MadSci Network: Chemistry
Query:

Re: What properties must a matieral have to be good absorber?

Date: Sun Nov 21 23:43:19 1999
Posted By: Martin Thomas, Post-doc/Fellow, Phyiscal Chemistry, Quantachrome Corporation
Area of science: Chemistry
ID: 940743865.Ch
Message:

Rene,

Your question is directed to liquid absorption, so the answer is:

a)contact angle
b)pore volume
c)hydration etc

Let me explain...

a)For the liquid to be drawn into pores or spaces within the absorber the 
liquid must like the solid more than itself.  That is the "adhesive 
forces" exceed the "cohesive forces".  This results in a contact angle of 
less than 90 degrees.  This defines a "wetting" liquid.  The lower the 
contact angle the better for an absorber.  Note that the contact angle is 
always measured through the liquid.

b)The amount of liquid which can be absorbed is a function of the total 
pore volume or space within the solid.

c)The above two parameters explain the physical nature of absorption.  Of 
course there are many absorbers which start off dehydrated in a chemical 
sense, the ANHYDROUS form.  These materials react with water molecules to 
form the HYDRATE.  You must know of white copper sulphate turning blue 
when water is added.  I'm sure you know why.  Try to find out what works 
so well in a modern diaper (nappy).

Now, if you want to know what makes a good ADsorber, as in GAS adsorption, 
then surface area controls the adsorption CAPACITY, at least in the 
absence of the gas condensing to the liquid phase inside the pores (in 
which case pore volume is critical).  For gases to adsorb the force of 
interaction with the surface must overcome the thermal energy.  When the 
pore become really small, approaching the size of gas molecules (just a 
few angstroms, a nanometer or less)then the surface area can become huge.  
One gram of activated carbon can, for example, exhibit the surface area 
equivalent to that of a football field - and it doesn't matter which 
football; American, Association(soccer), Rugby, not even Australian Rules!

Thanks for your question!


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