Molecular dynamics simulation of spherical-to -threadlike micelle transition in a cationic surfactant solution

The effect of sodium salicylate (NaSal) on the spherical-to-threadlike micelle shape transition in 3-hexadecyloxy-2-hydroxy-propyl trimethyl ammonium bromide (R₁₆HTAB) solution was studied using molecular dynamics simulation. The simulations were started from a preassembled infinitely long threadlike micelle of R₁₆HTAB. By analyzing the aggregation morphologies and structural details, we find that the preassembled threadlike micelle in the absence of NaSal was unstable and assembled into a spherical micelle. While in the presence of NaSal, the threadlike micelle exhibited fluctuations but remained the threadlike shape during the long simulation run. The Sal^? ions were found to penetrate inside the micelle, which promoted the junction between the surfactant and salicylate counterion. The aromatic Sal^? ions located in the surfactant headgroup region with their phenyl groups pointing toward the interior core region of the micelle. From another simulation started with two individual spherical micelles, we found that the Sal^? ions can link the two spherical micelles into a long threadlike micelle, in accordance with a mode proposed by experimental studies. Our studies showed that the H-bonds and electrostatic interactions between the Sal^? ions and the surfactants played an important role in micellar growth and stabilising the threadlike micelle.