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

Abstract

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.