Hydration of ions in water

Figure 4.6 Clustering of water molecules which has significant consequences on the hydration of ions in water. When salts (e.g., NaCl) are added to pure water, ionic bonds of NaCl are broken due to a primary hydration sphere that develops around each ion (hydration)—as shown here for Na+. (From Degens, 1989, with permission.)...

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... 

In this section we link our discussion in two previous sections dealing with structure of water and with ion-water interaction. In the discussion on ion-water interaction it was shown that ions in water arrange their immediate neighboring water dipoles into a local structure of the primary water of hydration. Between this local structure and the bulk water is the nonstructured secondary water of hydration. Thus, the presence of ions in water will change the number of water molecules in both the structured and unstructured regions. Any decrease in the number of water molecules in a cluster will result in a corresponding decrease in the value of g and thus a decrease in the dielectric constant of water [Eq. (2.4)]. 

The main question is whether the hydrated ions behave as hard spheres while this seems plausible for ions much larger than the water molecules, it is probably not entirely applicable to small ions, whose hydration shells continuously change. Marcelj a calculated recently the double layer interaction8 using the anisotropic hypemetted chain method and potentials of mean force between pairs of ions in water, provided by Monte Carlo simulations. This... 

It is more fruitful to compare complex formation in ionic liquids with the phenomenon of hydration of ions in aqueous solution (Chapter 2). It will be recalled that though an ion was seen as constantly nudged by the water molecules of the surrounding medium, a certain number of the water molecules—the hydration... 

Electronic Mechanism. According to Luder and Zuffanti, the use of iron and acids to reduce nitrobenzene to aniline can be explained as follows Any acid (using the word in the Lewis sense) will increase the concentration of hydrogen ions (hydrated, of course) in water solution. The salts (FeCh and RNH3CI) and cations that have appeared in the equations above are acids in this general Sense, and in the presence of iron and water a plentiful supply of both electrons and protons is available. 

In the above sections, coextraction of ions into water-immiscible solvents has been elucidated in terms of selective hydration of ions in mixed solvents. However, it has frequently been asked whether water molecules are actually bound with ions in organic solvent. Nevertheless, there seems to be no doubt about it, since the selective hydration of hydrophilic ions in water-miscible and -immiscible solvents has been confirmed by solubility... 

Solvent-extraction experiments combined with Karl Fischer coulometry have revealed that hydrophilic inorganic ions and some charged groups (e.g., -CO2 and -NH ) in organic ions have the ability to transport certain water molecules into water-immiscible NB. Such phenomena can be elucidated in terms of selective hydration of ions in organic solvent. It has been undoubtedly shown by the NMR study that the hydration of hydrophilic ions proceeds by a successive reaction mechanism. For this reason, the hydration number of ions in organic solvent is not necessarily an integer. 

Aqueous Solutions. - Over the years MD has been applied to investigate the solute structure and dynamics of alkali halides in water. It is well-known that in water at 25 °C the residence time of the water molecules around the ions decreases dramatically as the ion increases in size. MD simulations have been carried out to explore the hydration of ions in supercritical water solutions. They calculated a residence time correlation function. 

The attraction of ions for water molecules is called hydration. Hydration of ions favors the dissolving of an ionic solid in water. Figure 12.9 illustrates the hydration of ions in the dissolving of a lithium fluoride crystal. Ions on the surface become hydrated and then move into the body of the solution as hydrated ions. 

The properties of ions in solution depend, of course, on the solvent in which they are dissolved. Many properties of ions in water are described in Chapters 2 and 4, including thermodynamic, transport, and some other properties. The thermodynamic properties are mainly for 25°C and include the standard partial molar heat capacities and entropies (Table 2.8) and standard molar volumes, electrostriction volumes, expansibilities, and compressibilities (Table 2.9), the standard molar enthalpies and Gibbs energies of formation (Table 2.8) and of hydration (Table 4.1), the standard molar entropies of hydration (Table 4.1), and the molar surface tension inaements (Table 2.11). The transport properties of aqueous ions include the limiting molar conductivities and diffusion coefficients (Table 2.10) as well as the B-coefficients obtained from viscosities and NMR data (Table 2.10). Some other properties of... 

Bromine is moderately soluble in water, 33.6 g/L at 25°C. It gives a crystalline hydrate having a formula of Br2 <7.9H2 O (6). The solubiUties of bromine in water at several temperatures are given in Table 2. Aqueous bromine solubiUty increases in the presence of bromides or chlorides because of complex ion formation. This increase in the presence of bromides is illustrated in Figure 1. Kquilibrium constants for the formation of the tribromide and pentabromide ions at 25°C have been reported (11). 

Once a matrix of particles is formed, whether filter cake, thickened underflow, or soil, applying a current to the fluid causes a movement of ions in the water and, with the ions, water of hydration. The phenomenon is called electro osmosis. The pressure generated on the fluid is given by (127) ... 

The internal structure of a liquid at a temperature near its freezing point has been discussed in Sec. 24. Each molecule vibrates in a little cage or cell, whose boundaries are provided by the adjacent molecules, as in Fig. 20, and likewise for each solute particle in solution in a solvent near its freezing point. It is clear that the question of the hydration of ions no longer arises in its original form. In aqueous solution an atomic ion will never be in contact with less than three or four water molecules, which in turn will be in contact with other water molecules, and so on. There is an electrostatic attraction, not only between the ion and the molecular dipoles in immediate contact with it, but also between the ion and molecular dipoles that are not in contact with it. For solvent dipoles that are in contact with a small doubly charged ion, such as Ca++,... 

Fig. 17-13. Hydration of ions orientation of water dipoles around ions in aqueous solutions.

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