Relationship between surfactant structure and dispersibility
Surfactants are commonly used in water-dispersed systems as dispersants. In this paper, the relationship between the structure of surfactants and dispersibility is analyzed using water systems as a benchmark. As a hydrophobic solid particle, the hydrophobic group of a surfactant can be adsorbed, and if it is an anionic surfactant, the outwardly-facing hydrophilic genes have the same charge and repel each other. Obviously, the adsorption efficiency of the surfactant increases with the increase of the length of the hydrophobic group, so the long carbon chain has better dispersibility than the short carbon chain.
If the hydrophilicity of a surfactant is increased, its solubility in water tends to increase, thereby reducing adsorption on the particle surface. This effect is greater if the interaction between the surfactant and the particles is weak. For example, when preparing a water dispersion system for dyes, a highly sulfonated lignin sulfonate dispersant for strongly hydrophobic dyes can form a dispersion system with good thermal stability; and the same dispersant for hydrophilic dyes has thermal stability Inferior, but using a lignin sulfonate with a lower degree of sulfonation as a dispersant, a dispersion system with better thermal stability can be obtained. The reason is that the dispersant with a high degree of sulfonation has a high solubility at high temperature, and therefore it is easy to separate from the surface of the hydrophilic dye which is weak in nature, thereby reducing the dispersibility.
If the dispersed particles are charged, and a surfactant with an opposite charge is selected, flocculation may occur before the charged charges of the particles are neutralized. Only after the second layer of surfactant is adsorbed on the charge-neutralized particles can it be well dispersed. If a surfactant with the same charge is used, it is difficult for the particles to adsorb the surfactant. Similarly, only at a high concentration, there is sufficient adsorption to stabilize the dispersion. In fact, the ionic dispersant used often contains multiple ionic groups and is distributed over the entire surfactant molecule, while the hydrophobic group contains an unsaturated hydrocarbon chain with polar groups such as aromatic rings or ether bonds.
The highly hydrated polyoxyethylene chain of the polyoxyethylene nonionic surfactant molecules stretches into the water phase in a curled shape, forming a good spatial barrier for the aggregation of solid particles. At the same time, there are many thick hydrated oxyethylene layers. It greatly reduces the van der Waals gravitational force between particles, so it is a good dispersant. In particular, the block copolymer of propylene oxide and oxyethylene has a polyoxyethylene chain length that increases solubility, while the polyoxypropylene hydrophobic group grows and increases the adsorption of solid particles, so both are long. It is very useful as a dispersant. suitable.
When using ionic and non-ionic surfactants, on the one hand, the molecules are stretched into the water phase, forming a spatial barrier, preventing the particles from approaching each other; on the other hand, the interface film strength of the solid particles is enhanced. Therefore, after mixing, as long as their solubility in the water phase does not significantly affect the adsorption of the particle surface, the dispersant with a longer hydrophobic group is stronger.