Solubilization of disperse dyes in cationic gemini surfactant micelles

The solubilization of three disperse dyes, 1-(4-nitrophenylazo)-4-N,N-diethanolaminobenzene (NPDEAB), p-aminoazobenzene (4-PAA) and 1,4-diaminoanthraquinone (1,4-DAA) into the micelles formed by two cationic gemini surfactants, propanediyl-alpha,omega-bis(dimethyldodecylammonium bromide) (DC3 - 12) and hexanediyl-alpha, omega-bis(dimethyldodecylammonium bromide) (DC6-12), was investigated and compared with that achieved using two conventional surfactants, dodecyltrimethylammonium bromide (C12C1NBr), and ethyldodecyldimethylammonium bromide (C12C2NBr). The solubilization power of the DC6-12 micelle was greater than that of the DC3-12 micelle, suggesting that the gemini surfactant micelle, which has a longer chain, enables greater solubilization power. The solubilization power of the gemini surfactant micelles was greater than that of the corresponding conventional surfactant micelles for NPDEAB and 1,4-DAA, while the opposite result was obtained for 4-PAA. The amounts of dye solubilized in the micelles increased in the order of NPDEAB < 1,4-DAA < 4-PAA, indicating that smaller dye molecules are solubilized more readily in micellar solutions. Thus, solubilization power is strongly dependent not only on the nature of the surfactant micelles but also on the structure of the solubilized dye molecules. The aggregation number of the surfactant micelles was determined using steady state fluorescence quenching. Although the aggregation number was different for the gemini and the conventional surfactant micelles, the number of the alkyl tail groups included in the gemini micelle was similar to that in a conventional one. The solubilization capacity was calculated from the solubilization power and the aggregation number. The solubilization capacity was significantly dependent on the dye structure. The visible absorption spectra of the disperse dyes revealed that the dye solubilized was not located in the inner part of the micelle core but in the outer regions. (C) 2000 Elsevier Science Ltd. All rights reserved.