[seanspotatobusiness]

I bought some toluene for cleaning electronic flux off circuit boards. It came in the same kind of plastic bottle as a few other chemicals I ordered at the same time. It seems to be evaporating from the bottle despite it being tightly closed because the bottle is collapsing in on itself, suggesting that the toluene is leaving but not being replaced by air. Could it be going through the plastic itself? I did once have some in a jar but it seemed to knacker the seal and evaporate from there too. Do I need a glass bottle with a glass stopper? Would that be tight enough? They're kind of expensive and fragile which are two of my least favourite things! I'll store most of it in a shed outside but a smaller amount will be on my electronics bench.

[Enthalpy]

Toluene does not evaporate through the plastic walls. What you observe, the collapsing walls, can be the cooling of toluene after you have used the bottle and closed it.

When toluene is warmer, more of its vapour fill the bottle, pushing the air away. If toluene cools in the closed bottle, the vapour pressure drops, and the external atmospheric pressure lets the walls collapse.

You can observe this with a plastic water bottle. Pour a bit of hot water in it from the tap, let the hot water move in the bottle and wet the surfaces for some time, then close the bottle and wait, let optionally cold water flow on the bottle outside. Less water in the bottle makes it faster. The walls collapse.

Your toluene isn't supposedly as hot as tap water, and its vapour pressure resembles water. Maybe your bottle is less stiff than a water bottle, especially if it has no ripples and is designed to be squeezed by hand.

[P]

Does it stay crushed though when you open the bottle?  I have some plastic bottles that have crushed themselves but when opened they remain crushed - which puts doubt in my mind over the expansion/contraction explanation due to temperature change.