The relationship between structure and dispersion of surfactants

Water dispersion system is commonly used to analyze the relationship between surfactant structure and dispersion. As a hydrophobic solid particle, it can adsorb the hydrophobic group of the surfactant, and if it is an anionic surfactant, the outward-facing hydrophilic genes have the same charge and repel each other. Obviously, the adsorption efficiency of surfactants increases with the increase of the length of hydrophobic groups, so the long carbon chain has better dispersion than the short carbon chain.

If the hydrophilicity of the surfactant is increased, its solubility in water is often increased, thus reducing the adsorption on the particle surface. This effect is even greater if the interaction between the surfactant and the particles is very weak. For example, when the dye water dispersion system is prepared, the highly sulfonated lignosulfonate powder for strong hydrophobic dyes can form a good thermal stability dispersion system. When the same dispersant is used for hydrophilic dyes, the thermal stability is poor, but the dispersive system with low sulfonation degree can be obtained. The reason is that the dispersant with high sulfonation degree has a large solubility at high temperature, so it is easy to separate from the surface of the hydrophilic dye which has a very weak effect, thus reducing the dispersion.

If the dispersed particle itself has an electric charge, and a surfactant with an opposite charge is selected, flocculation may occur before the charge of the particle is neutralized. Only after the second layer of surfactant is adsorbed on the charge-neutralized particles can it be dispersed well. If the surfactant of the same charge is selected, it is difficult for the particles to adsorb the surfactant, and also only at high concentrations, there is enough adsorption to stabilize the dispersion. In fact, the ionic dispersants used often contain multiple ionic groups and are distributed over the entire surfactant molecule, while the hydrophobic groups contain unsaturated hydrocarbon chains of polar groups such as aromatic rings or ether bonds.

The highly hydrated polyoxyethylene chain of polyoxyethylene non-ionic surfactant molecules extends to the aqueous phase in a curly shape, forming a good spatial barrier to the aggregation of solid particles. At the same time, the very thick multiple layers of polyoxyethylene hydrate greatly reduce the van der Waals attraction between particles, so it is a good dispersant. Especially the block copolymer of propylene oxide and oxyethylene, the polyoxyethylene chain length increases the solubility, and the hydrophobic group of polyoxyethylene increases the adsorption of solid particles, so both are long and are very suitable for dispersants.

When ionic and non-ionic surfactants are combined, on the one hand, the molecules extend into the water phase, forming a spatial barrier to prevent particles from approaching each other; On the other hand, the strength of solid particle interfacial film was enhanced. Therefore, after mixing, as long as the increase of their solubility in the water phase does not significantly affect the adsorption of the particle surface, the dispersant with a long hydrophobic base has a strong dispersion performance.