Ionic dye-surfactant nanoassemblies: interplay of electrostatics, hydrophobic effect, and π-π stacking

The interplay of several non-covalent interaction forces is used as key to supramolecular structures. Combining cationic alkyltrimethylammonium bromide surfactants and the divalent anionic azo dye Acid Red 26 (Ar26) as small building blocks in aqueous solution, electrostatic interactions of the oppositely charged building blocks in combination with hydrophobic effect and pi-pi interactions play a major role in aggregate formation. Static and dynamic light scattering and small-angle neutron scattering (SANS) revealed different sizes of aggregates in the range of 2 nm a parts per thousand currency signaEuro parts per thousand R (H) a parts per thousand currency signaEuro parts per thousand 420 nm depending on surfactant length, concentration and of dye to surfactant loading ratio. A strong relationship of assembly size with surfactant concentration has been found, where initial surfactant monomers and micelles influence the aggregate formation differently. The stability of dye-surfactant aggregates which also shows a dependency on surfactant tail length has been related to zeta-potential measurements. Small-angle neutron scattering elucidated that dye-surfactant aggregates possess cylindrical shapes with different aspect ratios. UV/Vis spectroscopy gave information on the dye-dye pi-pi stacking geometry and extent, while the thermodynamic parameters for micellization and dye-surfactant binding Delta H, Delta G, and Delta S as well as stoichiometry and binding constant obtained by isothermal titration calorimetry revealed insight into the interplay of interactions.