This, however, doesn't happen in an orbital freefall condition though because there is no gravitational field to define which direction is up and which direction is down.
According to a Newtonian physical model, there needs to be a change in momentum, an impulse, to cause the less dense object to slide through the denser object at a faster rate, this occurs in the presence of a gravitational field which provides that impulse because gravitation is a body force related to mass which thereby acts on the denser object more than it acts on the lighter object.
That is at least part of the cause of buoyancy, however that doesn't exactly explain all of it though. Buoyancy is also statistically related to the principle that a less massive object, perhaps a soccer ball for example, will be knocked higher into the air than a more massive object, perhaps a bowling ball, assuming that the same amount of impulse (force integrated over time) is provided to the objects, perhaps by a kick of the foot. Statistically speaking, more massive objects have more mass inertia than the lighter objects do, and so the same amount of energy will drive one higher than the other.
On the molecular scale of collisions, every collision is like different masses being kicked around against each other. Statistically, the lighter masses should end up higher than the heavier masses provided that the same amount of impulse is given to all of them. Unfortunately for the model, the Earth is an open system and thereby is exposed to the light from the sun, so different molecules will absorb different wavelengths and get different levels of impulses than other molecules do.
So you can therefore get the paradox of more massive molecules going higher than a lighter molecules as you go up.
Sources:
Water P-V-T
Gas Absorption Spectrum
