The total cumulative charge is directly proportional to the total surface area of the crystal. As particle size decreases, the overall surface area increases, and thus the particles become more highly negatively charged. This property makes these materials very interesting for nanotechnological research and applications.
Natural or chemical?
A positive metallic ion (usually either sodium or calcium and sometimes magnesium) is associated with each crystalline face. These are known as exchangeable ions, as they vary according to the source of the mineral. The effect of these cations is to significantly reduce the overall negative charge of the platelets. Montmorillonite clay is quite hydrophilic and readily attracts and adsorbs water to itself. When hydrated, bentonite crystals aggregate together to form a three-dimensional structure known as a house of cards, with a layer of water between each card.
When the positive counterion is sodium, these clays retain a higher degree of negative charge and are thus extremely hygroscopic. In this form they swell tremendously due to absorption of large quantities of water molecules, which can be hazardous in biological applications. However, sodium bentonite can be very useful in many other applications, particularly in the mining and petroleum industries. Calcium bentonite is the preferred form for most cosmetic and pharmaceutical uses, as it still swells in aqueous systems, but it is not quite so hygroscopic as the sodium form.
The unique ionic, three-dimensional, sandwich-like structures of calcium bentonite impart extremely interesting and useful properties to these materials. They are excellent colloid dispersant agents and highly effective emulsifiers as they can trap oils in the layers between platelets. They also can enhance the structures of micelles in solution and aid in the development of liquid crystalline structures, such as vesicles.
Bentonite clays are often used as viscosity modifiers for fluid systems that require thickening but that are also expected to be easily spreadable or dispensable. Consider toothpaste. It must be sufficiently viscous to fill out and remain inside the tube, but it must also be easily squeezed and dispensed onto a toothbrush. Bentonite clays are pseudoplastic materials (those with non-Newtonian properties) that exhibit this type of thixotropic behavior, which means that the viscosity of the system decreases over time with a constant applied shear force.