Micelles hydrophobic bonding

This large entropy increase on micellization in aqueous medium has been explained in two ways (1) structuring of the water molecules surrounding the hydrocarbon chains in aqueous medium, resulting in an increase in the entropy of the system when the hydrocarbon chains are removed from the aqueous medium to the interior of the micelle— hydrophobic bonding (Nemethy, 1962) (2) increased freedom of the hydrophobic chain in the nonpolar interior of the micelle compared to the aqueous environment (Stainsby, 1950 Aranow, 1960, 1961, 1965). Any structural or environmental factors that may affect solvent-lyophobic group interactions or interactions between the lyophobic groups in the interior of the micelle will therefore affect AG nic and consequently the value of the CMC. 

The micelles can be dispersed (dissociated) by urea or SDS, suggesting the involvement of hydrogen and hydrophobic bonds in micelle integrity. 

Although the submicellar model of the casein micelle readily explains many of the principal features and physicochemical reactions undergone by the micelles and has been widely supported, it has never enjoyed unanimous support and two alternative models have been proposed recently. Visser (1992) proposed that the micelles are spherical conglomerates of individual casein molecules randomly aggregated and held together partly by salt bridges in the form of amorphous calcium phosphate and partly by other forces, e.g. hydrophobic bonds, with a surface layer of K-casein. Holt (1992, 1994) depicted the casein micelle as a tangled web of flexible casein... 

With alkali halide-TBA-W or alkali halide-PD-W systems, the parameters Bne are negative for volumes and heat capacities (see Figures 1-5 and 10). This sign seems to be the one usually observed for the interaction of a hydrophobic with a hydrophilic solute (6). At intermediate cosolvent concentration, AYe°(W — W + TBA) and AYe°(W — W + PD) deviate in the direction we would expect for hydrophobic association the volume increases sharply, and the heat capacity decreases further. Inorganic electrolytes lower the critical micelle concentration of surfactants by salting out the monomers, thus favoring micellization (25) in a similar way, in the co-sphere of a hydrophilic ion, the hydrophobic bonding between the cosolvent molecules may be enhanced. 

The effect of low concentrations of urea (2M) on the large dihydroxy bile salt micelles is striking, while similar concentrations have no effect on the small trihydroxy or dihydroxy micelles. The effects of urea on micelle formation and aggregate size are undoubtedly complicated (10) and involve changes in solvent structure and thus hydrophobic bonding and hydration of polar groups. For large micelles of dihydroxy bile salt... 

Secondary Micelles. These micelles are formed only by dihydroxy bile salts in the presence of increased counterions. Secondary micelles are probably formed by the aggregation of primary micelles. Since the surface available for hydrophobic interaction is expended in the formation of the primary micelles, the bonding that takes place is probably between some of the hydrophilic parts of the bile salts. It is suggested (Figure 11c) that in the presence of increased counterion concentrations... 

A model of the micelles of the polyethylenoxide derivates would contain a nucleus of the hydrophobic iso-octylphenyl groups and the water soluble polyethylenoxide chains. The micelle nucleus can solve other organic molecules for example dyestuffs and induce a solubility effect of solutes by a hydrophobic bond effect. These complex compounds dyestuff micelle-nucleus have a buffer effect on the dyestuff and play an important role in textile dyeing processes129,134 ... 

Hydrophobic forces The hydrophobic effect is the name given to those forces that cause nonpolar molecules to minimize their contact with water. This is clearly seen with amphipathic molecules such as lipids and detergents which form micelles in aqueous solution (see Topic El). Proteins, too, find a conformation in which their nonpolar side chains are largely out of contact with the aqueous solvent, and thus hydrophobic forces are an important determinant of protein structure, folding and stability. In proteins, the effects of hydrophobic forces are often termed hydrophobic bonding, to indicate the specific nature of protein folding under the influence of the hydrophobic effect. 

The size-dependent term ngfkT includes la) the decrease in attractive hydrophobic bonding between hydrocarbon tails of amphiphiles, due to their partial exposure to the aqueous medium, and (b) the repulsive interaction between the hydrophilic head groups, of the amphiphiles. This repulsive interaction is caused by Steric repulsion between head groups in nonionic micelles and by electrical repulsion between the ionic head groups in ionic micelles. ... 

Hydrophobic bonding caimot occur in apolar solvents. Micelles in such media have the hydrophilic groups inside, the hydrophobic ones sticking out. Their formation must be primarily erithalpically determined. 

So. at the minimum, where ln]c.m.c.] is - independent of T, AH(mic) = 0. Below this temperature, AH(mic)>0 (micelle formation endothermic), above it, AH(mic) < 0 (exothermic). Both the low magnitude and the sign change are in line with hydrophobic bonding as the main driving force. For cationic surfactemts not so many detailed studies are available, but the same trend is expected. In order to observe the minimum, careful measurements are required over a long temperature... 

At higher ionic surfactant concentrations. Van der Waals interaction between hydrocarbon chains and hydrophobic bonding results in aggregation, forming clusters called hemi-micelles (12.17-21). The aggregation numbers of hemi-micelles are not well established estimates range from 2 to -250 (15.22.23). Surface charge is reduced more rapidly than at lower solution concentrations and is ultimately reversed as solution concentration and adsorption increase so the adsorption bond includes a chemical or specific contribution. 

Ionic surfactants are strong electrolytes in dilute aqueous solution, and non-ionic surfactants are monomers, but above the so-called critical micelle concentration (cmc) they spontaneously self-associate to form micelles [15]. Micellization in water is an example of the hydrophobic effect at work [18]. The phenomenon is more properly called the solvophobic effect, because it is important in associated solvents which have three-dimensional structure, and normal micelles form in 1,2-diols, or formamide [19] and micelles with a carbocationic head group form in 100% sulfuric acid [20], for example. However, we live in an aqueous world, and most normal micellar systems are studied in water, so we can reasonably retain the term hydrophobic with the hydrophobic bond dictated by water association. 

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