When to drop the “ad” and “ab” prefixes

The prefixes ad and ab are generally used with the meaning toward and away from, respectively. But not within every context is the meaning and distinction that clear. An interesting example comes with the terms adsorption and absorption—near homophones. The noun adsorption refers to the attraction of something toward the interface of an object, while absorption means that something has been drawn into the bulk of the object through the interface. But where does the interface end and where does the bulk begin? And what if the subject of interest is found both at the interface and inside the object?

Adsorption and absorption phenomena are studied in physical chemistry. They play a key role in many chemical engineering processes. Chemical species such as neutral molecules and ions may stick to a material surfaces and/or diffuse deeper into the substrate material. Robert Kunin, who worked in the field of ion exchange, thoughtfully elaborated on the “ad-versus-ab controversy” in the 1950s:  

If we consider adsorption processes strictly as confined to changes occurring at an interface or surface and absorption as those processes involving solids engulfing substances throughout their entire structure, then ion exchange phenomena may fall into both categories and might therefore be more safely considered as a sorption process, a term that avoids the controversy between the devotees of the ad and ab prefixes. --- [Robert Kunin, 1958].

Kunin elected to use the term adsorption within his book on ion-exchange resins, considering that “energy considerations of ion exchange are more closely related to true adsorptive processes.”

Often, your particular context will clarify whether to speak of  ad- or absorption. When in doubt or with excusable lack of commitment to detail, the unprefixed word sorption—as implied by Kunin—should be the word of choice. The expression “ad/ab-sorption” is possible, but may confuse more than help.

Keywords: terminology, prefix use, physicochemical vocabulary.

 

Reference

Robert Kunin: Ion Exchange Resins. John Wiley & Sons, Inc., New York, 1958 (Second Edition); page 5.