Sentence:  Whereas the usual organic <WSD>surface-active</WSD> agent is strongly sorbed at oil-water interfaces , the highly charged ions 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 chemical agent capable of reducing the surface tension of a liquid in which it is dissolved