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The question is simple: What human-consumption safe liquid has the best caffeine solubility?

It's 200mg/10mL of water. That's a lot of water for one dose!

I found some ramblings on a fourm stating it's "low" solubility in propylene glycol and glycerine (glycol) but no sources or exact numbers, so that doesn't mean much.

In boiling water, it seems much higher, 66g/100mL, which is more of the range I am hoping. So what happens if I boil a liter of water, dump 100g into it, and then let it cool?

I am hoping for something in the range of 200mg in 1mL-3mL, but as low as possible, and relatively easy to acquire.

All help very much appreciated.

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    $\begingroup$ Why do you think 200mg/10mL water is a lot of water for one dose? Also what do you mean by consumption-safe? Water is the only liquid that is safe to take for a large quantity. Glycol is moderately toxic. The safest liquid besides water is likely alcohol, but you do not want to take it with caffeine. $\endgroup$ – Xiaolei Zhu Dec 5 '13 at 18:02
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    $\begingroup$ It probably goes without saying, but please be cautious when ingesting anything that you've created in a lab. $\endgroup$ – jonsca Dec 5 '13 at 21:56
  • $\begingroup$ Thanks for the tip with glycol. I was just hoping to get it down to 1-3mL so it could fit in a smaller dropper bottle. It's easier to carry 10mL around than 30mL. $\endgroup$ – A.B. Carroll Dec 6 '13 at 18:58
  • $\begingroup$ I hope you are also aware that caffeine itself is moderately toxic. It's not usually a problem in caffeinated beverages because it is present in rather small amounts, but it's all too easy to ingest a couple of grams of the pure substance. Your body also may develop some resistance to the effects of caffeine after some time, and increasing the dose to compensate may lead you into unsafe consumption territory. $\endgroup$ – Nicolau Saker Neto Dec 8 '13 at 3:06
  • $\begingroup$ For reference sake the LD50 of caffeine is estimated at ~150 mg/kg. Since there's a slow process of removing caffeine from your system, this can also be exceeded by consuming caffeine through the day. Reference $\endgroup$ – Geoff Hutchison Oct 8 '14 at 21:12
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Safety Warning

Caffeine is a toxic substance. When following the advice below, always keep in mind the factors that will help you kill yourself if you are not careful! For more details you can start out on Wikipedia.

Supersaturation

Your idea of preparing a supersaturated solution is not that bad, but it might be that you have some nucleation sites in the bottle where you want to store it. Furthermore, it is probably not that easy to transfer the supersaturated solution from the container where you prepared it to the vial.

Acidic Medium

I've come across this PDF that mentions something interesting:

Solubility in water is increased by adding dilute acid (e.g. HCl or citric acid).

It would be very easy to prepare an acidic solution (I'd aim at a pH of about 3 or so, in the general vicinity of energy drinks and Coca Cola, beverages known to contain large amounts of caffeine) from some citric acid¹ and dissolve caffeine in there. Maybe you could even conduct a study about the solubility of caffeine at different pH values!

Water/Ethanol Mixture

There is one dissertation (PDF) that touches on the solubility of caffeine in a mixture of water and ethanol. I wouldn't trust it too much, since it was written using MS Word and contains several formatting errors (it says $K_{ow}$ for example, instead of $K_\text{ow}$). The graphic containing the data can be found on page 52, but for your convenience it is reproduced below:

Solubility of caffeine (diamonds) against the volume fraction of ethanol in the solvent mixture.

(Seriously, the figure caption is useless, and the plot is so bad I'm tempted to redo it just for this answer.)

Never mind the linear relationship that is indicated by the line, the diamonds are the important part here. As you can see, there seems to be a solubility maximum at around $x=0.5$, where $x$ is the volume fraction of ethanol in the water-ethanol mixture. So that's a whopping 50 vol.-% of ethanol, about equivalent to a cask-strength whisky!

However, it is probably not a good idea to use this knowledge to its fullest, because that will be quite a lot of ethanol, and as already mentioned in the comments it is probably not very wise to combine the consumption of caffeine with ethanol.

Does it have to be dissolved?

There are also caffeine tablets and pills that you can take. They come with the additional guarantee that some FDA-like agency has looked into the production of these products, so you can assume they are safe to ingest. Also, they come with instructions and more than enough warning labels so you can educate yourself further.

Conclusion

If you really want a caffeine solution, it is probably your safest and best bet to create an acidic solution where you can dissolve above-normal (i.e. pH=7) amounts of caffeine.


¹ Though please do not ask here how to calculate the amount you need, that would be considered a homework question and get deleted pretty quickly...

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