Sentence:  Polyphosphates gave renewed life to soap products at a time when surfactants were a threat though expensive , and these same polyphosphates spelled the decline of <WSD>soap</WSD> usage when the synergism between polyphosphates and synthetic detergent actives was recognized and exploited . 
Meaning of the word marked with <WSD> tags:  a cleansing agent made from the salts of vegetable or animal fats