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

Abstract

Micellization brought about by nonclassical hydrophobic effect invokes enthalpy as the driving force. Thus, the underlying molecular phenomena differ from the entropically dominated hydrophobic effect. In quest for a molecular-scale understanding, we report on the molecular arrangement of nonamphiphilic structures of an anionic boron cluster compound, COSAN. We synergistically combine experimental (NMR and calorimetry) and theoretical (molecular dynamics and quantum chemical calculations) approaches. The experimental data support the mechanism of closed association of COSAN, where the self-assembly is driven by the enthalpy contribution to the free energy. Molecular dynamics simulations in explicit solvent show that water molecules form a patchy network around COSAN molecules, giving rise to the strong hydrophobic self-association. In the second solvation shell, water forms a slightly hydrophilic "spot" close to the C-H segments of the cluster.

Atomistic Simulations of COSAN: Amphiphiles without a Head‐and‐Tail Design Display “Head and Tail” Surfactant Behavior

“…Theaverage aggregation number as afunction of concentration obtained from our MD simulations (Figure 4a)i s consistent with the experimentally determined [6,7,10,13] critical aggregation concentration for micelles of Na + COSAN À , which is ca. 10 mm.…”

“…It should be noted that, experimentally, the micelles are observed to coexist with vesicles of ca. 20 nm in radius with aggregation numbers of about 12 500 molecules . Attempts to directly simulate these vesicles in water require the use of coarse‐grain models instead of the all‐atomic MD simulations discussed here.…”

“…Thed etailed molecular mechanism underlying this nonclassical amphiphilic behavior is still unknown. Some proposals emphasize the possibility of an on-classical hydrophobic effect [7,9,10] or as o-called chaotropic effect [7,11] as ad riving force for the association of COSAN in water, intrinsically different from the association mechanism of classical surfactants.…”

“…It should be noted that, experimentally,t he micelles are observed to coexist with vesicles of ca. 20 nm in radius [4,6,10] with aggregation numbers of about 12 500 molecules. [4] Attempts to directly simulate these vesicles in water require the use of coarse-grain models instead of the all-atomic MD simulations discussed here.H owever,t he ordering of cis-COSAN molecules in the vesicle walls can be analyzed by MD simulations of aplanar vesicle patch in atensionless state, as done in MD studies of surfactant vesicles [30] and lipid membranes.…”

The Chaotropic Effect as an Assembly Motif in Chemistry

“…Indeed, the previously determined thermochemical signature in the solid-phase binding of dodecaborate cluster anions (enthalpically driven binding with large entropic penalty) [182] fully agrees with this new interpretation. Moreover,i nt wo independent lines of investigation, Matějíček and co-workers have established the operation of the chaotropic effect for boron cluster anions, [11,40,53,190,191] while the group of Bauduin and Diat have expanded the observed self-assembly and surfaceassembly tendencies to POMs as superchaotropic anions. [15,16,18,192] In detail, POMs adsorb at micellar surfaces and on monolayers of nonionic surfactants,a sshown by using small-angle X-ray scattering (SAXS) and ion flotation techniques.…”