Association Colloids Micelles, Vesicles, and Membranes

Previous chapters have discussed the formation of colloidal particles by various mechanisms, including commutation, nucleation and growth, and emulsification. There exists another very important class of colloids that differ significantly from those discussed previously. Their formation, for example, does not result from the input of energy such as in commutation or emulsification it is a spontaneous association process resulting from the energetics of interaction between the individual units and the solvent medium, as is crystallization. However, the size, shape, and basic nature of the associated structure are controlled by a complex series of factors distinctly different from those involved in crystalhzation. The size, in particular, will be much more limited than that of a normal crystal. This class of colloids is generally referred to as association or self-assembled colloids. 

This class of association colloids can be further divided into several subgroups, which include micelles, vesicles, microemulsions, and bilayer membranes. Each subgroup of association colloids plays an important role in many aspects of colloid and surface science, both as theoretical probes that help us to understand the basic principles of molecular interactions, and in many practical applications of those principles, including biological systems, medicine, detergency, crude-oil recovery, foods, pharmaceuticals, and cosmetics. Before undertaking a discussion of the various types of association colloids, it is important to understand the energetic and structural factors that lead to their formation. 

The adsorption of surfactants at interfaces has been, and will be, discussed in specific contexts. However, surfactants also have a life of their own within 

The physical manifestation of one such mechanism is the crystalUzation or precipitation of the surfactant from solution—that is, bulk-phase separation. An alternative is the formation of molecular aggregates or micelles that remain in solution as thermodynamically stable, dispersed species with properties 

FIGURE 15.1. A surfactant in solution has various options in terms of its surface activity. Depending on the system composition, surfactants can (and usually do) play the field, completing various functions at the same time. Usually, the multirole playing is advantageous, although there are situations in which such flexibUity can be counterproductive. For that reason, surfactant selection can be an important decision in many applications. 

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Although the impact of this molecular-level effect is quite small (negligible in most cases), it can, under some circumstances, produce an appreciable energetic effect—repulsive in the case of two approaching surfaces—that is, it will be difficult to displace the last molecular layers separating the surfaces—or attractive in the separation of two contacting (adhesive) surfaces. These topics will appear again in Chapters 10 (on colloids and colloidal stability), 15 on association colloids micelles, vesicles and membranes), and 19 on adhesion). 

There is, however, a large and important class of colloids in which nuclcation is absent. Growth is spontaneous, but the structures so formed are usually limited by geometric and energy factors to a finite size often towards the lower end of the colloid size range. This class comprises association colloids, or in more general terms self-assembly systems, it includes not only micelles but many more complex forms, e.g. vesicles with, as extreme examples, biological structures such as cell membranes. 

Although the vast majority of surfactants form micelles of some kind in aqueous solution, some materials, because of their special structure or composition, will not associate in the normal way described above. They will, however, take part in other association processes to form equally interesting and important association colloids, including especially vesicles and bilayer membranes. 

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