The aggregation structure of surfactant 4-(5 -16 alkyl) benzene sulfonate (5C16) at the gas/liquid surface was investigated by molecular dynamics simulation. The simulated results showed that with the addition of CaCl₂ to the solution, Ca^2＋ will replace the position of Na^＋ and bind with the polar head at the surface. In the aggregation structure, two types of angles between two alkyl chains of surfactant molecule were found, and the ratio of angles changes from 1.9∶1 to 4.0∶1 with the addition of CaCl₂. The mean square displacement data show that there are different types of water around the polar head, and water molecules around the headgroup have weak diffusion.

Key words: 4-(5 -16 alkyl) benzene sulfonate, air/liquid surface, molecular dynamics simulation, Ca^2＋