Hydration, ionic, Gibbs energy

Table 3.7. Real potentials of monovalent ions in aqueous solution, after Randles, ionic Gibbs energies of hydration taken from table 1.5.4, and the difference between them. All data in kJ mole" (25 mV in x " and a corresponds with 2.4 kJ mole" in Fx and Faj). The sign convention is such that a positive value of x " implies that water is positive with respect to air.

For a comprehensive compilation of Gibbs energies of solvation, see C. M. Criss and M. Salomon Thermodynamic Measurements - Interpretation of Thermodynamic Data. In A. K. Covington and T. Dickinson (eds.) Physical Chemistry of Organic Solvent Systems, Plenum Press, London New York 1973, p. 253 ff. - Cf also D. W. Smith Ionic Hydration Enthalpies, J. Chem. Educ. 54, 540 (1977). - A critical selection of standard molar heat capacities of hydration, AhydC°/ (J K") moF ), of single ions has been given by M. H. Abraham and Y. Marcus, J. Chem. Soc., Faraday Trans. 1 82, 3255 (1986). 

Subsequently, there has been no lack of Improved methods to narrow down the uncertainty margin. These approaches invoke information on standard potentials, on improved models to establish individual ionic activities (to determine x " only for one ion is such a datum required) or individual (Gibbs) energies of hydration. For instance, Alfenaar and de Ligny" experimented with... 

Non-Ionics of the C E -type have a very typical solubility behaviour, which is related to the EO-water interaction, hydration for short. First, poly(ethylene oxide), (PEO)jj is fairly soluble in water at room temperature, but polylpropylene oxide) (PPO) is not (as expected), and neither is poly(methylene oxide) (PMO), (unexpected). This irregular trend reminds us that solubility is not only determined by hydration in solution, but also by the Gibbs energy in the crystalline phase, which will be related to the molecular packing therein. Based on this difference in solubility, and hence in adsorbability, surface active polymers of the PEO-PPO type have been synthesized [Pluronics]-, they have a wide scope of application. 

Note that the excess Gibbs energy carries an asterisk to denote that it pertains to the set of components with formally hydrated solutes. Thus, w w is the number of kilograms of "free" water (w w = WwD), and I is the molal ionic strength, calculated in the usual way but with the nominal molalities replaced by molalities corrected for hydration (thus I = I/D). If it were desired to place less reliance on the Debye-Huckel model, other terms could be added to the right hand side in the usual way. We will presently ignore this as we are immediately interested in the hydration correction itself. 

A much-discussed electrostatic treatment of hydration based on the Born equation relates the Gibbs energy of solvation AG to the ionic radius and the solvent dielectric constant [128], For the hydration process [Eq. (1)] we take hydration as a specific case of solvation and set AGh = AGs- The change in Gibbs energy may be obtained readily if the ions are treated as hard spheres in a continuous dielectric ... 

Tabulated are single-ion entropies of about 110 diatomic and polyatomic ions in water Gibbs energies, enthalpies, and entropies of hydration of monatomic ions at 25 C partial molar volumes of about 120 common ions at 25 C ionic partial molar heat capacities of ions Gibbs energies of transfer of inorganic electrolytes from HjO to 020 and calorimetrically determined enthalpies of solution of salts in H2O and 020. 

To circumvent this difficulty, the Bom process has traditionally been used for the estimation of individual ionic standard molar Gibbs energy of ion hydration, A G,, dealing, however, only with the direct electrostatic effects involved. It results from... 

Estimation of reasonably correct A,G" values [8] results from the sum of the electrostatic terms, Equations 4.13 and 4.14, and the cavity formation term, Equation 4.10. Such values are shown in Table 4.1. It should be noted that the ionic standard molar Gibbs energies of hydration need to be compatible with the corresponding enthalpies and entropies of hydration (see the following text) according to A G, = A /f,°°-TA Sj . The latter quantities are more directly available from experimental data, so that values adopted from Ref 8 are presented in Table 4.1... 

The ionic hydration energies (enthalpies and Gibbs free energies) of metals are consequently roughly a linear function of the square of the oxidation state divided by the effective ion radius (z /r ff. Figure 8.9).It may be added that AH y and AG y, respectively, of the individual alkali halide ion pairs form two linear branches with a maximum at Csl (KBr). 

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