Colloid association

The surface-active agents (surfactants) responsible for wetting, flotation and detergency exhibit rather special and interesting properties characteristic of what are called association colloids or, in the older literature, colloidal electrolytes. These properties play an important role in determining, at least indirectly, the detergency of a given surfactant and are therefore considered here... 

Other properties of association colloids that have been studied include calorimetric measurements of the heat of micelle formation (about 6 kcal/mol for a nonionic species, see Ref. 188) and the effect of high pressure (which decreases the aggregation number [189], but may raise the CMC [190]). Fast relaxation methods (rapid flow mixing, pressure-jump, temperature-jump) tend to reveal two relaxation times t and f2, the interpretation of which has been subject to much disagreement—see Ref. 191. A fast process of fi - 1 msec may represent the rate of addition to or dissociation from a micelle of individual monomer units, and a slow process of ti < 100 msec may represent the rate of total dissociation of a micelle (192 see also Refs. 193-195). 

The traditional association colloid is of the M R" type where R" is the surfactant ion, studied in aqueous solution. Such salts also form micelles in nonaqueous and nonpolar solvents. These structures, termed inverse micelles, have the polar groups inward if some water is present [198] however, the presence of water may prevent the observation of a well-deflned CMC [198,199]. Very complex structures may be formed in nearly anhydrous media (see Ref. 200). 

All these results are consistent with the hypothesis that aryl cations react in aqueous media at diffusion-controlled rates with all nucleophiles that are available in the immediate neighbourhood of the diazonium ion. On this basis Romsted and coworkers (Chaudhuri et al., 1993a, 1993b) used dediazoniation reactions as probes of the interfacial composition of association colloids. These authors determined product yields from dediazoniation of two arenediazonium tetrafluoroborates containing ft-hexadecyl residues (8.15 and 8.16) and the corresponding diazonium salts with methyl groups instead of Ci6H33 chains. ... 

Staudinger relentlessly championed the molecular, or primary valence, viewpoint in the years which followed. He supported his original contentions with the observation that hydrogenation of rubber, as well as its conversion to other derivatives, does not destroy its colloidal properties. In contrast to association colloids, high polymers (or macromolecules as he chose to call them ) exhibit colloidal properties in all solvents in which they dissolve. Polyoxymethylenes were ex-... 

Colloids are introduced in the second half of the chapter. The various classifications of colloid types are discussed, together with ways of forming, sustaining and destroying colloids, i.e. colloid stability. Finally, association colloids ( micelles ) are discussed. 

The weak physical forces that hold together self-assembled nanoparticles are, of course, susceptible to disruption under the influence of thermodynamic and/or mechanical stresses. Hence some workers have investigated ways to reinforce nanoscale structures via covalent bonding. For instance, improved stability of protein nanoparticles, in particular, casein micelles, can be achieved by enzymatic cross-linking with the enzyme transglutaminase, which forms bonds between protein-bound glutamine and lysine residues. By this means native casein micelles can be converted from semi-reversible association colloids into permanent nanogel particles (Huppertz and de Kruif, 2008). 

In fact, even in pure block copolymer (say, diblock copolymer) solutions the self-association behavior of blocks of each type leads to very useful microstructures (see Fig. 1.7), analogous to association colloids formed by short-chain surfactants. The optical, electrical, and mechanical properties of such composites can be significantly different from those of conventional polymer blends (usually simple spherical dispersions). Conventional blends are formed by quenching processes and result in coarse composites in contrast, the above materials result from equilibrium structures and reversible phase transitions and therefore could lead to smart materials capable of responding to suitable external stimuli. 

Before we proceed, however, it is important to review briefly the roles thermodynamic and kinetic considerations play in determining the structure. In some cases, the distinction is easy to establish. In the case of the association colloids we discussed in Chapter 8, thermodynamics determined the formation and the structure of the colloidal particles and their subsequent transformations to more complex structures at higher concentrations of the particles. In... 

The phenomena of association colloids in which the limiting structure of a lamellar micelle may be pictured as composed of a bimolecular leaflet are well known. The isolated existence of such a limiting structure as black lipid membranes (BLM) of about two molecules in thickness has been established. The bifacial tension (yh) on several BLM has been measured. Typical values lie slightly above zero to about 6 dynes per cm. The growth of the concept of the bimolecular leaflet membrane model with adsorbed protein monolayers is traceable to the initial experiments at the cell-solution interface. The results of interfacial tension measurements which were essential to the development of the paucimolecular membrane model are discussed in the light of the present bifacial tension data on BLM. 

It is convenient to classify sols into three types (I) tvophilii (solvent loving) colloids, for example, are solutions or gelatin or starch in water (2 association colloids, of which a solution of Soap in water at moderate concentration is an example and (3) Iwphohic (solvent repelling) colloids, for example, sulfur in water. Both lyophilic and association colloids can be prepared in thermodynamic equilibrium, so that when solvent is removed and then returned to the system, the original properties of the system are regained. 

The dimensionality of the lattice depends on the physical nature of the phase. For transitions in linear biopolymers and associative colloids, a one-dimensional lattice is used because these materials become ordered in a onc-dimcnsional manner even though they actually exist in three dimensions. For absorption of gas onto a surface, a two-dimensional lattice is sufficient. For bulk phase changes, however, a three-dimensional lattice must be used. 

For binary solutions including linear associative colloids, the 0 and 1 represent solvent and solute respectively ivn and woo are the potential energies between like molecules and 1 3 is the potential energy between unlike molecules. 

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