Micelles. Physical chemistry of surfactant association

Molecules or ions which are amphiphilic, i.e. contain at the same time a hydrophilic and a hydrophobic (normally an alkyl chain) part frequently display in aqueous solutions a characteristic strongly cooperative association into colloidal aggregates termed micelles. A general treatment of micellization and related phenomena is given in order to provide an understanding on a molecular level. Hydrophobic interactions are the driving force in micellization but the inclusion of hydrophilic group interactions is required in the thermodynamic description. Intra- and intermicellar electrostatic repulsions are discussed in connection with the changes in micelle shape occuring at higher concentrations or at addition of electrolyte etc. The kinetics of micelle formation is characterized by two greatly different time constants, the rapid one referring to the micelle-monomer exchange and the other to the dissolution and formation of micelles. A large number of physico-chemical methods applicable to the study of micelle formation are surveyed with special emphasis on recent spectroscopic approaches which give a detailed insight into structure, dynamics and interactions. For example, the hydrocarbon chains are characterized by a rapid but slightly hindered motion. The polar head-groups are hydrated while otherwise no appreciable water-amphiphile contact occurs in the micelles. The binding of small counter-ions to micelles of ionic amphiphiles is characterized by a marked specificity affecting both spectroscopic and macroscopic properties. The phenomena of solubilization, mixed micelle formation and micellar catalysis are discussion on a molecular basis as well as the formation of reversed micelles and amphiphile aggregation in non-aqueous media. The biological implications of micelle formation are briefly indicated.