Simulation of Dynamic Behavior of Surfactants on a Hydrophobic Surface Using Periodic-Shell Boundary Molecular Dynamics

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  • Minami Daiki
    Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science
  • Horikoshi Satoshi
    Research Institute for Science and Technology, Tokyo University of Science
  • Sakai Kenichi
    Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science Research Institute for Science and Technology, Tokyo University of Science
  • Sakai Hideki
    Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science Research Institute for Science and Technology, Tokyo University of Science
  • Abe Masahiko
    Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science Research Institute for Science and Technology, Tokyo University of Science

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Abstract

The adsorption and aggregation behaviors of sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) on a hydrophobic graphite surface were examined using a novel molecular dynamics (MD) simulation with the periodic-shell boundary condition (PSBC). Differences in the adsorption behavior of SDS and CTAB molecules were clearly shown on the hydrophobic surface. Unexpectedly, the SDS molecules approached the graphite surface with their hydrophilic head groups. This unexpected approach mode was thought to be due to the aqueous layer on the graphite surface. The hydrophobic moiety of SDS molecules repeatedly adsorbed and desorbed on the graphite surface. In addition, SDS molecules kept moving on the graphite surface; thus, they did not form a stable adsorption layer. In contrast to SDS, the hydrophobic moiety of CTAB molecules approached the graphite surface at the primary step of adsorption. The hydrophobic moieties of CTAB molecules came close to each other, whereas the hydrophilic groups separated from one another. This result suggests that the CTAB molecules form molecular assemblies with a curved structure. The simulation results were consistent with the experimental observations. A clear difference between the adsorption behavior of SDS and CTAB molecules was revealed by MD simulations with PSBC.

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