Desolvation and precipitation

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. 

Ikegami Imai (1962) made a study of precipitation and hydration using turbidity, conductance, refractive index and viscosity measurements. The following account is based on their description. 

Ikegami Imai (1962) explained their results by assuming that divalent ions can be bound to PAA in two forms, which they represented as 

According to these workers the formation of COO-Me causes a small degree of dehydration, while that of COO-Me-OOC is accompanied by considerable dehydration. The experimental results showed that, when a = 10, divalent ions are bound as COO-Me-OOC, a form which favours precipitation. However, as a decreases, the COO-Me form becomes more apparent. 

The ratio of the two forms depends on the cation as well as on a. Ba has a greater tendency to make linkages of the COO-Me-OOC type than Mg and this difference is accentuated when the density of COO in the polyanion is low. Thus, at a = 025 more Ba ions are in the COO-Ba-OOC form than in the COO-Ba form, while the reverse is true for Mg ions. Moreover, the structure COO-Mg is more stable and soluble than COO-Ba because Mg is more hydrophilic than Ba. For these reasons, Ba is precipitated at a = 0-25 while Mg is not. This interpretation is supported by titration experiments in the presence of divalent cations (Jacobsen, 1962). Magnesium forms very stable hydrates and would be expected to be more difficult to desolvate. 

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Lyophilic colloids can also be desolvated (and precipitated if the electric double layer interaction is sufficiently small) by the addition of non-electrolytes, such as acetone or alcohol to aqueous gelatin solution and petrol ether to a solution of rubber in benzene. 

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