Aggregation of azo dyes with cationic amphiphiles at low concentrations inaqueous 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 amphiphi les (C(m)pyI, m = 8, 10, 12, and 14) induced by low concentrations of azo d yes in aqueous solution has been investigated by means of ultraviolet (UV) spectroscopy. It was observed that aggregation takes place at surfactant co ncentrations far below the cmc of C(12)TAB, C(14)TAB, C(16)TAB, and C(18)TA B with methyl orange (MO), ethyl orange (EO), and para-methyl red (pMR). Ag gregation 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 agg regation. Interactions at low surfactant concentrations have also been obse rved in aqueous solutions of C(10)pyI, C(12)pyI, and C(14)pyI with MO. Bind ing of methyl red (MR), methyl yellow (MY), and azobenzene sulfonate (ABS) with cationic surfactants below their cmc did not occur. Aggregation was re flected by the appearance of a blue-shifted absorption band in the spectra of the dyes. Precipitates formed in aqueous solutions from the cationic sur factants 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 experiment s with EO, MR, MY, or ABS as the solvatochromic dye molecules, no precipita tion occurred, although surfactant-EO salts precipitated at high EO concent ration. Dye-surfactant salts formed from C(n)TAB and MO were found to form myelins in phase penetration experiments. Temperatures at which myelins sta rt 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 pres ence and position of the ionic group in the dye molecule is important in de termining the association. The importance of hydrophobic interactions is re vealed by the chain length dependence on the aggregation process and the ob servation that interactions are absent in ethanol. Electrostatic interactio ns also play an important role, as shown by the effect of NaCl on the bindi ng process.