NB: This question has been edited. Originally, some of the confusion was caused by me not knowing that condensed phase could refer to solid phases as well. This has been removed, but the confusion caused by the other elements still remains and even the first answerer to this post was unable to clear that up.

The Gold Book's definition of adsorption

An increase in the concentration of a dissolved substance at the interface of a condensed and a liquid phase due to the operation of surface forces. Adsorption can also occur at the interface of a condensed and a gaseous phase.

What I've learned is that adsorption is when a fluid, or the solute(s) within a liquid, adhere to a surface. The IUPAC definition however, only recognizes the case when the solute(s) within a substance adhere to a surface. A more limited, and to my knowledge, less useful definition. Furthermore, the formulation is very ambiguous. It does not state if it is the condensed phase or the fluid phase that contains the solute(s), or if either can contain the solute(s). The middle option would necessarily be in contradiction to the IUPAC's definition of a solution:

The Gold Book's definition of solution (shortened):

A liquid or solid phase containing more than one substance, ...

This definition does not count gaseous phases as solvents. The fluid phase can be a gas, but if the dissolved substance is always within the fluid, then in cases where the fluid phase is a gas, it would be a solvent gas. This is assuming that dissolved substance is the same as solute.

Now, for part two of this mess:

Furthermore, the IUPAC definition of absorption encroaches onto the semantic territory of adsorption, at least the territory that I've seen and been taught.

The Gold Book's definition of absorption

The process of one material (absorbate) being retained by another (absorbent); this may be the physical solution of a gas, liquid, or solid in a liquid, attachment of molecules of a gas, vapour, liquid, or dissolved substance to a solid surface by physical forces, etc.

The last part there is the, or a part of the, definition of adsorption. It described surface attraction, which I thought was clearly within the domain of adsorption. The first part of the definition says that the absorbate and absorbent create a physical solution. I cannot find any definition of that, but I assume they are referring to a homogeneous mixture where the solute isn't disassociated, but still kept within a solution complex. However, an absorbate isn't (necessarily) homogeneously distributed within the absorbent. This means they're violating their own (and countless other) defintion(s) of solution.

As mentioned, a solution is a single phase containing more than one substance, and a phase, as defined by the IUPAC, is homogeneous.

The Gold Book's definition of a phase:

An entity of a material system which is uniform in chemical composition and physical state.

Are these IUPAC definitions outdated? If so, what is the most authoritative definition of adsorption and absorption. I like to use words consistently, but given how inconsistent and/or ambiguous scientific literature can be, I want to pick the best definition in order to influence the literature, if however minutely, towards more consistency and away from ambiguity.

  • 3
    $\begingroup$ That's a lot of writing to criticise what is perfectly reasonable description. $\endgroup$
    – Mithoron
    May 27, 2021 at 0:10
  • $\begingroup$ @Mithoron I think the same and I've also upvoted the answer below. Yet, I am now confused by the overlap between Adsorption and Absorption, as the definition of the latter can include the former. Perhaps the key - as I was insisting few days ago commenting a related question - is that absorption does not results in gradients. It is merely "volumic". $\endgroup$
    – Alchimista
    May 27, 2021 at 10:28
  • $\begingroup$ @Mithoron The definition is perfectly reasonable in one regard; the fact that the surface adhered to is that of a condensed substance. I did not know that "condensed" could refer to both liquid and solid phases, and not knowing that caused a part of the confusion. However, as user theorist said in his answer, he, nor me, are aware of why the definition necessitates that the adhering substance be a dissolved substance. Most cases of adsorption I've heard of involves a fluid adhering to the surface, not solutes within them (or dissolved substances within them, it appears there's a difference). $\endgroup$
    – user110391
    May 27, 2021 at 10:34
  • $\begingroup$ @Mithoron: solved. Indeed as I said before, in Absorption, there is no any potential for a concentration gradient per sé. It is like a fluid trapped in a sponge, which (let aside eventual Adsorption) is identical to the liquid outside. Now there is not even this point to justify so much words, indeed. $\endgroup$
    – Alchimista
    May 27, 2021 at 14:27
  • $\begingroup$ "Adsorption can also occur at the interface of a condensed and a gaseous phase". Why does not fixes your trouble but opposite causes one? $\endgroup$
    – Alchimista
    May 27, 2021 at 14:29

2 Answers 2


The IUPAC defintion isn't more limited than the ones you've read. Instead, it is more general.

Your confusion results from not being familiar with the term "condensed phase". You think a condensed phase is a 'more condensed liquid', and consequently think IUPAC is referring to a surface formed between a liquid and a 'more condensed liquid': "where there is an interface of two liquids, one containing a solute, and one being more condensed".

No. A condensed phase means a liquid or a solid. Thus, when IUPAC says the following, what they mean is that adsorption can occur at liquid-liquid, liquid-solid, gas-liquid, and gas-solid interfaces.

