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

Area of Science:

• Physical Chemistry
• Supramolecular Chemistry
• Materials Science

Background:

• Aqueous mixtures of cationic surfactants like cetyltrimethylammonium bromide (CTAB) and hydrotropes such as sodium salicylate (NaSal) exhibit complex phase behavior.
• Understanding the structural transitions of these aggregates is crucial for applications in formulation and materials science.
• Previous experimental studies have observed pH-dependent micelle formation in CTAB/NaSal systems.

Purpose of the Study:

• To investigate the pH-induced structural transitions of aqueous CTAB/NaSal mixtures using molecular dynamics simulations.
• To elucidate the molecular mechanisms underlying the formation of different micellar structures at varying pH values.
• To provide atomic-level insights into the role of salicylate protonation and ion interactions in micelle shape changes.

Main Methods:

• Molecular dynamics (MD) simulations were performed on aqueous mixtures of CTAB and NaSal at different pH conditions (pH 7, 2, and 0).
• Analysis of micellar structural properties, including the distribution and molecular orientation of surfactant (CTA+) and hydrotrope (Sal-) species.
• Investigation of hydrogen bonding, electrostatic interactions, and cation-π interactions within the micelles.

Main Results:

• Simulations reproduced experimental observations of rigid cylindrical micelles at pH 7, spherical micelles at pH 2, and flexible cylindrical micelles at pH 0.
• Protonation of salicylate at lower pH alters its binding interactions with CTAB, weakening electrostatic attraction and promoting headgroup repulsion, leading to spherical micelles.
• At pH 0, strengthened cation-π interactions between salicylate and CTAB, along with chloride ion association, facilitate the formation of flexible cylindrical micelles.

Conclusions:

• The study provides a detailed atomic-level understanding of pH-induced micelle shape transitions in CTAB/NaSal systems.
• Salicylate protonation and its subsequent interaction changes with CTAB and water are key drivers of structural transitions.
• The findings highlight the importance of hydrotrope-surfactant binding and counterion effects in controlling aggregate morphology.