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Dumbbell- and bola-shaped amphiphiles are commonly expected to self-assemble into vesicles with condensed hydrophobic domains due to the dominant hydrophobic interaction. In this work, we examined the assemblies of the dumbbell-shaped AC60-AC60 amphiphile, with two carboxylic acid-functionalized fullerenes (AC60) polar head groups linked by an organic tether, and found that they assemble into hollow, spherical blackberry-type structures with porous surfaces, judged by their smaller assemblies in organic solvents with higher polarity and in aqueous solutions with high pH. We attribute the formation of blackberry structures to the organic tether that may be too short to fill up a condensed hydrophobic domain, as confirmed by all-atom simulations. This is further proved by noticing that several bola-type macromolecules with hydrophilic polyethylene glycol (PEG) chain being the linker and no hydrophobic components, AC60-PEG-AC60, can also self-assemble into hollow, spherical assemblies and demonstrate similar pH response as the assemblies from AC60-AC60 dumbbells. Therefore, we conclude that the driving force of the self-assembly for these dumbbell- or bola-shaped molecules is counterion-mediated attraction from the two AC60 head groups rather than the hydrophobic interaction due to the organic linkers. The so-formed blackberry structures here, as well-studied before in other systems, possess porous surfaces, making these charged amphiphiles a valuable model for designing stable nanocontainers with controllable porosity to the change of environment. 

Macroionic solutions behave quite differently from small ions in solution or colloids in suspension, representing a previously missing and very important transitional stage, and can further be connected to solutions of polyelectrolytes, including proteins and DNA ( e.g. , similarities between “blackberry” formation and virus capsid formation). While synthesis and characterization have produced an immense database regarding the self-assembly behavior of macroions in solution resulting in many empirical rules and guidelines, theory and simulations are sorely needed to connect these disparate threads into a cohesive and coherent narrative of macroionic solution theory and to provide guidance for future work. We recently developed a versatile coarse-grained model specifically designed for modelling the self-assembly of macroions in solution and have answered some of the most outstanding questions about the solution behavior of macroions including the source of the attractive force between like-charged macroions and how they self-assemble into a 2D monolayer structure.