Clays divalent cations’ effect

A number of processes affect the solubility characteristic of metal-hu-mate and metal-fulvate complexes in soils, as well as in natural waters. A major factor is the extent to which the complex is saturated with metal ions. Other factors affecting solubility include pH, adsorption of the complex to mineral matter (e.g., clay), and biodegradation. Under proper pH conditions, trivalent cations, and to some extent divalent cations, are effective in precipitating humic substances from very dilute solutions monovalent cations are generally effective only at relatively high particle concentrations. 

The Donnan equilibrium theory implies that dilution of a clay/water system containing monovalent and divalent cations displaces the equilibrium in such a manner that the absorption of divalent ions increases, whereas the absorption of monovalent ions decreases. The ionic charge is not the only determining factor in the absorption effect. Factors such as temperature, pH, and specific ions also play important roles. Hydration energy, which appears to be one of the most important factors for the absorption and fixation of cations, displaces the ionic equilibria in a manner opposing the Donnan equilibrium theory. According to Sawhney (1972), "cations with low hydration energy such as Ca, Mg and Sr, produce expanded interlayers and are not fixed". 

Salinity Salinity plays at least two important roles, namely it maintains the integrity of the reservoir and it balances the physicochemical environment so that surfactant formulation stays close to optimal. Thus, ultra-low interfacial tension and oil solubilisation are very sensitive to salinity. Mixing of the surfactant slug with connate water may alter the surfactant formulation mainly due to dilution and to the incorporation of new electrolytes to the formula. Adsorption and desorption of electrolytes, particularly divalent cations, onto or from solid materials such as clay, will also change the salinity of the aqueous phases to some extent and may cause surfactant precipitation, which is however not always an adverse effect [151]. In order to attenuate the undesirable salinity effects on formulation, surfactants able to tolerate salinity changes [109], high salinity [150] and the presence of divalent ions [112] maybe used. 

In the dry clay, the cations are in hole position. Each monovalent cation in this position can effectively neutralize each charge resulting from substitution in the octahedral sheet. (The mineral studied did not have tetrahedral substitution according to the analysis of the authors.) A divalent cation, however, must neutralize two charges in the lattice simultaneously. It can do so effectively only when two substitution sites happen to be neighboring sites close to the hole where the cation is located. Random distribution of substitution sites in the octahedral sheets was assumed, and it was computed that about 30 percent of the exchange positions would be favorable for simultaneous neutralization by a divalent cation. It is reasonable to assume that (a) in replacing sodium by calcium, first the 30 per cent favorable positions for the calcium ions are occupied, and (b) in these positions, the hydration behavior of the calcium ions is the same as that of the sodium ions. 

Equation 3.108 predicts a higher local concentration of cations near a negatively charged clay surface than in bulk solution, and a lower concentration of anions near the surface than in solution. Figure 3.24 shows this predicted distribution of monovalent cations and anions near the clay surface for two different concentrations of electrolyte in solution. More modem statistical mechanical models of this clay interfacial region have predicted that ion-ion correlation (electrostatic) effects should cause deviations from this classical picture, such as the positive adsorption of anions at intermediate distances from the surface when the cation is divalent or multivalent. 

Another factor which affects IFT is hardness or the presence of divalent metal cations, because they can couple with the surfactants forming less active species in solution. Sodium silicates help maintain the lowest IFT possible, since they are very effective in sequestering and removing these ions. In most cases these divalent ions can never be completely removed from the reservoir systems because clays are present which continually reintroduce hardness into solution... 

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