Importance of head group polarity in controlling aggregation properties of cationic gemini surfactants

Cationic gemini surfactants have been extensively studied in the recent past and the effect of chain length, spacer length and nature on aggregation behavior has been examined. But the effect of variation in head group polarity on micellization has not been examined. Hence, the effect of head group polarity of the butane-1,4-bis(dodecyldimethylammonium bromide) surfactants on aggregation properties is studied through conductance, surface tension, viscosity, and small-angle neutron scattering (SANS) measurements. The critical micellar concentration (cmc), average degree of micelle ionization (βave), minimum area per molecule of surfactant at air–water interface (Amin), surface excess concentration (Γmax) and Gibbs free energy change of micellization (ΔG°mic) of the surfactants were determined from conductance and surface tension data. The aggregation numbers (N), dimension of micelle (b/a), effective fractional charge per monomer (α) were determined from SANS and hydration of micelle (hm) from viscosity data. The increasing head group polarity of gemini surfactant having spacer chain length of 4 methylene units promotes micellar growth, leading to decrease in cmc, βave, ΔG°mic and increase in N and b/a. This is well supported by the observed increase in hydration (hm) of micelle with increase in aggregation number (N) and dimension (b/a) of micelle. The Kraft temperature (kT), foamability and foam stability as a function of head group polarity of gemini surfactants were also examined.