Self-Assemblies Driven by the Hydrophobic Effect
Water is a simple molecule but is an essential part of life. One key aspect of the properties of water is the hydrophobic effect, and whilst there is an appreciation of this phenomenon at the macro-scale (raindrops falling off leaves) and the micro-scale (the structure of cellular systems), a complete understanding at the molecular level still eludes science. Addressing this issue, our studies involve synthetic supramolecular compounds that assemble in water via the hydrophobic effect. First of all, a novel water-soluble deep-cavity cavitand was synthesized. It possesses four endo methyl groups on top rim of the cavitand, eight water-solubilizing carboxylic acid groups coated on the cavitand exterior, and a relatively large hydrophobic interior. Compared to a previous well-studied water-soluble deep-cavity cavitand octa-acid (OA), this novel cavitand (TEMOA) possesses a non-monotonic assembly profile in the presence of a homologous series of straight-chain alkanes. Three supramolecular species were observed: 1:1, 2:2, and 2:2 and they are approximately isoenergetic. Second, we examined the guest-controlled self-sorting in assemblies. A mixture of OA and TEMOA formed hetero-capsular complex driven by the hydrophobic effect. The extent of homo- or hetero-dimerization is intimately tied to the size of the guest being encapsulated. TEMOA is less predisposed to dimerize than OA, thus TEMOA possesses the ability to form various self-assembled states, such as tetrameric and hexameric assemblies. Furthermore, we also discussed our observation of how external stimuli such as changing the nature or concentration of a co-solute salt influences a unique, unusual transition from one assembled state to another.
self-assembly, hydrophobic effect, diffusion constant, hydrodynamic volume, salt, non-monotonic, Organic Chemistry
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