Investigations of the dynamic behaviour of counterions of anionic micellar systems by fluorescence quenching experiments

Abstract

The mixing of sodium and nickel lauryl sulphate [SLS and Ni(LS)₂, respectively] has been investigated using 2-phenylbenzoxazole (1) as a fluorescence probe. Measurements of the fluorescence quenching efficiency above and below the c.m.c. of SLS/or Ni(LS)₂ and analysis of the dependence of the absorption and fluorescence spectra upon solvent polarity lead to the conclusion that the probe is solubilized at the micellar interface and can be used to monitor Ni²⁺ counterions at the solubilization site. A Stern–Volmer analysis yields non-linear plots reaching a limit of saturation and reflecting the strong adsorption of Ni²⁺ to the micellar system. Repeating the quenching experiment at constant ionic strength (0.1 mol dm^–3) reduces the fluorescence efficiency from 72% to 31% and gives a linear plot. The rate constants of fluorescence quenching have been found to be 4.2 × 10¹⁰ dm³ mol^–1 s^–1 when NiSO₄ is added to SLS micelles and 4.7 × 10¹⁰ dm³ mol^–1 s^–1 when Ni(LS)₂ is mixed with SLS. These values are in good agreement with the theoretical diffusion rate constant (3.3 × 10¹⁰ dm³ mol^–1 s^–1) obtained from model calculations using the Debye–Smoluchowski equation. The results obtained fit an intramicellar static quenching process allowing evaluation of the association constant of the Ni²⁺ counterions with lauryl sulphate micelles.