Molecular dynamics simulations of the structure–property relationships of DDA/anionic surfactant mixtures at the air/water interface

Abstract

In this study, three anionic surfactants (sodium oleate (NaOl), sodium dodecyl sulfonate (SDS) and naphthenic acid (NA)) were combined with dodecylamine (DDA) with molar ratios of 1:1 to investigate the interfacial behaviors of DDA/anionic surfactant mixtures at the air/water interface. The self-assembly processes and dynamic properties of the monolayers were then clarified by using molecular dynamics simulations to explore the effects of surfactant structure on surface activity. Compared with the pure DDA system, the mixtures that were oriented more perpendicularly at the air/water interface and had thicker hydrophilic films exhibited reduced surface tensions and improved hydrophobicity. The interactions between polar groups of the anionic surfactants and DDA reduced the electrostatic repulsions between DDA cations and increased the adsorption capacities of the mixtures at the air/water interface. The molecular dynamics simulations showed that different DDA/anionic surfactant mixtures had different surface activities. Due to the strong electronegativity of the sulfonic group, the interactions between SDS and DDA were the strongest, the amine groups showed a higher hydration degree, and the compatibility of the surfactant tails was better than those of the other anionic surfactants with DDA, which exhibited stronger synergistic effects. The simulation results were consistent with our previously reported data on flotation and surface tension, which may provide guidance for selection of collectors and foaming agents used in the flotation process.