Chapter 3 79 shape of a micelle can be explained on the basis of the principle of opposing forces (see Figure 1). Two or three amphipathic molecules alone cannot form a stable micelle because micellization is essentially a cooperative process that requires the participation of many amphipathic molecules bound together by hydrophobic interaction. However, if hydrophobic interaction accounted solely for the formation of micelles, then the association would continue until phase separation occurred, as in oil separating from water. Therefore, there must be a force that opposes the hydrophobic association and controls the size of the micelles. This force is the repulsion between the head groups that could arise from ion-ion repulsion and/or hydration of the head groups.12 Theoretically the repulsive surface terms are difficult to handle from a thermodynamic perspective but the presence of micelles has been extensively validated experimentally. If micelle structure was determined solely by thermodynamics, spherical micelles would always be favored over other shapes. However, real micelles are not restricted to a spherical shape in fact, spherical structures account for only a small minority of micelles. Figure 1. The shape of a surfactant micelle is determined by the balance between the mutual repulsion between hydrophilic groups at the micelle surface and the cohesion due to hydrophobic interaction. This has been dubbed the principle of opposing forces.