A Study of the Solvent Properties near the Surface of a Surfactant Micelle by Merocyanine Dye

Abstract

Absorption spectra of a merocyanine dye have been measured in aqueous solutions of anionic (NaC12S), cationic(C12TAC), and nonionic surfactant(C12(EO)7)under various conditions, and in the mixed solvents of water and organic solvents with different polarites. The spectrum of the merocyanine dye is highly sensitive to changes in the polarity of solvents aad, therefore, can be used as a sensitive probe toestimate. the polarity of solvent near the surface of surfactant micelle.
The transition energies, ET, and apparent dissociation constants, K, of the dye in micellar solutions were calculated from changes in the absorption maxima of the spectm. The difference between K and K (intrlnsic dissociation constant at the micell surface) has been interpreted in terms of the electrostatic potential ψ at the micelle surface where the dye is located. The values of ψ are -157, 33, and 12 mV for NaC12S, C12TAC, and C12(EO)7, respectively. The "effective" dielectric constants ε of the solvents near the micelle surfaces, estimated from the ε vs. ET plot(Fig.7), are 31, 72, and 78 for NaC12S, C12TAC, and C12(EO)7, respectively. These dielectric constants are in good agreement with the values expected from the Booth's theory of dielectric saturation. In addition, the mechanism of color changes ia micellar solutions has been discussed.