Micellization colloidal aggregates

Surfactants have a unique long-chain molecular structure composed of a hydrophilic head and hydrophobic tail. Based on the nature of the hydrophilic part surfactants are generally categorized as anionic, non-ionic, cationic, and zwitter-ionic. They all have a natural tendency to adsorb at surfaces and interfaces when added in low concentration in water. Surfactant absorption/desorption at the vapor-liquid interface alters the surface tension, which decreases continually with increasing concentrations until the critical micelle concentration (CMC), at which micelles (colloid-sized clusters or aggregates of monomers) start to form is reached (Manglik et al. 2001 Hetsroni et al. 2003c). 

The use of micelles or similar aggregates in isotopic separations on the basis of nuclear spins is an especially interesting example of the way in which colloidal aggregates can keep reactive intermediates in close proximity (Turro and Kraeutler, 1980 Gould et al., 1984b Herve et al., 1984). 

The highly dynamic colloidal structures described in this chapter result in considerable complexity in behaviors. This complexity has resulted in relatively slow improvement in our understanding of colloidal systems despite the fact that the structure of micelles was in essence described almost a century ago already. Results from a series of relatively recent approaches to describe colloidal aggregates are now beginning to coalesce into a model of colloidal structures incorporating the dynamic and nonhomogeneous structures of these aggregates. 

The whey proteins are molecularly dispersed in solution or have simple quaternary structures, whereas the caseins have a complicated quaternary structure and exist in milk as large colloidal aggregates, referred to as micelles, with particle masses of 106-109 Da. 

The morphology obtained by crystallizing an undiluted polymer can be considerably more complicated. The simplest model proposed consists of a system in which small crystallites exist, with amorphous chains attached to them like fringes on a colloidal aggregate. Such a system is called a micelle. This model is probably too simple except for a few polymers which form relatively few, very small crystallites. 

Chlorhexidine is a strong base (Lewis acid-base theory) because it reacts with acids to form salts of the RX2 type, and it is practically insoluble in water (<0.008% wt/vol at 20°C). The water solubility of the different salts varies widely as demonstrated in Table 2.13. Chlorhexidine is moderately surface-active (a net+chare over its surface) and forms micelles (molecular aggregates form colloidal particles) in solution the critical micellar concentration of the acetate is 0.01% wt/vol at 25°C (Heard and Ashworth 1969). Aqueous solutions of chlorhexidine are most stable within the pH range of 5-8, and above pH 8.0 chlorhexidine is precipitated because conditions for a base (>pH 7) reaction are present. 

Fig. 6. g-micelle volumes ( aggregation numbers = acid residues per aggregate) in ml as determined by fluorescence depolarization measurements versus sulfonate concentration in g-equiva-lents per liter of dinonylnaphthalene sulfonates at 25 °C in benzene saturated with water [J. Colloid Sci. 12, 465 (1957)]... 

In the various sections of this chapter, I will briefly describe the major characteristics of FT-IR, and then relate the importance of these characteristics to physiochemical studies of colloids and interfaces. This book is divided into two major areas studies of "bulk" colloidal aggregates such as micelles, surfactant gels and bilayers and studies of interfacial phenomena such as surfactant and polymer adsorption at the solid-liquid interface. This review will follow the same organization. A separate overview chapter addresses the details of the study of interfaces via the attenuated total reflection (ATR) and grazing angle reflection techniques. 

Effect of Pressure on Micelles. While temperature studies of the phase transitions of bilayers and micelles have been performed for some time now, the utilization of pressure as a variable is a more recent development. Variation in temperature of a colloidal aggregate such as a bilayer causes simultaneous changes in thermal energy and volume, whereas isothermal variation in pressure (up to 50 kbar) yields spectroscopic changes due only to volume effects. A review of high pressure vibrational spectroscopy of phospholipid bilayers has recently appeared (74). in which the surprisingly rich barotropic phase behavior of these compounds is explored in detail. 

A micelle is a colloidal aggregate of amphiphilic molecules (50-100 molecules per micelle) which forms at a specific concentration termed the critical micelle concentration. As illustrated in Fig. 1, in polar media such as water, the hydrophobic part of the amphiphilic molecule tends to locate away from the polar phase while the polar groups of the molecule tend to locate in the water phase, forming the micelle aggregate. Micellar systems are able to solubilize both hydrophobic and hydrophilic compounds. 

Surfactants are organic molecules that possess a nonpolar hydrocarbon tail and a polar head. The polar head can be anionic, cationic, or nonionic. Because of the existence of the two moieties in one molecule, surfactants have limited solubility in polar and nonpolar solvents. Their solubility is dependent on the hydrophile-lipophile balance of their molecular structure. At a critical concentration, they form aggregates in either type of solvent. This colloidal aggregation is referred to as micellization, and the concentration at which it occurs is known as the critical micelle concentration. The term micelle was coined by McBain (7) to designate the aggregated solute. In water or other polar solvents, the micellar structure is such that the hydrophobic tails of the surfactant molecules are clustered together and form the interior of a sphere. The surface of the sphere consists of the hydrophilic heads. In nonpolar solvents, the orientation of the molecules is reversed. 

Micelle An aggregate of surfactant molecules in solution. Such aggregates form spontaneously at sufficiently high surfactant concentration, the critical micelle concentration. The micelles are of colloidal dimensions. 

Li, F. Li, G-Z. Chen, J.-B. Synergism in mixed zwitterionic-anionic surfactant solutions and the aggregation numbers of the mixed micelles. Colloid Surface A 1998, 145, 167-174. 

Solubilization is the increase in solubility of a poorly water-soluble substance with surface-active agents. The mechanism involves entrapment (adsorbed or dissolved) of molecules in micelles and the tendency of surfactants to form colloidal aggregations at critical micelle concentration levels. Thus, the critical micelle concentration is the minimum surfactant concentration that begins solubilization of the insoluble molecule. Increases in the concentration of micelles lead to increases in drug solubility. 

Submicroscopic, colloidal aggregates can influence chemical reactivity. Aqueous micelles are the most widely studied of these aggregates, and these micelles form spontaneously when the concentration of a surfactant (sometimes known as a detergent) exceeds the critical micelle concentration, cmc (1-3). Surfactants have apolar residues and ionic or polar head groups, and in water at surfactant concentrations not much greater than the cmc, micelles are approximately spherical and the polar or ionic head groups are at the surface in contact with water. The head groups may be cationic, (e.g., trimethylammonium), anionic, (e.g., sulfate), zwitterionic (as in carboxylate or sulfonate betaines), or nonionic. The present discussion covers the behavior of ionic and zwitterionic micelles and their effects on chemical reactivity. 

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