[durrantulah]

Hi everyone, I was hoping someone might be able to provide a bit of insight into the surface tension of propylene glycol. I am a biologist and we use propylene glycol in pit traps when catching ants, but we are keen to make sure that any ants that go into the propylene glycol sink straight away, and don't float on the surface. Now, I can see that we can either alter/mix the propylene glycol directly, and potentially change the surface tension, or we can put a surfactant on top to act more quickly in getting the ants into the liquid. But my understanding is that propylene glycol does have a low surface tension already, and is already used in some surfactants. So, in the first instance, would it make any difference to put detergent on top of propylene glycol…would it even work, or would it just mix in? Is there something that could be used as a surfactant that is non-toxic…maybe something like liquid paraffin?
Any help that can be provided would be very much appreciated.
Regards

[curiouscat]

Interesting application.

Why does toxicity matter? The ants die anyways, right?

A viscosity of 0.042 Pa·s sounds pretty low already. Water would be 0.001 Pa·s

How about Ethylene Glycol? 0.01 Pa.sec so a bit lower.

You could always try H2O + Glycol blends to get the viscosity you desire.

Why do you prefer PG? Are there other specific properties you are looking for?

[curiouscat]

Glycerol monostearate, Glycerol palmitate seem food safe surfactants. You could try those. Anyways, with the tiny doses typically used I doubt most surfactants would be very toxic.

Can you use heat to lower viscosity?

[billnotgatez]

@curiouscat
This is a wild guess, but Propylene glycol has a faint sweet taste
From WIKI

Water-propylene glycol mixtures dyed pink to indicate the mixture is relatively nontoxic are sold under the name of RV or marine antifreeze.

Ants are attracted to sweet, but I still wonder if the ants will drown
Well if another critter like a pet drinks it, there is a chance it will not die - unlike  if it drank Ethylene glycol
As you said

Interesting application.

I wonder if @durrantulah mixes in sugar to enhance the trap effect?

@durrantulah
why not experiment directly by dropping in ants and seeing how well they are trapped.
or create a small arena and see how it works with ants in the arena?
Some pre-experiment experiments.

[durrantulah]

Thanks very much for the prompt replies.

Ok, the toxicity matters because there is a chance that we will get some small reptiles falling/ going into the trap. We are using a number of design measures to reduce this, but there is still the chance that very small, juvenile skinks and blind snakes will still be able to get through. These animals are referred to as bycatch. If this happens, we need to make sure that the bycatch is capable of surviving, for ethical reasons.

To this end, we look at four things;
     making sure the level of the liquid is low enough that the bycatch can't drown,
     there is a surface that the bycatch can get onto to escape the ants (as some of the ants will be predatory),
     the liquid in the trap is not lethal to the bycatch, and
     the ants sink quickly, meaning they are less likely to escape or reach the bycatch.

The level of the liquid is fine, as the traps will only be out for about a week and we will be checking each trap every day, so that the liquid doesn't get saturated with ants and that any bycatch can be removed, and we can put a platform in for the bycatch to escape on to and await removal.

We use propylene glycol because it does an adequate job of preserving the ants, at least until we can get them into ethanol, and it appears to be unlikely to cause any issues for any bycatch that might go in the trap (propylene glycol is used as a carrier liquid in reptile keeping to administer a number of mite/ parasite sprays and medications). In the past, we have used ethylene glycol in our traps but this is more toxic and also affects our ability to use the samples for molecular work (whereas propylene glycol appears to more suitable for this too).

We don't use any sort of attractant as that would bias our results. Unfortunately, if the propylene glycol is acting as some sort of attractant, it is an unavoidable compromise.

We have experimented with ethylene and propylene glycol and there are some ants, particularly the small ones, that struggle to break the surface tension, so they sit on the surface. If they are on the surface, they can potentially make it to the bycatch or the sides of the container to escape.

So, the upshot is, the toxicity is important to keep the bycatch alive and the surface tension is important to get the ants under the surface of the liquid straight away, so as to not impact on the bycatch or escape.

Thanks again everyone.

[billnotgatez]

@durrantulah
I really enjoyed your explanation of your experiment.
It is interesting that, whatever you use in the trap, you do not want an attractant or a deterrent.
Because I was so interested, I decided to do GOOGLE on
Food safe surfactants
emulsifiers
wetting agents

And got many answers back like
Lecithin
Quillaia
polyoxyethylenesorbitanmonoleate

As mentioned before, some pretesting may be needed to get the best one.

[durrantulah]

Thanks @billnotgatez

I'm glad you enjoyed the explanation, it is always an interesting exercise to explain what you are doing to those who are not familiar with your field, an exercise I'm sure you know well.
Thanks for the suggestions, it gives me a few things to focus on.
Cheers

[curiouscat]

My guess is PG plus a few drops of any routine detergent will work just fine.

If it is ok to let humans touch it & wash clothes in it a few drops ain't going to be toxic to animals either.

[durrantulah]

Thanks curious cat. I suppose I wasn't sure whether using detergent would make much difference given they generally have PG in them already.

[HomeStation]

Pharma formulations chemist here.  Surfactants and/or detergents might damage living tissue or cause irritation.  Add Cyclomethicone to the PG to lower the surface tension or use it neat.  It is used in cosmetic, Rx, and other topical applications so it has little toxicity.  it should have a similar effect as PG but with very low surface tension.  It is cheap too.

[pgk]

It's polypropylene glycol copolymers that are used in some nonionic surfactants (e.g. pluronics, poloxamers} and not propylene glycol. Polypropylene glycol is a polymer of low toxicity, while the monomer’s “propylene glycol” toxicity is significant. Besides, propylene glycol is a solvent (that’s how the low surface tension is explained} and not a surfactant (there is a lack of lipophilic moieties: surfactants have both hydrophilic and lipophilic moieties}. As a conclusion, propylene glycol cannot be used as a surfactant. Contrary, fatty monoesters of propylene glycol, in combination with polyethylene glycol, might work. Avoid ionic surfactants that are cytotoxic, as mentioned above.
PS: Ethylene glycol behavior, is similar.