Premicellar aggregation of amphiphilic molecules

We revisit the problem of amphiphilic aggregation using a simple two-state (monomer-aggregate) thermodynamic model, which allows the study of metastable aggregates of variable size. A sequence of well separated concentrations emerge: c₁, where a metastable aggregated state appears; c₂, above which an appreciable amount of metastable aggregates forms; and c ₃, where the aggregated state becomes stable. Of these, c₃ is shown to correspond to the critical micelle concentration (cmc) as commonly measured in macroscopic experiments. Thus, appreciable premicellar aggregation is predicted in the concentration range between c₂ and c₃. We show that, so long as the micelles are not too large, the extent of premicellar aggregation is much larger than that expected from mere finite-size effects. It stems from the variability of the micelle size and the small free energy difference between the metastable state, containing monomers and aggregates, and the pure monomeric one. The aggregate size is found to weakly change with concentration below and above the cmc. The existence of premicellar aggregates and their concentration-insensitive size are in agreement with a recent experiment.