Surface Activity and Molecular Organization of Metallacarboranes at the Air–Water Interface Revealed by Nonlinear Optics

Abstract

Because of 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 liquid phase. Unlike classical surfactants, metallabisdicarbollides (MCs) do not have a well-defined amphiphilic structure. They are nanometer-sized inorganic anions with an ellipsoidal shape composed of two carborane semicages sandwiching a metal ion. 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. By combining second harmonic generation and surface tension measurement, we show here that cobaltabis(dicarbollide) anion {[(C2B9H11)2Co](-) also named [COSAN](-)} with H(+) as a counterion, the most representative metallacarborane, adsorbs vertically at the water surface with its long axis normal to the surface. This vertical molecular orientation facilitates the formation of intermolecular and nonconventional dihydrogen bonds such as the B-H(δ-)···(δ+)H-C bond that has recently been proven to be at the origin of the self-assembly of MCs in water. Therefore, it appears here that lateral dihydrogen bonds are also involved in the surface activity of MCs.

Stealth Amphiphiles: Self-Assembly of Polyhedral Boron Clusters

“…[16][17][18][19][20][21][22][23][24] Mechanism of formation of small aggregates (sometimes called "micelles") 20 has been already extensively studied, and it obeys the closed association model with critical aggregation concentration, CAC. 23 To reveal a mechanism of the aggregation, NMR spectroscopy is a suitable method.…”

How Nano‐Ions Act Like Ionic Surfactants

“…It should be noted that nano‐ions with the lowest charge densities, that is, with a more pronounced hydrophobic character, display some tendency to self‐assemble in water, for example, the Keggin‐type phosphotungstate, PW 12 O 40 3− , some giant POMs such as the ones from the Keplerate type, and halogenated dodecaborates . Among nano‐ions and boron clusters, metallabisdicarbollides stand out, as they exhibit the properties of surfactants: self‐assembly in water, surface activity, foaming, and formation of lyotropic phases presumably by combining high hydrophobicity, molecular polarity (or anisotropic distribution of charge), and intramolecular di‐H‐bond formation. Additionally, hydrophobic (organic) ions such as tetraphenylborate have also been termed superchaotropes regarding their tendency to adsorb to hydrophobic interfaces…”

Studies of Multicenter and Intermolecular Dihydrogen B–H···H–C Bonding in [4,8,8′‐exo‐{PPh3Cu}‐4,8,8′‐(μ‐H)3‐commo‐3,3′‐Co(1,2‐C2B9H9)(1′,2′‐C2B9H10)]

“…[59][60][61][62][63] This bonding can be interpreted in terms of individual bond description and the molecular electrostatic potential distribution [64] (ESP, Figure 5, a). It can be seen that the ESP is negative in the apical region of carborane and above the phenyl rings, and positive in the region of H(C) atoms; the ESP distribution in the region of two dicarbollide anions is different.…”