The Chaotropic Effect as an Assembly Motif in Chemistry

Total Description of Intrinsic Amphiphile Aggregation: Calorimetry Study and Molecular Probing

Isothermal titration calorimetry (ITC) is an apt tool for a total thermodynamic description of self-assembly of atypical amphiphiles such as anionic boron cluster compounds (COSAN) in water. Global fitting of ITC enthalpograms reveals remarkable features that differentiate COSAN from classical amphiphiles: (i) strong enthalpy and weak entropy contribution to the free energy of aggregation, (ii) low degree of counterion binding, and (iii) very low aggregation number, leading to deviations from the ideal closed association model. The counterion condensation obtained from the thermodynamic model was compared with the results of 7Li DOSY NMR of Li[COSAN] micelles, which allows direct tracking of Li cations. The basic thermodynamic study of COSAN alkaline salt aggregation was complemented by NMR and ITC experiments in dilute Li/NaCl and acetonitrile aqueous solutions of COSAN. The strong affinity of acetonitrile molecules to COSAN clusters was microscopically investigated by all-atomic molecular dynamics simulations. The impact of ionic strength on COSAN self-assembling was comparable to the behavior of classical amphiphiles, whereas even a small amount of acetonitrile cosolvent has a pronounced nonclassical character of COSAN aggregation. It demonstrates that large self-assembling changes are triggered by traces of organic solvents.

In situ synthesis of ultrafine metal clusters triggered by dodecaborate supramolecular organic frameworks

Supramolecular nano-assemblies with tunable morphology have attracted extensive attention in composite material manufacturing and many other fields. Herein, a new class of shape-controlled dodecaborate-based supramolecular organic frameworks (BOFs), decorated with diverse ultrafine noble metal clusters (Au, Pd, Pt, Ag), has been successfully fabricated via the tunable host-guest assembly of cucurbit[n = 5,6,7,8]uril and Cs2[closo-B12H12]. Due to the unique dodecaborate-cucurbit[n]uril chaotropic effect, a breakthrough has been made in modulating the supramolecular frameworks in the urchin-like, network-like or octahedron-like structures without tedious chemical modifications or additional additives. More interestingly, Cs2[closo-B12H12], a prominent component in supramolecular structures, can also play dual roles as an excellent reductant and capping agent for the formation of metal clusters. The final product, i.e., BOFs decorated with ultrafine metal clusters (metal/BOFs) can be precipitated from aqueous solutions rapidly and they show high catalytic activity and recyclability in Suzuki reactions and in the selective reduction of furfural (FAL) to furfuryl alcohol (FOL).

The Chaotropic Effect as an Assembly Motif in Chemistry

Following up on scattered reports on interactions of conventional chaotropic ions (for example, I- , SCN- , ClO4- ) with macrocyclic host molecules, biomolecules, and hydrophobic neutral surfaces in aqueous solution, the chaotropic effect has recently emerged as a generic driving force for supramolecular assembly, orthogonal to the hydrophobic effect. The chaotropic effect becomes most effective for very large ions that extend beyond the classical Hofmeister scale and that can be referred to as superchaotropic ions (for example, borate clusters and polyoxometalates). In this Minireview, we present a continuous scale of water-solute interactions that includes the solvation of kosmotropic, chaotropic, and hydrophobic solutes, as well as the creation of void space (cavitation). Recent examples for the association of chaotropic anions to hydrophobic synthetic and biological binding sites, lipid bilayers, and surfaces are discussed.