Abstract
Owing to their amphiphilic structure, surfactants adsorb at the water/air interface with their hydrophobic tails pointing out of the water and their polar heads plunging into the water phase. Unlike classical surfactants, metallabisdicarbollides (MCs), nanometer-sized inorganic anions composed of two carborane semi-cages sandwiching a metal ion, do not have a well-defined amphiphilic structure. However, MCs have been shown to share many properties with surfactants, such as self-assembly in water (formation of micelles and vesicles), formation of lamellar lyotropic phases and surface activity. We have shown here, by combining Second Harmonic Generation (SHG) and surface tension measurement, that cobaltabis(dicarbollide) anion ([(C2B9H11)2Co]-) also named [COSAN]- with H+ as counterion, the most representative metallacarborane, adsorbs vertically at the water surface by forming a monolayer. This vertical molecular orientation facilitates the formation of intermolecular di-hydrogen bonds such as B-Hδ-…δ+H-C that has recently been proved to be at the origin of the self-assembly process of MCs in water. Therefore it appears here that lateral di-hydrogen bonds are also involved in the surface activity of MCs.