Primary and secondary solvation

An analogy between the situation just described and those involved in ion-solvent and ion-ion interactions can be drawn. The solvent water, for example, normally has a particular structure, the water network. Near an ion, however, the water dipoles are under the conflicting influences of the water network and the charged central ion. They adopt compromise positions that correspond to primary and secondary solvation (Chapter 2). Similarly, in an electrolytic solution, the presence of the central ion makes the surrounding ions redistribute themselves—an ionic cloud is formed (see Chapter 3). 

On treating ion solvation it is useful to differentiate between primary and secondary solvation shell or between chemical and physical solvation, respectively The electrostatic calculation of ion solvation is quite often less accurate because specific ion-solvent interactions have to be considered. In the primary solvation shell specific ion-solvent interactions are of much more importance than those with solvent molecules... 

First, immediately after ionization, contact ion pairs are formed, in which no solvent molecules intervene between the two ions that are in close contact. The contact ion pair constitutes an electric dipole having only one common primary solvation shell. The ion pair separated by the thickness of only one solvent molecule is called a solvent-shared ion pair In solvent-shared ion pairs, the two ions already have their own primary solvation shells. These, however, interpenetrate each other. Contact and solvent-shared ion pairs are separated by an energy barrier which corresponds to the necessity of creating a void between the ions that grows to molecular size before a solvent molecule can occupy it. Further dissociation leads to solvent-separated ion pairs Here, the primary solvation shells of the two ions are in contact, so that some overlap of secondary and further solvation shells takes place. Increase in ion-solvating power and relative permittivity of the solvent favours solvent-shared and solvent-separated ion pairs. However, a clear experimental distinction between solvent-shared and solvent-separated ion pairs is not easily obtainable. Therefore, the designations solvent-shared and solvent-separated ion pairs are sometimes interchangeable. Eventually, further dissociation of the two ions leads to free, i.e. unpaired solvated ions with independent primary and secondary solvation shells. The circumstances under which contact, solvent-shared, and solvent-separated ion pairs can exist as thermodynamically distinct species in solution have been reviewed by Swarcz [138] and by Marcus [241],... 

Net Effect on Solubility of Influences from Primary and Secondary Solvation... 

S as written in Eq. (2.154) has taken into account the primary and secondary solvation and can be identified with the solubility of the nonelectrolyte after addition of ions to the solution. Hence,... 

Only nuclei with a net nuclear spin will give a signal. However, this is no problem for hydration studies since it is the chemical shift of the proton, H, signal of H2O which is being measured. In certain cases the signal from in Hj O is used. In a solution the O atom is the atom closest to the cation and the H atom is the one closest to an anion. Essentially the experiment looks at the environment of the atom whose resonance is being studied. In an electrolyte solution there are three species of interest the solvated cation, the solvated anion and bulk water. Coordinated water will exchange with secondary solvation of the ion under study, with the primary and secondary solvation of the counter ion and with the bulk water. 

It is best if hydration can be studied using the two peaks corresponding to coordinated water and the rest of the water which comprises secondary solvation of the cation and primary and secondary solvation of the anion and the bulk water. When two signals are observed it is possible to determine individual hydration numbers. 

Gusev assumed that the point of intersection indicates the absence of free (i.e., non-solvating) water, so there is minimal proton transport through the solution. The water cation mole ratio at that point is presumably the hydration number of the cation, including both primary and secondary solvation. This method gave a value of h = 20 for Th(IV) which can be compared to the value of 22 obtained from compressibility measurements (Bockris and Saluja 1972a, b, 1973) based on the lower compressibility of solvated solvent molecules (Passynski 1938) as a result of electrostriction. 

The most common model of ion solvation is the concentric shell model, postulating the existence of several coordination shells around the solvated ion (Enderby and Neilson 1981). However, there remains controversy over the precise definition of primary and secondary solvation shells and numbers, especially for those ions where movement of solvent molecules between shells and bulk solvent is very fast. To avoid ambiguity, the term inner (or first) coordination sphere is used here to refer to both the solvent and the ligand molecules which are in direct contact with the central ion. 

Oximes are another solute type having nitrogen and oxygen as basic sites. From the one available example (24), the primary and secondary interactions appear to be at oxygen and nitrogen, respectively. Nonequivalence shown by the three methyl resonances of the oxime derived from This example suggests a... 

In general, the observation of opposite senses of nonequivalence for substituents on opposite faces of the plane defined by primary and secondary interactions will be the hallmark of a normal solvation model. Deviations are of no consequence for enantiomeric purity determinations but should raise questions concerning the validity of the usual model for the assignment of absolute configuration based on the observed senses of nonequivalence. Since knowledge of solute structure often allows anticipation of such third interactions, ... 

Plasticizers should be relatively nonvolatile, nonmobile, inert, inexpensive, nontoxic, and compatible with the system to be plasticized. They can be divided based on their solvating power and compatibility. Primary plasticizers are used as either the sole plasticizer or the major plasticizer with the effect of being compatible with some solvating nature. Secondary plasticizers are materials that are generally blended with a primary plasticizer to improve some performance such as flame or mildew resistance, or to reduce cost. The division between primary and secondary plasticizers is at times arbitrary. Here we will deal with primary plasticizers. 

We have also carried out experiments in secondary alcohols. For example. Fig. 5 shows the spectra in primary and secondary octanol (1-octanol vs. 2-octanol). These data show that while the initial spectra are approximately the same in the two alcohols, the final spectra are considerably different. There is much less shift in the absorption spectrum in the 2-octanol, suggesting that 2-octanol is less effective at solvating the benzophenone molecule. 

Figure 8 (a) Temperature dependence of the solvation of the benzophenone anion in primary and secondary propanol and n-butanol. (b) Temperature dependence of the solvation of the electron in n-propanol and 2-propanol. 

These structural changes in the primary and secondary regions are generally referred to as solvation (or as hydration when, as is usual, water is the solvent). Since they result from interactions between the ion and the surrounding solvent, one often uses the term solvation and ion-solvent interactions synonymously the former is the structural result of the latter. 

Noncovalent MTPA Derivatives. The enantiomeric purity of some chiral amines can be determined by H NMR with (S)- or (J )-MTPA as a chiral solvating agent. The method is particularly useful for chiral tertiary amines that are not amenable to conversion into MTPA amides, e.g. (18) and (19), although it has been utilized for primary and secondary amines as well, e.g. (20). ... 

Basic, protic solvents include ammonia and primary and secondary amines. These solvents are primarily of interest for an organic electrochemist because of their ability to solvate electrons, and solvated electrons have special reducing properties (Chapter 29). They also permit reductions in a protic medium in the presence of a very strong base, the conjugate base of the solvent. 

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