Superchaotropic or Hydrophobic Behavior? How Borate Clusters Assemble with Nonionic Surfactants

This work investigates the interaction of anionic borate clusters with self-assembled surfactant systems. Anionic borate clusters, such as closo-dodecaborates B12X122- (X = H, Cl, Br, I) and cobaltabisdicarbollide (COSAN-), bind hydrated solutes via a water-mediated (super)chaotropic effect. Cloud point measurements on the micellar phase of the nonionic surfactant C8E4 reveal a continuous series of binding affinity from chaotropic (B12H122-) over superchaotropic (B12Cl122-, B12Br122-) to hydrophobic ions (B12I122-, COSAN), extending seamlessly the Hofmeister series of classical ions and in line with data for ten additional nanometer-sized ions gleaned from the literature. Small-angle X-ray and neutron scattering revealed a structural criterion distinguishing superchaotropic from hydrophobic ions. Superchaotropic ions form a saturated interfacial monolayer between surfactant headgroups. Hydrophobic ions, however, localize in the apolar interior of the micelle, causing a curvature transition into bilayer structures in C8E4, a surfactant system that does not form bilayers on its own, leading to micellar destabilization and phase separation at higher concentrations.