Micelles shape factor

The viscosity starts to increase above the CMC and it is well established that the viscosity of a colloidal solution can give information on size and shape of the particles. From studies of the viscosity as a function of micellar concentration, the intrinsic viscosity may be obtained by extrapolation. The intrinsic viscosity depends on a shape factor, and the micelle specific volume and viscosity studies are therefore used to determine micelle shape and hydration. In many cases, these factors appear to be quite constant over a wide concentration range above the CMC. In other cases, such as hexadecyltrimethylammonium bromide (Fig. 2.9), dramatic increases in viscosity are observed at higher concentrations35). Studies of surfactants with low... 

There is a vast body of data concerning the influence of third components on surfactant liquid crystals. Because of the potentially great complexity of the inherent mesophase behaviour, this array of data can appear to be enormously difficult to rationalize. However, if we consider the simple concepts described above (micelle formation, micelle shape/packing constraints, volume fractions and the nature of intermicel-lar interactions), then a reasonably simplified picture emerges, at least for the water-continuous phases. This present section does not attempt to be comprehensive - it simply reports selected examples of behaviour to illustrate the general concepts. The simplest way to show the changes in mesophase behaviour is to employ ternary phase diagrams. The reader should recall that the important factors are (i) the behaviour as a function of surfactant/additive ratio, and (ii) the volume... 

There must be some kind of connection between the structure of the soap micelles and the mechanical properties of soap solutions. We have already seen that many soaps can form elastic or gelatinous systems and there is therefore every reason for establishing the connection between the properties and the X-ray observations. We have to thank Philippoff in particular for attempts in this direction. To what extent — he asks himself — will the viscosity depend on the aggregation Since the specific volume of the dissolved material will undergo little or no change through association of the particles, the shape factor must be influenced. In this connection various possibilities can be envisaged ... 

Micelle shapes are determined to a large extent by a packing factor p) defined as p = v/a l, where v is hydrodynamic volume of surfactant molecule, 4 is the length of the tail, and is the head group cross-section area (see Figure 13.1). Whenp is equal to 1/3 or less, the surfactant will be cone shape and the micelles will be spherical. This is the most commonly encountered micelle shape. For p close to or equal to 1/2, the micelles are of cylindrical shape (rod like). Figure 13.1 shows different micelle shapes and the corresponding p values. 

FIGURE 13.1 Different micelle shapes with corresponding packing factor (p) values. (From Zhang, Y., Correlations among surfactant drag reduction, additive chemical structures, rheological properties and microstructures in water and water/co-solvent systems, PhD thesis. The Ohio State University, Columbus, OH, 2005.)... 

Figure 7.11 Absorption of thioridazine in goldfish in the presence of increasing concentrations of various non-ionic detergents, the rate of absorption being proportional to the reciprocal of the death time of the fish, reciprocal death time is plotted on the ordinate concentrations of surfactants (% w/v) are marked. Lack of enhancement of absorption by some surfactants is probably due to poor ability to penetrate lipid membranes because of shape factors. Decrease in absorption is due to non-ionic micelle formation. From Florence and Gillan [41] with permission. The surfactants are all Atlas products (Honeywill-Atlas, UK).

The MAC is increased by addition of excess salt. " This effect is due to (1) a decrease in CMC (as mentioned above), (2) an increase in micelle size and/or a change in micelle shape, and (3) fortified hydrophobicity of the palisade layer owing to reduced repulsion between hydrophilic groups. These factors lead to the higher Ki value. However, the salt effect is relatively small for micelles with small aggregation numbers, for example bile salts. ... 

A qualitative description of the factors responsible for micelle shape has been put forward by a number of different workers. " At the micelle surface there exists a balance of forces between head group repulsions and hydrocarbon-water repulsions. The former are particularly prevalent for surfactants containing ionic head groups. Head group repulsions serve to increase the average surface area per molecule (a). The... 

Micelles are extremely dynamic aggregates. Ultrasonic, temperature and pressure jump techniques have been employed to study various equilibrium constants. Rates of uptake of monomers into micellar aggregates are close to diffusion-controlled306. The residence times of the individual surfactant molecules in the aggregate are typically in the order of 1-10 microseconds307, whereas the lifetime of the micellar entity is about 1-100 miliseconds307. Factors that lower the critical micelle concentration usually increase the lifetimes of the micelles as well as the residence times of the surfactant molecules in the micelle. Due to these dynamics, the size and shape of micelles are subject to appreciable structural fluctuations. 

In order to be exploitable for extraction and purification of proteins/enzymes, RMs should exhibit two characteristic features. First, they should be capable of solubilizing proteins selectively. This protein uptake is referred to as forward extraction. Second, they should be able to release these proteins into aqueous phase so that a quantitative recovery of the purified protein can be obtained, which is referred to as back extraction. A schematic representation of protein solubilization in RMs from aqueous phase is shown in Fig. 2. In a number of recent publications, extraction and purification of proteins (both forward and back extraction) has been demonstrated using various reverse micellar systems [44,46-48]. In Table 2, exclusively various enzymes/proteins that are extracted using RMs as well as the stability and conformational studies of various enzymes in RMs are summarized. The studies revealed that the extraction process is generally controlled by various factors such as concentration and type of surfactant, pH and ionic strength of the aqueous phase, concentration and type of CO-surfactants, salts, charge of the protein, temperature, water content, size and shape of reverse micelles, etc. By manipulating these parameters selective sepa-... 

Statistical mechanics was originally formulated to describe the properties of systems of identical particles such as atoms or small molecules. However, many materials of industrial and commercial importance do not fit neatly into this framework. For example, the particles in a colloidal suspension are never strictly identical to one another, but have a range of radii (and possibly surface charges, shapes, etc.). This dependence of the particle properties on one or more continuous parameters is known as polydispersity. One can regard a polydisperse fluid as a mixture of an infinite number of distinct particle species. If we label each species according to the value of its polydisperse attribute, a, the state of a polydisperse system entails specification of a density distribution p(a), rather than a finite number of density variables. It is usual to identify two distinct types of polydispersity variable and fixed. Variable polydispersity pertains to systems such as ionic micelles or oil-water emulsions, where the degree of polydispersity (as measured by the form of p(a)) can change under the influence of external factors. A more common situation is fixed polydispersity, appropriate for the description of systems such as colloidal dispersions, liquid crystals, and polymers. Here the form of p(cr) is determined by the synthesis of the fluid. 

We have already alluded to geometric limitations which place restrictions on the allowed shapes of micelles, and it is clear that packing constraints must be invoked for a proper treatment of self-assembly, for, in the absence of any such restrictions, spherical micelles will always be thermodynamically favoured over other shapes like cylindrical micelles or bilayers. There must then be some overriding factor that... 

We have examined the stmcture of both ionic and nonionic micelles and some of the factors that affect their size and critical micelle concentration. An increase in hydrophobic chain length causes a decrease in the cmc and increase of size of ionic and nonionic micelles an increase of polyoxyethylene chain length has the opposite effect on these properties in nonionic micelles. About 70-80% of the counterions of an ionic surfactant are bound to the micelle and the nature of the counterion can influence the properties of these micelles. Electrolyte addition to micellar solutions of ionic surfactants reduces the cmc and increases the micellar size, sometimes causing a change of shape from spherical to ellipsoidal. Solutions of some nonionic surfactants become cloudy on heating and separate reversibly into two phases at the cloud point. 

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