Thermodynamic and Aggregation Properties of Gemini Surfactants with Hydroxyl Substituted Spacers in Aqueous Solution

Abstract

Critical micelle concentration (cmc), degree of micelle ionization (α), headgroup area (a0), mean aggregation number, enthalpy of micellization (ΔHM), and volume of micellization (ΔVφ,M) properties are reported for a series of gemini surfactants, 12−4(OH)n−12 (n = 0, 1, or 2), with increasing hydroxyl substitution within the spacer group. The cmc and mean aggregation numbers are observed to decrease with increased hydroxyl substitution consistent with increasing hydrophilicity of the headgroup. Both a0 and α are unaffected by hydroxyl substitution. ΔHM° is observed to decrease (become more exothermic) with increased substitution of the spacer group, while ΔVφ,M is observed to increase. These observations are rationalized in terms of changes in the solvation and conformation of the spacer group at the micelle−bulk solution interface. Free energies of micellization, ΔGM°, and entropies of micellization, ΔSM°, are calculated from the ΔHM°, cmc, and α values. The results indicate that the micellization process for all surfactants is entropy-driven.