Effect of head group polarity and spacer chain length on the aggregation properties of gemini surfactants in an aquatic environment

The aggregation behavior of cationic gemini surfactants with respect to variation in head group polarity and spacer length 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 the air-water interface (Amin), surface excess concentration (Γmax), and Gibb's free energy of micellization (ΔGmic) of the surfactants were determined from conductance and surface tension data. The aggregation numbers (N), dimensions of micelles (b/a), effective fractional charge per monomer (α), and hydration of micelles (hE) were determined from SANS and viscosity data, respectively. The increasing head group polarity of gemini surfactant with spacer chain length of 4 methylene units promotes micellar growth, leading to a decrease in cmc, βave, and ΔGmic and an increase in N and b/a. This is well supported by the observed increase in hydration (hE) of micelles with increase in aggregation number (N) and dimension (b/a) of micelle.