Structure-breaking salts

As further evidence of the importance of hydrophobic interactions in these systems, we examined the partition coefficient of methyl orange in the presence of water structure-forming and water structure-breaking salts above and below the transition temperature [70], Methyl orange is an easily detected, hydro-phobic dye which has been sulfonated to improve water solubility. Water structure-breaking salts like tetraethylammonium chloride (TEAC) are known to minimize hydrophobic interactions while water structure-forming salts like ammonium sulfate are known to increase hydrophobic interactions [165, 166]. 

Previous research regarding soluble salt flotation has suggested that interfacial water structure may play a significant role in surfactant adsorption (Hancer et al. 2001 Veeramasuneni et al. 1997). For example, even in the absence of collectors, structure-breaking salts such as KCl and KI have been shown to have a hydrophobic surface character, whereas structure-making salts are completely wetted by their saturated brine. Table 4.7 is the summary of contact angles for saturated brines at the surfaces of selected alkali halide salt surfaces in the absence of a surfactant (Hancer et al. 2001). 

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,... 

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... 

The structure-breaking influence of urea accounts for a remarkable set of solubility data for hydrocarbons in aqueous solutions (Wetlaufer et al., 1964), Added urea salts-in propane so that transfer of one mole of propane from water to an aqueous solution of urea (7 mol dm-3) is thermodynamically favourable at 298 K (Table 6). However, the transfer is endothermic, increase in solubility... 

Hydrophobic effects are on a list of special phenomena. They are closely tied to salting in because one of the reasons for hydrophobic effects (water pushing-out effects, one could say) is that the ions of the solute tend to attract each other or other nonelectrolytes present and push the water between them out. Structure breaking in a solution, some part of which rejects water in the rearrangements formed, also gives hydrophobic effects. 

Density measurements on solutions of alkali-metal salts in methanol from 0 to 60 °C over a wide concentration range show that the structure-breaking effect of the salts decreases in the order NaC104 > Nal, NaBr and KI> Nal > Lil.94... 

In the fluoride salts of alkali metals, those cations considered structure-breaking by Frank and Wen, such as cesium, show the highest apparent distribution coeflBcients (Figure 10). Conversely, lithium, a structure-maker in water, is taken into the ice lattice with greater diflBculty. From a consideration of Figure 10 we arrive at the following tentative series, in order of decreasing acceptability ... 

An interesting situation arises in mixed solvents that are predominantly aqueous. A small addition of, for instance, alcohol, enhances the hydrogen-bonded structure, whereas at larger alcohol concentration, the structure breaks down and the solvent becomes nonaqueous (54). In the range of small ethanol concentrations, J// of the 5 1 solvolysis of t-butyl chloride has been compared (7, 8) with the heat of solution of that substance on the one hand, and with the heat of solution of salts of comparable molecular size, assumed to resemble the transition state, on the other. It is concluded (8) that it is the initial state energy and not that of the transition state which is influenced by the structure of the solvent. 

Luck (1965,1975) used infrared spectroscopy to obtain the fraction of non-bonded OH groups at the wavelength of the peak of free OH in water at 200 °C. He then derived values of Tst ranging from 10-85 °C for salt solutions at T from 20-65 °C, but shown only in diagrams. Leyendekkers (1983) used these data at 25 °C to obtain the A r = Tsti - T values shown as Ar(IR) in Table 3.4. When the structure breaking properties of a salt dominate over the structure making ones AT is positive but is negative otherwise. 

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