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Introduction to the Structure and Cloud Point of Surfactants

Surfactant refers to a substance that can significantly change the interface state of the solution system by adding a small amount. Surfactants are generally organic amphiphilic molecules with hydrophilic and hydrophobic groups, which are soluble in organic solutions and aqueous solutions. It has a fixed hydrophilic and lipophilic group, and can be aligned on the surface of the solution. Surfactants are divided into ionic surfactants and nonionic surfactants. It is a large class of organic compounds. Their properties are very unique, and their applications are extremely flexible and extensive. They can reduce the surface tension of water by adsorption at the gas-liquid two-phase interface, and can also reduce the oil-water interfacial tension by adsorbing between the liquid interfaces.


1. The structure of surfactants


(1) Gemini surfactants all have two hydrophobic chains and hydrophilic head groups.


(2) Linking group: It can be a short-chain group, a rigid group, a flexible group, a hydrophilic group, or a hydrophobic group.


(3) Hydrophilic head group: It can be anionic (sulfonate, sulfate, carboxylate) or cationic (ammonium salt), or non-ionic (sugar, polyether).


(4) Most of the reported gemini surfactant has symmetrical structures, and gemini surfactants with asymmetric structures have also been reported.


(5) There are also reports on the synthesis of surfactants with multiple hydrophilic head groups and hydrophobic chain structures.


2. The cloud point of surfactants


For non-ionic surfactants, the hydrophilicity depends on the number of ether bonds, and the binding of ethers to water molecules is an exothermic reaction. When the temperature rises, the water molecules gradually break away from the ether bond, and the phenomenon of turbidity occurs. The temperature when the turbidity just appears is called the cloud point. At this point the surfactant loses its effect. The higher the cloud point, the wider the temperature range used.


Conventionally, surfactants are a class of substances that can significantly reduce surface tension even at very low concentrations. With the in-depth study of surfactants, it is generally believed that as long as the substances that can significantly change the surface properties or related and derived properties at lower concentrations can be classified as surfactants.


No matter what kind of surfactant, its molecular structure consists of two parts. One end of the molecule is a non-polar lipophilic hydrophobic group, sometimes called a lipophilic group; the other end of the molecule is a polar hydrophilic hydrophilic group, sometimes also called an oleophobic group or vividly called a hydrophilic head. Two types of molecular fragments or groups with completely opposite structures and properties are located at both ends of the same molecule and are connected by chemical bonds to form an asymmetric and polar structure, thus endowing this special molecule with both hydrophilic properties, and lipophilic, but not the overall hydrophilic or lipophilic properties. This characteristic structure of surfactants is often referred to as "amphiphilic structure", and surfactant molecules are therefore often referred to as "amphiphilic molecules".


Depending on the required properties and specific applications, sometimes surfactants are required to have different hydrophilic-lipophilic structures and relative densities. The desired balance of hydrophilic and lipophilic can be achieved by changing the types, proportions and positions of hydrophilic or lipophilic groups in the molecular structure. After years of research and production, many types of surfactants have been derived, and each type contains many varieties, which brings difficulties to identifying and selecting a specific variety. Therefore, it is necessary to scientifically classify thousands of surfactants, which is conducive to further research and production of new varieties, and facilitates the screening and application of surfactants.

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