The density of liquids and possible volume contration

[Kurt]

Yesterday I measured the density of a 36 mg PG liquid known to be only nic and PG to be 1.128 g/mL. The density of pure nicotine liquid is 1.009 g/mL, and the density of pure PG is 1.036 g/mL. Thus if there is no change in volumes upon forming a solution, the final liquid density should be somewhere between these two densities of the pure substances.

What this implies is if a 250 mL 36 mg liquid is made from .036*250/1.009 = 8.92 mL pure nicotine, and the volume is brought to 250 mL with the remaining 241.08 mL, when the solution is finished forming, its volume will only be 223 mL, and the actual nic concentration will be about 40 mg/mL, not 36 mg/mL.

I do not work with pure nicotine, and have not made any liquids from PG or VG and pure nic myself, so I cannot verify that the solution actually did contract. I only measured the density of a known liquid sent to me by a vendor. That liquid titrated to 43.5 mg, so perhaps this is was much of the issue, along with the usual error associated with any volume dispenser. Not saying who the vendor is, since there have been no complaints, he just wanted me to test his. He makes liquids individually per bottle, using a volumetric syringe to deliver the pure nic to the bottle, dilute it up to the level for that bottle with PG, and shakes it up for a while.

Is such a contraction known with these liquids? Something is increasing the density, and a post diluting contraction is the only thing I can come up with. Such an effect would not surprise me, since both nic and PG are hydrogen bonding and a solution may disrupt some of them. For a similar reason a volume of solid ice will contract when it melts, the result of some of the H-bonds breaking in the melt.

 

[36tinybells]

Have you tried a vg + nic solution yet? Might be interesting to have a comparison result. And what about just the vg and the pg? I am no chemist- but this is all very interesting.

 

[Kurt]

I did try a VG-nic liquid, and while I got a similar result, I don't know for sure what the composition is. Many vendors add a little water to their VG-nics. I did this originally to see if by measuring the volume and mass of some quantity of liquid I could use the densities of pure PG and nic to algebraically work back to the nic content. If there was no total-liquid volume change, I would have been able to do it, but with a density higher than either component, no go.

PG and VG have known densities at room temp: 1.036 and 1.261 g/mL, respectively.

The algebra is two equations, two unknowns:

Let mnic = mass of nic, mPG = mass of PG, Vnic = volume of nic, VPG = volume of PG, dnic = density of nic, dPG = density of PG.

mnic + mPG = mT, with mT = total mass

Vnic + VPG = VT, with VT = total volume

If we sub mnic = Vnic*dnic, and mPG = VPG*dPG, we can get to an expression for mnic:

mnic = (mT - VT*dPG)/(1 - dPG/dnic)

My VT = 12.00 mL
My mT = 13.544 g

This gives a negative number for mnic. The expression assumes that the total volume of the liquid will be the sum of the volume of the nic and the volume of the PG. And this clearly is not the case. I think my algebra is correct, but logically, the density of the liquid cannot exceed that of either component...unless it shrinks when it is fully formed as a solution, which does happen with some water-based solutions.

 

[AzPlumber]

Could this additional density be caused by PG's tendency to attract moisture (humidity) or does that just increase the volume?

 

[SiBurning]

For the practical purpose of making liquids, this is the difference between molarity and molality. For mixers--and authors of mixing software and guidelines--it's whether or not to measure separately or "volumetrically", which is to add the solute (nicotine) and then fill to the mark with solution (PG). There seems to be a practical lesson for all of us in your observations.

But you asked something far more interesting. I'm no chemist--just a guy that does a bit of etching and the tad of chemistry needed to balance the bath--so I don't pretend to understand or be able to tell which factors are important. Also, everything I could find was about the chemistry on nicotine in water or with the more usual reactants, such as metal salts, acids, etc.

At least one factor is that nicotine is amphiphilic (obviously in water), and will form micelles at higher concentrations. In Chemistry 001 terms, it means they can take up less space when combined. In Chemistry 101 terms: At low concentration, the hydrophyllic ends will snuggle close to the water, but at higher concentrations there won't be enough water, and the nicotine will bunch together, staying away from the water, and take up more or less the same space it requires as pure nicotine. But if the nicotine molecules are snuggled up with the water (in low concentration), they're packed in pretty neatly in the space the water doesn't need, and the solution takes up less space, making the solution denser at low concentration. That corresponds to what you observe.

There do seem to be a lot of other complicated factors, but they're way over my head, and I can't tell if any matter. Certainly not in an hour. One that does matter is that there's a low point along the concentration/density curve, which seems to mean that below that level of concentration, the density will begin to decrease again.

