Solvation sheath, sphere

There has recently been increasing interest in mixed solvation using either the chemical shift (29, 36-43) or the relaxation (27, 44-49) as an experimental variable. Either method bears the potential to provide information on preferential solvation as a function of solvent composition. The chemical shift method is based on the assumptions (a) that the shielding of the solvated ionic nucleus varies linearly with the composition of the solvation sheath and (b) that outer-sphere... 

Interchange (I) A concerted process (i.e., one without detectable intermediates) in which the departure of the leaving group and the entry of the incoming ligand occur on a timescale that is short relative to the lifetime of the encounter complex or second coordination sphere (solvation sheath, etc.). If the reaction... 

The primary solvation number may be determined by means of various mutually independent methods. However, it must be noted that the different methods do not yield identical values in every case. Padova [Pa 63b, Pa 64a] calculated the solvation numbers (n) of certain electrolytes from the molar volumes. He used the assumption that the solute ion gives rise to such a strong electrostatic field that the solvate sheath consisting of solvent molecules bound in the first coordination sphere becomes incompressible. Thus, the molar volume (cm /mole), of the solvated electrolyte can be described by the equation... 

As a consequence of the preferential solvation of the dissolved ions in a solvent mixture, one or other solvent may become enriched in the solvate sphere of a given ion, and this changes the relaxation conditions reflecting the solvent-solvent interaction. Equations describing these have been elaborated by Capparelli et al [Ca 78a]. Their method was used to draw conclusions on the changes in the compositions of the solvate sheaths in solutions of magnesium perchlorate, potassium iodide and rubidium iodide in water-methanol solvent mixtures, purely from the influence of the interaction of the water molecules alone on the rate of intermolecular relaxation. It was found that all three cations coordinate water in a higher proportion than corresponds to the composition of the solvent mixture, i.e., they are preferentially hydrated. 

Even these few data show that there is a sudden change in the composition of the solvate sheath at a methanol to dimethylformamide ratio of 0.64 1. In systems with a lower dimethylformamide content, methanol clearly predominates in the inner ligand sphere. 

The chemical characterization of a solvent mixture is much more difficult because it is not the mixture, but its components, that take part in the solvation reactions. However, the solvating powers of the components are affected by their interactions with one another. Depending on the chemical properties of the system, the solvating powers of the individual components may differ widely. Hence, the effects of the components in forming a solvate sheath will not be proportional to their ratio in the mixture. An understanding of the system is made even more complicated by the fact that, besides solvates that contain only one or the other solvent, the solute can also form mixed solvates in the solvate spheres of which both solvents are present together, possibly in various ratios. This topic is treated in greater detail in Chapter 8. 

In Passynski s theory, the basic assumption is that the compressibility of water sufficiently bound to an ion to travel with it is zero. Onori thought this assumption questionable and decided to test it. He used more concentrated solutions (1-4 mol dm ) than had been used by earlier workers because he wanted to find the concentration at which there was the beginning of an overlap of the primary solvation spheres (alternatively called Gurney co-spheres) of the ion and its attached primary sheath of solvent molecules. 

Ions and charged surfaces can break down the ice slurry structure of water. The electric charges are stronger than dipole forces and tend to pull water molecules away from their groups by attracting the positive or negative ends of the water dipoles. Solutes and water molecules are constantly in motion, but they remain in the vicinity of each other for some period of time. If water molecules remain near an ion longer than the time required for the water molecules to dissociate from the water structure, the ion will have a sphere of water molecules (a solvation sphere or sheath) around itself. The number of water molecules in the closest solvation sphere is called the primary hydration number. 

For a long time, the following model was used for the characterization of the solvation of ions [Fr 57]. The dissolved ion is surrounded by a strongly binding primary sheath of solvent molecules. Around this is situated the still ordered secondary sheath, less firmly bound, which is separated from the bulk of the solvent by a non-ordered layer. Symons [Sy 75c] cast doubt on the existence of such a non-ordered, third layer in solutions formed with protic solvents. In his model, the entire system is characterized by continuous hydrogen bonding interactions, the solvent molecules of the ordered solvate spheres being connected directly to the similarly ordered bulk solvent. 

The term complexation refers to the reaction of a metal ion with an electron donor group (ligand), and entails the transformation of a solvated metal ion into a complex ion. In aqueous solutions of metal salts, the metal cations are surrounded by a sheath of aquo groups which are collectively called the primary hydration sphere. These solvated metal ions are not referred to as complex ions. Instead, a complex ion is formed if one or more of the solvent molecules within the primary hydration sphere of the metal ion is replaced either by ions, or by... 

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