Micellization of a Series of Dissymmetric Gemini Surfactants in Aqueous Solution

A series of dissymmetric gemini surfactants, [C m H 2 m +1 (CH 3 ) 2 N(CH 2 ) 6 N(CH 3 ) 2 C n H 2 n +1 ]Br 2 (designated as C m C 6 C n Br 2, with constant m + n = 24 and m = 12, 13, 14, 16, 18), have been investigated by electrical conductivity measurement, steady-state fluorescence measurement, and time-resolved fluorescence quenching. The critical micelle concentration (CMC) and the micelle aggregation number ( N ) were determined and the micropolarity of micelle was characterized. The micelle ionization degree (α) was obtained using a combination of electrical conductivity data and N . Furthermore, the Gibbs free energy of micellization (Δ G mic ) and the entropy of micellization (Δ S mic ) were studied. The results have shown that the degree of dissymmetry ( m / n ) has an important effect on the micellization in aqueous solution. As the m / n ratio increases, the CMC value decreases linearly, the N value at the CMC increases slightly (from 22 to 30), and α decreases slightly down the series. The value of Δ G mic becomes more negative for greater m / n ratios, which supports the belief that the micellization will be more spontaneous and the contribution, per CH 2 unit, to micellization increases as the m / n ratio increases. The calculated thermodynamic parameter | T Δ S mic | is much larger than |Δ H mic |, which indicates that the micellization of the C m C 6 C n Br 2 series is entropy-driven. However, the micropolarity of the micelle that is sensed by pyrene varies little, irrespective of the m/n ratio.