The Effect of Salt on the Interactions Between Droplets in Water-in-Oil Microemulsions

The interaction enthalpies, entropies and Gibbs energies between microemulsion droplets for {water/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/n-decane}, {water/NaCl/AOT/n-decane} and {water/CaCl2/AOT/n-decane}, with the molar ratio ω of water to AOT being 26.3 and the concentration of salt in water being 0.02 mol·L–1, have been investigated by measurements of liquid–liquid phase equilibrium and isothermal titration microcalorimetry. The interaction enthalpies determined from isothermal titration microcalorimetry and liquid–liquid equilibrium are consistent. The interaction was found to be enhanced as the ionic strength increases. From the data of liquid–liquid equilibriums 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 and supported the pseudo binary solution proposal for these multiple microemulsions. A thermodynamic approach based on the Carnahan–Starling–van der Waals type equation was improved and used to deduce the interaction properties between droplets in the microemulsions. Salt effects on the microemusion phase equilibria are also discussed.