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This is a model of two ways to filter a toxin out of the air. The purple molecules make up an active filter -- they have a strong, specific attraction to the toxin. The concentration of the toxin in the air declines as it sticks to the filter, then levels off as the filter saturates.
Van der Waals forces are weak, but sufficient to make things adsorb to a surface. A more complex surface area can absorb more molecules.
Additional Related Concepts
The two models are identical, except for the arrangement of the molecules in the filter. As the models run, you can see that one of the filters gets more of the toxin out of the air, because it has a higher surface area. In the real world, activated charcoal is a good filter of this type because it has an enormous surface area (it's full of tiny pores). The charcoal doesn't really need a specific attraction to whatever impurities it's filtering -- that was just used in this model to make it work. Apparently, van der Waals forces and hydrogen bonds are sufficient to make things adsorb to it. Activated charcoal is used in things like gas masks, Brita water filters, and even in the process of removing caffeine from coffee.