Unraveling the Differential Aggregation of Anionic and Nonionic Monorhamnolipids at Air–Water and Oil–Water Interfaces: A Classical Molecular Dynamics Simulation Study

The molecular structure of a surfactant molecule is known to have a great effect on the interfacial properties. We employ molecular dynamics simulations for a detailed atomistic study of monolayers of the nonionic and anionic form of the most common congener of monorhamnolipids, α-rhamnopyranosyl-β-hydroxydecanoyl-β-hydroxydecanoate ((R,R)-Rha–C10-C10), at the air–water and oil–water interfaces. An atomistic-level understanding of monolayer aggregation is necessary to explain a recent experimental observation indicating that nonionic and anionic Rha–C10-C10 show surprisingly different surface area per molecule at the critical micelle concentration. Surface-pressure analysis, interface formation energy calculations, and mass density profiles of the monolayers at the air–water interface show similar properties between nonionic and anionic Rha–C10-C10 aggregation. It is found that there is a significant difference in the headgroup conformations of Rha–C10-C10 in the nonionic and anionic monolayers. Hydrogen ...