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A number of bottles of pop (two varieties) in unopened plastic bottles have sat for a few years in a room-temperature environment. All of the bottles have now collapsed a certain amount (all to about the same degree).

They all started out under pressure from the carbonation, but now, not only is that gone, but the internal pressure has dropped below ambient. They all still have an "air" space above the liquid contents; not sure if it is or is not more than when originally purchased. If one of these bottles is opened, it draws in air and regains its original shape.

I have the following questions:

  1. What happened to the carbonation? Did it leak out, or did it undergo some sort of chemical reaction, or perhaps both?

  2. How could the pressure inside the bottle drop below ambient? What chemical or physical process is causing it, and what is driving the process? Why would it not simply reach equilibrium with ambient?

  3. What would be the ultimate result if these bottles were left even longer?

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    $\begingroup$ The carbonate diffuses out until its partial pressure is the same in the bottle and outside. Strictly speaking the same is true for the water in the bottle. $\endgroup$ – Karl May 25 '16 at 1:54
  • $\begingroup$ @Karl Diffuses through the plastic? If the CO2 and H2O diffuse out, why doesn't O2/N2 diffuse in? $\endgroup$ – Anthony X May 25 '16 at 3:39
  • $\begingroup$ A net transport by diffusion depends on a gradient in partial pressure. Also the diffusion coefficients of the gases are very different. $\endgroup$ – Karl May 25 '16 at 8:23
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    $\begingroup$ comment: This happens to me with sealed plastic water bottles (non-carbonated) at work, where the indoor temperature is set by a thermostat to the same temperature (day and night, including weekends and holidays, summer winter). Only takes a few months for the sides of the bottle to cave in. I've noticed this for occurring for over a decade, in different buildings at work. Happens with many different brands of bottled water. (I keep the water bottles a year as part of my emergency supply). I wonder if the walls of the container weaken, and became stretched? $\endgroup$ – Russ G Jun 7 '17 at 21:16
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  1. It is most likely the CO2, H2O and air in the pop bottle headspace diffused (leaked) out of the pop bottle.

  2. You are correct: the pressure has reached ambient (not dropped below ambient). Because the pressure in a freshly bottle pop bottle was originally higher than the pressure in the room (normally atmospheric pressure), and the bottle's plastic walls were smooth and strong, any decrease in pressure inside the pop bottle would cause the pop bottle walls to collapse.

  3. If the bottles were left longer, the CO2 and water would continue to evaporate from the liquid phase until the headspace contained approximately the same pressure as the air in the room. If somehow all the water evaporated, you'd be left with sugar and a bunch of other solids (that were dissolved). I doubt full evaporation of water would occur.

Also, if the bottle is made from PETE (polyethylene terephthalate), prolonged contact with water (including pop) will result in many by-products. Two of them are:

a. UV exposure, air and water (together) promote peroxide formation on polymer end-groups (OH, COOH). Another way of saying this is that the bottle walls degrade.

b. The PETE plastic could leach some trace antimony (if that was used in the polymerization process) and/or phthalates, suspected to be endocrine disruptors.

IF water permeates through the bottle walls, it can allow for microoganisms to diffuse in/out of the bottle. You might get some mould in there!

Those are just some things that could happen.

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    $\begingroup$ Re: point #2: The pressure inside the bottles is slightly less than ambient - the partially-collapsed bottles "want" to return to their normal shape and do so as soon as they are opened. The diffusion process has evidently maintained a less-than ambient internal pressure. $\endgroup$ – Anthony X May 26 '16 at 0:43
  • $\begingroup$ re: micro-organisms... I was really only aiming to understand the limiting condition of the collapse process. $\endgroup$ – Anthony X May 26 '16 at 0:45
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The weakest point in a bottle with a lid is usually the lid - note that differences in temperature can see the lid loosen and gas escape (I'm going to guess there's no wax or similar over the lid to prevent this). Alternatively, some other form of slow leak exists. I'm also going to guess that there's some temperature variation in the room they're stored in. In this case, when it's warmer, more gas will escape and when it's cooler the pressure will be lower than external ambient . There is a famous experiment where you take an empty bottle or similar, and place it in hot water with no lid, then place it cold water with a lid on, whereupon it collapses, illustrating this last idea.

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  • $\begingroup$ I know the demonstration. Boil water in a can, tightly cap it and allow to cool. As the water condenses, atmospheric pressure crushes it. But in this case, temperature variation is just a few degrees, ranging from minimum of maybe 19C to a maximum of 25C, and it is liquid volume which has been lost. I haven't measured, but "guesstimate" that they have lost perhaps 25% or more of their liquid volume. $\endgroup$ – Anthony X May 26 '16 at 0:50
  • $\begingroup$ Over a few years, across multiple change of seasons? Okay, that was a long way from my experience. In my place, it would be from 10 C night time winter to 30 C when we get home from work in summer (i.e. before we switch the A/C on), based on observing the thermostat. $\endgroup$ – Robert de Graaf May 26 '16 at 0:59
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CO2 diffused through the plastic or possibly leaked out the closure. PET is the most common "plastic" bottle in the USA but others are used. However, when PET was developed many things were stored for extensive periods in bottles to satisfy FDA rules and provide technical information. As I remember ,carbonated beverages went over 2 years with no change ( this may be searchable under Amoco Chemicals after about 1970.) I suspect you had a different plastic ; For example , H2S totally diffuses through vinyl in 24 hours.

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protected by Community Apr 11 at 18:29

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