Association colloids-micelle formation

Muckerjee, P. Differing patterns of self-association and micelle formation In Physical chemistry Enriching topics from colloid and surface science. Van Olphen, H., Mysels,K. J. (eds.). La Jolla, Calif. 1975... 

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 structure of the lyotropically mesomorphous lattice is made up of multimolecular units called mesoaggregates. These are surrounded by an intervening liquid. Lyotropic mesomorphism is therefore closely related to the tendency of lipids to accumulate at interfaces. The surface activity is a consequence of the same dualistic polar/non-polar molecular structure that causes the formation of micelles in solutions of association colloids (I, 2, 3, 4, 5, 6). 

Association colloids are formed by fairly small molecules that associate spontaneously into larger structures. A clear example is the formation of micelles, i.e., roughly spherical particles of about 5nm diameter, by amphiphilic molecules like soaps see Figure 2.8. At high concentrations, such molecules can in principle form a range of structures, called mesomorphic or liquid crystalline phases, which are briefly discussed in Section 10.3.1. To be sure, the whole system is called a mesomorphic phase, not its structural elements. 

The ease of emulsion formation increases and the droplet size achievable decreases as the interfacial tension falls. Systems in which the interfacial tension falls to near zero j<10-3 mNm (dyne cm-1)] may emulsify spontaneously under the influence of thermal energy and produce droplets so small (<10 nm diameter) that they scatter little light and give rise to clear dispersions. The micro emulsions so formed occupy a place between coarse emulsions and micelles. They are usually effectively monodispersc and unlike coarse emulsions are thermodynamically stable. Microemulsion droplets have sometimes been classified as swollen micelles. In fact, there probably exists an essentially continuous sequence of states from association colloids to coarse emulsions,... 

Flynn and Lamb have reported that solvolysis of meihylprednisolone-21-phosphate in dilute aqueous solution (less than 0.005 M) is qualitatively similar to that observed for the methylphosphate and other simple monoalkyl phosphates, particularly in the pH range 3-8. In more concentrated solutions (greater than 0.02 M), however, there is an acceleration of reaction velocities and marked deviation from the expected pH dependency. This change in chemical behavior is attributed to association colloid formation, and this interpretation is supported by independently determined critical micelle concentration values. 

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. 

Micellar colloids are in a dynamic association-dissociation equilibrium, and the kinetics of micelle formation have been investigated for a long time. " In 1974, a reasonable explanation of the experimental results was proposed by Aniansson and Wall, " and this conception has been accepted and used ever since. The rate of micelle dissociation can be studied by several techniques, such as stopped flow, pressure jump, temperature jump, ultrasonic absorption, NMR, and ESR. The first three methods depend on tracing the process from a nonequilibrium state brought about by a sudden perturbation to a new equilibrium state— the relaxation process. The last two methods, on the other hand, make use of the spectral change caused by changes in the exchange rate of surfactant molecules between micelle and intermicellar bulk phase. 

First of all, a few words on the importance and practical implications of intermolecular and intcrpar-licle forces related to colloids and interfaces. In surface science, we see the intermolecular forces in the discussion of surface and interfacial forces that are directly connected to forces between molecules. Actually, as we will see in Chapter 3, the surface component theories of interfacial tension take into account exactly this connection to some of the most important forces, the dispersion, polar and hydrogen bonding ones. Of course, the high surface tension of water with all its implications (e.g. insects walking on water and spherical droplets) is due to the extensive hydrogen bonds of water and to the associated hydro-phobic phenomenon. The latter has a cmcial role also in micelle formation in surfactant solutions. The nature and value of interfacial tension in liquid-liquid interfaces is connected to the extent of miscibility in these systems, which is in itself linked to the... 

DTBP as a proton trap in hexane/CH2Cl2 at 0°C. Their properties and behaviors in aqueous solution were investigated in terms of micelle formation, self-association, colloidal dispersion, and pressure-enhanced dynamic heterogeneity. 

Around the turn of the last century, chemists were reluctant to accept the idea of rubber, starch, and cotton as long, linear chains connected by covalent bonds. A popular alternative was the idea of an associated colloidal structure. As a matter of fact, some small molecules do exhibit such behavior. Soap molecules will associate into complex liquid crystalline structures and are used as the basis for the formation of mesoscopic solids. Other surfactant molecules such as the phospholipids present in the wall of many living cells will form micelles and vesicles. However, the effective molecular weight of such structures varies with concentration and temperature, whereas the molecular weights of true polymers with covalent links do not. 

---