Sentence:  Whereas the usual organic surface-active agent is strongly sorbed at oil-water interfaces , the highly charged ions are most strongly sorbed at interfaces between water and insoluble <WSD>materials</WSD> exhibiting an ionic structure ( see Table 26 - 2 on p. 1678 ) . 
Meaning of the word marked with <WSD> tags:  the tangible substance that goes into the makeup of a physical object