Molecular Dynamics Simulation of the Effects of NaCl on Electrostatic Properties of Newton Black Films

The effects of NaCl on electrostatic properties of Newton black films (NBF) coated with two monolayers of sodium dodecyl sulfate were investigated by using molecular dynamics simulations. Film systems without salt and with 1.0 mol/L NaCl were studied separately. Charge density profiles of salt ions are calculated and fitted using a function we proposed. Fitting results show that adding NaCl leads to more positive charge adsorbed to the surface and less diffusing in the middle of the film. More positive charge adsorbed to the surface leads to the destruction of solvation shells around amphiphilic groups, so that water molecules arrange less regularly. Adding NaCl leads to a decrease of both ions and water contributions to the electrostatic field, which are controlled by ionic charge distributions and water orientations. The relative decrements of ions and water contributions determine the difference in electrostatic potential. On the basis of the investigation of electrostatic potential and species distribution, mixing energy, electrostatic energy, and electrostatic Helmholtz free energy are derived. The electrostatic Helmholtz free energy begins to increase exponentially as the thickness of the film decreases to a certain value, which is 22 Å for films without NaCl and 18 Å for films with NaCl. By fitting the exponentially increasing part of electrostatic Helmholtz free energy, the electrostatic component of disjoining pressure is derived, and we found it is lower in films with NaCl than that in films without salt, in agreement with experimental results, but as film thickness decreases to <10 Å, the adsorbed and diffusing charges of salt ions merge together, the differences in water orientations and electrostatic field and potential caused by NaCl almost disappear, which explains the experimental result that adding salt does not change the stable thickness of NBF.