Structure-making ions

To measure the effect of the presence of ions on the hydrogen-bond structure of bulk water, we studied the concentration dependence of the orientational relaxation of bulk water in aqueous solutions of Mg(C104)2. We chose Mg(C104)2 because Mg2+ is considered to be a strong structure-making ion the viscosity of 1 M Mg(C104)2 is about 30 %... 

The structure of Li and K ammonia solution has been recently studied by neutron diffraction experiments [36]. The results show, for saturated lithium-ammonia solutions, that the cation is tetrahedrally solvated by ammonia molecules. On the other hand, from the data of the microscopic structure of potassium-ammonia solutions, the potassium is found to be octahedrally coordinated with ammonia molecules. The Li+ is a structure making ion and K+ is a structure breaking ion in alkali metal-ammonia solutions [37, 38]. 

A simple model that accounts for the large specific interactions between ions and surfaces is based on the long-known role of ions in structuring the water around them the structure-making ions interact preferentially with the bulk water, hence they are effectively repelled by the surface, while the structure-breaking ions disturb the... 

Keywords Specific ion effect Ion hydration Structure-making ion Structure-breaking ion... 

The surface tension can either increase or decrease with electrolyte concentration, depending whether the depletion of the structure-making ions in the vicinity of the surface or the adsorption of the structure-breaking ions dominates the process. The distribution of ions in the vicinity of the interface could be calculated if accurate expressions for AlT,(x) would be available. 

Hence, one can describe the hydration interaction of a structure-making ion by a parameter, representing a distance of closes) approach to the surface. It is plausible to consider dt related to the hydrated radii of the ions of species T however, they are not the same because the structure of water is different in the vicinity of the interface from that in the bulk. 

The change in the ion hydration energy between the bulk and the water-air interface for structure-making ions is so large compared to kT, that these ions practically do not approach the interface. The change is not so steep when a solid interface is immersed in water, particularly when the surface has sites which can bound structure-making ions. In what follows, it will be assumed that the anions are structure breaking and hence interact with the surface via an attractive potential of the type (Wx >0, see Fig. 3a) ... 

These repulsive ion-hydration forces acting on the structure-making ions decrease their concentration in the vicinity of the interface. In contrast, the structure-breaking ions have unfavorable interactions with the water molecules, because the bulk water molecules can better organize their hydrogen bonding network in their absence. Consequently, the structure breaking ions are expelled from the bulk toward the interface, where their concentration is increased [10]... 

Chapter 5 is concerned with specific ion effects due to ion hydration forces and is based on the Structure Making / Structure Breaking (SM/SB) model. The structure making ions prefer the bulk water and are repelled by the surface, while the structure breaking ions disturb the order of bulk water and are expelled toward the interface. The model was used to calculate the force between two parallel plates. There are numerous experiments that point out that the... 

LiCl, on the other hand, which has a positive B in water, shows no minimum in fact, B changes very little with composition in the water-rich region. Thus, structure-breaking ions find more structure to break in water that contains a little methanol than in pure water. Structure-making ions, however, find little scope for their talents. 

Further linear correlations of the values with independent properties of the ions, such as their molar refractivity (polarizabihty), their softness, and their effects on the structure of the water in their vicinity, have also been reported by Marcus [131], Water-structure-breaking ions (see Chapter 5) have small positive or even negative values and structure-making ions are desorbed from the water surface with positive values. On the whole, however, hardly any correlation exists when all the ions for which both k values and also values are known are considered. Thus,... 

Structural Entropy A molar structural entropy of ions in solution, A 5, is obtained from the standard molar entropy of solvation of the ion, when certain irrelevant quantities are subtracted from the latter. These include the compression entropy (change of available volume) on transfer from the gas to the solution and contributions from the formation of the ionic solvation shell and possible limitation of the ionic rotation of a polyatomic ion in the solution compared with the gas. The terms kosmottopes for water structure-making ions and chaotropes for water structure-breaking ions were introduced by Collins [48] (Section 5.1) according to whether A S < or respectively. This view sfressed the competition... 

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