Classical Amphiphilic Behavior of Nonclassical Amphiphiles: A Comparison of Metallacarborane Self‐Assembly with SDS Micellization

Abstract

The self-assembly of metallacarboranes, a peculiar family of compounds exhibiting surface activity and resembling molecular-scale Pickering stabilizers, has been investigated by comparison to the micellization of sodium dodecylsulfate (SDS). These studies have shown that molecules without classical amphiphilic topology but with an inherent amphiphilic nature can behave similarly to classical surfactants. As shown by NMR techniques, the self-assembly of both metallacarboranes and SDS obey a closed association model. However, the aggregation of metallacarboranes is found to be enthalpy-driven, which is very unusual for classical surfactants. Possible explanations of this fact are outlined.

Nonclassical Hydrophobic Effect in Micellization: Molecular Arrangement of Non‐Amphiphilic Structures

“…[28] Our very recent contribution to this controversy aimed at rationalizing the solution behavior of metallacarboranes. [27] We demonstrated that COSAN aggregation obeys the rules established for classical surfactants like SDS (the closed association model with critical aggregation concentration (CAC)) pointing to the hydrophobic effect as a general driving force for the selfassembly of all surfactants and amphiphiles. The most important results are briefly outlined: The aggregates (micelles) were visualized by electron microscopy as dark round stains with average diameter 2.59 ± 1.34 nm in 24.5 mm COSAN solution.…”

“…As a consequence, metallacarboranes aggregate in water. [26,27] Even though metallacarboranes are considered rather exotic compounds, the observed morphologies of the aggregates (large vesicles, layered and cylindrical structures, and small spherical associates) [26][27][28] have their parallels in the field of classical surfactants. Unfortunately, the theories on metallacarborane aggregation usually suffer from the tendencies to exclude boron cluster compounds from classical amphiphiles.…”

“…The θ -shaped amphiphile [26] concept aimed at describing the behavior of H[COSAN] in aqueous solution was invoked for an explanation of unilamellar vesicles, structures that are unparalleled in colloid science (although the abundance of the COSAN vesicles is extremely low in solution). [27] However, such a view does not consider the details of molecular structure of COSAN cluster as follows. The idea that the polar part is simply due to +3 formal charge of the central cobalt ion is misleading for the following reasons: (i) the Co ion is buried in the COSAN structure (see Figure 1B), (ii) the Co ion accepts around 2e of charge (2.7e for RESP and 1.8e for Natural Population Analysis, see Experimental Section), which renders it far less positive.…”

“…A very small fraction of large nanoparticles (diameter larger than 20 nm) interpreted as monolayer vesicles [26] was also observed. [27] The aggregation number of small aggregates, N agg , was determined by small angle X-ray scattering (SAXS) to be around 5. Furthermore, the appearance of a correlation peak in the SAXS curve indicates an ordering of COSAN clusters within the aggregate.…”

“…Besides the many similarities, there are however several differences between COSAN and SDS micellization. Only a small fraction of counterions condenses onto metallacarborane aggregates, [27,28] and the main driving force is enthalpy contribution. [27] In this work, we aim at elucidating the molecular level formation of small aggregates of metallacarborane salts in water by concerted use of experiment (NMR and isothermal titration calorimetry (ITC)) and multiscale modeling (molecular dynamics (MD) and quantum mechanics (QM)).…”