To explore how the solvent characteristics influence the self-aggregation of the cationic dimeric surfactant hexamethylene-1,6-bis(tetradecyldimethylammonium bromide) (14-6-14, 2Br(-)) and to obtain various energetic parameters, conductometric experiments were performed on the binary mixtures of three organic solvents--viz., 2-methoxyethanol (ME)-, acetonitrile (AN)-, and formamide (FA)- water (WR) at different temperatures ranging from 298.15 to 323.15 K. The steady state fluorescence measurements allowed us to calculate the average aggregation number (N(agg)) and the air-bulk phase surface tension helped us to correlate the solvent cohesive energy density with Gibbs energy of micellization (DeltaG(0)(m)). The results showed that, although micellization process becomes less favorable with the increase in volume % of the ME, AN and FA in the binary mixed media, the increment in the cmc below 20% (v/v) of the organic solvents is comparatively less showing the predominance of WR character in the bulk phase. Because of the decrease in Gibbs interfacial energy contribution (DeltaG(0)(interf)) to DeltaG(0)(m), the average aggregation number (N(agg)) decreased with the increase in the volume % of the organic solvent in the mixed media. As the enthalpy of micellization became more negative, the corresponding entropy change became less positive and enthalpy-entropy compensation phenomenon was observed for the micellization of 14-6-14, 2Br(-) in all the mixed media in the studied temperature range.

Micellar parameters such as critical micelle concentration (cmc), degree of counterion dissociation (R), and aggregation number (N agg ) of the cationic gemini surfactant hexamethylene-1,6-bis(tetradecyldimethylammonium bromide) (14-6-14, 2Br -) have been determined in aqueous binary mixtures of dioxane (DO), dimethylformamide (DMF), and ethylene glycol (EG) by employing conductivity and fluorescence methods. An increase in the cmc and R values and a decrease in the N agg were observed when volume fractions (φ) of the studied organic solvents were varied (0 to 0.80 for conductivity and 0 to 0.50 for fluorescence studies) at 303.15 K. The temperature dependence of the cmc values provided information about various thermodynamic parameters and its effect on the selected compositions (φ ) 0, 0.10, 0.20, 0.30, and 0.50) of the mixed media on varying the nature of the medium. The Gibbs energy of micellization (∆G 0 m ) increased with the increase in the φ of the studied organic solvents. It was also observed that the increase in the cmc with the rise in temperature was more in the media containing a higher φ of the organic solvents. † Part of the "Josef M. G. Barthel Festschrift".

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