Chapter 3 81 The fraction, v/al, is known as the packing factor (see Figure 3). When the packing factor has a value of 1/3, the surfactant molecule can be approximated by a conical shape and the molecules pack into a sphere (Figure 4). When the packing factor has a value of ½, the micelles become cylinders (Figure 5), and when the packing factor has a value of 1, the surfactant molecules pack as planar bilayers in a so-called lamellar structure (Figure 6). For ionic surfactants, the area per head group can be decreased by adding soluble salt to the solution to lessen the ionic repulsion between the head groups. (Salt also enhances the hydrophobic interaction.14) Increase in salt and/or surfactant concentration causes spherical micelles to transition to rods and then to long worm-like micelles.15 The wormlike micelles behave like polymers in solution.16 Figure 3. Surfactant molecules having a packing factor of 1/3 have a shape that can be approximated by a cone. Figure 4. These conical molecules pack naturally into a sphere.