Kinetics of the interactions between dyes and micelles

Abstract

The rates of association of three azo dyes, methyl red (MR), methyl orange (MO), and pyridine-2-azo-p-dimethylaniline (PADA), with sodium dodecylsulfate (SDS) and cetyltrimethylammonium bromide (CTAB) micelles in water were measured at 25 °C over a wide pH range by the continuous flow method of integrating observation (CFMIO). The association was considered in 3 steps: bulk solution encounter, pH jump, and electrostatic and hydrophobic interactions. Either the first or third step is rate limiting depending on the charge of the dye. The rate constant of association of neutral PADA, MR, and MO with SDS micelles is approximately 2 × 106 dm3mol−1s−1 while charging the dyes positively increases k to 109 dm3mol−1 s−1. Increasing the hydrophobicity of dye or micelle increases k for the interaction with the neutral dye species. Changes in k with pH for the dye–CTAB association were less pronounced.