Micellization steric factors

Estimation is easier and less time-consuming because use is made of empirical relationships between the BCF and physicochemical properties of the compound, such as water solubility (S) [42-48], Km, (solid organic carbon/water partition coefficient) [48], Kmw (membrane water partition coefficient), iipw (liposome water partition coefficient) [49], critical micelle concentration (CMC) [45], steric factors, molecular weight [47,48], and others. The most common regression method is the estimation of BCF from the octanol-water partition coefficient (Kovl) [18,42,44-48,50,51],... 

The 1,3-dipolar cycloadditions of benzonitrile oxides with tertiary cinnamides yield the 5-phenyl and 4-phenyl regioisomers in a reversal of the expected regioselectiv-ities shown with methyl cinnamate. Calculations have shown that steric factors are responsible for this reversal of regioselectivity." The 1,3-dipolar cycloadditions of benzonitrile oxide with electron-rich and electron-poor dipolarophiles are accelerated by sodium dodecyl sulfate micelles. Phenyl nitrile ylides react with electron-deficient alkenes to produce five-membered -heterocycles where measured rate constants are between 4 x 10 and 7 x 10 lmoP ... 

The micelles formed in the non-polar solvents would seem in general to be relatively small, the aggregation number being of the order of 4 to 30. When the micelles are nearly spherical, as they seem to be at low concentrations, the low aggregation numbers are due to steric factors. The space in a spherical soft-core micelle permits only the accommodation of a limited number of polar groups this also applies to the number of bulky hydrocarbon groups in the outer parts of the micelle (Eicke, 1980 Ekwall, 1972 Kertes and Gutman, 1976 Rounds, 1976). 

Steric factors in the surfactant molecule, such as the presence of a bulky hydrophilic or hydrophobic group, inhibit micellization more than they do adsorption at the aqueous solution-air interface. On the other hand, electrical factors such as the presence of an ionic, rather than a nonionic, hydrophilic group in the surfactant molecule, appear to inhibit adsorption at the aqueous solution-air interface more than they do micellization. 

The complexity of MLC is much greater dmn that of conventional RPLC with aqueous-organic solvents, because of Ihe number of possible interactions with both mobile and stationary phases (Fig. 5.1). Hie solutes in the mobile phase can interact electrostatically with the charged outer-layer of ionic micelles, and hydrophobically with their lipophilic interior. The steric factor can also be important. The modification of the stationary phase by adsorption of surfactant monomers, which creates a "micelle-like" surface, gives rise to similar interactions with the solutes. The combination of these interactions cannot be duplicated by any traditional pure or mixed solvent system. While micellar solutions will never totally replace traditional aqueous-organic eluents, they offer several interesting alternatives to separation work. 

Neutral solutes eluted with all types of surfactants and charged solutes eluted with non-ionic surfactants will only be affected by non-polar, dipole-dipole, and proton donor-acceptor interactions. Besides these interactions, charged solutes interact electrostatically with ionic surfactants. In any case, the steric factors can be important, owing to the difficult access of micelles to the bonded-phase pores. 

The existence of ion pairs of hydroxy aromatic anions with polar groups of cationic micelles was proposed by Zaitsev et al. [66] to explain the effective charge of anions close to zero, observed in acid-base photoreactions of hydroxyaromatics in CTAB solutions. Such a value for the effective charge was found by simulation of the values of the diffusion rate constants of hydrogen ions to excited anions of hydroxyaromatics to make the calculated diffusion-controlled protonation reaction of the excited anions rate constants close to experimentally observed ones. In aqueous solution, the excited anions are protonated with diffusional values of the rate constant with some nonsignificant steric factor [67,68]. The three-phase model can help to interpret the reactivity of polar and charged substances in micellar solutions. 

Size and steric factors make the orientation of the larger polycyclics in the micelle more formidable than for a simple straight-chain compound. From results showing that the solubility of the steroid hormones was much greater than those of the polycyclic hydrocarbons in surfactant solutions, Ekwall et al [154] concluded that the latter, like the smaller apolar solubilizates, were situated in the hydrocarbon parts of the micelles, whereas the hormones were situated in the palisade layers of the micelles. 

This may be caused by two factors. First of all, in the case of pyridinium salts there may be a contribution from the hydrophobic interactions between neighbouring bound headgroups (an effect which would not contribute to the free energy of micelle formation). Secondly, a steric hindrance effect may prevent the positive chrge on the trlmethylammonium head group from approaching close to the polylon charge. 

Van der Waals forces There has been some success in relating these forces to micellar stability. However, the steric stabilization has been found to be also of some importance. Especially, the hairy layer interferes with the interparticle approach. There are several factors that will affect the stability of the casein micelle system ... 

Since the micelles are closely packed, intermicellar collisions are frequent however, the micelles do not normally remain together after collisions. The micelles are stabilized by two principal factors (1) a surface (zeta) potential of c. —20 mV at pH 6.7, which, alone, is probably too small for colloidal stability, and (2) steric stabilization due to the protruding K-casein hairs. 

With respect to size and shape of inverted micelles the former is, as a rule, considerably smaller than aqueous micellar aggregates. Since no thermodynamically determining factor, like the hydrophobic effect, exists in the case of inverted micelles it appears to be accepted that steric restrictions are important, i.e. the ratio of the cross-sectional areas of the hydrocarbon to the polar moieties essentially determines size and shape of the aggregates (19). 

---