Aggregation of azo dyes with cationic amphiphiles at low concentrations in aqueous solution

The aggregation of a series of n-alkyltrimethylammonium bromide (C(n)TAB, n = 10, 12, 14, 16, and 18) and 4-n-alkyl-1-methylpyridinium iodide amphiphiles (C(m)pyI, m = 8, 10, 12, and 14) induced by low concentrations of azo dyes in aqueous solution has been investigated by means of ultraviolet (UV) spectroscopy. It was observed that aggregation takes place at surfactant concentrations far below the cmc of C(12)TAB, C(14)TAB, C(16)TAB, and C(18)TAB with methyl orange (MO), ethyl orange (EO), and para-methyl red (pMR). Aggregation below the cmc of C(10)TAB was also induced by EO. In the case of MO and pMR, however, higher dye concentrations were necessary to induce aggregation. Interactions at low surfactant concentrations have also been observed in aqueous solutions of C(10)pyI, C(12)pyI, and C(14)pyI with MO. Binding of methyl red (MR), methyl yellow (MY), and azobenzene sulfonate (ABS) with cationic surfactants below their cmc did not occur. Aggregation was reflected by the appearance of a blue-shifted absorption band in the spectra of the dyes. Precipitates formed in aqueous solutions from the cationic surfactants and IMO and pMR are C(n)TA-MO, C(n)TA-pMR, and C(m)py-MO salts and consist of an equimolar ratio of surfactant and dye. In similar experiments with EO, MR, MY, or ABS as the solvatochromic dye molecules, no precipitation occurred, although surfactant-EO salts precipitated at high EO concentration. Dye-surfactant salts formed from C(n)TAB and MO were found to form myelins in phase penetration experiments. Temperatures at which myelins start to form increase upon increasing alkyl chain length of the surfactant as revealed by optical microscopy. The formation of vesicles from C(10)TA-MO and from C(12)TA-MO crystals was indicated by electron microscopy. The presence and position of the ionic group in the dye molecule is important in determining the association. The importance of hydrophobic interactions is revealed by the chain length dependence on the aggregation process and the observation that interactions are absent in ethanol. Electrostatic interactions also play an important role, as shown by the effect of NaCl on the binding process.