohydrate-based gemini surfactants as well as for the physisorption of hydroxide ions to interfaces in general.