[Skiznibbler]

What gives a substance a greater surface area than other substances. For example substances like activated charcoal and perlite supposedly have very large surface areas. Why does a 10cc cube of activated carbon have a greater surface area than a 10cc cube of iron? Is it simply the fact that the charcoal has many little cracks and pores that exposes more surface area?

Also why is it that a substance with a greater surface area can absorb greater amounts of other substances? Is it intermolecular forces that makes activated charcoal absorb and retain other substances? If so would I be right in assuming activated charcoal absorbs highly polar and ionic substances more readily than non polar substances?

[Arkcon]

Is it simply the fact that the charcoal has many little cracks and pores that exposes more surface area?

Yes, carbon is naturally very porous, but activated carbon is specifically treated to have a large amount of microporosity - pores within pores withing pores.

Is it intermolecular forces that makes activated charcoal absorb and retain other substances?

Yes, in many ways the surface of activated carbon can be considered, in the short range to be like graphite -- hexagonal molecules of carbon laid flat.  You can think of it as benzene-like, without the hydrogens, or semi-metallic electron sharing (carbon is a metalloid,) and molecules are attracted by dispersion forces.  Activated carbon has a special affinity for aromatic compounds, in preference to more polar compounds.  Envision the pi electrons stacking if you will, and once lost in the maze of pores, unlikely to get out.

[Skiznibbler]

That explains some of it. Thanks. Carbons a metalloid?? I was sure carbon, phosphorous and selenium didn't fit into the metalloid family. Are you sure about that? None of these elements make good semi conductors. I don't know about P or Se but they use carbon to manufacturer resistors so its definitely no semi conductor.

I know activated carbon is very porous but what I was asking is if thats the reason it has a large surface area or are there other factors involved? I know thats the case with perlite. It only has a large surface area because its packed full of cracks and gaps. What I'm wondering is why does a large surface area absorb substances more readily. When talking about surface area and absorbing I'm guessing they are referring to very porous substances.

What do you mean molecules are attracted by dispersion forces? Thats the first time I've heard the term dispersion forces. I don't understand how they have an affinity for aromatic compounds either. You say activated carbon is bonded hexagonally like benzene. When you say think of a pi bond to you mean pi bonds between the carbon atoms?

[Arkcon]

That explains some of it. Thanks. Carbons a metalloid??

True enough, carbon isn't a metaloid, but it's close.   If there's such a thing as a "poor metal" maybe there's such a thing as a "poor non-metal"

I know activated carbon is very porous but what I was asking is if thats the reason it has a large surface area or are there other factors involved? I know thats the case with perlite. It only has a large surface area because its packed full of cracks and gaps. What I'm wondering is why does a large surface area absorb substances more readily. When talking about surface area and absorbing I'm guessing they are referring to very porous substances.

Cracks, gaps, pores, interstitial spaces, aren't those all the same thing?  Just different names?

What do you mean molecules are attracted by dispersion forces? Thats the first time I've heard the term dispersion forces.

Oh, I don't know, what are the kids calling it these days   London forces?  Or Van Der Walls?  Again with the names.

I don't understand how they have an affinity for aromatic compounds either. You say activated carbon is bonded hexagonally like benzene. When you say think of a pi bond to you mean pi bonds between the carbon atoms?

Look at a graphite structure, see it as stacks of hexagonal carbon atoms.  That is one, of the many, natural carbon associations in amorphous carbon.

Don't just stick with carbon either.  Finely divided platinum, called platinum black, is very useful for hydrogenation.  Hydrogen has a special affinity for a platinum surface, but to get the best yeild, you need as much surface as possible.