Microstructrues of aqueous sodium chloride solution at different concentrations have been investigated by molecular dynamics simulation at 298 K. Concentration has little effect on the short- range hydration structure of the ions. Two kinds of association modes between Na^+ and Cl^- are observed by simulation , i. e. , the contact ion pairs and solvent-separated ion pairs. The simulation results indicate that when constructing molecular thermodynamics models for highly concentrated electrolyte solutions, the contribution of ion association should be considered.

Key words: SODIUM CHLORIDE, MOLECULAR DYNAMICS, MICRO-STRUCTURE, ASSOCIATION, ELECTROLYTE