Molecular insights into the pH-induced self-assembly of CTAB/PPA system

Abstract

Self-assembly systems of stimuli-responsive molecules possess the capability of altering its morphologies and properties when receiving external signals (variations in pH value, temperature, light, etc), which has generated emerging applications, and therefore, attracted great interests. However, revealing mechanisms of stimuli-induced self-assembly morphology transition is still in its infancy and experimentally challenging. In this work, we studied the pH-induced self-assembly of cetyltrimethylammonium bromide (CTAB) and potassium phthalic acid (PPA) mixtures by coarse-grained (CG) molecular dynamics (MD) simulation, aiming to give insights into the pH-responsive self-assembly behavior. We simulated the self-assembly of CTAB/PPA mixtures, and obtained spherical, rod-like, and threadlike micelles depending on pH value and PPA/CTAB ratio. Besides, we found that the length of rod-like micelles fluctuated with the increase of PPA/CTAB ratio, and that the self-assembling morphology shifted between rod-like and threadlike micelles when fixing the PPA/CTAB ratio at 4:6 and varying the pH value. With analyzing the net charge number around the CTAB micelles, the arrangement and distribution of CTAB and PPA molecules in micelles, we concluded that the electrostatic shielding and space location-obstruct of PPA play a key role in the pH-induced self-assembly. This work revealed molecular-level information and mechanisms of the pH-induced self-assembly of CTAB/PPA mixtures, which is expected to trigger further understanding and studies on pH-responsive self-assembly behavior.