Photophysical and Photochemical Processes in Micellar Systems

TLDR

Micelles form above the critical micelle concentration, creating hydrophobic cores and charged interfaces that generate microheterogeneous environments with distinct electrostatic potentials, and functional micelles can incorporate photoreactive groups that exhibit cooperative effects upon self‑assembly. The study aims to use micellar systems to organize reactants at the molecular level, enabling comparison with homogeneous solutions to elucidate reaction pathways and conditions favoring specific mechanisms. Reaction rates and efficiencies in surfactant systems are governed by local electrostatic potentials and reagent compartmentalization within micelles, where probe distribution statistics influence fast photochemical events.

Abstract

Abstract A fascinating feature inherent to aqueous surfactant solutions is the phenomenon of self‐organization: above a certain critical concentration (the critical micelle concentration, CMC) detergent molecules associate spontaneously to build up structural entities of colloidal dimensions called micelles. The architecture of these agglomerates is such that the interior contains the hydrophobic alkyl chain of the amphiphile while the hydrophilic head groups are located at the surface and are in contact with bulk water. In the case of ionic micelles the interface is charged giving rise to an electrical double layer and a potential difference of up to several hundred millivolts between the micellar pseudophase and water. Thus micellar systems are microheterogeneous in character: the electrostatic potential and polarity prevailing in the interior of the aggregate differ from those of the bulk aqueous phase. A particularly attractive aspect of photochemical studies in micellar systems is the possibility of organizing the reactants at a molecular level: by comparison of the data in micelles with similar data in homogeneous solution one can learn about the molecular details of a given reaction and establish which conditions favor one pathway or another. In simple surfactant systems differences in rate and efficiency of a reaction will often be controlled by local electrostatic potentials and the compartmentalization of the reagents within the surfactant aggregates. Through the latter effect the statistics of probe distribution over the micelles becomes important in controlling fast photochemical events. Functional micelles are distinguished by the fact that the surfactant molecule contains a group which itself participates in the photoprocess. These units are unique in that self‐assembly often introduces striking cooperative effects.