Solvation of cations and anions

In mixed solvent systems the difference in the solvating abilities of solvent molecules S and S2 causes a selective solvation of cations and anions [119,120],... 

The solvation of cations and anions by water molecules is possible due to the dipolar nature of water in the sense that oxygen and hydrogen have partial negative and positive charges respectively. 

Another problem with the MSA is that it does not distinguish between the solvation of cations and anions of the same size. Thus, although the K and F ions have approximately the same radius, the F anion is more strongly solvated than the K cation (table 3.4). This can be understood in terms of the effect that each ion has on local water structure. The K ion disrupts this structure more so that the stabilizing effect of the local ion-dipole interactions is offset by the work done to break up the water structure, that is, to disrupt attractive dipole-dipole interactions and hydrogen bonding between local water molecules. This means that the parameter 8s should be different for cations and anions in the same solvent (table 3.5). 

The value of X for a typical polar solvent is approximately two. This equation was introduced earlier in the development of the MSA for ion-solvent interactions (section 3.5). It was seen that the MSA gives an improved description of ion solvation parameters with respect to the Born model. However, it fails to distinguish between the solvation of cations and anions of the same size. In other words, it fails to distinguish between the short-range chemical interactions which stabilize ions of differing charge. 

The enthalpy of solution connected with the solution process is relatively small because it is just the difference between the lattice enthalpy and the enthalpy of solvation of cations and anions. For several years the dissociation theory explained most of the experimental... 

Z is the valence state of the ion. In a polar medium with a high dielectric constant, K becomes very large and the positive and negative ions can be considered to move rather independently. This solvation of cations and anions by polar solvent molecules is responsible for dissociation reactions as... 

Equation (31) is true only when standard chemical potentials, i.e., chemical solvation energies, of cations and anions are identical in both phases. Indeed, this occurs when two solutions in the same solvent are separated by a membrane. Hence, the Donnan equilibrium expressed in the form of Eq. (32) can be considered as a particular case of the Nernst distribution equilibrium. The distribution coefficients or distribution constants of the ions, 5 (M+) and B X ), are related to the extraction constant the... 

If the system separates, it can be extended to a model for the interface between two solutions by introducing ions. In the basic case the system contains a salt composed of cations and anions which is preferentially solvated by the solvent 5], but badly solvable in solution 2, and a salt K2A2 that is preferentially dissolved in solvent 2. This can be achieved by choosing suitable interaction parameters between the ions and the two solvents. 

During the migration of cations and anions towards their respective electrodes, each ion tends to carry solvated water along with it. As cations are usually more solvated than anions, a net flow of water towards the cathode occurs during the separation process. This effect, known as electro-osmosis, results in a movement of neutral species which would normally be expected to remain at the point of application of the sample. If required, a correction can be applied to the distances migrated by ionic species by measuring them... 

Although solvents may be classified as donor solvents (Lewis bases) and acceptor solvents (Lewis acids), most of the more widely used nonaqueous solvents are donor solvents. Some acceptor solvents, such as S02, BrF3, AsC13, or the liquid hydrogen halides, have proved to be useful in coordination chemistry.13"16 Ionization is promoted in a donor solvent by solvation of cations and in an acceptor solvent by solvation of anions. For example, arsenic(m) iodide is ionized in a donor solvent D according to the reaction... 

At pHs above the pJfa of the acid, it will also be more soluble in water. Hydrocarbons are insoluble in water—oil floats on water, for example. Unless a compound has some hydrophilic groups in it that can hydrogen bond to the water, it too will be insoluble. Ionic groups considerably increase a compound s solubility and so the ion A- is much more soluble in water than the undissociated acid HA. In fact water can solvate both cations and anions, unlike some of the solvents you will meet later. This means that we can increase the solubility of a neutral acid in water by increasing the proportion of its conjugate base present. All we need to do is raise the pH. 

The Born equation for the solvation of ions provides a means of determining the hydration energy of a charge in an aqueous medium. When two ions in an aqueous medium react (as in the case of cations and anions in an acid-base reaction), the reaction may be considered as occurring between the dissociated ions whose energy is modified due to this hydration. 

The dissociation constants of trityl and benzhydryl salts are KD 10 4 mol/L in CH2C12 at 20° C, which corresponds to 50% dissociation at 2-10-4 mol/L total concentration of carbocationic species (cf. Table 7) [34]. The dissociation constants are several orders of magnitude higher than those in analogous anionic systems, which are typically KD 10-7 mol/L [12]. As discussed in Section IV.C.l, this may be ascribed to the large size of counterions in cationic systems (e.g., ionic radius of SbCL- = 3.0 A) compared with those in anionic systems (e.g., ionic radius of Li+ 0.68 A), and to the stronger solvation of cations versus anions. However, the dissociation constants estimated by the common ion effect in cationic polymerizations of styrene with perchlorate and triflate anions are similar to those in anionic systems (Kd 10-7 mol/L) [16,17]. This may be because styryl cations are secondary rather than tertiary ions. For example, the dissociation constants of secondary ammonium ions are 100 times smaller than those of quaternary ammonium ions [39]. 

No systematic study of the effect of different solvation models has been performed. A few reports have compared specific cases such as the study of cationic and anionic alanines, which shows a significant improvement in the chemical shift prediction using polarized continuum method (PCM) or better stiU a hybrid solvation approach (Section 1.4.3). However, the linear scaling correction discussed below can often account for the systematic solvent effect and so sometimes one can get away without any solvent computation at all. 

Fig. 3.3 Stereo views of the unit cells of the gold (upper) and red (lower) forms of the salt of 3-XVI and 3-XVII. In both cases the view is on the plane of the three central atoms of the oxonol molecule, with the bisector of the angle formed by the three oriented vertically for the oxonol molecule in the lower right hand comer of both figures. Chloroform molecules of solvation have been eliminated for clarity. Both figures indicate the relative orientation of cation and anion which is one distinguishing feature of the overall packing.

In addition to dipole-dipole interactions, polar protic solvents are capable of intermolecular hydrogen bonding, because they contain an O - H or N - H bond. The most common polar protic solvents are water and alcohols (ROH), as seen in the examples in Figure 7.6. Polar protic solvents solvate both cations and anions well. 

Polar protic solvents like H2O and ROH solvate both cations and anions well, and this characteristic is important for the SnI mechanism, in which two ions (a carbocation and a leaving group) are formed by heterolysis of the C-X bond, The carbocation is solvated by ion-dipole interactions with the polar solvent, and the leaving group is solvated by hydrogen bonding, in much the same way that Na" and Br are solvated in Section 7.8C. These interactions stabilize the reactive intermediate. In fact, a polar protic solvent is generally needed for an SnI reaction. SnI reactions do not occur in the gas phase, because there is no solvent to stabilize the intermediate ions. 

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