Molecular insight into photoresponsive surfactant regulated reversible emulsification and demulsification processes

Photoresponsive emulsions have attracted growing attention and exhibited enormous potential in a wide range of applications related to their green and efficient characteristics. Herein, molecular dynamics simulations were performed to elaborate the reversible emulsification/demulsification processes regulated by a mixture of an anionic surfactant sodium dodecyl sulfate (SDS), and the photoresponsive cationic surfactant 2-(4-(4-butylphenyl)diazenylphenoxy) ethyltrirnethylammonium bromide (AzoTAB). The simulation results indicate that the oil/water interface dominated by trans-AzoTAB molecules was more stable than that dominated by cis-AzoTAB molecules at specific concentrations, during which emulsification/demulsification can be reversibly regulated by the trans-cis isomerization reaction. The van Hove function results suggest that the cis-AzoTAB molecules possessed a faster diffusion rate than the trans isomer, contributing to the mutual dissolution of the oil and water phases. Furthermore, the role of SDS in regulating emulsification/demulsification processes was revealed. The addition of a small amount of SDS inhibited the desorption of AzoTAB from the oil/water interface and highlighted the influence of the structural difference between trans- and cis-AzoTAB molecules on interfacial stability. This work provides a molecular understanding of the interfacial stability and fluidity of photoresponsive emulsions, which are fundamental principles for the rational design of emulsions and other dispersion systems. (C) 2021 Elsevier B.V. All rights reserved.