Adsorption and micellization of gemini surfactants with pyrrolidinium head groups: effect of the spacer length

A new family of gemini surfactants with pyrrolidinium head groups, 1,1'-(alkane-1, s-diyl)bis(1-dodecyl pyrrolidinium) bromide (C-12-C-s-C12PB, s = 3, 4, 6, 8, 10, 12, 14, and 16), were synthesized. Their adsorption and micellization in aqueous solutions were investigated by various techniques such as equilibrium surface tension, fluorescence, and conductivity in detail. It was found that the effect of the spacer length on surfactant aggregation in aqueous solutions is similar to that of the well-known gemini surfactants with quaternary ammonium head groups (m-s-m), whereas the occupied limiting area per molecule (A(min)) at the surface increases linearly with s rather than going through a maximum value at a medium value of s like that of m-s-m. The thermodynamic parameters suggest that the micellization of C-12-C-s-C12PB is an entropy driven process regardless of the spacer length s. Moreover, a second breakpoint appeared in both gamma-log c and I-1/I-3-c curves of C-12-C-14-C12PB and C-12-C-16-C12PB as the surfactant concentration increases, which can be attributed to the formation of larger aggregates. The morphology of the aggregates is confirmed by employing dynamic light scattering (DLS) and cryo-transmission electron microscopy (cryo-TEM) techniques.