Elucidation of Interactions between l-Ascorbic Acid and Mixed Micellar Aggregates of Catanionic {Sodium Dodecylsulfate plus Cetyltrimethylammonium Bromide} Surfactants via Physicochemical and Spectroscopic Studies

Surfactant micelles mimic the microenvironment present in biological systems and can act as a medium for antioxidant studies. Moreover, the thermodynamic profile of micellization and spectroscopic studies provides very good information about interactions in these systems. Thus, the mixed micellar behavior of sodium dodecylsulfate (SDS) and cetyltrimethylammonium bromide (CTAB) at varying mole fractions of SDS was studied in (0.01, 0.02, and 0.03) mol kg(-1) -ascorbic acid((aq)) solutions with the aid of various techniques viz., conductivity, density and sound velocity, and spectroscopy. From the CMC values of the mixed surfactants, the degree of ionization (beta) and thermodynamic parameters (Delta Gm degrees, Delta Hm degrees, and Delta Sm degrees) were evaluated at 298.15, 308.15, and 318.15 K. The UV absorption spectra were recorded in (1-3) x 10(-4) mol kg(-1) -ascorbic acid((aq)) solutions at various mole fractions of SDS. The proton (H-1) NMR spectra of mixed (SDS + CTAB) surfactants were studied in (0.01-0.03) mol kg(-1) -ascorbic acid solutions. Hydrodynamic diameters (D-h) of mixed micellar aggregates were obtained from the dynamic light scattering (DLS) studies. The present studies suggest the predominance of ionic-hydrophilic interactions between the ionic head groups {O-SO3- or N+ (CH3)(3)} of surfactants and the polar (-OH, -C=O and -O-) sites of -ascorbic acid.