Abstract

The adsorption behavior of mixed cationic dodecylamine (DDA)/anionic sodium oleate (NaOL) at different molar ratios at the air/water interface were investigated using molecular dynamics (MD) simulations. Some parameters such as the distribution of headgroups, carbon chains, counterions and water, as well as the headgroups-water radial distribution function, were calculated. The simulation results show that compared to pure DDA and NaOL, their mixtures are more compact and can form a tight monolayer at the air/water interface, indicating higher surface activity. The polar groups of surfactants are tilted into the liquid phase because of strong interactions with water; meanwhile, the hydrophobic carbon chains stretch towards the air phase at a smaller angle than pure surfactant. Water molecules can gather easily and aggregate around the headgroups in the mixed surfactants system. The order of the surface activity is as follows: DDA/NaOL=1/3>DDA/NaOL=1/1> DDA/NaOL=3/1. These results demonstrate a strong synergistic interaction between DDA and NaOL at the air/water interface.