Surface properties and aggregation behavior of cationic gemini surfactants with dipropylammonium head-groups

A series of cationic gemini surfactants, butane-α,δ-bis-(dipropyl alkyl ammonium bromide), [CnH2n+1(CH3CH2CH2)2N(CH2)4N(CH3CH2CH2)2CnH2n+1]Br2 (where n is the tail chain length, n = 10, 12, 14, and 16), referred to as CnC4Cn(Pr) were synthesized. The surface properties and aggregation behavior of these surfactants were investigated by measuring their surface tension, electrical conductivity, fluorescence, dynamic light scattering (DLS), and transmission electron microscopy (TEM). These gemini surfactants CnC4Cn(Pr) have a lower critical micelle concentration (CMC) than their corresponding monomer surfactants. Solution electric conductivity measurements indicated that premicellar aggregations were found in the C16C4C16(Pr) solution. Steady-state fluorescence measurements revealed that the C16C4C16(Pr) surfactant formed noncompact aggregate structures in solution over a wide concentration range. Interestingly, dynamic light scattering and transmission electron microscopy studies showed that the aggregations of the four surfactants were vesicle, which is not in accordance with the critical packing parameter (P) values obtained from Tanford's equation. To explain this unusual phenomenon, the volume of the component groups was used to calculate the hydrophobic volume and it was assumed that this phenomenon probably resulted from the face-to-face hydrophobic chains of these gemini surfactant molecules were inclined to be interdigitized in the aggregate.