Just saw this.
I think your freezer should be just fine at -18C. This is about 0 F, and cooler than my freezer (10F). I have only limited data from freezing a VG-nic juice with 20% water, from years ago, but it did not separate or have solid ice in it...it just got thicker. It was only one attempt at this, but it makes sense to me from a chemistry view: through cross H-bonds, the water solvates completely with the VG, and cannot recombine back into pure water with cold.
As for those that are obsessed with purging headspace air from the stored bottle with a gas like Ar, let's do a little calculation. Let's assume 4 mL of headspace in a bottle of nic. If O2 is 25% of the air (rough estimate) that is 1 mL of O2. We will ignore vapor pressure of VG or water. If we assume 1 atm pressure and 25 C (298K), we can use standard PV=nRT high school chemistry math, where n is moles, and R is 0.0821L-atm/mol-K. So 0.001L O2 is
0.001L x 1 atm/(0.0821L-atom/mol-K * 298K) = 4.08x10^-5 moles of O2.
If one mole of O2 reacts with one mole of nicotine (it is in fact a 1:1 molar ratio in the reaction), then 4.08x10^-5 moles of nicotine will react if all that O2 reacts. Molar mass of nicotine is 166.22 g/mole, so
4.08x10^-5 moles nic x 166.22g/mole x 1000 mg/g = 6.8 mg of nic oxidizing if ALL the O2 in 4 mL headspace reacts.
I think I may have overstated the importance of O2 from added water some years ago here. O2 solubility in water at room temp and 1 atm is about 8 mg/L (0.008g/L). Suppose we add 20% water to 100 mg VG-nic, and we are storing 50 mL of the mixed base in a glass bottle. mg of nic that will oxidize if all the O2 reacts from the added water (10 mL) will be
0.008 g O2/L * 0.010L * 1mole/32g * 1mole nic/1 mole O2 * 166.22 g/mol * 1000 mg/g = 0.4 mg nic oxidized if all the O2 in the water reacts with nic. Less than 1 mg.
O2 solubility in VG is about 2 mg/L. In our 50 mL bottle of VG-nic with 20% water, we have 40 mL of VG (about, there is nic too, but to make it simple). This will lead to oxidation of nic of:
0.002 g O2/L * 0.040L * 1mole/32g * 1mole nic/1 mole O2 * 166.22 g/mol * 1000 mg/g = 0.4 mg nic oxidized if all the O2 in the VG reacts. Same amount as from the 10 mL added water. O2 solubility in PG is about the same as in VG.
Thus, if no O2 gets into the VG-nic from outside, the intrinsic O2 present in the headspace, 20% water and VG will oxidize less than 8 mg of nic total in a 50 mL bottle with 4 mL headspace. Might be enough to give a very slight yellow, but most probably not enough to detect in a DIY e-liquid made from it. And even at room temp, this will take some time.
You cannot remove this intrinsic O2. But it is certainly not enough to be worried about even if it all reacts. Most plastics we use are fairly porous to O2 transport through it from the atmosphere, but while glass is virtually impervious to O2, the plastic caps are not, although they are better than typical dropper bottle plastic (LDPE). This is why even at room temperature, VG-nic in sealed glass bottles lasts a long time without significant coloration. But probably not years. So we cool the nic to slow any reaction from O2 invading through the cap to almost zero. Shrink wrap or electrical tape around the cap might help some, but maybe not much. I don't know, but I don't do this myself.
I store undiluted VG-nic mostly at 100 mg/mL for space reasons, and only in glass. For my VG-nics stored cold after 6.5 years there has been no change. I don't know why one batch of PG-nic I had years ago oxidized to deep yellow-orange after about a year, but to be honest it was not as pure as the nic we generally use today, and might have been oxidized somewhat upon getting it (2009 Totally Wicked Red Label). It was in a deep amber glass 30 mL dropper bottle (rubber surrounding) with child-proof cap that might not have been good at keeping O2 out. Rubber is not good at blocking O2 transport. Other purer PG-nic I have had in better sealed glass did fine for the couple of years I stored it. I haven't had PG-nic in my possession as long as my VG-nic, since I don't vape PG-nic. But others here that have stored pure PG-nic have had good results for multiple years in glass in the freezer.
Conclusion: no matter the DIY-nic you get, get it into well-sealed glass, give a bit of headspace for possible expansion (VG/PG expands with heat), put it in the freezer, and worry about it no more.
