Effect of a Clathrate-Forming Counterion on Micellar Solubilization

Clathrate hydrates are supramolecular solids in which small, hydrophobic species are trapped inside a network of hydrogen-bonded water cages. While the best known examples are the natural gas hydrates, similar crystalline structures can also be formed from salts of tetra-n-butylammonium hydroxide (TBAOH). When TBAOH is reacted with medium to long chain carboxylic acids it produces surfactant compounds capable of forming either clathrate hydrates or micelle solutions in aqueous solution. However, little is known about how this dual nature affects the functional properties of the surfactants. Here we show a direct comparison of the solubilization of the solvatochromic dye, Nile Red (NR) by sodium 10-undecenoate (NaU) and tetrabutylammonium 10-undecenoate (TBAU). We found from the fluorescence emission spectra that the environment around NR was more polar for TBAU micellar solutions than for NaU. Molecular dynamics simulations indicated that for both systems, the NR had partial exposure to water at the surface of the micelle. The primary difference was a significant structuring of the counterions and water molecules as part of the micelle/NR complex for TBAU. Our results demonstrate an active role of role of the clathrate-forming counterion in the formation of the micelle and the solubilization of the substrate. We anticipate that the more polar environment near the surface of the micelle induced by the TBA/H2O structuring will enhance the solubilization power of the micelle for more polar substrates. Furthermore, we expect the more structured shell around the micelle core to enhance the isolation of the substrate from the environment, a valuable feature for applications such as targeted drug delivery.