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 ( a 0 ), mean aggregation number, enthalpy of micellization (Δ H M ), and volume of micellization (Δ V φ,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 a 0 and α are unaffected by hydroxyl substitution. Δ H M ° is observed to decrease (become more exothermic) with increased substitution of the spacer group, while Δ V φ,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, Δ G M °, and entropies of micellization, Δ S M °, are calculated from the Δ H M °, cmc, and α values. The results indicate that the micellization process for all surfactants is entropy-driven.