FIG. 5 An oil/water microemulsion droplet composed of an oil core dressed with a surfactant film. When an excess phase is present (Winsor I case), the chemical potential of the oil in the fully equilibrated droplet is the same as in the excess oil phase.

doi:10.1201/9780203021262-168

Chapter

FIG. 5 An oil/water microemulsion droplet composed of an oil core dressed with a surfactant film. When an excess phase is present (Winsor I case), the chemical potential of the oil in the fully equilibrated droplet is the same as in the excess oil phase.

Chapter

ABSTRACT

The stability (minimum) condition with respect to a deformation mode as described by a spherical harmonics Ylm has been derived by Ou-Yang and Helfrich [29]. It has the form:

(116)

The most unstable case occurs for for which we get:

(117)

where the left-hand side of the inequality is normally on the order of 2kT whereas the right-hand side typically amounts to 6kT. Thus, we may conclude that by introducing the notion of curvature free energy, supposedly depending on curvature to second order in agreement with the Helfrich expression, we can readily account for the thermodynamic stability of spherical microemulsions as opposed to the instability exhibited by ordinary droplets and bubbles of very small size.