Surfactant Behavior of “Ellipsoidal” Dicarbollide Anions:  A Molecular Dynamics Study

Abstract

We report a molecular dynamics study of cobalt bis(dicarbollide) anions [(B(9)C(2)H(8)X(3))(2)Co](-) (XCD(-)) commonly used in liquid-liquid extraction (X = H, Me, Cl, or Br), showing that these anions, although lacking the amphiphilic topology, behave as anionic surfactants. In pure water, they display "hydrophobic attractions", leading to the formation of aggregates of different sizes and shapes depending on the counterions. When simulated at a water/"oil" interface, the different anions (HCD(-), MeCD(-), CCD(-), and BrCD(-)) are found to be surface active. As a result, the simulated M(n+) counterions (M(n+) = Na(+), K(+), Cs(+), H(3)O(+), UO(2)(2+), Eu(3+)) concentrate on the aqueous side of the interface, forming a "double layer" whose characteristics are modulated by the hydrophobic character of the anion and by M(n+). The highly hydrophilic Eu(3+) or UO(2)(2+) cations that are generally "repelled" by aqueous interfaces are attracted by dicarbollides near the interface, which is crucial as far as the mechanism of assisted cation extraction to the oil phase is concerned. These cations interact with interfacial XCD(-) in their fully hydrated Eu(H(2)O)(9)(3+) and UO(2)(H(2)O)(5)(2+) forms, whereas the less hydrophilic monocharged cations display intimate contacts via their X substituents. The results obtained with the TIP3P and OPLS models for the solvents are confirmed with other water models (TIP5P or a polarizable 4P-Pol water) and with more polar "oil" models. The importance of interfacial phenomena is further demonstrated by simulations with a high oil-water ratio, leading to the formation of a micelle covered with CCD's. We suggest that the interfacial activity of dicarbollides and related hydrophobic anions is an important feature of synergism in liquid-liquid extraction of hard cations (e.g., for nuclear waste partitioning).

Insights into Uranyl Chemistry from Molecular Dynamics Simulations

“…In the presence of synergistic anionic species like cobalt dicarbollides, though, UO 2 2 + can be attracted to the interface. [56] To simulate biphasic TBP/water solutions, a mixing-demixing protocol was designed to avoid as much as possible trapping near a metastable state. Demixing of randomly mixed solvents and solute molecules occurred in less than one nanosecond, so that, in reality, water and oil are never mixed at the microscopic level and delineate interfaces, of about 1-2 nm in width, with chloroform or supercritical CO 2 (sc-CO 2 ) as "oil phase" in which TBP is dissolved.…”

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

“…[20] In the former case, closely apposed pairs of COSAN molecules accompanied by two Na + counterions bind in the HIV protease active site. [17] The latter application of metallacarboranes is underlaid by their unique solution behavior which led to the computational prediction [21][22][23] and the experimental evidence [24,25] that they behave as surfactants. Various substituted forms (Me, Cl, Br) and the parental cobalt bis(dicarbollide) anions coupled with several counterions (Na + , K + , Cs + , H 3 O + , UO 2 2+ , Eu 3+ ) showed surface activity.…”

Metallacarboranes and their interactions: theoretical insights and their applicability

“…In 42,73 The major difference from previous studies was the use of other organic solvents as "oil" phases. Instead of chloroform, the use of octanol and nitrobenzene allowed the possibility to extend the knowledge to new biphasic systems.…”

“…In this case, the electrostatic potential is fundamental to properly describe these compounds in the simulations at liquid-liquid extraction interfaces with a set of cations. 42 As discussed above, an important feature extracted from atomic charge calculations is the bonding nature and electronic distribution in the molecule. For metallacarboranes, the polarity between the carbon and boron vertices due to the higher electronegativity of carbon (2.55 vs. 2.04, respectively) causes that electrons are concentrated near the carbon.…”

“…Other examples where the non-bonding interactions play an important role are the molecular conductors based on the cobaltabisdicarbollide anion, 51 their surfactant behaviour 42 and interactions with biomolecules 52 (see section 4.a).…”