Structure-breaking ions

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

Typical MS/MS configuration. Ions produced from a source (e.g., dynamic FAB) are analyzed by MS(1). Molecular ions (M or [M + H]+ or [M - H]", etc.) are selected in MS(1) and passed through a collision cell (CC), where they are activated by collision with a neutral gas. The activation causes some of the molecular ions to break up, and the resulting fragment ions provide evidence of the original molecular structure. The spectrum of fragment ions is mass analyzed in the second mass spectrometer, MS(2). 

Effect of Cs+, a structure-breaking type cation In acidic medium, the silica anions will interact preferentially with the smallest positively charged entities which appear to be the hydrated Cs+ ions. Consequently, a small number of nuclei (because silicate-Pr N interactions are less preferred), essentially Si-rich (becau -se silicate-Al(H20) + interactions are less favoured) and containing very little Na (because silicate-Na(H20)x+ interactions are... 

The explanation given here seems more feasible than that using the concepts of co-sphere overlap (29), and the structure-making or structure-breaking characteristics of ions (SO). This is unsatisfactory because the order shown by the curves does not depend on concentration, and is evident at very low concentrations where the co-spheres would have to be enormous in order to have any overlap. Moreover, the physical nature of the forces arising from such overlap is rather intangible. 

Molecular rotation In a normal crystal every atom occupies a precise mean position, about which it vibrates to a degree depending on the temperature molecules or polyatomic ions have precisely defined orientations as well as precise mean positions. When such a crystal is heated, the amplitude of the thermal vibrations of the atoms increases with the temperature until a point is reached at which the regular structure breaks down, that is, the crystal melts. But in a few types of crystal it appears that notation of molecules or polyatomic... 

NOJ > J" > SCN". The reason can be understood by our experiments with the influence of ions on the water content in organic systems of two phases organic/ aqueous (Chapter lie.). The structure breaking ions increase the water content and imply an increase of the bulk water like H20 molecules which can solve ions (Chapter lie.). 

Typical results for the evolution of the distances R of ions from the center of the box and corresponding snapshots of the dissolution process are shown in Fig. 4. Water molecules are omitted in this figure. The dissolution process of NaCl and CsF crystals has been recorded on video tape (27), in which the movement of water molecules is included. From these the rotational, librational, and translational motions of water molecules are observed. The video pictures show the behavior of water molecules around the structure breaking chloride ions, and the slower motion of water molecules around the structure making fluoride as opposed to chloride can be seen. Color photographs of the snapshots for the dissolution of an NaCl crystal, together with water molecules, within 7 ps are shown in Fig. 5. 

Leaching of ions and break-down of silicates During leaching of a mineral an ion is removed from a site in a crystal structure to an aqueous phase. Most transition metal ions are present in six-coordinated sites in silicate minerals and exist in aqueous solutions as hexahydrated ions, [A/(H20)6]"+. The crystal field splittings summarized in table 2.5 indicate that CFSE s of ions in aqueous solutions and oxide structures are comparable,... 

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... 

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. 

A similar simple treatment is less accurate for the structure-breaking ions. The change in the total free energy between surface and bulk cannot exceed a few kT, because otherwise the ions would be trapped at the interface and generate a large surface potential, possibility ruled out by experiment. Since AW,-(x) is not very large, it cannot be well approximated, in general, by a step function. However, in the spirit of the above approximation and in the absence of more accurate information, we will assume that it can be described by a potential well and will examine the consequences. 

More difficult is to treat the case of structure-breaking ions, which are pushed toward the interface, because they have there more favorable interactions with water. The consequence of a potential well for the anions in the vicinity of the interface is therefore investigated. However, the depth of the well should not be larger than a few kT, otherwise huge surface potentials would be generated at high ionic strengths, and this was not observed experimentally. 

The behavior of surface tension at high ionic strength also can be understood on the basis of changes in ion hydration changes between bulk and interface. The tendency of the structure-breaking ions to accumulate at the surface can lead to a positive surface adsorption. However, if the cations cannot approach the interface, the asymmetry of the ion distributions generates a potential, which repels the anions from the interface, and the total adsorption becomes negative. Consequently, the surface tension increases with electrolyte concentration this occurs for simple salts (NaCl, KC1). If the cations can approach the interface, the accumulation of anions in the vicinity of the interface is also followed by an accumulation of cations,... 

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) ... 

Structure Making/Structure Breaking (SM/ SB) Model for the Short-Range Ion-Hydration... 

For structure-breaking ions, it was suggested that their attraction toward the interface is governed by a simple surface potential well, described by two adjustable parameters (the width and the depth of the surface potential well).6 When this simple model was employed for the air/ water interface, it could explain the dependence of the surface potential on the electrolyte concentration and on pH, the behavior of surface tension of salt and acids at high ionic strengths and the Jones-Ray effect (the existence of a minimum in the surface tension of salts at a small electrolyte concentration).7... 

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