Kinetics of fluorescence quenching of n(9-anthroyloxy) stearic acids by tertiary amines in non-ionic micelles of Triton X-100

Abstract

Fluorescence quenching phenomena were studied in non-ionic micelles of Triton X-100 using n(9-anthroyloxy) stearic acid probes nAS (n= 2, 6, 9, 12) and dimethylaniline (DMA) and triethylamine (TEA) as quenchers. The steady-state and transient data obtained adjusted well to a kinetic formalism deduced on the basis of a spherical micelle geometry with a reflecting boundary surface. Deviations from linearity in the experimental data were attributed to transient effects which enable the calculation of reactional distance–effective diffusion coefficient pairs (R_e–D_eff) of the order of (6–12)× 10^–8 cm (R_e) and (2–10)× 10^–7 cm² s^–1(D_eff) in these systems. In microregions of high viscosity the transient effects are clearly observed, supporting the existence of geometric confinement that increases the effective concentration of the donor (amine) thereby imposing a time dependence on these transient effects. The distances R_e increase with the decrease of [(ΔG^≠/k_BT)+ In D_eff] as expected for a charge-transfer diffusion-controlled interaction. Diffusion coefficients decrease as the fluorescence probe penetrates deeper into the micelle, pointing out to a lesser fluidity in the inner core. Estimated micro-viscosities depending whether the nAS probe is considered ‘fixed’ or ‘mobile’ inside the micelle, range from (13–43) cP or (22–71) cP.