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Michel de Vijlder
Influence of Solubilized Azo Dyes on the C.M.C. Value of Cetyl-Trimethylammonium Bromide
The spectral changes in the visible absorption range of the two anionic azo dyes Methyl an Ethyl Orange in neutral aqueous solution, induced by the presence of Cetyl-trimethylammonium bromide (CTAB) micelles, are attributed to the partial solubilization of the dyes rather than to the formation of mixed micelles. The same phenomenon is not observed with the larger Orange IV (C6H5NHC6H4N=NC6H4SO3−Na+).
The c.m.c.CTAB values in the presence of dyes were obtained from spectrophotometric as well as from surface tension measurements. The reduction of the electrostatic forces within the micelles due to the presence of dye counterions initially provokes the expected progressive lowering of the c.m.c.CTAB values. In a higher dye concentration range, however, appears a systematical increase denoting a renewed strengthening of the interactions between dye and surfactant monomers that also prevail below the c.m.c. (Buwalda et al.[1]). This assumption is supported by the observed corresponding decrease of the values for the association constant which characterize the interactions existing between the dye monomers and the surfactant micelles.
At fixed dye concentrations, increasing amounts of a strong electrolyte (NaBr) lower the c.m.c.CTAB in accordance with the requirements of the Corrin-Harkins relation.
Partition coefficient values have been estimated from the spectrophotometric data by using the relation proposed by Kawamura et al.[2]. They depend on the nature and on the concentration of the solubilizate as well as on the ionic strength of the solution but their order of magnitude invariably evidences the importance of the solubilization process.
Zeitschrift für Physikalische Chemie, Oldenbourg Wissenschaftsverlag
Print ISSN: 0942-9352
Volume: 217, 06/2003
Pages: 653