A theta-shaped amphiphilic cobaltabisdicarbollide anion: transition from monolayer vesicles to micelles

Abstract

Self-assembly is ubiquitous in nature and in surfactant science.1 Globular micelles or vesicles in water are usually obtained with surfactants2 or amphiphilic block copolymers3 composed of a hydrophilic part and a hydrophobic part. The aggregation mechanism is then controlled by the tendency of the nonpolar part to avoid contact with water, known as the hydrophobic effect,4 while the polar part tends to be strongly hydrated. Consequently, the minimum free energy is reached during the aggregation process by minimal contact of the hydrocarbon chains with water and by maximal entropy that lead to the formation of the smallest possible aggregates, that is, spherical micelles for single-chain surfactants. As the concentration increases, the shape changes from spherical micelles to rods (hexagonal phase) to a lamellar phase, that is, from high to low curvature toward the apolar part.5 Double-chained surfactants, or some block copolymers with a pronounced hydrophobic tendency, cannot pack into spheres because of steric reasons, and are forced immediately into closed bilayers (vesicles) of near-zero curvature at the surfactant/copolymer scale.6 Recently, the formation of vesicles has been reported for more exotic, that is, non-amphiphilic chemical systems such as amino acid derivatives, oligopeptides, cyclodextrins, macrocycles, fluorofullerenes, and polyelectrolytes.7

Herein we show that the cobaltabisdicarbollide (mono-) anion (H+[3,3′-Co(1,2-C2B9H11)2]− (H+COSAN−; Figure 1) forms monolayer vesicles at low concentrations in water. An increase in concentration leads to a phase transition from vesicles to small micelles (Figure 1) and results in the coexistence of both aggregation states at higher concentrations. The formation of monolayer vesicles and small micelles was hitherto unknown for water-soluble carborane derivatives.