Hello Jon,

any "plastic" permits some diffusion of gases.

Because of water molecules smaller molecular

weight (compared to nitrogen/oxygen) water

vapour will diffuse faster that air throug the plastic.

So because more water vapour diffuses outward

of Your bottle as air diffuses inward, the bolltle will

deflate. But beware! The influence of temperature

on the partial vapour pressure is very strong!

To get meaningful results, You have to have the

temperature in the bottle constant within some

tenths of a degree.

This diffusion depends on the number and dimension

of microscopic voids in the plastic.

Rather high diffusion rates one finds for hardened

plastics (eg. Bakelite) and for rubber. 

(Much to the disappointment of children, deflating their

helum-filled balloons)

For thermoplastics like PE, or PET diffusion is

is less, but not zero.

Georg

PS

there are some new (to me) sort of balloons sold on fairs

in Germany since some years, evidently not made from rubbery material.

They are fused from two or more pieces of plastic foil,

maybe PET (my guess) and the foil looks like metallized.

I think the idea is to block the outward diffusion of helium

as much as possible.

No, there is very little air in the bottles, I filled them as full as I could.

Of course, water also shrinks a bit when it's cold (if not under plus four), and it IS quite cold even indoors at the moment, but that would affect the second bottle too, because it too was filled when it was warmer.

Hello ecstatist

an addendum to "cellophane":

Pure cellophane is very permeable to water, when wetted it 

becomes limp.

For that reason cellophane for packaging got a thin coat

of nitrocellulose clear laquer.

But: is there some cellophane still in use? Here in Germany

cellophane seams to be outdated. There is a special kind of HDPE which

now is used to have that "crispy" touch, even on cigarette packages.

Georg