Association Colloids and Self-Assembly Systems

The formation of colloidal dispersions by growth from the molecular state is often controlled by a nuclcation step. An embryo has to achieve a certain minimum size before growth can proceed spontaneously (Chapter 4) but is then limited by the availability of material in the bulk phase from which the particle is growing. 

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. 

This chapter outlines the main features of systems of this kind and indicates in broad terms the factors leading to their formation, and controlling their size and shape. Discussion of another group of spontaneously formed colloids — gels - will be deferred to Chapter 13. 

VVe observe that surface-active substances that form micelles possess one common feature, namely they are amphipathic, which means that the molecule consists of two parts, one of which is highly soluble in the medium concerned and the other insoluble. We shall deal mainly with aqueous solutions, although it is important to realise that micellisation is not confined to water as solvent but can occur in many non-polar media. In the case of aqueous systems the surfactant molecule consists of a hydrophilic group (head group) to which is attached a hydrophobic hydrocarbon group (tail). 

Two main types of such compounds may be distinguished ionic (including anionic, cationic. and ampholylic) and non-ionic Ionic 

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On the basis of interaction between the particles or macromolecules of the dispersed phase with the molecules of the dispersion medium, colloidal systems are classified into three groups 1) lyophilic, solvent loving colloids, in which the disperse phase is dissolved in the continuous phase 2) lyophobic, solvent hating colloids, in which the disperse phase is insoluble in the continuous phase and 3) association colloids, in which the dispersed phase molecules are soluble in the continuous phase and spontaneously self-assemble or associate to form aggregates in the colloidal size range. 

Surfactant molecules commonly self-assemble in water (or in oil). Even single-surfactant systems can display a quite remarkably rich variety of structures when parameters such as water content or temperature are varied. In dilute solution they form an isotropic solution phase consisting of micellar aggregates. At more concentrated surfactant-solvent systems, several isotropic and anisotropic liquid crystalline phases will be formed [2]. The phase behavior becomes even more intricate if an oil (such as an alkane or fluorinated hydrocarbon) is added to a water-surfactant binary system and the more so if other components (such as another surfactant or an alcohol) are also included [3], In such systems, emulsions, microemulsions, and lyotropic mesophases with different geometries may be formed. Indeed, the ability to form such association colloids is the feature that singles out surfactants within the broader group of amphiphiles [4]. No wonder surfactants phase behavior and microstructures have been the subject of intense and profound investigation over the course of recent decades. 

Figure 19.10. Illustrations of the effects of hydrophobic interactions, i. e. the tendency to eliminate contacts between water and nonpolar molecules or surfaces (a) water and oil are immiscible, with a strong driving force to expel hydrocarbon molecules from water (b) self-assembly of surfactant molecules (c) other types of association of hydrocarbon chains (d) folding of proteins (e) strong adhesion between non-polar surfaces in water (f) non-wetting of water on hydrophobic surfaces (g) rapid coagulation of hydrophobic particles in water (h) attachment of hydrophobic particles to air bubbles (mechanism of froth flotation). (Redrawn from J. Israelachvili, Intermolecular and Surface Forces, with Applications to Colloidal and Biological Systems, Academic Press, London, 2nd Edn, 1991)...

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