Determination of the Critical Micellar Concentration (CMC) of a Cationic Micelle from Stokes Shift Data

This is a new method for the determination of critical micellar concentration (CMC) based on the measurement of Stokes shift of a solvent-sensitive fluorescent probe located at the micellar aggregate. The variation of the Stokes shift with respect to bulk water is a result of the formation of micelles from the surfactant monomer, here, cetyl trimethyl ammonium bromide (CTAB) and is measured from the difference in the maxima of emission and excitation spectra of the probe. Earlier methods reported in literature were based on surface tension measurement and micellar catalysis. Stokes shifts can be further correlated to solvent reorganization energy (as Stokes shift is proportional to reorganization energy). The reorganization energy results from the creation of a transient solute dipole (by an exciting photon) in a solvent atmosphere. This induces a reorganization of the solvent molecules (dipoles; for nonpolar solvents there is no reorganization) about the dipole in a manner that stabilizes the energy of the system, and the energy term so involved is referred to as the reorganization energy.