For what little the terms amphiphilic and micelle are worth, and whether or not it even works in PG... Well, maybe it does and maybe it doesn't, but if it does, it supports your observation. It certainly fits with your ideas of H bonds. We obviously need a real chemist here.

The one thing that puzzles me is that the curves only show about 0.7% of a change. Maybe I don't understand the meaning of the graph. For what it's worth, the math & discussion gives 9cc/mole. (I read that at the end after my eyes recovered.) Maybe that will make more sense to you than to me.

There was also stuff online about how to calculate the concentration of these kinds of solutions, but it was way beyond me, between theory, math, and technique, but there was also mention about something not quite stable about nicotine's density in water, so there seemed to be a lot of extra stuff to consider. Hopefully, there's other ways. Perhaps optical deflection? Or just do the titration. Do enough of those and you could make a density graph for future quick reference. (But what about added water?)

reference 1
reference 2

 

[Kurt]

Thanks for the links, SiBurning. Generally hydroxylic compounds will increase their density as H-bonds are broken. Witness water when it melts: its density as liquid is less than that of ice. Actually I lectured on that today in my general chem course!

But for this, regardless of the theory behind the why, the fact remains that this seems to occur, at least in PG-nic solutions. The vendor whose liquid I did the measurements on is now trying to do some experiments to see if he can actually see this contraction of the solution. I don't know how long it takes to manifest, but given the higher viscosity of PG compared to water, it might take some time.

Yes, these liquids we take for granted now do evidently have some properties that make them a bit unpredictable. Your thoughts about molality vs molarity are correct, but the final concentration we use is closer to molarity (moles of soluter per L of solution) than molality (moles of solute per kg of solvent).

I would make a plot of density vs nic conc, but I would need a bunch of verified liquids, and I'm not sure what it will actually do for us. My point here was that it seems there may be a contraction of the liquid volume after the solution process is complete at the molecular scale. So to alleviate this, more PG or VG should be added to make sure the total volume is what it is supposed to be. How long it takes for this contraction to occur is not known right now, but the density shows it can occur, and may affect the nic concentration by about 10% (maybe more, maybe less, depending on the liquid).

Perhaps other vendors that mix from pure nic can make notes of this. I think if the volume is adjusted to maintain correct level, this problem will be largely solved.

 

[AzPlumber]

Are any of this vendors using PEG 400 for their nic solutions?

 

[SiBurning]

My point here was that it seems there may be a contraction of the liquid volume after the solution process is complete at the molecular scale. So to alleviate this, more PG or VG should be added to make sure the total volume is what it is supposed to be. How long it takes for this contraction to occur is not known right now, but the density shows it can occur, and may affect the nic concentration by about 10% (maybe more, maybe less, depending on the liquid).

I missed that point entirely.

Thanks for taking the time. It clarifies what a DIY liquid maker can expect, and points to some things to play with. Naturally, it's kind of secondary-level working with 10% nicotine in VG, with some unknown water content after sitting around. But I don't need much excuse to learn something new or brush up on technique.

Looking forward to reading some more fundamental results.

 

[Kurt]

Are any of this vendors using PEG 400 for their nic solutions?

None that I am working with, or at least the liquids I am analyzing. I just want to figure out PG-nic right now. Then maybe VG-nic. Those are coming up too dense too, but I don't have verified compositions yet. Some add water to them.

 

[Killjoy1]

Very few use PEG-400 for their nic bases. In fact, the only one I have personally come across that does is Decadent Vapors

 

[kinabaloo]

... Thus if there is no change in volumes upon forming a solution, the final liquid density should be somewhere between these two densities of the pure substances.

Something is increasing the density, and a post diluting contraction is the only thing I can come up with. Such an effect would not surprise me, since both nic and PG are hydrogen bonding and a solution may disrupt some of them. For a similar reason a volume of solid ice will contract when it melts, the result of some of the H-bonds breaking in the melt.

One could try to measure the temperature rise that would ensue from the exothermic mixing (increased order / decreased entropy). It is almost certainly down to the hydrogen bonds - though differently to the effect of apolar solutes, this might be enhanced spatial arrangements between the two as both glycols and nic are polar and of somewhat comparable size (so perhaps spatially and H bond position congenial).

I wonder at what concentration (proportion) the effect is greatest; presumably not too far from the 4% nic that gives the 10% increase in density. Might the effect become even stronger at say 10%? Possible, and would begin to explain some of the discrepancies found in some high strength nic base liquids; depending on how the dilution was done.

