What exactly is negative adsorption?

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Summary:

My book just gave a brief description about negative adsorption. Now that I have read it and didn’t understand it properly, I can’t forget it and want to know what it is.

Main Question or Discussion Point

My book states the following: When the concentration of the adsorbate is less on the surface relative to its concentration in the bulk, it is called negative adsorption.

I googled it and there are barely any source that talk about negative adsorption. I saw one or two videos and I believe that they are spreading the ‘wrong’ knowledge.

According to that video, when the concentration of the adsorbate on the surface is less than its concentration in the bulk of the adsorbent it is called negative adsorbtion. But according to me that definition is very vague. It gives us an idea that not only does the adsorbate penetrate through the surface but some of it is alos retained on the surface of the adsorbent. Then the concentration of the adsorbate inside and outside the adsorbent is compared to determine if the the adsorption is negative or positive.

I read one example online. When a dilute solution of potassium chloride is shaken wih blood charcoal concentration of the KCl in the solution (bulk) reduces.
Is it because the solvent (dipolar water molecule) is adsorbed on the charcoal and hence the amount of solvent in the bulk of the solution reduces due to which concentration of KCl increases. Is it?

Could you please help me in understanding negative adsorption?

(The video is in my native language and hence I haven’t shared it here.)
 

Answers and Replies

  • #2
Borek
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Never seen the term. First thing that is difficult to stomach for me is the comparison of bulk concentration with surface concentration, apples and oranges.

Could be there is a way of defining both things in a way that will allow comparison, but I don't know it.
 
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Never seen the term. First thing that is difficult to stomach for me is the comparison of bulk concentration with surface concentration, apples and oranges.

Could be there is a way of defining both things in a way that will allow comparison, but I don't know it.
I suppose one could compare the relative % cross section of the adsorbate for a slice "at the surface" to a similar (parallel) slice "in the bulk". Not a term of art with which I am familiar, either.
 
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jim mcnamara
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Is this perhaps a translation issue? If your textbook is in English or you can direct us to some reference we can read it sure would help. There are advisors here here like @TeethWhitener and @chemisttree who may be acquainted with the term. I do not know about it.
 
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  • #5
TeethWhitener
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I’ve never encountered the term but the concept seems straightforward enough. If you add a sorbent and the concentration of your solute in the solution goes down, it’s positive adsorption; if you add a sorbent and the concentration of the solute goes up, it’s negative adsorption.

The reason is presumably that positive adsorption occurs when the solute has a higher affinity for the sorbent than the solvent (thus causing preferential adsorption of the solute onto the sorbent surface and lowering the concentration of the solute in the solution phase) and negative adsorption occurs when the solvent has a higher affinity for the sorbent than the solute.
 
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  • #6
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I’ve never encountered the term but the concept seems straightforward enough. If you add a sorbent and the concentration of your solute in the solution goes down, it’s positive adsorption; if you add a sorbent and the concentration of the solute goes up, it’s negative adsorption.

The reason is presumably that positive adsorption occurs when the solute has a higher affinity for the sorbent than the solvent (thus causing preferential adsorption of the solute onto the sorbent surface and lowering the concentration of the solute in the solution phase) and negative adsorption occurs when the solvent has a higher affinity for the sorbent than the solute.
Thanks!

I found this on the internet and it is almost the same as your explanation. Now what I don’t understand is what do they mean by bulk (in the definition of positive adsorption). Is it the adsorbent’s bulk or the solution’s bulk?
 
  • #7
Borek
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if you add a sorbent and the concentration of the solute goes up, it’s negative adsorption.
That was my first intuition, but it doesn't make sense. The only way to make the concentration to go up is to add the solute itself, just adding a sorbent can't do that.
 
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The only way to make the concentration to go up is to add the solute itself, just adding a sorbent can't do that.
Unless the the sorbent adsorbs the solvent. Does "concentration of the adsorbate is less on the surface relative to its concentration in the bulk" mean that the solute/solvent ratio is less on the surface compared to the solution?
 
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  • #9
256bits
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Summary:: My book just gave a brief description about negative adsorption. Now that I have read it and didn’t understand it properly, I can’t forget it and want to know what it is.

I googled it and there are barely any source that talk about negative adsorption. I saw one or two videos and I believe that they are spreading the ‘wrong’ knowledge.
There are hordes of sites about adsorption mentioned positive and negative.

What do you think adsorption is if not a surface effect?
Adsorption is a surface process that leads to transfer of a molecule from a fluid bulk to solid surface. This can occur because of physical forces or by chemical bonds. Usually it is reversible (the reverse process is called desorption); then it is responsible not only for a subtraction of substances but also for release.
https://www.sciencedirect.com/topics/earth-and-planetary-sciences/adsorption

Either the concentration of the adsorbate on the surface, or rather within the small interstitial layer adjacent to the surface, is greater or less than its concentration in the bulk ( ie farther away from the surface ). That's all it is.
 
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  • #10
256bits
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Summary:: My book just gave a brief description about negative adsorption. Now that I have read it and didn’t understand it properly, I can’t forget it and want to know what it is.

When a dilute solution of potassium chloride is shaken wih blood charcoal concentration of the KCl in the solution (bulk) reduces.
That doesn't explain much.
Versus what happens with a concentrated solution of KCl, should have been added to the quote


Page 180
https://books.google.ca/books?id=QoErDAAAQBAJ&pg=PA180&lpg=PA180&dq=dilute+solution+of+potassium+chloride+is+shaken+with+blood+charcoal+experiment.&source=bl&ots=I_EqM1ZBcf&sig=ACfU3U34gzqAX69vRZA1CY52OajB4aw2qw&hl=en&sa=X&ved=2ahUKEwj-xubDq8LqAhX7oHIEHRVaBE0Q6AEwA3oECAkQAQ#v=onepage&q=dilute solution of potassium chloride is shaken with blood charcoal experiment.&f=false

1594372856883.png
 
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  • #12
TeethWhitener
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That was my first intuition, but it doesn't make sense. The only way to make the concentration to go up is to add the solute itself, just adding a sorbent can't do that.
@DrStupid in post 8 has the general idea I was getting at here. If the sorbent adsorbs solvent very strongly, then the relative concentration of the solute in the solution phase will go up.
 
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  • #13
TeethWhitener
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Thanks!

I found this on the internet and it is almost the same as your explanation. Now what I don’t understand is what do they mean by bulk (in the definition of positive adsorption). Is it the adsorbent’s bulk or the solution’s bulk?
“Bulk” on that website refers to what I’ve been calling the solution phase.
 
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  • #14
chemisttree
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All kinds of things potentially going in here for the blood/KCl/charcoal example. Blood is not simply a solute in a solvent. It is a mixture of solutes in a solvent. The fact that high concentrations of KCl causes a different type of adsorption can simply be due to a competitive binding between two or more solutes with the bound species modifying the surface chemistry of the sorbent. An increased salt content can cause a species in solution to adopt a different tertiary structure which would cause a change in how it presented its hydrophobic parts to the sorbent’s surface. Whatever the mechanism, the end result is neatly described with the terms “positive adsorption” and “negative adsorption.”

You might describe solute/stationary phase interactions in affinity chromatography to be an extreme example of positive adsorption and hydrophilic solute/reverse phase chromatographic interactions to be an example of negative adsorption.
 
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  • #15
Borek
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Unless the the sorbent adsorbs the solvent.
Good point, missed that.
 

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