Understanding the Balance between Ionic and Dispersion Interactions in Aqueous Micellar Media

We have studied the interaction of selected rhodamine alkyl esters with cationic, anionic, and neutral micelles to elucidate the balance between ionic and dispersion forces operative in these systems using steady-state and time-resolved optical spectroscopies. The strongest intermolecular interactions with the cationic probes are seen in anionic SDS micelles, where ionic and dispersion interactions are expected to both operate as attractive forces. Weaker interactions are seen between the probes and cationic CTAB micelles, owing to the repulsive ionic interactions counteracting dispersion interactions. Interactions of the probes with the neutral Thesit micelle are intermediate between results for SDS and CTAB, and we observe the general trend that as the rhodamine aliphatic ester chain is lengthened, the interaction of the chromophore with the micelle is stabilized to a greater extent.