If the shrinkage was fast (immediate; as one would reasonably expect - why would it take time?*), this would lead either to over or under strength liquid depending on what liquid was being added (assuming the added liquid was simply added to a predetermined overall volume). Otherwise, the shrinkage would effectively lead to increased strength by volume; (anyone checked if their 30ml / 100ml of liquid is actually of lower quantity?; of course a vendor might reasonably be expected to err a bit over with this as it is not critical).

After all, when we say that an polar inorganic liquid is hydrophilic this is not just about solubility in water but an attraction for polar molecules whch 'fit' in the htdrogen bonding sscheme; water molecules, being very small and of simple shape are an ideal; but nicotine molecules seem to be a good 'fit' too.

It is certainly a different mechanism to dissolving salt in water whereby the volume of the solution is barely increased as the ions just 'fill in the gaps'.

The figure for the 36/40+ liquid is ~8% more dense (similar to the change in density of ice to water; i.e. very significant).

Certainly this is cause for further analysis ...

---

* If this is a slow process, the reason might be, strangely enough, the absorption of water - increased mass with little increase in volume. Though this would not alter the nic strength by volume. So it is most likely instant volume shrinkage due to enhanced H bonding.

Interestingly, life depends on H bonding making water ~15% denser than it would be otherwise; without this, water would not remain liquid to anywhere near 100C and life out of water at least probably could not have evolved..

 

[SiBurning]

Along with water absorption, nicotine absorbs CO2. It also oxidizes.
Autoxidation of Nicotine mentions the following products of autoxidation: ammonia, methylamine, nicotinic acid, myosmine, cotinine, nicotyrine, and others. Perhaps there's something in there to account for the density change.

Reading further... there's nicotine-N-1'-oxide. Also, if cotinine is present, its concentration relative to nicotine will increase over time. (From "Analytical Determination of Nicotine..." Gorrod, Jacob)

I wonder what some of these changes might do to a titration test.

 

[AriM]

Also keeping in mind that some base contains PGA. Do any of you think that PGA could lead to test contamination, due to the fact that is hydrophilic, and could possibly change the density of the test sample (if left unsealed)?

Also is there any pool of data showing possible trace amounts of Petroleum ether contamination (leftover from the nicotine extraction)?

I can't imagine it would be cost effective for any e-liquid maker to run MS analysis (for Pe) on each and every batch of nic base.

 

[AriM]

I wonder what some of these changes might do to a titration test.

I have personally had quite a problem with this, and it has forced me to do the titrations in an extremely controlled and tedious manner. The nic, and trace PGA seem to pull enough H2O into the solution to change the potency of the HCl solution (as it drips into the vessel). The solution has been to do the tirtations in a "closed system" if you will. It's not really possible to avoid some level of contamination, but I have found that keeping the testing system as closed as possible, does lead to different end results. I'm not sure how relevant it is though, because the swing in the result is below the acceptable margin of human error (IMO). It did drive me crazy for a few weeks though.

Do any of you think this notion is totally off base? It's not a repeatable result for me, nor do I have the equipment to judge that problem beyond the margin of error.

 

[Rocketman]

kurt,
I changed to mixing to a predetermined final volume after noticing I would usually come up short.
I've noticed about a 6% loss in volume when mixing a 10% nic/pg base with equal amounts of pg and vg down to a 1.5% nic then measuring total final volume. Using grade A cylinders and a grade A 500ml volumetric flask I also came up short and had to add about 30ml of pg to 'hit the mark'. Checked the cylinders against the flask and came up well within 5ml (less than 1%).
Time lapse to do the mix was about 45 minutes, with no further change noticed hours later. Several times in the past I added water (5% of the VG volume) and thought this caused the shrinkage. The target for my last mix was 500ml without the water and got close to the same shrinkage.
( mixed for strength/volume, then thin and flavor).
I don't think it's water absorption (or CO2) from the air. If I had just used the input volumes I would get a calculated nic strength 6% higher than using total volume.
'Blue' titration comes out closer to the nic/total volume value.

EDIT: Did a quick check for shrinkage. Dispensed two 50ml quantities of PG into a 100 ml cylinder and got about 49.8ml and 99.8ml (extrapolated). Emptied, washed and dried.

Then dispensed 50ml of 10% Nic (PG base) using all the same labwear then added 50ml of PG. Result was 98.0ml within seconds. 98.2ml after bubbles and drainage. Covered with plastic wrap and will check later in AM.

Still 98.2ml

 

[Kurt]

Rocketman, this is interesting info! I would not have thought making a 15 mg juice would have done this. The scenario I was suspecting this with was pure nic into PG or VG to make 36-48 mg liquid.

