Molecular Dynamics Simulation of Sodium Dodecyl Sulfate Micelle in Water:  Micellar Structural Characteristics and Counterion Distribution

An all-atom 5 nanosecond molecular dynamics simulation of a water-solvated micelle containing 60 sodium dodecyl sulfate monomers was performed. Structural properties such as the radius of gyration, eccentricity, micellar size, accessible surface area, dihedral angle distribution, carbon atom distribution, and the orientation of the monomers toward the micelle center of mass were evaluated. The results indicate a stable micellar system over the duration of the simulation. Evaluation of the structure and motion of the sodium counterions show (1) a long equilibration time (1 nanosecond) is required to achieve a stable distribution of counterions and (2) approximately 25% of the sodium ions are located in the first shell and 50% are located in the first two shells of the micelle during the course of the simulation. The structure of the micelle oxygen−sodium ion radial distribution function reveals two distinct peaks which divide the counterions into those close to the micelle (first shell) those far from the micelle (bulk) and those between (second shell). Finally, values of the diffusion coefficient for sodium ions followed a decreasing trend for ions in the bulk of the micellar system (D = 1.9 × 10 -5 cm2/s), ions in the second shell of the micelle (D = 1.4 × 10 -5 cm2/s), and those in the first shell of the micelle (D = 1.0 × 10 -5 cm2/s).