Are large micelles rigid or flexible? A reinterpretation of viscosity data for micellar solutions

Surfactant solutions in which large cylindrical micelles form are characterized by two essential features, namely, they are polydisperse and the average micelle size is proportional to the square root of the micelle concentration. These features of the cylindrical micelles imply that the intrinsic viscosities of these micellar solutions should also depend on the concentration of micelles. This fact has been ignored in all past treatments of micellar viscosity data. Here, viscosity data available in literature were reexamined in terms of concentration-dependent intrinsic viscosity so as to evaluate the rigidity or the flexibility of the large micelles. It is found that the interpretation of viscosity data assuming that the large micelles are rigid rods is in very good agreement with the theoretically anticipated essential features of large micelles. It is concluded that large micelles can be satisfactorily represented by rigid rods rather than by flexible rods or coils.