Controls are very crucial to scientific research. In MD research an abstract mathematical approach is taken in order to acquire accurate controls. The video shows about 1ns of simulation time with 0.5femtosecond time steps. This video shows carbon tetrachloride (green on top) and water molecules on the bottom of the interface (red and white). The simulation shows two perfect cubes stacked on top of each other; the cubes are 40 angstroms in length, width and height. A 20x40x40 angstrom region is left to serve as a vacuum below the water box and above the carbon tetrachloride box. The volume of each cube was calculated and the density of carbon tetrachloride (1.59g/cm^3) and water (1.00g/cm^3) were used to calculate the necessary amount of molecules to put in each box with the observed density of each type of molecule.
The edges of this box are contained by a particle-mesh ewald (PME) system. This allows molecules traveling to the left to come back through on the right and molecules traveling out the front side to come back through the back side. This is a more representative system setup so that this simulation percieves itself as an infinite array of interfaces. Anything going out the top will come through the bottom, but this is prevented by the presence of a 20 angstrom vacuum (which invisible in the simulation). PME makes the system have more realistic movements; rather than molecules constantly bumping into the edge of a box.
A simulation like this one and just with a water cube are analyzed to check the smoothness of the surface of the water, or the interface of carbon tetrachloride and water. Interfacial width values given by a hyperbolic tangent function allow for comparison with the addition of surfactant molecules. Surfactant molecules have been shown to increase the interfacial width when analyzed using vibrations sum-frequency spectroscopy (VSFS).
In the computational data I have gathered so far I have significant evidence that sodium laurate surfactants do increase the interfacial width. I will be posting more videos of simulations with surfactants along the interface as long as some of the results I have gathered so far. I am continuing to investigate how the presence different of divalent cations (Ca,Zn, and Mg++) will affect the interfacial properties of the system; and how differing concentrations of these cations will affect the interface.