Ionic hydration, in the gas phase

To understand the dissolution of ionic solids in water, lattice energies must be considered. The lattice enthalpy, A Hh of a crystalline ionic solid is defined as the energy released when one mole of solid is formed from its constituent ions in the gas phase. The hydration enthalpy, A Hh, of an ion is the energy released when one mole of the gas phase ion is dissolved in water. Comparison of the two values allows one to determine the enthalpy of solution, AHs, and whether an ionic solid will dissolve endothermically or exothermically. Figure 1.4 shows a comparison of AH and A//h, demonstrating that AgF dissolves exothermically. 

In a solid-gas reaction involving a molecular crystal, the reactants are respectively the molecules in the crystalUne solid and the molecules in the gas phase and the product is the product crystal, which can be crystalUne or amorphous. Vapour uptake to generate a solvate crystal (e.g. hydration) is a related process. In fact the difference between a crystal solvation process and a solid-gas reaction leading to new molecular/ionic species is mainly in the energetic scale of the processes and in the fact that in solvation processes, molecules retain their chemical identity. On this premise there is a relevant analogy between the uptake of smaU molecules by a nanoporous material [16] and the reaction between a molecular crystal and molecules to yield a co-crystal or a salt (e.g. acid-base... 

The very negative value of the estimated enthalpy of hydration of the proton is consistent with a Born radius of 63 pm (from equation 2.43). Since that equation overestimates the ionic radius by —67 pm it follows that a bare proton (radius —0 pm) would be expected to have such a very negative enthalpy of hydration. Studies of the proton and water in the gas phase have shown that the stepwise additions of water molecules in the reaction ... 

Fig. 2. Plots testing simple ionic model equations for diatomic molecules in the gas phase, ionic crystals and the hydration of ions. The slopes of the lines coincide with those of the simple theory, see Phillips and Williams. U is the binding energy from free gas ions.

Fig. 2.29. Relative concentrations of ion hydrates, Na(H20), in the gas phase at (a) 300 K and (b) 400 K. (Reprinted from B. E. Conway, Ionic Hydration in Chemistry and Biophysics, Elsevier, New York, 1981.)...

The hydration of halide anions is of intrinsic interest to the process of solvation. The most important aspect of water, its hydrogen bond network, is directly perturbed by the anion in a simple and direct way. It competes for the protons with its own ionic hydrogen bond. The gas phase studies of the smallest hydrated ions show extremely large shifts in the 0-H stretch in the 0-H-X bond. This strong interaction must play a role in the bulk solvation process. Interesting implications will be discussed in the final section of this chapter. 

Although Li has a much smaller ionic radius than in the gas phase, the solvated radii of the two ions is reversed Li+ (aq) = 382 pm, whereas K (aq) = 328 pm. Explain these results in terms of the enthalpy of hydration and the numbers of water molecules that surround each ion in aqueous solution. 

As previously reported for Nafion, the membrane thickness has been reduced in order to decrease the ohmic drops within the membrane and to enhance the fuel cell performance. However, it requires a low gas permeation [ 141 ]. The gas barrier properties of Nafion are very good in the dry state [174], but they decrease when hydrated due to a higher gas solubility in the water phase than in the perfluorinated one. The gas permeation properties of SPIs have been determined as a function of the ion and water content [33,151,155,175,176]. While large differences in diffusivity and selectivity are observed in the dry state depending on the ion content [155], the gas permeation is significantly reduced when hydrated, which suggests the existence of a closed nanoporosity [151]. This porosity located in the ionic domains is then filled by water molecules, thus reducing the gas permeation. While the gas barrier properties of SPI can be considered as favorable for its use as a fuel cell membrane, they become a serious drawback for the use of this ionomer to prepare fuel cell electrodes (see Sect. 4) [177]. 

We list in Table 11 the Gibbs free energies for hydration (35) AG° and hydration numbers n for the alkali metal cations (the values for n are a combination of experimental determinations (17,36,37) and of personal choice between conflicting results ). Just like the gas-phase value (Table 8, for instance) the AG° values vary regularly with the ionic radius, the smallest ions being more strongly hydrated. Lithium ion owes to its very small radius an unusually small coordination sphere, consisting of only four water molecules in the primary hydration shell (37). 

My last comment concerns the reaction of palladium olefin complexes with carbon monoxide discovered by Tsuji. I agree that this is most likely to proceed by an insertion rather than an ionic mechanism. Chloride attack on coordinated olefin is rare however. Chloride ion is an inhibitor, for example in the palladous chloride catalyzed hydration of ethylene (0). I, therefore, wondered whether carbon monoxide was affecting the ease with which chloride attacks olefin. One can postulate that carbon monoxide participates in this insertion either as a gas phase reactant or by first forming a carbonyl olefin complex. Such complexes of the noble metals were unknown, but examining the reaction between carbon monoxide and the halogen bridged olefin complexes of platinum revealed that they are formed very readily... 

Obtaining the individual properties of ions with solvation numbers from measurements of ionic vibration potentials and partial molar volumes is not necessary in the study of gas phase solvation (Section 2.13), where the individual heats of certain hydrated entities can be obtained from mass spectroscopy measurements. One injects a spray of the solution under study into a mass spectrometer and investigates the time of flight, thus leading to a determination of the total mass of individual ions and adherent water molecules. 

With this kind of approach, it was also possible to conclude that, in aqueous medium, aU the cations (monovalent or bivalent), as well as the S04 anion, are electroadsorbed in the pores in hydrated state. On the contrary, it seems that the monovalent anions are basically adsorbed in a non-hydrated state. These results enable to establish a scale of ionic effective dimensions in aqueous medium. These dimensions are compared with the ones of the different molecules used for the calibration of the average pore size of carbons by gas phase adsorption (N2, CF4, SFe and methyl tert-butyl ether (MTBE)) ... 

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