Micellization surfactant structure relation

Markina, Z. N., G.A. Chirova, andN. M.Zadymova. 1998. Structure-related mechanical properties of hydrogels of micelle-forming surfactant oll. J. 60 568-572. 

On a molecular basis, the conflicting factors can be seen conceptually as arising from the different roles of the molecular structure in the adsorption process. Surfactant efficiency is related to the extent of adsorption at the interface as a function of bulk surfactant concentration. At a concentration well below that at which micellization becomes a factor, efficiency can be structurally related to the hydrophobicity of the surfactant tail and the nature... 

As might be expected, the structure of the reactive substrate can have as much influence on micelle-assisted rate enhancement as that of the surfactant. Since the catalytic effectiveness of the micelle can be related to the location and orientation of the substrate in the micellar structure, the more hydrophobic the substrate (and the surfactant), the more significant may be the catalytic effect. 

The first parameter acting on the cmc value is the surfactant structure. A longer hydrophobic chain will render the surfactant more hydrophobic and promote micelle formation. For the homologous surfactant series, the cmc can be related to the alkyl chain length by ... 

Fluorinated surfactants are both hydrophobic and lipophobic. For example, potassium per-fluorooctanesulfonate—an industrially important surfactant —forms a third phase with octanol and water, and it is impossible to determine its octanol-water partition coefficient. " Similar to fluorocarbon-hydrocarbon bulk solvent mixtures, mixed binary systems containing a perfluorocarbon surfactant and a structurally related hydrocarbon surfactant are known to behave nonideally, that is, exhibit phase separation in insoluble monolayers at the air-water interface or form two types of micelles simnltaneously in solution—one type is fluorocarbon-rich and the other is hydrocarbon-rich. This nonideal behavior of fluorocarbon-hydrocarbon surfactant mixtures is used in firefighting foams and powders—an important technical application of fluorinated surfactants. " ... 

The above refers to surfactants. However, it also holds for amphiphilic block copolymers. These compounds are structurally related to surfactants, but their hydrophilic and hydrophobic moieties are of polymeric nature macromolecular surfactants). They can form micelles, microemulsions, vesicles, and lyotropic mesophases, just like surfactants. Amphiphihc block copolymers are currently attracting much interest in view of their many potential and actual uses. ... 

The critical micelle concentrations of mixtures of ionic and nonionic surfactants has not been as fuUy explored as that of mixtures of structurally related materials, although it appears as if such systems are reasonably well behaved. Using the assumption that the mole fraction of ionic and nonionic surfactants in the micelle is the same as the bulk ratio, a good correlation can be made between micellar aggregation number, cmc, and the composition of the mixture. 

Chapter 4 discussed the formation of relatively small, uniform, or isotropic association structures or micelles in dilute surfactant solutions. We know, however, that surfactants and related amphiphilic molecules, including the naturally occurring lipids, some proteins, and a variety of combined natural chemical species, tend to associate into structures more extensive than simple micelles in both aqueous and nonaqueous environments. In many cases, such assembUes can transform from one type to another as a result of sometimes subtle changes in solution conditions such as (1) changes in the concentration of the amphiphilic components, (2) the addition of new active components, (3) changes in solvent composition, (4) the addition of electrolytes, (5) temperature changes, (6) changes in solution pH, and (7) unspecified influences from internal and external sources—such as the phase of the moon, or so it seems at times. 

This chapter focuses on three types of phenomenon that are closely related to the presence of amphiphUes and micelles in solution, and on the roles surfactant structures and other characteristics may play in their application. To exploit the micellar nature of surfactants and to realize their technological potential, it is necessary for the investigator to understand and very carefiilly control the many variables involved in the various phenomena. It is probable that in the near future we will see a dramatic increase in the use of micellar and related systems to produce better, more effective, more appealing, and (hopefully) cheaper products for the realization of a better world. 

In the past few years, a range of solvation dynamics experiments have been demonstrated for reverse micellar systems. Reverse micelles form when a polar solvent is sequestered by surfactant molecules in a continuous nonpolar solvent. The interaction of the surfactant polar headgroups with the polar solvent can result in the formation of a well-defined solvent pool. Many different kinds of surfactants have been used to form reverse micelles. However, the structure and dynamics of reverse micelles created with Aerosol-OT (AOT) have been most frequently studied. AOT reverse micelles are monodisperse, spherical water droplets [32]. The micellar size is directly related to the water volume-to-surfactant surface area ratio defined as the molar ratio of water to AOT,... 

Description of the different mimetic systems will be the starting point of the presentation (Sect. 2). Preparation and characterization of monolayers (Langmuir films), Langmuir-Blodgett (LB) films, self-assembled (SA) mono-layers and multilayers, aqueous micelles, reversed micelles, microemulsions, surfactant vesicles, polymerized vesicles, polymeric vesicles, tubules, rods and related SA structures, bilayer lipid membranes (BLMs), cast multibilayers, polymers, polymeric membranes, and other systems will be delineated in sufficient detail to enable the neophyte to utilize these systems. Ample references will be provided to primary and secondary sources. 

Table 6.2 Schematic representation of nanoscale structure and experimental data relating to self-assembly of sodium caseinate induced by interactions of the protein (1.0 % w/v) with micelles of food-grade surfactants (CITREM and SSL) in an aqueous medium (pH = 5.5, ionic strength = 0.05 M, 293 K) above the surfactant cmc. 

Chapter 8 has been revised to include a discussion of the critical packing parameter of surfactants and its relation to the structure of resulting surfactant aggregates. This simple geometric basis for the formation of micelles, bilayers, and other structures is intuitively easier to understand for a beginning student. 

---