Re: Is a potassium alum molecule too big to be absorbed into the (human) skin?

Hmmm... my answer would be "neither". Potassium Alum is not a molecular species. It is an ionic compound composed of a potassium ion, an aluminum ion (which is not the same thing as "aluminum") and two sulphate ions. It can be found in an anhydrous form (without waters) but is more typically hydrated with 12 water molecules.

As an ionic species, it forms a crystalline lattice with alternating potassium and aluminum ions interspersed and electrostatically bound to sulphate ions. The water molecules solvate the whole lattice.

When Potassium Alum is exposed to water, the lattice structure disintegrates resulting in the release of potassium ions, aluminum ions, and sulphate ions. The waters of hydration simply become part of the bulk water. So, in aqueous solution, there really isn't something that you could call "Potassium Alum" floating around - just its constituent parts. That said, the compound doesn't completely break up. Some of the aluminum and sulphate ions will still hang out together in something called "an ion pair" but the majority of ions go their own separate way (depending upon concentration of the salt in the water, this could range from 70% to almost 100%).

But that is not the only chemistry that is going on. Aluminum ions - with a +3 charge - are able to hydrolyze water and depending upon the pH of the solution, a wide variety of aquated aluminum ion species can be obtained. In its simplest form, aluminum typically occurs in aqueous solution as a hydroxide species. Further, aluminum can be chelated by naturally occuring organic compounds.

So, what I think you are really asking is: "is it safe to put potassium alum on your underarms?" The answer is "probably". Yes, some of the aluminum will dissolve in body sweat and form solvated aluminum ions which could potentially transport across the skin's membrane but not easily. Our skin has evolved to keep things out for the most part and that includes ionic species such as aluminum. But there are pores and other mechanisms which would allow for some transport.

Does it matter where you get your potassium alum (i.e. deodorant stones)? No. The body doesn't recognize potassium alum from a deodorant stone as being any different than any other potassium alum compound. The only advantage that might accrue from a stone is that very little in the way of material will be transferred to your arm pits. Yes, that means less aluminum on the skin but it also means that your arm pits will not be protected from wetness and that wetness allows the growth of bacteria which produce all those body odours that we are told to get rid of.

By the way, it is not "impossible through natural means to get water to permeate the skin", regardless of what moisturizers say. If you want to test this out, just go soak in a bath for half an hour.

Hope this answers your question!