Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: chris on 21/12/2006 23:11:05

Title: How does "silica gel" keep things dry?
Post by: chris on 21/12/2006 23:11:05

My new hard drive arrived vacuum packed and contained a small sachet of "silica gel".

What is this, how does it keep my hard drive dry, and if it soaks up water won't it swell up? If so, why don't the beads explode, or do they?

Chris

Title: Re: How does "silica gel" keep things dry?
Post by: neilep on 22/12/2006 00:04:59

There's this really good program on Discovery called ' How's It made ' it's just the kind of program that person like me (a saddo) lieks to watch.

yesterday, one of the things on ' How's It Made ' was sanitary towels !!

I am sure he made a reference to the silica gel packs and sanitary towels sharing the same ingredient..the silica.

Yes Chris, I think if it got wet it will expand !..it may well expand anything from 10-30 times it's original mass.

I believe it can be re-used after  'drying out ' and that cats enjoy it's qualities by crapping and weeing in it. ie: Cat litter

It is super-dooper absorbent and methinks you probably know all of this already...anyway...for the science side of things I'll leave it for a passing silica gel expert to dish out the real deal.

Title: Re: How does "silica gel" keep things dry?
Post by: Karen W. on 22/12/2006 00:15:08

I don't know, but you can also hold your real flowers upside down in a bucket and pour this stuff all around it and leave it like that it aides in drying your flowers and they retain their color better as I recall! I am also curious.. Silica coming from the word silicone? Is that related in some substance!

Title: Re: How does "silica gel" keep things dry?
Post by: neilep on 22/12/2006 00:39:54

As a firm believer in empirical research I went and found a girly with silicone implants and poured water on her boobies and rubbed it in.

Conclusion ?: I'm out on bail over the Christmas period !!







However...Wiki again has come to our rescue right here http://en.wikipedia.org/wiki/Silica_gel

Title: Re: How does "silica gel" keep things dry?
Post by: Karen W. on 22/12/2006 02:52:34

LOL>>> YOUR A SCOUNDRAL!! LOL

Title: Re: How does "silica gel" keep things dry?
Post by: eric l on 22/12/2006 08:01:16

I will not pretend to be the silica expert, but silica gel and silicones are about as different as - say soda water and petrol.
Silica is essentially sodium silicate, often with addition of cobaltchloride which makes its colour turn pink when moist.  (The colbaltchloride makes it slightly toxic or carcinogenic, I have to check - anyway, that's why silica gel is mostly packed in this little paper bags)
Silicones are long chain molecules, with an [-Si-O-Si-O-Si-O-]chain backbone.  (I admit, in petrol the chain is built exclusively with C-atoms)
Anyway, the chemistry of silica is inorganic chemistry, while the chemistry of silicones is much closer to organic chemistry.

Title: Re: How does "silica gel" keep things dry?
Post by: Karen W. on 22/12/2006 09:49:05

Cool I was wondering about that and wondering if they were all derivitives of each other. Thats good information. Thanks Eric!

Title: Re: How does "silica gel" keep things dry?
Post by: daveshorts on 22/12/2006 11:20:37

Silica gel is interesting stuff. Apparently you make it by taking Sodium Silicate and reacting it with Hydrochloric acid
Na₂SiO₃ + 2 HCl → H₂SiO₃ + 2 NaCl

the silicic acid (H₂SiO₃) then looses water
H₂SiO₃ → SiO₂ + H₂O

the SiO₂ then comes out of solution with each silicon bonding to other silicons via an oxygen

      |       |
      O       O
      |       |
- O - Si - O -Si - O -
      |       |
      O       
      |       |
- O - Si - O -Si - O -
      |       |
      O       O
      |       |

although it is 3D and probably roughly a tetrahederal structure and not very regular

These form loads of microscopic spheres between 0.5nm to 3μm which end up fusing together to form a structure wih loads of surface area - about 400m² per gram

[diagram=49_0]

It is then washed and dried out, and then baked to drive off the water. If you then put it in a moist condition the water will stick to the surface again, and because it has so much surface it can absorb a lot of water about 25% of it's weight.

Silicone is a different structure, CH₃ groups are added to stop it bonding away from the chain.

    CH3     CH3
    |       |
O - Si - O -Si - O
    |       |
    CH3     CH3

So you get long molecules (polymers) which can slide past one another, hence it is flexible