Bordeaux‐R interactions with surfactants: thermochromic investigations of proton ionisation in cationic, nonionic and anionic surfactant solutions

Abstract

Apparent thermodynamic parameters for the hydroxy proton ionisation of Bordeaux‐R were obtained in micellar solutions using the thermochromic method. The effect of addition of the surfactants CTAB, TX‐100 and SDS was investigated in buffered solutions in the pH range of 6–13. Ionisation constants in water and in the presence of surfactants were determined spectrophotometrically and the pK′ of Bordeaux‐R in water was found to be 10.90 in the absence of surfactant. However, at concentrations above critical micelle concentration, pK′ dropped to 10.14 with CTAB, increased to 11.29 with TX‐100 but was insensitive to SDS addition. Standard enthalpies and entropies of ionisation were obtained using the thermochromic method. This method, in conjunction with pK and spectral measurements and comparison with different azo dyes, has been shown to provide detailed information on the mechanism of dye–surfactant interaction.