Sentence:  Whereas the usual organic surface-active agent is strongly sorbed at oil-water interfaces , the highly charged <WSD>ions</WSD> are most strongly sorbed at interfaces between water and insoluble materials exhibiting an ionic structure ( see Table 26 - 2 on p. 1678 ) . 
Meaning of the word marked with <WSD> tags:  a particle that is electrically charged (positive or negative); an atom or molecule or group that has lost or gained one or more electrons