Molecular Dynamics Simulation of the pH-Induced Structural Transitions in CTAB/NaSal Solution.

We performed molecular dynamics simulations to study the pH-induced structural transitions for aqueous mixtures of a cationic surfactant (cetyltrimethylammonium bromide, CTAB) and a hydrotrope (sodium salicylate, NaSal). We obtained rigid cylindrical, spherical, and flexible cylindrical micelles at pH 7, 2, and 0, respectively, which agrees well with the experimental results of Umeasiegbu et al. (Langmuir 2016, 32, 655). By analyzing the different micellar structural properties, including distribution and molecular orientation of CTA and Sal inside the micelle, we found that the binding form of the protonated salicylate molecules with CTA is different from that of Sal ions. Because of the protonation of salicylate molecules with reduction in pH, their hydrogen bonding interactions with water molecules strengthened and the electrostatic interactions with CTA headgroups weakened. Thus, the repulsion of the CTA headgroups led to the breakage of the cylindrical micelle into spherical ones. At pH 0, the H-bond-strengthened cation-π interactions between salicylate and CTA were verified. We concluded that the penetration of salicylate molecules inside the micelle and the strong association of Cl ions on the micellar surface play a key role in the formation of a flexible cylindrical micelle. This work provides an atomic-level insight into the mechanism of pH-induced shape transitions in the CTAB/NaSal systems, which is expected to be helpful to understand the aggregate behavior of cationic surfactant-hydrotrope solution.