Surface charge: aluminosilicate ions

Aluminosilicates such as montmorillonite, kaolinite, illite, and vermiculite are solids that have structures readily accessible to counter ions. The excess negative charge resulting from isomorphic substitution of Al for Si is primarily distributed over the three adjacent surface O atoms of the layer, where it is electrically balanced by mobile, exchangeable cations- Thermodynamically, ion exchange can be interpreted in terms of the interlayer electrostatic interaction between surface charges and hydrated cations in accordance with the classical Eisenman theory (Eisenman, 1983). A comprehensive description has recently been given by Maes and Cremers (1986). 

However, it is still not clear why a negatively charged surface of an aluminosilicate should not exert an even more powerful attraction for the cationic groups of choline on cell membranes than does pure silica. It can only be assumed that both ionic attraction and hydrogen bonding are necessary. As shown by Her (253) the aluminosilicate ions prevent hydrogen bonding. It is on this basis, then, that a relatively low concentration of alumina must reduce the destructive attraction of the silica surface for cell membranes. 

Hydrogen ions participate in the cation-exchange processes of the interlayer space. As will be seen later (Section 2.7.1), they have a very large affinity for the layer charge. Hydrogen and hydroxide ions are potential-determining ions of the external surfaces via the protonation and deprotonation processes of aluminol and silanol sites. In acidic media, the degradation of aluminosilicates can be observed. 

The most abundant and important of the aluminosilicate minerals in the earth s surface are the feldspars, which result from the substitution of aluminum for silicon in three-dimensional silicate networks such as quartz. The Al ions must be accompanied by other cations such as sodium, potassium, or calcium to maintain overall charge neutrality. Albite is a feldspar with the chemical formula NaAlSi308. 

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