A Model of Microemulsion Formation and Percolation: Experimental Validation

Abstract

Experimental results afford strong support for a previously proposed mathematical model of the formation of a microemulsion system. Water-in-oil microemulsions with various straight chain hydrocarbons as the continuous phase, and nonylphenol ethylene oxide (NP(EO)n) condensates as surfactants were prepared using the titration method. The free energies of the system were determined using the Gibbs equation. Next, the free energy of formation of a single microemulsion droplet was calculated. Free energy value obtained from the titration method showed a high degree of agreement with the free energy predicted by the mathematical model when experimental value for the interfacial tensions at the water/oil interface and bare interface, free energy of formation of the interfacial sheath, natural radius of the microemulsion, flexibility constant of the interfacial sheath, and maximum and minimum radii of the microemulsion were inserted in the appropriate equation. This finding validates our mathematical model, and offers evidence of the important roles played by the positive interfacial tension at the water/oil interface and the flexibility of the interfacial sheath in microemulsion formation and stability.

Keywords:

• Gibbs isotherm
• microemulsion model
• microemulsion thermodynamics
• nonionic surfactants
• nonylphenol ethylene oxides
• surface tension

Acknowledgments

The authors are grateful to Dr. Liang Tsour of Unilever for allowing us to carry out quasi elastic light scattering measurements at Unilever's Edgewater, NJ, facilities.

Notes

Water 1% NaCl (1 ml), NP-6-EO primary surfactant, NP-4-EO cosurfactant.