Salting-out phenomenon

Figure 8,12 Salting-out phenomenon for aqueous CO2. Activity coefficient of neutral species increases with increasing salinity, determining decreased solubility of aqueous CO2 in water, T and P conditions being equal. Reprinted from Garrels and Christ (1965), with kind permission from Jones and Bartlett Publishers Inc., copyright 1990.

As ionic strength, in Figure 2.3, is increased, the solution again reaches a point where the solute molecules begin to separate from solvent and preferentially form self-interactions among themselves that result in crystals or precipitate. The explanation for this salting-out phenomenon is that the salt ions and macromolecules compete for the attention of solvent molecules, that is, water. Both the salt ions and the protein molecules require hydration layers to maintain their solubility. When competition between ions and proteins becomes sufficiently intense, the protein molecules begin to self-associate in order to satisfy, by intermolecular interactions, their electrostatic requirements. Thus dehydration, or the elimination and perturbation of solvent layers around protein molecules, induces insolubility. 

In preparing dilute solutions of a series of alk.ylbenzene sulfonates for adsorption experiments, it was observed that most of them became cloudy upon the addition of NaCl, and that a precipitate of salted-out surfactant formed in a considerable number. Since it was deemed necessary to use clear surfactant solutions only for adsorption measurements, a more detailed study of the salting-out phenomenon was undertaken, and some of these results are presented here. 

Aside from the significance of the salting-out phenomenon itself, these observations are important for adsorption measurements in that it appears that the surfactant concentration actually in solution is less than 0.1 wt.% when appreciable concentrations of NaCl are present. Not only does the dissolved surfactant concentration appear to be less than about 0.1 wt.% but there is the effect on the apparent adsorption if the salted out surfactant partially or completely separates with the clay or other adsorbent being studied. Complete separation of the salted-out surfactant leads to large values of apparent adsorption and low equilibrium surfactant concentrations negligible separation of the salted-out surfactant leads to low adsorption and large apparent equilibrium surfactant concentrations but the actual dissolved surfactant con-... 

A quantitative solution to the salting-out phenomenon has not been found, but the phenomenon is reproducible. The Setchenov Equation has most frequently been used to describe the salting-out effect of neutral molecules although the equation does not have a theoretical basis ... 

Roberts, D.D., P. Pollien, Dependence of salting-out phenomenon on the physical chemical properties of the compounds, in Frontiers of Flavour Science, P. Schieberle,... 

An alternative scenario is possible in the absence of these hydrophobic chains on the adsorbent/any adsorbent, namely hydrophobic patches in two neighboring protein molecules are bound together this leads to protein precipitation. This is the salting-out phenomenon for protein solutions, wherein a high concentration of a salt, typically ammonium sul te (1.5-3M), added to the protein solution, precipitates the protein molecules from the solution. Generally, the nature of the cation in the salt is not important However, the anions are all important The precipitation ability of the anions follow in decreasing order ... 

The explanation of the lower consolute temperature for the polyoxyethylene chain in water solution, and the salting-out behavior, based on earlier considerations outlined above, is likely incomplete. The earlier treatment pictured the polyoxyethylene chain, above some minimum molecular weight of about 1500, as a highly expanded random coil and the collapse of that coil, particularly in salt solutions, as leading to theta conditions of the Flory type. A critique of this interpretation was presented by Amu (17). The question is whether or not the random coil in water-salt solutions is simply collapsed, in the Flory sense, by a salting-out phenomenon that could be described by the reasoning of the Debye-McAulay equation. 

The selective extraction of proteins in aqueous solutions having different ionic strengths can be used for their fractionation and separation. The process is based on the salting-in and salting-out phenomenon of food proteins. 

Divide the saturated solution of n-butyl alcohol in water into three approximately equal parts. Treat these respectively with about 2-5 g. of sodium chloride, potassium carbonate and sodium hydroxide, and shake each until the soli have dissolved. Observe the effect of these compounds upon the solubility of n-butanol in water. These results illustrate the phenomenon of salting out of organic compounds, t.e., the decrease of solubility of organic compounds in water when the solution is saturated with an inorganic compound. The alcohol layer which separates is actually a saturated solution of water in n-butyl alcohol. 

Divalent and trivalent ions can precipitate PAA, and this phenomenon is related to the loss of a hydration region. Such precipitation is to be distinguished from salting-out effects which occur with high concentrations of monovalent ions. 

The solubility of any solid can be either increased or decreased by the addition of an electrolyte to the solvent, a phenomenon known as the salt effect. Salting-out describes the situation in which the solubility of the solid is decreased by the salt effect, whereas salting-in is the term used when the solubility is increased. Salting-out takes place when the added electrolyte sufficiently modifies the water structure so that the amount of water available for solute dissolution is effectively reduced, and it is a procedure convenient for the isolation of highly soluble substances. 

Gas solubility decreases with increasing salinity. This phenomenon is referred to as salting out. It is caused by the electrostatic forces exerted by the salt ions. These forces have to be overcome to create spaces between water molecules to accommodate a gas atom or molecule. So higher salinities lead to less favorable energetics for gas dissolution. The high salt content of seawater also leads to nonspecific interactions that cause gases to have activity coefficients on the order of 1.1 to 1.2 at a salinity of 35%o and temperature of 25°C. 

Salt concentration. The addition of a small amount of neutral salt usually increases the solubility of a protein, and changes the interaction between the molecules as well as changing some amino-acid charges. The overall effect is to increase the solubility. This phenomenon is known as salting in. However, at high concentrations of salts the solvating interactions between protein and water are reduced, and the protein may be precipitated from solution—a process termed salting out. 

As a secondary effect, this clustering also causes a decrease in the solubility of potential hydrate guest molecules in water, a phenomenon known as salting-out. Both ion clustering and salting out combine to require substantially more subcooling to overcome the stmctural changes and cause hydrates to form. 

For Nal a peculiar phenomenon is observed. After the polyelectrolyte precipitates at a certain concentration of added Nal ( salting out ) it dissolves again upon further increase of the salt concentration ( salting in ). For the polyvinylpyridinium system this phenomenon is only observed for added salt containing iodide, whereas long ago the salting in was also observed for anionic polyelectrolytes [21, 22]. 

Based on the position of an ion in the Hoftneister series, it is possible to foretell the relative effectiveness of anions or cations in an enormous number of systems. The rank of an ion was related to its kosmotropicity, surface tension increments, and salting in and salting out of salt solutions (see below) [25]. A quantitative physical chemistry description of this phenomenon is not far off. Molecular dynamics simulations that considered ionic polarizability were found to be valuable tools for elucidating salt effects [26,27]. 

The solubility of a dissolved non-electrolyte solute can be reduced by the addition of a salt. This phenomenon, known as the salting-out effect, is of practical importance for the isolation of organic compounds from their solutions. In the presence of a dissolved dissociated salt, a fraction of the solvent molecules becomes involved in solva-tional interaction with the ions of the electrolyte, whereby their activity is diminished, leading to salting-out of the dissolved non-electrolyte solute. In other words, the salting-out can be considered as the difference in solubility in two kinds of solvents, the ion-free and the ion-containing one [248]. 

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