Ions hydrophilic

In almost all theoretical studies of AGf , it is postulated or tacitly understood that when an ion is transferred across the 0/W interface, it strips off solvated molecules completely, and hence the crystal ionic radius is usually employed for the calculation of AGfr°. Although Abraham and Liszi [17], in considering the transfer between mutually saturated solvents, were aware of the effects of hydration of ions in organic solvents in which water is quite soluble (e.g., 1-octanol, 1-pentanol, and methylisobutyl ketone), they concluded that in solvents such as NB andl,2-DCE, the solubility of water is rather small and most ions in the water-saturated solvent exist as unhydrated entities. However, even a water-immiscible organic solvent such as NB dissolves a considerable amount of water (e.g., ca. 170mM H2O in NB). In such a medium, hydrophilic ions such as Li, Na, Ca, Ba, CH, and Br are selectively solvated by water. This phenomenon has become apparent since at least 1968 by solvent extraction studies with the Karl-Fischer method [35 5]. Rais et al. [35] and Iwachido and coworkers [36-39] determined hydration numbers, i.e., the number of coextracted water molecules, for alkali and alkaline earth metal... 

Based on these measurements, a new model of the transfer of hydrophilic ions across the 0/W interface was proposed (see Fig. 8). In this model, the hydrophilic ion transfers from W to O with some water molecules associated with the ion. A typical example in Fig. 8 shows that a sodium ion transfers across the NB/W interface with four water molecules. In theoretical treatment of of such a hydrophilic ion, therefore, the transferring... 

As seen in Table 2, the order of the magnitude of rjj for alkali metal ions is the reverse of that of the magnitude of r. This means that a more hydrophilic ion has a larger rji. However, this fact does contradict the expectation from Bornian electrostatic theories. As can be seen in the Born equation [Eq. (2)], it is expected that the larger the radius an ion has, the more positive the value the ion has, that is, the more hydrophobic it... 

FIG. 8 Proposed model of the transfer of a hydrophilic ion across the O/W interface. The illustration shows the transfer of Na+ from W to NB as a typical example. (From Ref 46. Copyright 1997 American Chemical Society.)... 

Then we made a new approach that recognizes short-range interactions of a hydrated ion with solvents. By this approach, for hydrophilic ions could be... 

Specific-ion electrodes are expensive, temperamental and seem to have a depressingly short life when exposed to aqueous surfactants. They are also not sensitive to some mechanistically interesting ions. Other methods do not have these shortcomings, but they too are not applicable to all ions. Most workers have followed the approach developed by Romsted who noted that counterions bind specifically to ionic micelles, and that qualitatively the binding parallels that to ion exchange resins (Romsted 1977, 1984). In considering the development of Romsted s ideas it will be useful to note that many micellar reactions involving hydrophilic ions are carried out in solutions which contain a mixture of anions for example, there will be the chemically inert counterion of the surfactant plus the added reactive ion. Competition between these ions for the micelle is of key importance and merits detailed consideration. In some cases the solution also contains buffers and the effect of buffer ions has to be considered (Quina et al., 1980). 

The situation is different for reactions of very hydrophilic ions, e.g. hydroxide and fluoride, because here overall rate constants increase with increasing concentration of the reactive anion even though the substrate is fully micellar bound (Bunton et al., 1979, 1980b, 1981a). The behavior is similar for equilibria involving OH" (Cipiciani et al., 1983a, 1985 Gan, 1985). In these systems the micellar surface does not appear to be saturated with counterions. The kinetic data can be treated on the assumption that the distribution between water and micelles of reactive anion, e.g. Y, follows a mass-action equation (9) (Bunton et al., 1981a). 

It seems possible that a very hydrophilic anion such as OH- might not in fact penetrate the micellar surface (Scheme 1) so that its interaction with a cationic micelle would be non-specific, and it would exist in the diffuse, Gouy-Chapman layer adjacent to the micelle. In other words, OH" would not be bound in the Stem layer, although other less hydrophilic anions such as Br, CN or N 3 probably would bind specifically in this layer. In fact the distinction between micellar and aqueous pseudophases is partially lost for reactions of very hydrophilic anions. The distinction is, however, appropriate for micellar reactions of less hydrophilic ions. 

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. 

These uncertainties as to the location of ions such as OH- or F" cast doubt on the validity of the quantitative models which are used to treat micellar rate effects. The problem is less serious for reactions of less hydrophilic ions which bind strongly and specifically to micelles, and it should be relatively unimportant for bimolecular reactions of non-ionic reagents. It is probable also that the volume element of reaction decreases as the concentration of ionic reagent is increased, which would speed reaction. 

An impressive body of evidence supports these generalizations. This evidence has been reviewed (Romsted, 1984) and it does not seem necessary to discuss it in detail here, but some examples will be given and some exceptions to these generalizations will be mentioned. Some reactions of OH- are shown in Table 3 for both inert and reactive ion surfactants, and Table 4 gives data for reactions of other hydrophilic ions. Reactions of hydrophobic nucleophiles are shown in Table 2. For all these reactions second-order rate constants in the micellar pseudophase are compared with those in water. For some reactions we also give values of krcl, i.e. the rate constant relative to that in water. These values depend upon the reactant concentration and are included merely to provide an indication of the micellar rate effects. Other examples of micellar rate effects are given in the Appendix. 

The value of k 2/kw depends upon assumptions regarding a, and the molar volume of reaction, KM, in the micellar pseudophase. In a few systems authors have postulated a role for direct reaction of a hydrophilic ion in the aqueous pseudophase with micellar bound substrate. 

Consequently, an ISE for nitrate for example, a strongly hydrophilic ion, must have a strongly hydrophobic ion-exchanger ion. This conclusion has been demonstrated experimentally for a series of NO3 ISEs based on tetra-alkyl-ammonium salts with long alkyl chains [161] (see fig. 7.2). It was found that, in the studied series of substances, the tetradodecylammonium ion which is... 

They enable transfer of hydrophilic ions across the lipid membranes of the cells and cell organelles [6, 29a, 69, 114, 140,148], across lipid bilayer membranes (BLM) [5, 29, 59, 72, 117,135, 207] and across relatively thick membranes of organic solvents [155,221]. 

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. 

It is merely an extension of these ideas to demonstrate the conditions that the same membrane, containing MY, should also be responsive, in a Nernstian fashion, to Y activities+in solution. These conditions are again a three-ion situation M, Y and N. The salt NY is the aqueous sample whose Y activity is to be measured. N is typically a hydrophilic ion such as Na. When aqueous NY activity is varied, the interfacial pd is again S-shaped (mV vs log[NY]). These responses are illustrated from a theoretical calculation in Figure 1. The assumed extraction parameters are given in the legend. The similarity with silver halide membrane electrodes are summarized below. 

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