Glycerine is the natural byproduct of soap (a sugar alcohol). It's moisturizing when put back into the soap and it's a moisturizing agent in a lot of bath and beauty products to keep the product and/or the person moisturized. It's also used in foodstuff for retaining moisture and sweetness.
Propylene Glycol is a manmade version of it. It's recently been criticized and in the news a lot because China-based products are using it foodstuffs (illegally), substituting glycerine for cheaper propylene glycol and it can be harmful to people if you eat huge amounts of it (some is fine - kind of like baking soda. it's non-toxic, but if you eat a lot of it, you'll get sick). Also, if you had massive amounts and stuck it on your skin, it would hurt.http://www.tomsofmaine.com/toms/ifs/...ene_glycol.asp
There are a few different types of alcohols, some are known to be drying, but some are known to be moisturizing. If you check out some bottles of commercial conditioners, you will see a few with different kind of alcohols - those are the moisturizing kinds.
Polarity, if my memory serves me correctly - attributes to the stability of the atom. So in essence, propylene glycol is more stable than glycerine.
Sometimes people on this forum are all about natural things, but don't really realize that natural stuff can be deadly or used in different ways.
Watch the movie Fight Club sometime. Even though glycerine is all natural byproduct of natural soap, you can turn it into a homemade potent explosive (as nitro glycerine)
And of course glycerine based stuff is used in healthcare for bloodwork too. Glycerine and Propylene Glycol can also be in anti-freeze, and it's a non-toxic solution to anti-freeze. But since people hear the word "anti-freeze" and don't consider that the anti-freeze now has glycerine or propylene glycol instead of the old toxic stuff, they will still say it's toxic because it sounds bad that something is in anti-freeze.
It doesn't mean that all applications of Glycerine or glycerine is bad. Sometimes people take one item or chemical compound and their argument is that it's in something that is considered toxic. It's a weak argument and I wish people wouldn't do that as a scare tactic.