Critical Behavior of {Water + AOT + Decane} Microemulsion with Small Molar Ratio of Water to AOT

Liquid-liquid phase equilibrium and the effect of a water-soluble polymer on the interaction between droplets in water-in-oil microemulsions

The liquid-liquid phase equilibria of {water/PEG200/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/n-decane} with the molar ratio of water to AOT being 37.9 and various concentrations cPEG of PEG in water were measured in this study. The critical exponent β corresponding to the width of the coexistence curve was determined, which showed good agreement with the 3D-Ising value and supported the proposal of the pseudo binary droplet solution for these multiple microemulsions. A previously developed thermodynamic approach based on the Carnahan-Starling-van der Waals type equation was improved and used to analyze the coexistence curve data of {water/PEG200/AOT/n-decane} microemulsions to deduce the interaction properties between droplets and further to investigate the effect of the additive PEG200 on these interactions. It was found that the addition of PEG200 into the {water/AOT/n-decane} microemulsion resulted in the decrease in the critical temperature and the interaction enthalpy and entropy. Both the interaction enthalpy and entropy decreased first, then increased with an increase of cPEG and had minimum values at cPEG = 25 g L-1, which showed the same tendency as the isothermal titration microcalorimetric results for the {water/PEG200/AOT/isooctane} microemulsion we reported very recently. The effects of the interaction enthalpy and the entropy on the phase separation and their dependences on cPEG were discussed and related to the effects of the additive on the rigidity of the interface layer of the microemulion droplet.

Liquid-Liquid Phase Equilibria and Interactions between Droplets in Water-in-Oil Microemulsions

The liquid-liquid phase equilibria of [water/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/n-decane] with the molar ratio w₀ of water to AOT being 37.9 and [water/AOT/ethoxylated-2,5,8,11-tetramethyl-6-dodecyne-5,8-diol(Dynol-604)/n-decane] with w₀ = 37.9 and the mole fraction α of Dynol-604 in the total surfactants being 0.158 were measured in this study. From the data collected in the critical region, the critical exponent β corresponding to the width of the coexistence curve was determined, which showed good agreement with the 3D-Ising value. A thermodynamic approach based on the Carnahan-Starling-van der Waals type equation was proposed to describe the coexistence curves and to deduce the interaction properties between droplets in the microemulsions. The interaction enthalpies were found to be positive for the studied systems, which evidenced that the entropy effect dominated the phase separations as the temperature increased. The addition of Dynol-604 into the (water/AOT/n-decane) microemulsion resulted in the decrease in the critical temperature and the interaction enthalpy. Combining the liquid-liquid equilibrium data for (water/AOT/n-decane) microemulsions with various w₀ values determined previously, it was shown that the interaction enthalpy decreased with w₀ and tended to change its sign at low w₀, which coincided with the results from the isothermal titration calorimetry investigation. All of these behaviors were interpreted by the effects of entropy and enthalpy and their competition, which resulted from the release of solvent molecules entrapped in the interface of microemulsion droplets and were dependent on the rigidity of the surfactant layers and the size of the droplet.

Effects of water content and chain length of n-alkane on the interaction enthalpy between the droplets in water/sodium bis(2-ethylhexyl)-sulfosuccinate/n-alkane microemulsions

The concentration-dependent enthalpies of mixing for water/sodium bis(2-ethylhexyl)-sulfosuccinate (AOT)/n-alkane microemulsions with different water contents ω0 and chain lengths n of n-alkane were determined by isothermal titration microcalorimetry (ITC) and flow-mixing microcalorimetry at 298.15 K and used to calculate the interaction enthalpies (-ΔH(C)) between the droplets. It was found that -ΔH(C) increased with ω0, and changed from negative to positive at about ω0 = 10. The investigation of the dependence of -ΔH(C) on n revealed that the values of -ΔH(C) were negative and had a minimum for ω0 = 5; while they were positive and had a maximum for ω0 = 15. These phenomena were discussed based on the competition of the overlapping contribution of the surfactant tails between two neighbouring droplets and the penetration contribution of the solvent molecules into the surfactant tails. These results indicated the important role of entropy in the stability of the microemulsion systems.