Sentence:  Whereas the usual organic surface-active agent is strongly sorbed at oil-water interfaces , the highly charged ions are most strongly sorbed at <WSD>interfaces</WSD> between water and insoluble materials exhibiting an ionic structure ( see Table 26 - 2 on p. 1678 ) . 
Meaning of the word marked with <WSD> tags:  (chemistry) a surface forming a common boundary between two things (two objects or liquids or chemical phases)