Thermodynamic and aggregation properties of gemini surfactants with hydroxyl substituted spacers in aqueous solution

Critical micelle concentration (cmc), degree of micelle ionization (alpha), headgroup area (a(0)), mean aggregation number, enthalpy of micellization (DeltaH(M)), and volume of micellization (DeltaV(phi,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 alpha are unaffected by hydroxyl substitution. DeltaH(M)degrees is observed to decrease (become more exothermic) with increased substitution of the spacer group, while DeltaV(phi,M) is observed to increase. These observations are rationalized in tern-is of changes in the solvation and conformation of the spacer group at the micelle-bulk solution interface. Free energies of micellization, DeltaG(M)degrees, and entropies of micellization, DeltaS(M)degrees, are calculated from the DeltaH(M)degrees, cmc, and alpha values. The results indicate that the micellization process for all surfactants is entropy-driven.