Hydration of cations

A useful diagnostic tool for investigating possible hydration of cations of bases for which pA is greater than about one is the measurement of their ultraviolet spectra in aqueous acid solutions and also in an anhydrous acidic solvent such as dichloroacetic acid (for which the Hammett acidity function, Hq, is — 0.9, and in which hydration of the cation cannot occur). This technique has been used with quinazoline to obtain spectra approximating those of the hydrated and anhydrous cations, respectively. For weaker bases, spectral measurements in sulfuric acid-water mixtures of increasing acid content may be used to reveal a progressive conversion of hydrated into anhydrous species as the thermodynamic activity of the water decreases. 

Cations in aqueous solutions have an effective radius that is approximately 75 pm larger than the crystallographic radii. The value of 75 pm is approximately the radius of a water molecule. It can be shown that the heat of hydration of cations should be a linear function of Z /r where is the effective ionic radius and Z is the charge on the ion. Using the ionic radii shown in Table 7.4 and hydration enthalpies shown in Table 7.7, test the validity of this relationship. 

This treatment illustrates the inherent difficulty of the problem. Any cycle of this type will allow the calculation of sums of enthalpies of hydration of cations and anions, but it will not allow the estimation of the separate quantities. There are two ways of dealing with the matter. One is to use the conventional reference zero that the enthalpy of hydration of the proton is zero, i.e. Ahyti// (H+, g) = 0. The second approach is to estimate an absolute value for A Z/tH h, g) that may be used to estimate the absolute values for enthalpies of hydration of any other ions. Both are exemplified in the text, but only the second is of general use in the study of the hydration of ions and the discussion of the factors that determine the values of enthalpies of hydration of individual ions. 

The discrepancy introduced by ignoring the compression term is only slight and represents no more than a 2% difference from the absolute values usually used. It is normally ignored in view of the uncertainties associated with the problem of dividing the conventional enthalpies of hydration of cation-anion pairs into individual values. 

Anions bind also to other metals, like gold, platinum, or silver [74,81], Why do anions adsorb specifically to metals, while cations do not The explanation is a strong hydration of cations. A cation would have to give up its hydration shell for an adsorption. This is energetically disadvantageous. Anions are barely hydrated and can therefore bind more easily to metals [82], Another possible explanation is the stronger van der Waals force between anions and metals. The binding of ions to metallic surfaces is not yet understood and even the idea that cations are not directly bound to the metal, was questioned based on molecular-dynamics simulations [83],... 

It should be emphasized that none of the methods in categories (ii) and (iii) that have been used to obtain the absolute enthalpies of hydration of ions is theoretically rigorous. For example, Conway and Salomon (54) have made a detailed critique of the Halliwell—Nyburg type of treatment. If the water dipole orientation is not exactly opposite at cations and anions, as seems to be indicated by various previous calculations (55, 56), then the assumption that the difference between heats of hydration of cations and anions of the same radius originates from the ion-quadrupole interaction could be inaccurate. However, the results given in Table 7 are probably reliable to within a few kcal mole-1, despite the fact that it is impossible to assess their accuracy specifically. They indicate that an anion has a more negative absolute heat of hydration than a cation of the same crystal radius. 

Generally, the heats of hydration of cations increase as the charges on the ions increase and as the sizes decrease. The heat of hydration of the Ln3+ ions show this trend very well as illustrated in Figure 18.3. 

The shift ratios are constant (i.e.) independent of Ln(HI) ions for the head groups of the bilayer but vary significantly for glycerol-phospho-choline in the second half of the lanthanide series as opposed to the first half of the series. This probably reflects the differences in hydration of cations in the head groups and at the membrane surface. 

The fact that this is so allows one to follow the hydration of cation-polyanion association. This also affects the assignment of changes of band frequency in the water molecule as it dissociates with the counterion groups. 

Dipoles tend to orient in an electric field, directed along the field gradient. Thus, dipoles are attracted to ions, and ion-dipole forces are in large part (if not solely) responsible for the tendency of cations and anions to hydrate in water, a compound with a large dipole moment. Hydration of cations and anions is depicted in Figure 1.2. The energy of an ion-dipole interaction is given by... 

Table 3. Selectivity and Hydration of Cation Resins With Different Degrees of Crosslinkingt ...

How does each of the following affect the solubility of an ionic compound (a) lattice energy, (b) solvent (polar versus nonpolar), (c) enthalpies of hydration of cation and anion... 

Thus, the heat of solution for ionic compounds in water combines the lattice energy (always positive) and the combined heats of hydration of cation and anion (always negative),... 

Figure 2.4 Hydration of cations and anions. Dipoles H O form with ions aqua complexes and prevent their interaction.

The possibility (c) involves two layers of water molecules between the first layer of cations and the metal. The first layer of water can be regarded as a hydration layer with a dielectric constant of 6. The second layer is not uniform, some of the water there will be water of hydration of cations (dielectric constant 6) but some of it will not be particularly associated with any ion. The dielectric constant of unassociated water in this second layer should be between 6 and the bulk value of 80 one estimate may be 35. In not too concentrated solutions most of the second layer will be composed of unassociated water molecules, and, thus, one may assume that the average dielectric constant of this second layer of water is 35. If we have a capacitor... 

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