Aggregation of Amphiphilic Molecules in Solution: Thermodynamics, Metastability, and Kinetics

Amphiphiles are molecules, which have covalently bonded hydrophilic (soluble in water) and hydrophobic (insoluble in water) groups. In aqueous solution such molecules self-assemble into aggregates of various shapes and sizes. Many amphiphiles (e.g., surfactants, block copolymers) form spherical aggregates called micelles above a certain amphiphile concentration, known as the critical micelle concentration (cmc). This thesis addresses current issues in the theory of micellar aggregation and aims to give a unified theoretical description of some of the “universal” features of micellar solutions. Throughout this work we use a simple free-energy formalism, which views micellization as restricted nucleation. That is, the micelles are treated as nuclei of an aggregated phase, with the difference between micellization and macroscopic phase transition being the finite size of the micelles. Despite its simplicity our model enables us to study a host of new issues related to amphiphilic aggregation in and out of equilibrium. The first issue we address is the phenomenon of premicellar aggregation (i.e., the possible appearance of micelles below the cmc). This phenomenon and its features have been a subject of dispute among scholars during the years. Sensitive spectroscopic techniques like FCS, as well as NMR measurements, show in some cases the appearance of micelles at concentrations as low as 3-4 times below the literature known value of the cmc (as measured by macroscopic techniques such as conductivity and surface tension). This effect has often been attributed to the presence of a third component in the system (eg., a fluorescent dye), which participates in the formation of micelles and therefore lowers the cmc. We propose a different explanation. We study micellar aggregation using our free-energy model. It accounts for metastable states in the system, which we identify as premicellar aggregates. We examine the characteristics of these premicellar aggregates (aggregate size, polydisperisity, kinetic stability). We find that, for certain realistic values of parameters and assuming strict equilibrium, there should be an