Synthesis, Surface Activity Study and Molecular Dynamics Simulations of a Cationic Quaternary Ammonium Salt Surfactant

ABSTRACT A simple cationic surfactant, N,N‐dimethyl‐N‐allyl‐N‐hexadecyl ammonium bromide (DHAB), was synthesized and subsequently characterized by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy. The relationship between DHAB concentration and the surface tension of its aqueous solution was experimentally determined, with a critical micelle concentration (CMC) of 1.2 mmol/L, at which DHAB can reduce the surface tension of water to 38 mN/m. Molecular dynamics (MD) simulations revealed that DHAB molecules spontaneously migrate to the water‐air interface and form a layered structure in aqueous solution, analogous to watermelon vines crawling along the ground, which underlies the fundamental mechanism by which surfactants lower the surface tension of water. At concentrations above the CMC, DHAB molecules can further self‐assemble into micelles in the aqueous bulk phase. In‐depth analysis based on the independent gradient model based on Hirshfeld partition (IGMH) indicated that only extremely weak van der Waals interactions exist between DHAB molecules and their surrounding molecules.