Effects of unsaturated double bonds on adsorption and aggregation behaviors of amide-based cationic Gemini surfactants

Abstract

Four cationic amide Gemini surfactants, containing unsaturated and saturated hydrophobic chains, were synthesized from oleamide propyl dimethylamine (PKO-18:1) and stearate amide propyl dimethylamine(PKO-18). The chemical structures of the synthesized compounds were characterized by FT-IR spectra and 1H NMR. The Krafft temperatures were experimentally determined.. Their critical micelle concentration(CMC) in the aqueous solutions were determined by surface tension and conductivity methods. Equilibrium surface tension parameters such as γcmc, pC20, Гmax, Amin, △G°mic(Gibbs free energy of micellization), △G°ads(Gibbs free energy of adsorption) were obtained. Their adsorption behavior was analyzed through dynamic surface tension. Also, their self-assembly behaviors were investigated by negative-stained transmission electron microscopy(TEM) and critical packing parameter calculation. The results showed that the unsaturated double bond in hydrophobic chains was favorable for lowering the Krafft temperature and CMC. The oleamide-based Gemini surfactants present a lower Krafft temperature, higher surface activity and the adsorption kinetic process were conformed mixed diffusion-kinetic control model. The TEM results showed that the oleamide-based Gemini surfactants containing unsaturated double bonds in hydrophobic chains preferentially form worm-like micelles in solution. However, the stearate amide Gemini surfactants whose hydrophobic chains are saturated alkyl chains tend to form spherical aggregates in solution.