But, the way I understand it (read: going off memory, I can’t be bothered to look it up ;)), total pressure is simply the sum of all partial pressures. In other words, the CO[sub]2[/sub] pressure is independent of the other pressures, and the additional air from the pumps serves only to maximize the volume of the container so additional CO[sub]2[/sub] is not needed to increase the container’s volume?
Re-reading the old thread, it looks like I never followed up on my original answer.
This analysis assumes that the bottle will remain “scrunched up.” If the flexible bottle re-expands to its original volume, the same amount of carbon dioxide will come out of solution as it would have if you had left the bottle closed up at its original volume.
Pressurizing the bottle with air depends on the fact that the gases are not, in fact, ideal, as well as the fact that some carbon dioxide is present in the pressurized air.
To maximize the carbonation of the drink, you would pressurize the air space with pure carbon dioxide.
My BIL recently claimed that compressing the bottle to reduce the head space created a pressure reduction in the head space as the bottle tried to expand - this would cause more CO[sub]2[/sub] to come out of solution to equilibrate the partial pressure. My theory is that a compressed PET bottle does not have enough elasticity to create a significant vacuum, and if you have a minimal remaining head space anyhow (maybe 1 cm at the top of the bottle) the amount of CO[sub]2[/sub] lost into that space (even under vacuum) will be less than the CO[sub]2[/sub] lost to the head space of a half empty non-compressed bottle.
Some simple experiments/math should give an answer pretty quickly.
When I was a kid, my dad started squeezing our 2 liter soda bottles and I remember thinking it made the soda go flat much sooner.
I later found in one of his old science magazines (Omni I think), an article where they mentioned having had a contest where people wrote in with some kind of ‘practical use of science in the home’ idea. One of the winners was someone who suggested squeezing the extra air out of soda bottles. They later decided that that was a mistake, and not only did it not help keep the soda carbonated, but that it made it worse.
This seems like it would be easy to test, if you have a way to measure the carbonation.
I would think that whether or not the bottle re-expands would depend on what partial pressure of CO2 is required to maintain equilibrium. If it’s over 1 atmosphere, the bottle will re-expand.
Following that trail of logic, that would mean that scrunching up bottles that are still highly carbonated won’t help (and might hurt) because they would re-expand. Scrunching up lightly- carbonated bottles, though, might prevent the contents from going totally flat.
In the interests of science, I squished a 2 litre PET soda bottle, inverted it, and stuck it mouth down in a sink of water.
The maximum height of water within the bottle was 10cm, but that took a very careful squeeze - compressing the middle of the bottle all round to avoid major creasing. The water level was much lower (5cm) with a flat “across the middle” crease.
So the internal pressure of a squeezed bottle is lower than atmospheric pressure, but not by much. If you have a squeeze and some head space (maybe 1/4 of a bottle), the partial pressure of the CO[sub]2[/sub] will drop and more will come out of solution.
A squeezed bottle with almost no head space will have much less impact on CO[sub]2[/sub] in solution.
Someone really needs to do some maths, comparing a 50% empty bottle with 50% head space, a 50% empty bottle squished with 25% head space, and a 50% empty bottle squished with 5% head space. It’s too late for me here to do that sort of work, but we should have enough info to calculate it out.
One factor perhaps ignored in many of these discussions (although perhaps not very significant - I’m not sure) - when you pour your drink, you agitate the contents of the bottle, causing some CO[sub]2[/sub] to come out of solution - and this continues after the bottle is righted for capping. The gas in the headspace of the bottle is therefore probably not plain air - and may already contain quite a high proportion of CO[sub]2[/sub].
It seems to me that there is much more gas being released that was already out of solution and sticking to the side of the container. Agitation increases the amount of nucleation sites; I guess it just depends on how rough you pour your soda.
Regardless, the equilibrium partial pressure is (mostly*) irrespective of the initial pressure or makeup of the headspace upon capping. It can even go the opposite way. For example, a flat keg can be purged with a high pressure of CO[sub]2[/sub] after it’s been filled with air, and it will regain its fizzyness again. The taste will be suspect, but this is usually done with PBR, Coors or Bud Light, or another lovers-in-a-canoe beer, so the taste part isn’t an issue.