Sentence:  Whereas the usual organic surface-active agent is strongly sorbed at <WSD>oil-water</WSD> 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:  an interface forming the boundary between the non-miscible liquids oil and water