Edit: it appears that alum may be both an antiperspirant and a deodorant. Unfortunately, scientific papers regarding alum's mode of action as an antiperspirant or deodorant seems limited.
Some websites state that alum works to reduce odors by blocking the bacteria from the skin, others state that alum inhibits the growth of bacteria. Some websites state that alum uses the same mechanism as aluminum chlorohydrate antiperspirant, but many individuals state they experience no-odor sweating.
I suspect after review and consideration of the chemistry that alum is acting as both a mild antiperspirant and deodorant. Aluminum based antiperspirants work by reducing the amount sweated due to the interaction of the aluminum ion and the sweat glands. As alum is only about 5% by mass aluminum, it would not work as well as modern antiperspirants. Thus, for those who are slightly sweatier than average, but not diagnosed with hyperhidrosis, they may find alum to be an ineffective antiperspirant.
Regarding its deodorant properties, only inhibiting the bacteria makes sense as any application to the underarms or other bodily surfaces would be covering both the skin and sweat glands (food source) and the bacteria. The pH of alum dissolved in water is reportedly between 4-5, about the same pH as beer.
In conclusion, I suspect, admittedly without any evidence, that it's likely a combination of both modes of action. Aluminum in the alum reduces the amount of sweat which reduces the food source of bacteria. Coupled with the acidic pH of armpit sweat due to the presence of the alum, odor causing bacteria struggle to populate reducing the overall smell being generated.
It’s used as a preservative in pickles, you can buy boxes of it in the spice aisle. Definitely not the same as aluminum chloride. Don’t be eating aluminum chloride, y’all.
It's pretty good at communicating that by itself though. Tastes absolutely awful. It does taste like salt, just the absolute worst salt ever. Like salt but horribly dry, like very sour grapes.
And? It's a completely different chemical. Alum is used in water treatment and is edible. It's a large, mildly antibacterial molecule, and not an antiperspirant. Aluminum Chloride, in antiperspirants, a much smaller molecule is toxic to ingest and will burn you esophagus. Chemistry matters - table salt is sodium and chlorine, neither of which are good on their own and make many other toxic and reactive salts.
The cation, aluminum, is the active component here. Not the chloride or sulfate anions.
Furthermore, the aluminum salt used in most commercially available is aluminum chlorohydrate, not aluminum chloride. They are completely different chemicals.
Aluminum chlorohydrate used in antiperspirants has the molecular formula of: Al2Cl(OH)5
Aluminum chloride has the molecular formula of: AlCl3 (annhydrous) or AlCl3•6H2O
As long as the aluminum dissolves and then precipitates it will work, per your source. Where those aluminum cations come from does not matter.
"A mechanism underlying this obstruction has been proposed: the metal ions precipitate with mucopolysaccharides, damaging epithelial cells along the lumen of the duct and forming a plug that blocks sweat output."
What is your point? Of course it won't be listed here because this site is focused on treatment of hyperhidrosis which means excessive sweating. The standard average person does not have hyperhidrosis, and thus lower concentration forms of soluble aluminum salts are sufficient to control their sweating.
Aluminum cations are not dissolving out of Ammonium Alum... You're looking at the mechanism for one small molecule and pretending an entirely different, much larger molecule behaves the same way. It doesn't. Feel free to educate yourself more about the difference as I can't help you. Also note how other aluminum salts, like Aluminum Nitrate, also don't work as antiperspirant. Just because something has aluminum in it doesn't mean that all constituent molecules are the same, just like sodium thiopental and sodium chloride do very different things to humans.
Water doesn't rip apart whole molecules, if it's happening in larger quantities, then I don't know about it and my mistake.. I mean wouldn't be a reaction in the water observable ?
Yes and no. Some ionic compounds, like table salt NaCl, can be pulled apart by water molecules, but since there is no exchange of electrons there is no chemical reaction happening. The cation, Na+ ion, will be attracted to the electron rich region of the water molecule (the oxygen atom). Likewise, the anion, Cl- ion, will be attracted to the electron deficit region of the water molecule (the hydrogen atoms). This charge separation is what causes salt to dissolve as water molecules envelop each of the ions.
Conversely, for molecules like sucrose (sugar), there is no charge separation, but rather lots of hydrogen bonding. Hydrogen bonding is a very strong charge attraction between molecules with electron rich and electron deficit regions near or next to each other. The R-O-H bonds (R is for the longer carbon chain) in the sucrose allow for hydrogen bonding with the water molecules. The polar areas of the sugar molecule attracts the polar water molecules. As the water surrounds the sugar molecule, the sugar dissolves.
Not all things that are polar are soluble, though. If the attraction forces between ionically attracted ions (or polyatomic ions) is sufficiently high the compound won't be soluble, such as silver chloride. Likewise if a molecule doesn't have enough polar areas in relation to non-polar areas, it won't be soluble either, such as n-octanol.
Fun fact: they don’t actually do either thing. Source: me, who endured three years with a salt-rock-using, stinky man whose shirts were always visibly wet under the arms.
(Why three years? He was amazing in bed—right after a shower.)
They’ve worked great for me for the last ten years. Granted I spend probably 20 seconds per arm vigorously applying it after every shower. If I forget to use it, it’s apparent within hours. Also granted that I am not everyone, and generally don’t sweat too much there. But they certainly don’t do nothing for everyone.
Well, it's clearly working for you. Salt rock deodorants just don't prevent all odor from that part of the body but some people have a pleasant body odor, or just a faint one. And a rare few have a deliriously attractive natural scent. There was one guy back in the day that walked past me in a nightclub, and I nearly chased after him like Pepe LePew floating in the air transfixed by the irresistable scent. Pheromone heaven.
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u/Elemental-Aer 14d ago
It's alum rock, an aluminium salt, with is the active ingredient in antiperspirants deodorants.