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.