The fix for that vendor was straight up: use an analytical balance to measure amount of pure nic in grams, dilute some with PG or VG, dissolve, then bring volume up to total volume. This method gave accuracy of nic level to within less than 1%, measured by titration.

There is also the issue of the density of pure nic, which is supposed to be 1.009 g/mL. But I saw spec sheets of pure nic that was showing up as 1.2 g/mL. Another strike against using volume and a "known" density to measure it, however convenient that is. Seems the pure stuff can be much more than 1.009 g/mL. How I do not know, but there it is.

@arim: I don't think that the solution absorbed water. They were sealed, and would over time if opened, but these were pretty fresh.

You do bring up a good point, however, although not perhaps along the lines you were thinking. Pure PG or VG is significantly more acidic than water. Thus a titration of a PG or VG juice will be affected by this. In fact, the pH is redefined, since pH is assuming a mostly aqueous solution. I have not worked out the numeric details, but there might be an effect with kit titration of 2 mL of juice, rather than diluting the 2 mL of juice and titrating that. Probably not a big effect, since the kit uses an indicator. But with my pH curves, and higher precision, its something I wish to avoid entirely. So I dilute a set amount of the juice to 100.00 mL with distilled water, and titrate 25.00 mL of that solution, then work backwards to the original juice nic level. This way I am always titrating a dilute aqueous solution, not a pure juice.

I think the density issue can be avoided with accurate balances, rather than volume, dilute it some, let sit, then bring it up to the final total volume. Its not clear if this issue was across the board with vendors that mix high-nic liquids this way, but using the balance definitely fixed the one vendor's issues with this situation. I've tested several of his since he got the balance, and they all are spot on...and he was averaging about 20+% high before that. So one battle won!

And Rocket, mixing to a final total volume is really the only way a chemist would make a solution, with or without this contraction issue. Its just good technique. Mixing with two different fractions of the total volume only works if the solution has exactly the same interactions between molecules as the separate starting liquids do, and that is not at all always the case. Ethanol and water will do the same thing, for largely the same reasons...disruption of H-bonds.

 

[Rocketman]

could that have been a typo? (1.02?)

plus a 1% analytical error?

and it really was 1009 mg/ml?

 

[Kurt]

I saw several different analytical lab reports that had d > 1.009 g/mL. Maybe a typo, but using mass got to the right nic level. To me this avoids the d of nic issue as well as the possible contraction issues. We did several mixes using mass of nic, and they all came out fine.

I don't know why nic would have a higher density. Its an oil, but that should not be an issue in general. Haven't had the time to track that down, and using a balance solved it anyway.

 

[Rocketman]

Your final strength tests showed expected results based on the reported density of the source nic?

or from your specific gravity determinations?

The average DIY mixologist would not be able to do either. And once diluted by a vendor would be somewhat futile to try.
I think I take a little more care in mixing than the average DIYer but only consider the measures I take to insure not vaping a surprise 'HOT" mixture. Lots of percentage room when you vape at 12 to 15mg/ml.

I hope you can identify the root cause of the density variations in pure nicotine.
Hope it doesn't turn out to be related to 'heavy water' .

Another late question.

The compaction of nic/pg mixtures occur with initial dilution or also with subsequent dilutions that I seem to see (could just be user error)? It does seem strange than this would not peak with a 50/50 molarity (did I pick the right one?) between the nic and Pg. Or maybe a 2:1 PG/nic based on the PG molecule.

 

[Kurt]

To be able to predict that would be rather complicated. In theory, I think it is because nicotine is H-bonding less to VG or PG than those two are to themselves. So the pure VG or PG is fairly H-bonded, and this pushes the molecules apart a bit (why ice is less dense than liquid water and expands upon freezing: more H-bonds in ice). Introduce nic, and it disrupts some of the VG-VG H-bonds, and they can get closer together. That's the idea I am going on now, but I might be wrong.

So your idea of 50:50 might be correct, but I would think it would be a mole-fraction rather than a volume fraction thing. Since the mol weight of VG or PG are so much less than nic, one gram of nic has a lot less molecules than one gram of VG. Make sense? Well, this is also speculation, and the answer might be something else.

I would think if a juice is going to contract to "form" the solution, the rate that this takes place would be dependent on the viscosity of the juice made. And shaking can introduce microbubbles, so there are several variables that would need to be controlled out. Too much work for me right now.

My main interest in this is pure nic + PG or VG. Especially at the 100 mg level, and with vendors that dilute pure nic in retail bottles. Less of an issue with DIYers, since a 6% deviation will generally not be a dangerous thing.