The first simulation illustrates the mixing of 2 ideal gases, which mix readily. Since the gas particles don't interact you can think of the mixing as each gas expanding to fill both containers independently of each other. As I have shown in this simulation, the driving force for this expansion is an increase in entropy. Therefore, the driving force for mixing two ideal gasses is also purely entropic.
The second set of simulations illustrates the mixing of 2 liquids. Since they are liquids there must be attractive interactions between the atoms. If there were no interactions they would be (ideal) gasses. The strength of the interactions (and the temperature) determine whether they mix or not.
In the first liquid simulation, the attraction between two green atoms (εGG), between two blue atoms (εBB), and between a green and a blue atom (εGB) are the same.
ideal mixture (or ideal solution). The two liquids will mix at any temperature.
In the second liquid simulation, the attraction between two blue atoms (εBB) is stronger than between two green atoms (εGG) and between a green and a blue atom (εGB).
Salt and water is another example of on idea mixture, but here ΔmixH < 0 so salt and water almost always mixes spontaneously. The interpretation is that the interactions between the salt ions and water is stronger than the average interaction between salt ions and between water molecules.
The definition of ΔmixH in terms of the ε's suggest that liquids should also mix if
This is tested in the third liquid simulation. As you can see the liquids mix more than in the second simulation, but not quite as much as in the first simulation. This is mostly because of the simulation runs only for 100 picoseconds, which is to short to mix fully. But another reason is that there is less space (on average) between the blue atoms compared to the green atoms, because the blue atoms attract each other more. The next effect is that the blue particles tend to stay together to lower the enthalpy. More mathematically, z (the number of contacts between particles in solution) is not exactly the same for the blue and green particles so the interpretation of ΔmixH in terms of the ε's breaks down. The "safest" definition of an ideal mixture thus remains:
Accessing the simulations
You can play around with the simulations here and here, or you can download the models here and here if you have Molecular Workbench installed on your computer.