A water vapour proof film that is permeable to organics ?

[Peter Apps]

Hi All,

I need to make controlled-release dispensers for African wild dog chemical signals. Permeation through a membrane should work, but some of the components are hygroscopic, so I need a membrane that is permeable to organic vapours (with molecular weights up to about 250) but impermeable to water so that atmospheric moisure cannot get into my carefully crafted mixtures.

Oxygen permeability is not an issue, high or low will make no difference.

What I am looking for is the opposite of Nafion film and Goretex - I need selective IMpermeability to water vapour.

I do not want to re-invent wheels (even if I could). Is there an existing membrane that lets organics through, but stops water ?, digging around on the web shows siloxanes to be permeable to everything, and anything waterproof to be proof against everything else as well.

What I am looking for would be singularly useless as a food or beverage packaging because it would let all the flavour leak out.

Thanks in advance for any ideas.

Peter

[AICMM]

Peter,

I will confess ignorance on the water side but what about just a teflon septa? I am using them to make inexpensive (in-house) permeation tubes (get trace organics out.) You can get these from Grace Alltech in packs of 100 I believe.

Best regards,

AICMM

[Peter Apps]

Thanks AICMM

I have done the same myself with Teflon and silicones, but the permeation rate is too low (or the devices would have to be too big) for what I need to do; scent dispensers out in the field emitting micrograms per hour. On paper Teflon is the least permeable of all the common polymers, and methyl silicone the most permeable (which I suppose is why they put Teflon faces on silicone vial seals).

Low density polyethylene looks good on paper with regard to selectivity against water vapour, but it seems to do a reasonably good job of keeping food flavours in the bag, so I am doubtful it will work.

This is probably going to turn into one of those suck it and see projects !

Peter

[Andy F]

Have you had a look at EPTFE membranes? Should still be hydrophobic, but more porous.

[Peter Apps]

Have you had a look at EPTFE membranes? Should still be hydrophobic, but more porous.

Thanks Andy, I just Googled it and ePTFE is the "active ingredient" in Goretex. It stops liquid water but lets water vapour through.

Peter

[AICMM]

Peter,

Was going to suggest Tyvek but just learned too much like Goretex. Why not silicone? (too low an emission rate?)

Best regards,

AICMM

[Peter Apps]

Peter,

Was going to suggest Tyvek but just learned too much like Goretex. Why not silicone? (too low an emission rate?)

Best regards,

AICMM

It's hard to believe, but methyl silicone is 3000 times more permeable by water than polyethylene, and water permeates through silicone 60 times faster than oxygen does.

I start to wonder why they use it for vial seals - low (?) contamination and easy to deform to seal to the vial rim I suppose.

Peter

[gcguy]

Peter,
Many years ago I watched a documentary about sampling marsh gases being generated well below the surface of a marsh. They were using long pipes pushed down into the marsh. Each one had a condom placed over the end. Apparently they are permeable to low molecular weight volatiles but impermeable to water.
Not sure if this would help and it would certainly look a bit odd trying to order them from the stores!

GCguy

[Peter Apps]

Peter,
Many years ago I watched a documentary about sampling marsh gases being generated well below the surface of a marsh. They were using long pipes pushed down into the marsh. Each one had a condom placed over the end. Apparently they are permeable to low molecular weight volatiles but impermeable to water.
Not sure if this would help and it would certainly look a bit odd trying to order them from the stores!

GCguy

Thanks GCguy - a quick google shows that condoms are now available in various high tech materials besides latex. As to the ordering - When you make a living analysing poo and pee you get used to people giving you funny looks.

I am now wondering whether there are passive samplers that have already solved my problem - they just have to work in reverse; releasing chemicals into the air instead of absorbing chemicals form the air.

Peter

[gcguy]

Peter,

Perhaps this might work, something very simple! Or have I got the wrong idea about what you are trying to achieve?

http://www.airwick.co.uk/

[Peter Apps]

Peter,

Perhaps this might work, something very simple! Or have I got the wrong idea about what you are trying to achieve?

http://www.airwick.co.uk/

They do exactly what I need to do - but cost too much. The technologies are well worked out, and could probably be adapted and costs might come down for huge orders, but for experimental scale when we are still mixing and matching odour compounds I need something very straightforward - a screw-top vial with a membrane is easy to swap around. I am not sure that the commercial non-aerosol diffusers can work with hygroscopic compounds.

When we get closer to a final recipe for the odour, and need to do large scale deployments of hundreds of dispensers I plan to ask the air freshener industry for help.

Peter

[gcguy]

A thought just crossed my mind....could you mix the compounds with something like petroleum jelly or a melt of paraffin wax. That would keep the water away from them until released into the air. My wife is a fan of scented candles these tend to have a strong smell even when not lit.

You have probably thought of this and dismissed it already!

There are quite a few industrial applications for membranes utilising pervaporation, to reduce water content during distillations, which depending on the choice of membrane, will work either way.

[Peter Apps]

Putting the compounds into a permeable matrix could work, but the problem is that the emission rates drop sharply as the surface gets depleted of compound. The gel scent dispensers use a matrix that shrinks more or less a fast as the scent dissipates, so emission rates stay more or less constant. To get a uniform emission rate would mean tailoring the gel to each scent compound, which would be a lot of fiddle until we have a final recipe for the wild dog signals.

Membrane permeation is very well studied, both from the point of view of pervaporation and barrier films in packaging, and fairly straightforward to model, which is why it is attractive for the experimental dispensers.

Peter

[chemstation]

What about a portable Infusion pump ? eg Graseby-MS26
http://www.ebay.com/itm/Graseby-MS26-Da ... 3cf9bbe657
coupled with a LCMS infusion kit/adapter for proof of concept.

I had a similar idea as gcguy, in using a battery Airwick, but the filling of an
aersol can would be tedious and expensive. I then though that instead of an
aersol can, a syringe with a spring could be configured in its place.
The syringe would have a needle with a side hole/s, then on the programmed interval, the lever would drive down the syringe into a vial, the liquid would fill the hole/s , and then be retracted, the liquid in the needle hole would then evaporate.
The issue would then be, would the hydroscopic chemicals cause a water droplet
to form in the needle.

Kind Regards
Alex

[Peter Apps]

Thanks Alex

I am sure that these would work, but the constraint is that we need to put out dozens of them at the experimental stage, and in the final implementation several thousand, so costs very quickly become prohibitive.

Aerosol puffers as repellants are already in use in South Africa with good results reported on the web, but no independent rigorous assessments, and using them here to keep big predators away from penned cattle is a distinct possibility if someone would fund it. But that is a different scale of application from producing artificial home range boundaries along hundreds of km of protected area perimeter.

Peter