Kinetics and Mechanism of the Alkaline Fading of Brilliant Green in Aqueous Solutions of a Double-tailed and Some Single-tailed Cationic Surfactants

Owoyomi Olanrewaju,* Jide Ige, Oladega Soriyan, Ogunlusi Grace, Olaseni Segun Esan and Oladimeji Olanrewaju

Department of Chemistry, Obafemi Awolowo University, Ile Ife, Nigeria
Tel.: +234803-5066-940,
E-mail: oowoyomi@oauife.edu.ng

Abstract
The alkaline fading of Brilliant green (a triphenylmethane dye) was studied in aqueous solutions of dodecyltrimethylammonium bromide (DTAB), didodecyldimethylammonium bromide (DDAB) and cetyltrimethylammonium bromide (CTAB) at 25 °C. The pseudo-first order rate constant variation with surfactant concentration shows a sigmoid shaped curve, which is analogous to the positive cooperativity in enzymatic reactions. The catalytic factors k_ψ / k_w (the ratio of the rate constant in presence of surfactant additives to that in the pure water) of 1.8, 11.9 and 30.4 were obtained for DDAB, DTAB and CTAB respectively. The kinetic analysis shows that the bifurcated surfactant does not affect the observed rate appreciably as compared with the single-tailed surfactants. This is consistent with the length of the carbon chain being the dominant influence on the rate enhancement. The data obtained fit the Pieszkiewicz model with the value of the cooperativity index n greater than unity in all cases, but less than the number of surfactant molecules found in micelle, which indicates the presence of pre-micelle aggregates.

Keywords: Alkaline fading; brilliant green; cationic surfactants.