Tuning the Surface Activity and Micellization of closo -Dodecaborate-Based Dianionic Surfactants via Linker and Counterion Selection

To understand the impact of alkaline counterions on the self-assembly of atypical dianionic surfactants, we synthesized a series of novel amphiphilic compounds featuring a bulky dianionic head based on closo-dodecaborate and a hydrophobic alkoxy tail connected via short linkers of different complexation ability toward alkaline cations. The synthetic strategy relied on the opening of cyclic oxonium derivatives by activated alcohol, enabling the attachment of hydrophobic chains to the boron cluster framework. The synthesis of novel surfactants with high cationic purity free from contaminating ions or additional inorganic salts is described. Self-assembly of the dodecaborate-based surfactants at the air-water interface and in aqueous solutions was investigated using tensiometry, various NMR spectroscopy techniques, dynamic light scattering, and all-atom MD simulations. Interestingly, despite their dianionic heads, the surfactants form compact monolayers in cases in which efficient binding of counterions to surfactant dianions occurs. MD simulations suggested relatively small surfactant micelles above the critical micelle concentration, which was confirmed by light scattering analysis. There is evidence that their aggregation number and morphology are strongly dependent on counterion immobilization. The mobility of counterions was observed directly by ⁷Li, ²³Na, ³⁹K, and ¹³³Cs NMR spectroscopy, which allowed us to understand the binding abilities of linkers and dianionic heads toward counterions.