Aggregation of dicationic surfactants with methyl orange in aqueous solution

The interactions of Methyl Orange (MO) with a series of gemini bis(quaternary ammonium bromide) amphiphiles (12-n-12, it = 4, 8, 12), a pyridinium-based gemini amphiphile (10p-4-p10), a bolaform amphiphile (C20Me6), and a dicationic amphiphile lacking a second alkyl tail (12-4) in aqueous solution have been investigated by means of UV-vis spectroscopy. It was observed that aggregation of surfactant and dye takes place at surfactant concentrations far below the critical micelle concentration of the individual surfactants. Aggregation was reflected by a ca. 80 nm hypsochromic shift of the pi --> pi* absorption band of MO. Precipitates formed in the aqueous solutions of surfactants and dyes were isolated and consist of a surfactant to dye molar ratio of 1:2. Krafft temperatures (TK) Of surfactant-dye salts were determined by differential scanning calorimetry (DSC) and in phase penetration experiments using optical polarizing microscopy. The solution behavior of 12-n-12 2MO surfactants as studied by optical microscopy and DSC is dependent on the spacer length: 12-12-12 2MO dissolves at T-K whereas 12-8-12 2MO forms myelins at T-K Moreover, 12-4-12 2MO does not dissolve at T < 100 <degrees>C. Similarly, 10p-4-p10 2MO does not dissolve at T < 100 <degrees>C. Myelins are observed for C20Me6 2MO and for 12-4 2MO. Surfactant-dye salts 12-n-12 2MO (n = 4, 8, 12), 10p-4-p10 2MO, C20Me6 2MO, and 12-4 2MO form vesicular structures in aqueous solutions as indicated by transmission electron microscopy. Vesicles are also formed dupon mixing aqueous solutions ofsurfactants and MO. The types of aggregates formed in aqueous solutions of 12-n-12 2MO are independent of n since vesicles are formed in aqueous solution in all cases.