What’s the difference between absorb and adsorb?

[Crafter_Man]

According to this site,

Absorb
To take in and hold, not just on the surface, but deep into the material, usually forming a mixture out of two or more substances.

Adsorb
To take in and hold liquid and gas on the surface of a solid but porous material without causing a change in the basic structure of the material.

So does that mean a sponge does not absorb water? That it instead adsorbs water? Have I been using the term “absorb” wrong all my life?

This site gives opposite definitions. Is it incorrect?

[Surreal]

I don’t agree with those definitions.

Absorb is used to describe a situation where a substance is mechanically drawn into something else, i.e. capillary action.

Adsorb is used to describe a situation where something is drawn into something else via some type of chemical attraction.

[Crafter_Man]

Thanks Surreal, but it looks like we have dueling definitions here…

These sites say “adsorb” is when a material is “deeply entrenched” or “molecularly bonded” to another, while “absorb” is when a material simply fills the voids of another:
http://www.tomgriffin.com/aquasource/filtersdopt3.shtml
http://www.nanfa.org/archive/nanfa02/nanfajul02/0496.html
These sites say just the opposite, i.e. : “absorb” is when a material is “deeply entrenched” or “molecularly bonded” to another, while “adsorb” is when a material simply fills the voids of another:

http://www.greaseinspectors.com/pictures_info/gi_roof/roof_info.lasso
http://www.pioneerair.com/htdocs/glossary.htm
http://www.agresearch.co.nz/scied/search/tools/Elisa/answers.htm

The score is 3 to 2 with the above links. Your definition is all-together different.

I’m really confused.

So where do you go for accurate info nowadays??

[Nametag]

Surreal, that’s absolutely wrong.

Crafter_Man, bread absorbs gravy, sponges absorb water, and silica gel absorbs water. CO adsorbs onto the surface of platinum, mercury adsorbs onto the surface of gold, and just about everything adsorbs onto activated charcoal. Absorption is deep, but not necessarily strong, and adsorption is on the surface, but not necessarily weak (usually pretty strong, as weak interaction with a surface is usually called “touching” )

Either chemical interaction, mechanical trapping, or both might be involved in either process. The absorption of water into silica gel (or phosphorous pentoxide, or other desiccants) is a matter of chemical interaction (hydration), whereas a sponge relies on a more mechanical process, but both involve the deep penetration of water and intimate comingling with the sorbent molecules (sponges, unlike most sorbents, does not dissociate when saturated; instead, the polymer molecules “stretch”).

Adsorption, on the other hand, is a purely surface phenomenon. This is apparently the only thing the sites cited understand about it. Adsorption can occur by simple trapping of material on the surface (I am reminded of water “wetting” almost any surface; there’s an electrostatic attraction between water and all hydrophilic materials, and if the material is porous, it will adsorb water), or by a more chemical sort of interaction, as when a crystal surface, already irregular, accepts atoms of a doping agent.

[Crafter_Man]

I just found this in my ASHRAE Handbook in the “Sorbents and Desiccants” chapter:

The process of attracting and holding moisture is described as either adsorption or absorption, depending on whether the desiccant undergoes a chemical change as it takes on moisture. Adsorption does not change the desiccant, exception by the addition of the mass of water vapor; it is similar in some ways to a sponge soaking up water. Absorption, on the other hand, changes the desiccant. An example of an absorbent is table salt, which changes from a solid to a liquid and it absorbs moisture.

The ASHRAE Handbook is rather authoritative. Should I use this definition for my paper?

[Crafter_Man]

Nametag: Appreciate the response. But does your definition jive with ASHRAE’s definition as it applies to desiccants? If not, might ASHRAE be wrong?

[Terminus_Est]

I think Nametag’s got it. Adsorption involves some form of chemical bonding to a surface. Absorption when one substance is drawn into another substance, and may or may not involve a chemical change. I think part of the confusion comes because adsorbent materials are usually porous - the better to increase their surface area. [So do sponges absorb or adsorb water?]

[barbitu8]

Van Nostrand’s Scientific Encyclopedia gives the following definitions. I’ll give adsorption first, since that’s the easier one.

Adsorption is a type of adhesion which takes place at the surface of a solid or a liqid in contact with another medium, resulting in an accumulation or increased concentration of molecules from that medium in the immediate vicinity of the surface.

Absorption has several definitions, depending upon whether it is energy, physiology, or a process.

Absorption (Energy). The process whereby the total number of particles emerging from a body of matter is reduced relative to the number entering as a result of interaction of the particles with the body. Also, the process whereby the kinetic energy of a particle is reduced while traversing a body of matter…
Absorption (Physiology). The process by which materials enter the living substance of which the organism is composed. Materials including food and oxygen are taken into special organs by ingestion and respiration, but they must pass through the cell wall to become an integral part of the organism by absorption. The basic physical forces involved are those of osmosis and diffusion.
Absorption (Process). Absorption is commonlyused in the process industries for separating materials, notably a specific gas from a mixture of gases; and in the production of sol;utions such as hydrochloric and sulfuric acids. … The absorption medium is a liquid in which (1) the gas to be removed, i.e., absorbed is soluble in the liquid, or (2) a chemical reaction takes place between the gas and the absorbing liquid.

[andy_fl]

Absorption is used very loosely by all scientific disciplines. Adsorption is not used that loosely but still used somewhat loosely. I can see ASHRAE’s definition to be not entirely correct. But again it works for a moisture system - from a Chemical Engineer’s standpoint it’s wrong. Here are somethings to note :

Absorption

1. Usually refers to a physical phenomenon where liquids or gases fill up pores due to surface tension. It also refers to soluble gases dissolving in liquids. Having said that, sometimes low entropy change reactions may occur during absorption - like absorbing CO2 in a NaOH solution.

2. Refers to gases absorbed in liquid. And liquids absorbed in solids.

3. The entropy change is usually not much.

4. When designing a absorption system - you are concerned with the solubility of the gas/liquid being absorbed and hence the volume of the liquid needed.
   Adsorption

5. Usually is a surface activity - surface means not only what you see but both macro and microscopic pores. It is almost a chemical bond.

6. Usually refers to gases/vapors absorbed on a solid surface.

7. Adsorption is very selective - only particular gas/vapor molecules will be absorbed in a mixture.

8. Usually reversible - and reversed by changing pressure/temperature.

9. When designing a Adsorbing system, you are concerned about how much surface area is available for adsorption.
   I can point out many other differences if you want. Oh another thing a adsorbing system is much more expensive than a absorbing system.

[Enola_Straight]

ADsorb means something adheres to the surface of something else.

ABsorb means something is drawn into the interior volume of something else.

[moriah]

Not getting into how a discipline may have adapted (correctly or incorrectly) these words in a specialized way, in general, we can say:

The prefix ad- means to or toward. And so adsorb would be like adhesion. Material X sticks to Material Y. Water vapor adsorbs to cold surfaces. Cold surfaces adsorb water vapor.

The prefix ab- means away or away from. And so absorb would be like suction. Material Y absorbs Material X from Medium Z. Sponges absorb water from counter tops. Water is absorbed from counter tops by sponges.

Hope that helps.

Peace

Now, for adductors and abductors…

[Apollyon]

Merriam-Websters concurs:

Absorb, from Latin ab- + sorbEre, to suck up: to suck up or take up <a sponge absorbs water> <charcoal absorbs gas> <plant roots absorb water>

Adsorption (noun form of adsorb): adhesion in an extremely thin layer of molecules (as of gases, solutes, or liquids) to the surfaces of solid bodies or liquids with which they are in contact.