Stern layer binding

In the case of the chloride and iodide Table I shows that Stern layer binding is probably responsible. It is interesting to observe that the electro-osmotic coefficients correlate well with the (hydrated) hydrodynamic size of all ions. It is evident further that in spite of the extremely high molality of these systems (their internal molality is of the order of 5-7 M) there is apparently not a substantial dehydration of the lithium ion. The equal electro-osmotic coefficients of the chloride and iodide ions show that by whatever mechanisms they are transported through the membrane, the volumes transported are very much the same as for the free ions themselves. ... 

Calculations based on non-specific coulombic interactions between the micelle and its counterions gave reasonable values of a, which were insensitive to the concentration of added salt (Gunnarsson et al., 1980). Although these calculations do not explain the observed specificity of ion binding, they suggest that such hydrophilic ions as OH- and F- may not in fact enter the Stern layer, as is generally assumed. Instead they may cluster close to the micelle surface in the diffuse layer. 

Similar considerations apply to situations in which substrate and micelle carry like charges. If the ionic substrate carries highly apolar groups, it should be bound at the micellar surface, but if it is hydrophilic so that it does not bind in the Stern layer, it may, nonetheless, be distributed in the diffuse Gouy-Chapman layer close to the micellar surface. In this case the distinction between sharply defined reaction regions would be lost, and there would be some probability of reactions across the micelle-water interface. 

The problem may be a semantic one because OH- does not bind very strongly to cationic micelles (Romsted, 1984) and competes ineffectively with other ions for the Stern layer. But it will populate the diffuse Gouy-Chap-man layer where interactions are assumed to be coulombic and non-specific, and be just as effective as other anions in this respect. Thus the reaction may involve OH- which is in this diffuse layer but adjacent to substrate at the micellar surface. The concentration of OH- in this region will increase with increasing total concentration. This question is considered further in Section 6. 

DIASTEREOMERS STEREOSELECTIVITY STEREOSPECIFICITY STERIC-APPROACH CONTROL STERIC HINDRANCE Steric hindrance effect on ligand binding, SCATCHARD RLOT Stern layer,... 

Table 3 presents the results for the analysis of the homologue series of the alkyl sulfate surfactants. The maximum adsorption, Poo, increases, together with the increasing munber of carbon atoms in the hydrophobic tail. Consequently, there is an increase in the attraction forces the stronger attractions lead to smaller areas occupied by the surfactant ions. This increases the number of the counterion bindings (except the last homologue-tetradecyl sulfate). The model has not been able to best fit the data for tetradecyl sulfate in the presence as well as in the absence (A/r = 0) of a Stern layer. 

The micellar surface has a high charge density and the stability of the aggregate is heavily dependent on the binding of counterions to the surface. From the solution of the Poisson-Boltzmann equation one finds that a large fraction (0.4—0.7) of the counterions is in the nearest vicinity of the micellar surface300. These ions could be associated with the Stern layer, but it seems simpler not to make a distinction between the ions of the Stern layer and those more diffusely bound. They are all part of the counterions and their distribution is primarily determined by electrostatic effects. 

The high concentration of bromide counterion in the Stern layer of a cationic microemulsion droplet results in the binding of Cd(II)... 

Figure 4-34. Wall effects in capillary electrophoresis. The capillary walls, which are usually made of fused silica contain a small proportion of dissociated silanyl groups which bind positively charged counterions. In the region close to the wall, defining the Stern layer, these ions are relatively immobile mobility increases beyond this layer in a region which defines the Guoy-Chapman layer. The distribution of positively charged ions is termed the Zeta-potential it is fairly constant in the Stern layer, but falls off sharply with distance from the wall in the Guoy-Chapman layer.

The two limiting cases are again specific binding and atmospheric condensation. Specific binding, as exemplified by the divalent cations Ca and Mg ", Implies entrapment within a Stern layer and attachment to a specific group, such as carboxylate or phosphate. Atmospheric condensation refers to the presence of the fully hydrated Ion In a diffuse double layer or In a Gouy-Chapman shell. 

Added salts decrease the rate of the CTAB micelle catalysed alkaline hydrolysis of benzylpenicillin (Gensmantel and Page, 1982a). The salt effect can be considered to be due to competitive binding of the anions with the micelle. Increasing the unreactive anion concentration displaces hydroxide ion bound in the Stern layer leading to a reduction in the observed rate. 

Micellar catalysis of bimolecular reactions usually involves the concentration of reactants in the Stern layer, but the rates are often no faster at the interface than in bulk water. On the other hand, for unimolecular reactions that are catalysed by micelles, the binding energy is used directly to lower the free-energy difference between the initial and transition state. The vast majority of reactions affected by micelles are simple or degradative in nature. 

The adsorption of the coions of the nonamphiphilic salt is expected to be equal to zero, T3 = 0, because they are repelled by the similarly charged interface [26,38-40], However, the adsorption of surfactant at the interface, Fj, and the binding of counterions in the Stern layer, F2, are different from zero (Figure 4.1). For this system the Gouy Equation 4.33 acquires the form... 

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