The Importance of Surface Area in Adsorbents
Large Surface Area is Key to a Valuable Adsorbent
Why is surface area key to a quality adsorbent?
Before we talk about surface area it’s helpful to understand how adsorption works.
Adsorbents work by adsorbing liquids or vapors into pores on their surface. The adsorption process doesn’t truly absorb the vapor or liquid that’s running through it (meaning the the liquid or vapor isn’t turned into a solid with the adsorbent), rather molecules from the vapor or liquid are adsorbed and thus they get stuck on to the adsorbent. In short an adsorbent acts like a magnet.
The pores on an adsorbent are where adsorbed molecules are kept. The pores can have diameters between a couple of nanometers to hundreds of nanometers. The purpose of the pores is to not only store molecules but sometimes to separate certain molecules by size. The pore sizes can differ by nanometers or Angstroms (1 Angstrom = 1/10,000,000,000th of a meter) so you can separate liquids and gases at a molecular level.
For example if you wanted to separated methane from water you would use a 3A molecular sieve because the pore size on 3A is 3 Angstrom. Water molecules have diameters up to 2.9 Angstrom and methane molecules have diameters up to 3.8 Angstrom. The molecular sieve adsorbs the water and doesn’t adsorb the methanol thus separating the two molecules from one another.
Surface area measures how much exposed area there is on solid objects. It’s important to distinguish that surface area and volume are not the same. As long as the width, length, and height of an object remain the same the volume will never change. Surface area, on the other had, can change if you break the object into smaller pieces. See the example with the cube below.
Surface Area of a Cube = l*w*6
Volume of a Cube = l*w*h
Cube Length: 10mm
Cube Width: 10mm
Cube Height: 10mm
Cube Volume = 10*10*10=1,000mm3
Cube Surface Area = 10*10*6=600mm2
The volume of an object will remain the same, but surface area can expand. For example if you break the cube above into 5 parts you would find the following.
Number of Cube Shaped Boxes: 5
Cube Surface Area:
(2*10*10) + ( 4*2*10)*5=1,400mm2
Cube Volume: (2*5)*10*10=1000mm3
By breaking the cube up into smaller sections, the surface area of the cube increases while the volume remains constant.
Surface area in adsorbents can be large. 1 gram of activated carbon for example has a surface that’s usually around 500m2
The pores on most adsorbents go only a few molecules deep so what you need is a lot of pores if you want to adsorb a lot of material. Since pores are on the surface that is why you need a lot of surface area. More surface area means more pores which means more liquid/gas is adsorbed.
Size of methane molecule, Slide 16 http://www.epa.gov/lmop/documents/pdfs/conf/12th/gladstone.pdf