Actually I'm not sure, they are in HCl salt form and seemed quite stable, they sat out under low heat for a little less than an hour with no noticeable decomposition (freebase nicotine turns yellow almost immediately upon exposure to air), but after that I dissolved them in glycerine and vapped it up. Since it is being carried by the glycerol vapor I saw no reason to use freebase nicotine which decomposes so rapidly, and it worked quite well (vapped like crazy after I added a little vodka to it), no residue in atomizer and none of the extra chemicals found in commercial e-liquids (it was slightly bland but perfectly smokeable, I need to find a well-researched safe-to-inhale flavoring..). It was also a very smooth extraction, no emulsions or crazy soaps formed or any of the stuff that usually gunks up an extraction (If I remember right I added some non-iodized salt to the initial water extraction to help avoid emulsion but I'm not sure if it was necessary).
I used:
Distilled water
Naptha (I want to try it again with d-limonene because it would be nice to have an all-organic extraction without anything flammable)
Citric acid and HCl (one could use either/or for the steps involving these, I started with citric but then used HCl for the final because I didn't want any extra citric acid in the product. In case you can't tell I was making it up as I went along but it turned out wonderful)
Sodium Carbonate (One could use sodium hydroxide or another base, I didn't have pH papers at the time and didn't want harsh pH spikes or a strongly basic solution)
OPTIONAL:
Sodium Polyacrylate (Non toxic stuff that binds chemically to water, it's what that "fake snow, just add water" stuff they sell in stores around christmas time is made of, also very cheap from chem suppliers but not totally necessary anyway and can be replaced with some other stuff if you want, use what you have I won't go into detail)
Alcohol (no water in it, denatured alcohol or 99% isopropyl available from drug stores, and by the way if you don't see it on the shelf they can order it for you, if they say they can't ask them to please check because every single one I've been to said that and then they looked and there it was, I used to have a sweet hookup at the local grocery store the pharmacist would sell me a carton (like 20) of pint sized bottles for 10 bucks because my friend worked there)
Procedure:
1. 2-3 soaks in hot distilled water with a bit of citric acid (having extra isn't really important because it wont lower the pH to dangerous levels no matter how much you add and it is gone in the next step anyway, but don't get carried away: couple of grams is more than enough to get everything, and this only needs to be added to the first soak the others can just use distilled water). Soak in hot water 10-15 min, filter through coffee filter (lab funnel filter was too slow and there are no tiny bits of sediment anyway so the filtered liquid was perfectly clear), return any tobacco chunks from filter to flask (best to use erlenmeyer flask because you can hold it upside down and the shredded tobacco clumps together and prevents any from falling out so you don't have to transfer anything back, though one could use any glass or possibly metal container) then add more water and repeat 1-2 times, combining all the liquid extracts)
Optional: I tossed some sodium polyacylate in there (not much) to soak up some of the water then filtered it out. I wanted to get the solvent amounts lower as I went along so that my final step would have less to evap and I would also be able to have more concentrated solutions to ensure a higher yield during solvent transfers. This is totally not necessary and you should have a good yield without, I was trying to speed up the process because I had ran out of juice and I had a jar of it handy. One could just use the first two tobacco water extractions as most of the nicotine is in them anyway, and that would also cut down on solvent amount (I wouldn't boil it down, nicotine doesn't like heat and it is a lot of wasted time and possibly messy), or just do with more water it won't make much diff and I'm probably just confusing the non-chemists reading this.
2. To tobacco-water solution added sodium carbonate (bubbles form from CO2 release so don't dump a whole bunch in, just add a little, let it fizz up while swirling, then once bubbles go down add a little more) until bubbling finished, then added another little scoop to give it the extra bump and release the acid from my precious nicotine.
3. Added Naptha, put a stopper on flask (for anyone without a chemistry set, this whole process can be done in empty, clean, plastic water or coke bottles with no problems since there is no more heating), and shook it up for a bit (don't be violent with it, may form emulsion, though mine didn't. In any case shaking it like a lunatic doesn't do more than gentle rocking and shaking), let separate (naptha and water form 2 layers like oil and vinegar, the naptha now has the nicotine in it), save naptha, add some more naptha, shake, sep, save, one more time, combine the 3 extracts (mine was about 60-75% of the already reduced water extract, reducing the amount of solvent makes it much easier to work with and much more efficient).
4. mixed a little citric acid in water (less than 1/2 of the naptha stuff, no need for much water here as the acid form of nicotine is very water-loving), then added half of the water (1/4 of the naptha amount) in a separatory funnel with the naptha and shook the hell out of it (no emulsion will form at this point so go crazy) for a couple min, saved the water layer, added the other half of the acid water and did the same (this can be done in a water bottle too, just mix the stuff up and save the bottom water layer, sep funnel is useful if you have one). I now had a rather small amount of water with nicotine citrate dissolved in it.
5. This part is a bit redundant, but I wanted to make sure I got the alkaloids as pure as I could with just acid/base extraction, plus I didn't want any citric acid in the product and didn't feel like evaporating a bunch of water and then redissolving in another more selective solvent and re-evapping, and it didn't take hardly any time or solvent because I was now working with drastically less water (probably about 100-150 ml). I redid steps 2-3 (further reducing the solvent amount as I now had the freebase nic dissolved in about 75ml of naptha which I put in my little 125ml sep funnel).
6. Almost done, I added like a drop and a half of "muriatic acid" (pool store acid, it is roughly 35% HCl in distilled water and suitable for this purpose. it is really HCl acid they just renamed it because HCl got a bad rep) to a bit of water (like 50 ml), added half to sep funnel, shook for a while then saved the water, added the rest and saved water again, think I added a little more distilled water and shook up to make sure I got the stuff of the sides of the funnel and added that too.
7. I now had Nicotine HCl dissolved in water, this could easily be evaporated and used but I started eyeballing the sodium polyacylate again and got an idea to speed things up. I put it in a tiny florence flask (I think 75 or 125ml size) tossed in enough poly to soak up all of the water (yea I should probably mention that with no water to dissolve in the HCl all turns into gas form, yowch on the nose. I saw the white vapor start rising and covered it with my hand {wear gloves foo} and went outside and let it air out for a couple seconds), then added some alcohol (honestly can't remember if it was denatured ethanol or 99% isopropyl but either should work fine) and swirled it around, poured alcohol off the top (flask kept the poly down at the bottom, this could be done with filters or just careful pouring, but the proper equipment really makes it all easier).
Put alcohol in a little metal measuring cup (like 1/4th or 1/8th of a cup) and set on hot plate on low heat, alcohol evaporated very very fast (water is like watching paint dry..), was careful not to boil to avoid having the popping bubbles send my concentrated nicotine solution over the side of the container, just a steady trail of alcohol vapor coming up. When the solvent was almost completely gone I took it off and let it finish evapping as it cooled, after a few min I checked it and bam, beautiful clear crystals were spiderwebbing across the bottom of the cup. I scraped them out and let them sit out for a bit to completely dry, then dissolved in glycerine with a bit of distilled water and some vodka (I did a couple small vials till I got the consistency where I wanted it then added the nicotine) and it vapped wonderfully. I had been out of fluid for nearly a day and it felt soooooo good, combined with the joy of a successful chem project I was in heaven.
Hope that helps, questions/comments/improvements welcome.