Synthesis, aggregation and adsorption behavior of benzyldimethylhexadecylammonium based double-chained metallosurfactants

Abstract

Benzyldimethylhexadecylammonium (BDHA) based double-chained metallosurfactants, BDHA2[MCl4] (M = Mn, Co, Ni, Cu, Zn) were synthesized and characterized by elemental analysis, NMR (1H, 13C) and FT–IR spectroscopy. Thermal gravimetric analysis showed more stability of synthesized metallosurfactants as compared to the precursor benzyldimethylhexadecylammonium chloride. Their critical micelle concentration (cmc) values were determined in aqueous medium by using surface tension, conductivity and fluorescence spectrophotometric methods. The cmc values follow the order: Mn > Co ≈ Cu ≈ Zn > Ni. The thermodynamic parameters of micellization were computed from the temperature dependence of the cmc. These metallosurfactants have surprisingly low counterion binding constant values (0.25 to 0.31) compared to benzyldimethylhexadecylammonium chloride and other ionic surfactants. Compensation of enthalpy and entropy of micellization of BDHA2[MCl4] metallosurfactants is observed in the aqueous medium. Adsorption behavior of BDHA2[MCl4] metallosurfactants was assessed by calculating surface excess and area per molecule at the air/water interface. Dynamic light scattering and zeta potential measurements revealed the small average hydrodynamic diameter of highly stable micelles of the metallosurfactants in aqueous solutions (hydrodynamic diameter = 4–6 nm with polydispersity index <0.13 and zeta potential >+45 mV). Transmission electron microscope measurements also showed the presence of small spherical micelles.