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 bonding interactions between the Rha-C10-C10 molecules in the monolayers is also significantly different between nonionic and anionic forms. Representative snapshots of the simulated system at different surface concentrations show the segregation of molecular aggregates from the interface into the bulk water in the anionic Rha-C10-C10 monolayer at higher concentrations, whereas in the nonionic Rha-C10-C10 monolayer, the molecules are still located at the interface. The present work provides insight into the different aggregation properties of nonionic and anionic Rha-C10-C10 at the air-water interface. Further analyses were carried out to understand the aggregation behavior of nonionic and anionic Rha-C10-C10 at the oil-water interface. It is observed that the presence of oil molecules does not significantly influence the aggregation properties of Rha-C10-C10 as compared to those of the air-water interface.