Solution of Petroleum Sulfonate Sulfonate and its Oil/Water Equilibrated System

Micelle and premicellar solution of petroleum sulfonate and its oil/water equilibrated system have been studied by ESR with nitroxide spin probe. The ESR parameters decrease at CMC(critical micelle concentration) and c* (efficient critical micelle concentration in water/oil system). Spectral overlapping due to probes in bulk solution and into micelle have been analyzed. The CMC of petroleum sulfonate (YM-3A) in 3% n-butanol, 0. 30% NaCl solution was found to be 0. 003% (mass fraction). However, upon equilibration with n-dodecane, the c* in the equilibrated aqueous phase was found to be 0. 02% similar to 0. 05%. It is proposed that upon equilibration with oil, the higher molecular weight species partition preferentially in the oil phase whereas low molecular weight species remain preferentially in the brine phase, Thus, after equilibration with oil, the average weight of surfactant remaining in the aqueous phase is considerable less than the average equivalent weight of the original surfactant. These results give an explanation that c* for SPS upon equilibration with dodecane is higher than CMC.