I was recently given a humidor and one of the recommendations is to use distilled water or propylene glycol (PG) to hydrate the sponges inside. Unfortunately, I have Dipropylene Glycol (DPG) on hand for making incense but not PG. From my experience and meager knowledge of chemistry, in general, it seems to me one could be substituted for the other for most applications but I have been able to find nothing about doing so in a humidor. I know DPG is slightly more viscous than PG and has a little higher boiling point but have no idea whether the difference is enough to significantly affect performance in a humidor or if DPG would actually even work. If anyone knows a definitive answer to the question of whether DPG can be used instead of PG or even where to begin researching this, feedback would be appreciated.

Why not use only water as recommendation. But if you want use chemicals for it, glycols any kind can be used. I would recommend glycerol.

21 hours ago, chenbeier said:

Why not use only water as recommendation. But if you want use chemicals for it, glycols any kind can be used. I would recommend glycerol.

It has to be distilled water or you likely end up with mold, according to what I have read. While not a big deal to get/make, it seems silly to bother for a couple tablespoons of liquid every week when you have copious quantities of a possible substitute on hand most of the time. Short of actually trying DPG, I was hoping to find out more first and have learned a lot about it and similar chemicals but none of the information really seems to address the question I am asking.

7 hours ago, npts2020 said:

It has to be distilled water or you likely end up with mold, according to what I have read. While not a big deal to get/make, it seems silly to bother for a couple tablespoons of liquid every week when you have copious quantities of a possible substitute on hand most of the time. Short of actually trying DPG, I was hoping to find out more first and have learned a lot about it and similar chemicals but none of the information really seems to address the question I am asking.

All I know about this comes from this Wiki article which I have just read: https://en.wikipedia.org/wiki/Humidor

I note this claims that a 50:50 mixture of water and propylene glycol is recommended due to the buffering effect on humidity. Having looked it up, it appears propylene glycol is hygroscopic. This presumably means that if the humidity is high a suitably concentrated mixture of propylene glycol and water can actually pull excess humidity out of the air, hence the buffering effect.

The same appears to be true of dipropylene glycol, but I can't find anything that compares the relative degree of hygroscopic behaviour of the two. So it looks to me as if both can work but you would be taking a bit of a chance, in that you would not know at what level of relative humidity dipropylene glycol would buffer the system.

Both appear to be non-toxic and odourless, so not problematic from that point of view.

11 hours ago, exchemist said:

The same appears to be true of dipropylene glycol, but I can't find anything that compares the relative degree of hygroscopic behaviour of the two. So it looks to me as if both can work but you would be taking a bit of a chance, in that you would not know at what level of relative humidity dipropylene glycol would buffer the system.

Both appear to be non-toxic and odourless, so not problematic from that point of view.

This is basically what I have found out and speculated about, nothing definitive, but pointing pretty strongly that one may be substituted for the other. Appreciate the feedback from someone I am sure knows way more about chemistry than I.

3 minutes ago, npts2020 said:

This is basically what I have found out and speculated about, nothing definitive, but pointing pretty strongly that one may be substituted for the other. Appreciate the feedback from someone I am sure knows way more about chemistry than I.

Why not try it then and see how it goes? Doesn't seem too risky to me.

I'm actually now intrigued by what partial vapour pressure of water is generated by a 50:50 mixture (presumably by weight) with these hygroscopic glycols. I imagine there will be a -ve deviation from Raoult's Law, i.e. it will be lower than expected on the basis of the mole fractions present. It's just not something I've ever thought about before.

In fact there could be something of a catch here. The molecular weight of your dipropylene glycol will be approx double that of simple propylene glycol. So a 50:50 mixture by weight, or by volume, will have about half the number of molecules of the glycol and more water in proportion. So you may find the humidity is higher with the di glycol than with the recommended mono version. But I have no idea how sensitive cigars are to all this. On that, you are the expert, not me.

I am going to try it. Will post in a few days whether there is a noticeable difference. What you say makes sense, since I believe the glycol is mostly to keep the water from evaporating too fast but have no idea what the reality will be.

6 hours ago, npts2020 said:

I am going to try it.

I wish you well.

But I would caution that just as ethylene glycol (MEG), diethylene glycol (DEG), and triethylene glycol (TEG) have very different enduse applications due at least in part to their very different hygroscopic strengths (TEG being by far the strongest dehumidifier), then you may find out why propylene glycol is a recognised humectant and DPG is not.

23 hours ago, sethoflagos said:

why propylene glycol is a recognised humectant and DPG is not.

Actually, DPG IS a humectant and commonly used in hair and skin care products. I just have no idea how it compares to PG but, so far, there seems to be little difference while curing the humidor.

17 hours ago, npts2020 said:

Actually, DPG IS a humectant and commonly used in hair and skin care products...

... where it is a part of the formulation and helps retain moisture within that product. So yes, it acts as a humectant in that context. But we are not talking about that context are we? The DPG is not going inside the... cigars(?)

As I said, I wish you well.

6 hours ago, sethoflagos said:

... where it is a part of the formulation and helps retain moisture within that product. So yes, it acts as a humectant in that context. But we are not talking about that context are we? The DPG is not going inside the... cigars(?)

As I said, I wish you well.

Sure, exactly the same use as propylene glycol, the recommended liquid for in a humidor. So far it seems like the DPG works fine but I just wondered if anyone here knew much more about it than I do.

Thanks, I was going to try it anyway unless somebody knew of a good reason not to.

8 hours ago, npts2020 said:

Sure, exactly the same use as propylene glycol

No. Not really.

Think about it. If DPG keeps cosmetic products moist under drying ambient conditions, this implies that at some concentration within the formulation, it is able to extract moisture from air at perhaps <50% RH. This makes it a humectant wrt to the facepack or whatever it is a component of, but wrt the ambient surroundings it is a dessicant, and this is how it is classified on eg. PubChem.

This does not sound compatible with a 50:50 aqueous solution buffering an equilibrium 70% RH atmosphere which is what you want in a humidor and what MPG provides.

11 hours ago, sethoflagos said:

No. Not really.

Think about it. If DPG keeps cosmetic products moist under drying ambient conditions, this implies that at some concentration within the formulation, it is able to extract moisture from air at perhaps <50% RH. This makes it a humectant wrt to the facepack or whatever it is a component of, but wrt the ambient surroundings it is a dessicant, and this is how it is classified on eg. PubChem.

This does not sound compatible with a 50:50 aqueous solution buffering an equilibrium 70% RH atmosphere which is what you want in a humidor and what MPG provides.

Well, these guys, and all of the other cosmetic sites I looked at, seem to think DPG is a humectant.

Skincare Lab

Dipropylene Glycol - Is It Safe | Skincare Lab

Dipropylene Glycol is a type of alcohol popular in skincare, cosmetics, and personal care products to improve the stability and texture of products.

Well, after a couple weeks DPG seems to be basically the same as propylene glycol. The humidity measurements have been within about 2% of expectations (just under 70%) and well within the recommended 65-75%.