Hemi-micelles formation

The aim is to investigate when and how the adsorbed surfactants aggregate with each other, forming inhomogeneous interfacial layers. The effects of the surfactant tail length, the ionic strength and the size of the counterion on the hemi-micelle formation are also analyzed. 

After reviewing various earlier explanations for an adsorption maximum, Trogus, Schechter, and Wade [244] proposed perhaps the most satisfactory one so far (see also Ref. 243). Qualitatively, an adsorption maximum can occur if the surfactant consists of at least two species (which can be closely related) what is necessary is that species 2 (say) preferentially forms micelles (has a lower CMC) relative to species 1 and also adsorbs more strongly. The adsorbed state may also consist of aggregates or hemi-micelles, and even for a pure component the situation can be complex (see Section XI-6 for recent AFM evidence of surface micelle formation and [246] for polymeric surface micelles). Similar adsorption maxima found in adsorption of nonionic surfactants can be attributed to polydispersity in the surfactant chain lengths [247], Surface-active impuri-... 

Single molecules or micelles associate spontaneously in a thermodynamic equihbrium at a definite critical micelle concentration within a biocoUoidal system [47]. Analogously to micelle formation in liquid systems, aggregation of surfactants at a surface depends on a critical hemi-micellar concentration [48, 49]. The removal of the hydrophobic molecular region from the hydrophihc interface... 

Fuerstenau, D.W. Streaming potential studies on Quartz in solutions of aluminum acetates in relation to the formation of Hemi-Micelles at the Quartz-solution interface. J. Phys. Chem. 1956,60(7), 981-985. 

The following mechanism was put forward [31] to explain this autocatalysis (1) permeation by cosurfactant (amide) of the water-AOT-toluene interfacial regions as a result of partitioning equilibria with concomitant increase in polarity and dielectric constant in these regions (2) diffusion of swollen micelle to proximity of electrode surface (3) collision of swollen micelle with the electrode surface (de facto hemimicelle formation) or with a hemi-micelle on the electrode surface and diffusion of amide through the AOT interfacial region within the electron transfer distance of the electrode (4) irreversible oxidation of amide. 

The model on which the above derivations are based is by no means unequivocal. There is no proof that micelles diffuse to the surface and adsorb, or, indeed, that hemi-micelles as depicted in Fig. 7.7 form, although Somasundaran et al. [32] have previously postulated their existence. The transfer of solute molecules to the micelle at the surface probably involves complex interactions between surfactant, fatty acid and water perhaps with liquid crystal formation as an intermediate stage following penetration of surfactant molecules. As the earlier steps in the process are not rate limiting their formulation is perhaps less important. Diffusion of the solubilizate4aden micelle is a process which must occur. 

In this chapter we demonstrate the power of molecular modeling by addressing the formation of surfactant hemi-micelles at the air-water interface. The choice of this topic is fortuitous, because it allows us to handle two problems simultaneously. 

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