There are two opposite characteristics in the molecular structure of surfactants

2022-07-13 09:32:33

Surfactant is a kind of soluble and concentrated on the surface of liquid or the interface between liquid and immiscible objects. Its molecules can adsorb two substances with different properties. Surfactants can be divided into oil soluble (such as oleic acid, petroleum acid, yellow flower petroleum acid, etc.) and water soluble (such as soap).

Surfactants are generally soluble in most liquids, but their solubility is not high. This is because it has a balanced polar and nonpolar structure. Part of the molecule is dissolved in polar solution, while the other part is in nonpolar liquid. In other words, there are two opposite characteristics in the molecular structure of surfactants: the alkyl end has affinity for oil, while the polar end has affinity for water. Therefore, this molecule is neatly arranged on the interface of two different but adjacent media, such as the interface between oil and pigment, or the interface between two mutually insoluble liquids.

In colloidal chemistry, the ratio of total surface area to total volume of solid dispersed phase is recorded as the specific surface area of solid dispersed phase. Because the pigment particles are very small, its specific surface area is quite large. For example, the diameter of carbon black particles is about 0.1nm. If there are 1015 carbon black particles per cubic centimeter of ink, the specific surface area of carbon black particles as the dispersed phase in the ink can reach 6x105cm-1. The specific surface area of the dispersed phase is large, and the dispersed phase particles will have a large specific surface free energy. The system is unstable, and the macroscopic performance is the condensation and sedimentation of the dispersed phase particles.

However, under normal circumstances, this phenomenon does not occur because the pigment particles in the ink have a protective shell. There are always some surfactants such as higher free fatty acids and alcohols in the ink. These surfactants are directionally arranged around the pigment particles to form a solid shell, so that the particles cannot contact the condensation precipitation.