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

Abstract

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