An increase in the concentration of a dissolved substance at the interface of a condensed and a liquid phase due to the operation of surface forces. Adsorption can also occur at the interface of a condensed and a gaseous phase.

Is the IUPAC definition completely general? No. It doesn't account for adsorption between supercritical fluids and solids. But it at least covers all the routine phase combinations.

As to the issue of whether IUPAC's defintion of solutions (which limits the solvent to be a condensed phase) contradicts its defintion of adsorption: While it could have been written more explicitly, a close read indicates there's not necessarily a contradiction. Let's take each sentence in turn:

The first sentence concerns a liquid phase in contact with a solid or liquid surface. Here the adsorbent is referred to as a "dissolved substance", which would make it the solute within a liquid solution:

An increase in the concentration of a dissolved substance at the interface of a condensed and a liquid phase due to the operation of surface forces.

Note that the solute can be (prior to its dissolution) a solid, liqud, or gas.

The second sentence concerns a gaseous phase in contact with a solid or liqid surface. Here no explicit mention is made of a dissolved substance:

Adsorption can also occur at the interface of a condensed and a gaseous phase.

Thus we could parse IUPAC's language to mean that adsorption involves either the solute within a liquid solution that is in contact with a liquid or solid surface, or a gas that is in contact with a solid or liquid surface (where that gas could be pure, or part of a mixture).

  • $\begingroup$ I see, I did suspect that perhaps "condensed phase" meant anything more condensed than the gaseous phase. However, why does the definition only involve the increase in concentration of dissolved substance at the interface. Can't it also be the liquid or gas that adheres to the surface, and not just solutes within them? According to the IUPAC definition, a gas cannot be a solvent, so according to the IUPAC, what is adhering to the surface when a gas is the adsorbate? Also, why does the absorption definition involve adsorption: ... attatchment of ... to a solid surface. $\endgroup$
    – user110391
    May 27, 2021 at 8:30
  • 2
    $\begingroup$ According to the IUPAC definition of a solution, the solvent must be a liquid or solid. However, the IUPAC definition of adsorption is not restricted to solutions. Thus you could have adsorption of, e.g., gaseous acetone dissolved in gaseous nitrogen, or gaseous acetone dissolved in liquid water, onto a graphite surface. As to why IUPAC's definition of adsorption is limited to dissolved substances, I can't say. Perhaps someone else here can. Pure acetone at a pressure of p*, and dissolved acetone at a partial pressure of p* in an approximately inert gas, should adsorb approx. equivalently. $\endgroup$
    – theorist
    May 27, 2021 at 9:06
  • $\begingroup$ I thought that if a substance A was dissolved in substance B, substance A is a solute and substance B is a solvent. But dissolution can happen outside of the context of solutions? If something is dissolved in something else, aren't we necessarily talking about a solution? $\endgroup$
    – user110391
    May 27, 2021 at 10:28
  • $\begingroup$ @user110391 What is solvent and what is solute is based on the chemist explaining the system. It is entirely* semantically. (*Well, there are cases where one makes more sense than the other, but it is still a choice) $\endgroup$
    – Stian
    May 27, 2021 at 13:08
  • $\begingroup$ @StianYttervik First of all, by mentioning the idiolectic differences in people's definition of solutions, are you then confirming that a substance that has a dissolved substance within it is necessarily a solution? If so, the chemist describing the system here is the IUPAC, and they're self-contradictory, since the absorbent can be a gas, and the absorbent (or absorbate, they're not clear on that so I assume both) is always a solvent according to the IUPAC. Thus, the IUPAC says that a gas cannot be a solvent but sometimes has to (not exactly though, depending on the absorb(ent/ate) thing). $\endgroup$
    – user110391
    May 27, 2021 at 13:27

In a short email correspondence with the IUPAC, where I raised these issues, I was told that efforts are being made to revise the Gold Book. Jim McQuillan, COB of the Commission on Physiochemical Symbols, Terminology and Units, wrote this in an email:

We looked at the terms Adsorption, Absorption, and Sorption and found issues with all of the Gold book entries which underlined the urgent need to have these terms revised.

Exactly why these issues exist hasn't been made clear, and that isn't really the main issue (although my question does make it seem a bit like that). It does seem however, that this may be the result of outdatedness and/or a lack of consistency (perhaps due to poor coordination between the authors of the Gold Book, or maybe a result of poor coordination between authors within chemistry literature in general).

However, the important thing here is that the question of whether or not these definitions are problematic, has now been resolved; the IUPAC has directly confirmed the definitions' need for revision, and that the definitions are misaligned with IUPAC recommendations (see project details).

  • 1
    $\begingroup$ Excellent job. IUPAC glossary is not a heavenly book. All these definitions, symbolism are decided by human beings. Ten or fifteen years ago, the definition of extensive and intensive variables were messed up. I wrote to them, recieved no response but it was corrected. $\endgroup$
    – AChem
    Dec 3, 2021 at 1:24

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