Non-ionic microemulsion mechanism and theory of formation

An investigation of the mechanism of formation of non-ionic microemulsions began with the preparation, using the titration method, of a series of microemulsions, both W/O and O/W, based on non-ionic surfactants of the form (NP(EO) n). Mixing a constant weight of surfactant with a constant volume of the dispersed phase and an initial volume of continuous phase produces an emulsion, which is titrated to clarity with another surfactant (cosurfactant). Plotting (a) the volume of cosurfactant necessary to transform an emulsion into a microemulsion containing a fixed volume of dispersed phase and a constant weight of surfactant versus (b) different initial continuous-phase volumes yields a straight line. Extrapolating from experimentally determined values for the cosurfactant volume to the value corresponding to a zero-volume continuous phase allows the determination of the surfactant molar composition and the average number of ethylene oxides (EO) per nonylphenol adsorbed at the interface. Using a surfactant with the same number of ethylene oxides yields a single-surfactant microemulsion. Measurement of surfactant(s) transmittance in the oil and water phases demonstrates that microemulsification occurs when the surfactant interfacial film is equally soluble in the two phases. Surface pressure and potential measurements reveal that oil penetration impedes formation of O/W microemulsions with n-tetradecane or n-hexadecane as dispersed phase.;These experimental results afford strong support for a previously proposed mathematical model of the formation of a microemulsion system. W/O 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. The 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 values 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, rigidity constant of the interfacial sheath, and maximum and minimum radii of the microemulsion were inserted in the appropriate equation.