Nature/mechanism of "fat solubility"

In medicine, at least, there are two prototypes of solubility - water soluble (hydrophilic or polar) and fat soluble (lipophilic or hydrophobic or non-polar).

I think I have a fair intuitive sense of how and why ‘water solubility’ works. In my mental model, I view water as having an asymmetrical distrubution of charge such that even though the whole H[sub]2[/sub]0 molecule is electrically neutral, it becomes effectively charged. Hence, when other charged or ‘effectively charged’ molecules are placed in water, they become spread out through it by virtue of electrostatic forces. Bottom line is that there is an active mechanism (electrostatic) mixing the molecules. There would be mixing and miscibility even in the absence of an external kick to the system (i.e. the system doesn’t need to be shaken up or stirred to achieve complete mixing).

On the other hand, I have much more difficulty understanding what is going on in fat soluble situations and, in particular, difficulty understanding what “force” or driver leads to the complete intermixing of the constituent molecules.

If I imagine a bunch of long fatty acid molecules places placed in, say, a container containing a bunch of another type of fatty acid molecules, I don’t see what causes the former to spread out into the latter. The molecules of each are neutral, both in an absolute and ‘effective’ sense. There is no electrostatic force causing one molecule to move towards or away from another. So, it seems to me, that no dissolution or dispersion of the two types of molecules into one and other should occur unless one applies a mechanical input into the system (e.g. shaking it up).

Is this basically correct? Or, more likely, am I missing something here?

Thanks!

Brownian motion, random walks in three dimensions:

As opposed to random walks in two dimensions, where the probability of a particle returning to its starting point goes to 1 with an increasing number of steps.

Another thing that really helped me to understand fat solubility was looking at it from an entropic level.

When you have a few “hydrophobic” fat molecules hanging out by their lonesome in solution, it’s relatively positive H’s that surround it will tend to like to be buddied up with the slightly negative O of H20. This reduces the entropy of the water and of course raises the chemical potential of the solution.

If these fat molecules can come together in a droplet, the fat will have less overall surface area and will be imparting order on a smaller number of water molecules. As you know, systems move towards more entropy, so it’s this arrangement that makes the formation of fat droplets spontaneous. Think of it more as the fat-soluble components being pushed out of the polar solvent, water, than actively dissolving one another.

Once the fat-soluble compnents are grouped together, it’s the Brownian motion and further increases in entropy that leads the Vitamin K, THC, or whatever to diffuse.

Apologies for all of the personification, but it’s how I think of chemistry. I hope you weren’t imagining the entire post told by hand-puppets.

Weird typo: Both KarlGauss and threemae typed H[sub]2[/sub]0 (H-sub-2-ZERO) and H20 (H-2-ZERO) respectively instead of H[sub]2[/sub]O and H2O. It’s an odd typo for two people to make independently, especially since they both know what the notation means and aren’t doing it from ignorance.

Wow, I don’t know how you noticed that. You’re right, though.

Thanks. Both your answers helped.

The entropy approach was very neat.

Play around with fonts and font sizes until you can see it as well. (Naming fonts is futile: Every OS has its own bunch of them with their own names. Just open up some font-picking application and fool around a bit.)