Molecular insights into the motion of oil droplets in aqueous solutions of ester- and amide-containing cationic surfactants

Abstract

• Different motility of oil droplets in solutions of ester- and amide-type surfactants. • Ester and amide linkages in surfactants modulate the interfacial tension. • Ester-type surfactants have a higher affinity for oil due to dispersion forces. • Amide-type surfactants exhibit slower adsorption and weaker interactions with oil. • Molecular-level interactions are key to understanding macroscopic motility. The study of self-propelled motion in soft matter systems has garnered significant interest owing to its potential applications in microfluidics, soft robotics, and autonomous system design. Understanding the molecular mechanisms underlying motility is crucial for advancing these applications. This study investigates the self-propelled motion of lauronitrile oil droplets in aqueous surfactant solutions, focusing on the impact of different surfactant molecular structures on droplet dynamics. This study compares surfactants with ester and amide linkages, highlighting their critical role in modulating interfacial tension and driving Marangoni convection, a key factor behind droplet movement. Surfactants with ester linkages exhibit a high affinity for lauronitrile and rapidly adsorb at the oil–water interface, generating strong Marangoni flows and driving fast droplet motion. In contrast, amide-containing surfactants exhibit slower adsorption and weaker interactions with lauronitrile, leading to reduced or absent motion. These findings provide new insights into the molecular mechanisms underlying the self-propelled droplet behavior in non-equilibrium systems and contribute to a deeper understanding of self-organizing phenomena.