Sentence:  Polyphosphates gave renewed life to soap products at a time when <WSD>surfactants</WSD> were a threat though expensive , and these same polyphosphates spelled the decline of soap usage when the synergism between polyphosphates and synthetic detergent actives was recognized and exploited . 
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