Sentence:  Physicochemical investigations of anionic surfactants , including the soaps , have shown that there is little polymerization or agglomeration of the chain anions below a certain region of concentration called the <WSD>critical</WSD> micelle concentration . 
Meaning of the word marked with <WSD> tags:  at or of a point at which a property or phenomenon suffers an abrupt change especially having enough mass to sustain a chain reaction