title: Role of electrostatic and hydrophobic forces in the interaction of ionic dyes with charged micelles.
abstract: The nature and strength of the interactions between a cationic fluorophore, Rhodamine 123 (R123), and surfactants of different head charge are investigated. Series of absorption and fluorescence emission spectra and of fluorescence decays are measured. R123 does not interact with the monomers of both nonionic and cationic surfactants but it presents affinity to their micelles. A partition equilibrium model was proposed and the corresponding equilibrium constants were obtained, as well as the photophysical properties of the dye bound to the micelles. In the case of the cationic surfactants, changes of the fluorescence properties were already observed below the critical micellar concentration (CMC) due to dynamic quenching caused by the free counterions. In the presence of anionic surfactants R123 shows a very different behaviour with dramatic spectral changes below the CMC. The observed variations are attributed to a first, strong interaction between R123 and the surfactant monomers, which yields ionic pairs of singular photophysical properties and dominates at low surfactant concentrations, followed by the association of R123 with the surfactant premicellar and micellar aggregates at higher surfactant concentrations near the CMC. This behaviour results from the competition between the strong electrostatic interactions of the cationic dye with the anionic surfactant head groups and the hydrophobic forces stabilizing the dye inside the micelles. The results of this work illustrate the complex physicochemical and photophysical behaviour of a charged dye in micellar systems, which resembles the expected situation in similar systems such as biological membranes.
authors.affiliation: Departamento de Química Física, Facultade de Ciencias, Universidade de Santiago de Compostela, E-27002, Lugo, Spain.
authors.forename: Sonia
authors.initials: S
authors.lastname: Freire
authors.forename: Jorge
authors.initials: J
authors.lastname: Bordello
authors.forename: Daniel
authors.initials: D
authors.lastname: Granadero
authors.forename: Wajih
authors.initials: W
authors.lastname: Al-Soufi
authors.forename: Mercedes
authors.initials: M
authors.lastname: Novo
mesh_terms: D005456:Fluorescent Dyes / Q000737:chemistry; D057927:Hydrophobic and Hydrophilic Interactions; D007477:Ions; D008823:Micelles; D015394:Molecular Structure; D020112:Rhodamine 123 / Q000737:chemistry*; D013501:Surface-Active Agents / Q000737:chemistry*
chemical_list: D005456:Fluorescent Dyes; D007477:Ions; D008823:Micelles; D013501:Surface-Active Agents; D020112:Rhodamine 123