Surface and aggregation properties of heterogemini surfactants containing quaternary ammonium and guanidine moiety

A novel gemini surfactant with nonidentical hydrophilic groups containing guanidine group and quaternary salt, N,N-dimethyl-N-[3-(N′,N′-dimethyl-N″-alkylguanidiumhydrochloride)propyl]-1-alkyl ammonium chloride (diCnGQ, where n represents hydrocarbon chain lengths of 8, 10, and 12) was successfully synthesized. The adsorption and aggregation properties of diCnGQ in aqueous solution have been investigated through surface tension, conductivity, steady-state fluorescence. The critical aggregation concentration (cac) obtained from different techniques showed fairly good agreement. Surface tension measurements have been used to derive surface adsorption properties such as adsorption efficiency and effectiveness (pC20 and cac/C20), the maximum surface excess concentration (Γmax) and minimum surface area permolecule (Amin) at the air–water interface. Temperature dependent conductivity measurements have been used to obtain the degree of counterion binding (β), and the thermodynamic parameters such as standard free energy (ΔGagg), enthalpy (ΔHagg), and entropy (ΔSagg) of aggregation. The aggregation number (Nagg) for diCnGQ has been derived by using the fluorescence quenching technique. As a comparison, the heterogemini surfactants showed a lower cac and higher efficiency in lowering the surface tension than the corresponding monomeric surfactants and their equimolar mixture.