pH responsive vesicles with tunable size formed by single-tailed surfactants with a dendritic headgroup

Abstract

The structure–activity relationship of surfactants is a topic that has been pursued for a long time in colloid science. We report a study on the self-assembling of a new type of surfactant with a dendritic headgroup in dilute aqueous solutions. Three surfactants, N-alkyldipropionic acids (ADPA) with different chain lengths (n = 14, 16, and 18), were synthesized. As a result of the carboxylic acids and one tertiary amine in the headgroup, the protonation state of the surfactants changes with pH. The morphology of the surfactant aggregates in aqueous solutions was studied at various pH values by transmission electron microscopy (TEM) and dynamic light scattering (DLS). Vesicles were formed spontaneously at pH > 5. Potentiometric titration, turbidity and surface tension were used to determine the effect of protonation state on aggregation behavior. The results show that the vesicle size depends directly on the pH of the solution. The size distribution of vesicles was analyzed by the model of spontaneous curvature. The thermodynamic stability of the vesicles can be attributed to long-range repulsive interactions caused by the low bending modulus of the bilayers.