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

Abstract

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.