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The effect of active substance on the interfacial tension of an alkali/inorganic salt/synthetic surfactants/ crude oil system was studied, through the ways of contact time, infrared scan and mass analysis. This system consisted of heavy alkyl benzene sulfonate, sodium chloride, sodium hydrate and Daqing crude oil. The result of the experiment indicate that active substance diffusion from oil/aqueous interface to aqueous phase and finally the process of diffusion has a equilibrium in the system with the prolonging of the contact time. At the same time,the value of the minimum IFT and the value of equilibrium IFT rises with the prolonging of the contact time and has a linear relationship between IFT and t-1/2 when IFT value approaching the minimum and after the minimum, the process is diffusion controlled. The oil and aqueous phase had been analysed by infrared scan and mass analysis, and then the phenomenon have been found that the content of active substance in the oil decreased and the content of active substance in the aqueous increased after alkali had a reaction with compound system. It is proved that the active substances in the oil play a very important role for reducing the IFT.

Key words: contact time; diffusion action; alkali; interfacial tension