Examples Using the SImulator
General Instructions
With the Controls tab selected in the simulator window, click on the n-Pentane (r) + n-Heptane (b) radio button representing one of the five systems in the System frame. Before starting the simulation, notice that this is an example of a system for which:
Ubb > Urb > Urr and Sb > Sr
This means that the blue-blue attractions (Ubb) are somewhat larger that the red-blue (Urb) which in turn are somewhat larger than the red-red (Urr) and that the size of the blue molecules (Sb) is slightly larger than the size of the red molecules (Sr). The reason that the blue-blue interactions are larger is not because of a difference in interactions between the sites, but simply because there are more of them. As mentioned in the section on Molecular Interactions, the attractions are often treated as a sum of pair interactions between sites within the molecule. Even though the C-C, C-H and H-H pair interactions are the same in both molecules, there are simply more of these pair interactions in the heptane molecule than in the pentane. For two molecules close together, this gives a larger net attraction. (Using what is called a united-atom model in which each -CHX group is treated as a single interaction site, there would be 49 pair interactions between two blue molecules and 25 between two red molecules. Likewise, there would be 35 pair interactions between a red and blue molecule.) How will this affect the boiling point of the two compounds? Use these same ideas to try and understand what you see in the simulator.
Start the simulation by clicking on the GO button. The MD simulation now integrates the equations of motion to display the motion of the interacting particles. At the default conditions (pure blue, 323 K), the system is all liquid - there is no vapor space because the pure component vapor pressure of the blue molecules is not high enough at this temperature to exceed the external pressure of the ambient atmosphere.
Tutorial Examples
