Micellization of a Series of Dissymmetric Gemini Surfactants in Aqueous Solution

Abstract

A series of dissymmetric gemini surfactants, [CmH2m+1(CH3)2N(CH2)6N(CH3)2CnH2n+1]Br2 (designated as CmC6CnBr2, 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 (ΔGmic) and the entropy of micellization (ΔSmic) 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 ΔGmic becomes more negative for greater m/n ratios, which supports the belief that the micellization will be more spontaneous and the contribution, per CH2 unit, to micellization increases as the m/n ratio increases. The calculated thermodynamic parameter |TΔSmic| is much larger than |ΔHmic|, which indicates that the micellization of the CmC6CnBr2 series is entropy-driven. However, the micropolarity of the micelle that is sensed by pyrene varies little, irrespective of the m/n ratio.