Correlation between the Mixing Ratio of Surfactants and the Water/Oil Ratio in Middle Microemulsions in Water/Mixed Surfactant/Hydrocarbon Systems

We studied the relationship between the mixing ratio of surfactants adsorbed at the water/oil interface ( C opt ) and the weight ratio of oil/(water + oil) ( R ow ) inside the middle microemulsion and the difference between the C opt and the mixing ratio of surfactants in the whole surfactant ( W opt ). The three-phase plane method was employed to determine the C opt and the monomeric solubilities of surfactant in water and oil ( S w and S °) in water/mixed poly(ethylene glycol) alkyl ethers (C m EO n )/hydrocarbon (oil) systems. A linear relationship can express the relationship between the C opt and the R ow over a range of 0.2 < R ow < 0.8, while the S w and S ° are almost unchanged. In the HLB number ( N HLB ) scale, the curves of C opt vs R ow lie between N HLB = 10 and 11 for the decane system and near N HLB = 12 for the cyclohexane system. The W opt deviates from the C opt: the deviation is a function of C opt and the secession ratio of surfactant, i.e., the ratio of (lipophilic) surfactant desorbed from the micro interface into oil to the total surfactant.