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

Abstract

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 Å 2 /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.

Surface tension model for surfactant solutions at the critical micelle concentration

“…The Kirkwood-Buff (KB) molecular modeling approach [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] is fully atomistic. It utilizes Kirkwood-Buff relation between σ and diagonal components of the pressure tensor [23].…”

Multiscale and Multiphysics Computational Frameworks for Nano- and Bio-Systems

“…It was also successful in our studies on various other molecular systems, such as the hydrated polymer electrolyte membranes [57,58,59] and the surfactant-mediated air-water interface [60,61].…”

Surface Interaction of Nanoscale Water Film with SDS from Computational Simulation and Film Thermodynamics

“…It has been reported [15] that, by using MD simulations, electrostatic energy could be obtained based on electrostatic potential profile, by taking into account the uniform characteristic of water. The electrostatic contribution of disjoining pressure [16,17] can thus be derived, and its value is in similar order of magnitude to that of the VDW component from experimental evaluation. Certain quantities and liquid film properties [18][19][20][21] can be readily observed or calculated using computational simulations.…”