Abstract: In this paper, we used molecular dynamic simulation method to study the diffusion process and diffusion coefficient of small molecules in reverse osmosis membrane, and made an analysis and discussion of relationship between the membranous structure monomer and the diffusion coefficients of small molecules in membrane. Among the eight kinds of membranes, diffusion coefficent of water changed obviously in different membranes and the changing direction was in accordance with experimental values. In eight kinds of membranes we simulated, the diffusion coefficient value of Na⁺ and Cl^- had much difference. The movement of NaCl molecule was restricted by the ions which moves slowly. Diffusion process of salt molecule in the membrane was only restricted by the membrane species, without relationship with salt molecules itself. Movement of molecule of water was not affected by the kinds of salt molecules in the same membrane.

Key words: Molecular simulation, Reverse osmosis membrane, Diffusion coefficient, Membrane material