Computational Investigations of a pH-Induced Structural Transition in a CTAB Solution with Toluic Acid.

Yingbiao Xu,Hui Yan,Han Jia,Li Lv,Lingyu Zhang,Tingyi Wang

doi:10.3390/molecules26226978

Abstract

In this work, molecular dynamics simulations were performed to study the pH-induced structural transitions for a CTAB/p-toluic acid solution. Spherical and cylindrical micelles were obtained for aqueous surfactants at pH 2 and 7, respectively, which agrees well with the experimental observations. The structural properties of two different micelles were analyzed through the density distributions of components and the molecular orientations of CTA+ and toluic acid inside the micelles. It was found that the bonding interactions between CTA+ and toluic in spherical and cylindrical micelles are very different. Almost all the ionized toluic acid (PTA−) in the solution at pH 7 was solubilized into the micelles, and it was located in the CTA+ headgroups region. Additionally, the bonding between surfactant CTA+ and PTA− was very tight due to the electrostatic interactions. The PTA− that penetrated into the micelles effectively screened the electrostatic repulsion among the cationic headgroups, which is considered to be crucial for maintaining the cylindrical micellar shape. As the pH decreased, the carboxyl groups were protonated. The hydration ability of neutral carboxyl groups weakened, resulting in deeper penetration into the micelles. Meanwhile, their bonding interactions with surfactant headgroups also weakened. Accompanied by the strengthen of electrostatic repulsion among the positive headgroups, the cylindrical micelle was broken into spherical micelles. Our work provided an atomic-level insights into the mechanism of pH-induced structural transitions of a CTAB/p-toluic solution, which is expected to be useful for further understanding the aggregate behavior of mixed cationic surfactants and aromatic acids.

Highlights

Introduction iationsThe controllable self-assemblies of the amphiphilic molecules in aqueous solution are hot issues in both scientific and technological areas [1,2]
Adding certain amounts of simple inorganic ions or aromatic anions into cationic surfactant solutions can lead the formation of long rod-like or wormlike micelles at a lower surfactant concentration [9,10,11,12,13]
We studied the structural transitions of a typical cationic surfactant/additive micelle solution induced by pH variation

Summary

Introduction

Introduction iationsThe controllable self-assemblies of the amphiphilic molecules in aqueous solution are hot issues in both scientific and technological areas [1,2]. The size and shape of the surfactant assembly mainly depend on the chemical structures of the surfactants, such as the lengths of the hydrocarbon chains, properties of the polar headgroups, and the counter ions [3,4,5,6]. Surfactants in solution form spherical micelles spontaneously above the critical micelle concentration (CMC) [7,8,9]. With a further increase in surfactant concentration, the spherical micelles may grow into rod-like or wormlike micelles, and even vesicles. Adding certain amounts of simple inorganic ions (such as Cl− and Br− ) or aromatic anions (such as salicylate and benzoic acid) into cationic surfactant solutions can lead the formation of long rod-like or wormlike micelles at a lower surfactant concentration [9,10,11,12,13]

Objectives

Results

Conclusion