Structures and Properties of Newton Black Films Characterized Using Molecular Dynamics Simulations

We used molecular dynamics (MD) simulations to investigate the structures and properties of Newton black films (NBF) for several surfactants:  sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (C16TAB), and surfactin using film thicknesses up to 10 nm. By calculating the interface formation energy for various packing conditions on the surface pressure−area isotherm, we found that the most probable surface concentration is ∼42 Å2/molecule for SDS and C16TAB and ∼170 Å^2/molecule for surfactin. We then used this most probable concentration of each surfactant to simulate NBF with various film thicknesses. From analyzing the disjoining pressure−film thickness isotherms with the density profiles and the solvation coordination number, we found that the increase of the disjoining pressure during the film thinning was coupled with the change in inner structure of the NBF (i.e., density profile and the solvation of ionic entities). In the range of film thicknesses less than ∼30 Å, the disjoining pressures for the SDS and C16TAB were found to be larger than that of the surfactin. We predicted the Gibbs elasticity (175 dyn/cm for surfactin; 109 dyn/cm for C16TAB; 38 dyn/cm for SDS) required to assess the stability of NBF against surface concentration fluctuations, and the shear modulus (6.5 GPa for the surfactin; 6.1 GPa for the C16TAB; 3.5 GPa for the SDS) and the yield stress (∼0.8 GPa for surfactin; ∼0.8 GPa for C16TAB; ∼0.4 GPa for the SDS) to assess the mechanical stability against the externally imposed mechanical perturbation.