Micellar aggregates, molecular packing

Micellization is a second-order or continuous type phase transition. Therefore, one observes continuous changes over the course of micelle fonnation. Many experimental teclmiques are particularly well suited for examining properties of micelles and micellar solutions. Important micellar properties include micelle size and aggregation number, self-diffusion coefficient, molecular packing of surfactant in the micelle, extent of surfactant ionization and counterion binding affinity, micelle collision rates, and many others. 

As we demonstrate in this chapter, FTIR spectroscopy can be an ideal technique for studying the molecular packing in micellar aggregates. The major... 

The reverse micelle phase behavior in supercritical fluids is markedly different than in liquids. By increasing fluid pressure, the maximum amount of solubilized water increases, indicating that these higher molecular weight structures are better solvated by the denser fluid phase. The phase behavior of these systems is in part due to packing constraints of the surfactant molecules and the solubility of large micellar aggregates in the supercritical fluid phase. 

The hydrophobic effect , firstly recognized by Tanford, is responsible of selfassociation phenomena. Amphiphilic molecules can form micellar systems provided that their packing parameter, vjal, is not too close to unity. Significant changes of NMR parameters are observed as a result of amphiphiles self-assembly. Indeed molecules experience strong intermolecular interactions due to the interplay of both electrostatic and van der Waals forces. Micellar aggregates usually form isotropic liquid systems, thus NMR experiments can be easily performed and modeled. Hence chemical shifts, relaxation, and self-diffusion NMR measurements can provide reliable information, at a molecular level, on critical micelle concentration (c.m.c.), molecular conformations and interactions, counterion binding, hydration also in mixtures of different amphiphiles. 

Figure 16.1 Relationship between molecular shape, aggregate structure in dilute dispersions, phase behavior and packing parameter. Micellar phase (L,), cubic micellar phase (I), hexagonal phase (H), bicontinuous cubic phase (Q), La lamellar phase. Subscripts I and II indicate normal and inverted phases, respectively. From M. Scarzello, Aggregation Properties of Amphiphilic DNA-Carriers for Cene Delivery, Ph. D. Thesis University of Groningen, p 6, 2006...

---