Inorganic thickeners can be divided into inorganic low molecular weight thickeners and inorganic high molecular weight thickeners according to their relative molecular weight.
(1) Inorganic low molecular weight thickener
Inorganic low molecular weight thickener is mainly an aqueous solution system composed of inorganic salt and surfactant. At present, the inorganic salts used mainly include NaCl, NH4Cl, KCl, Na2SO4 and Na3PO4, among which NaCl is the most commonly used, with remarkable thickening effect. The thickening principle is that in the aqueous solution, the surfactant will form micelles, and the electrolyte in the aqueous solution will increase the number of associated micelles. The originally spherical micelles will gradually change into rod micelles, and the motion resistance will increase, so that the viscosity of the system will also increase accordingly. However, if there is too much electrolyte in the aqueous solution, the structure of the micelle will be affected by the electrolyte, which will reduce the movement resistance and reduce the viscosity of the system, which is the so-called salting out effect.
(2) Inorganic high molecular weight thickener
Inorganic high molecular weight thickener is a kind of gel mineral, which is expanded by water absorption and moisture absorption, so that it has a thixotropic [9]. It mainly includes bentonite, attapulgite, aluminum silicate, etc., of which bentonite is the most commonly used and has the most commercial value. These minerals generally have special structures, such as layered structure and expanded lattice structure. The existing metal ions diffuse outward from the lamellae in the aqueous solution, and the metal ions swell due to hydration. Finally, they are completely separated from the lamellae to form a colloidal suspension. At this time, the surface of the flake separated from the metal ions has a negative charge, and the appearance of the lattice fracture surface makes the edges and corners of the flake have a small amount of positive charge. In dilute solution, the negative charge on the surface is greater than the positive charge on the edge, and the particles cannot be thickened due to the repulsion. However, in the concentrated solution with more electrolytes, the negative charge on the surface of the lamellae decreases, and the force between the particles changes from the mutual repulsion between the original lamellae to the mutual attraction between the negative charge on the surface of the lamellae and the positive charge on the corner. Originally, the repulsion force is parallel between the lamellae, but later, due to the mutual attraction of positive and negative charges, the lamellae turn to be perpendicular to each other and swell, Gel is produced to achieve thickening purpose. At present, the mainstream research direction of thickeners is to use inorganic and other organic compounds to synthesize thickeners. For example, M. chtourou and others have prepared a synthetic thickener by using inorganic substances (Tunisian clay) and organic substances (organic derivatives of ammonium salts), which has better thickening effect.
