How Does Sodium Lauryl Sulfate Surfactant Achieve Its Cleaning Effect?

Sodium lauryl sulfate is a commonly used anionic surfactant. As one of the surfactants, sodium lauryl sulfate has amphiphilic properties and is therefore widely used as a detergent in industry. They can be used for cleaning, wetting, dispersing, emulsifying, foaming and defoaming. What is the specific cleaning principle of sodium lauryl sulfate surfactant? How does it achieve its cleaning effect?

First, let’s understand how surfactants work. Surfactant, is a compound that can significantly reduce the surface tension or interfacial tension between two liquids, between a liquid and a gas, or between a liquid and a solid.

The molecular structure of surfactants is amphiphilic: one end is a hydrophilic group and the other end is a hydrophobic group; the hydrophilic group is often a polar group, such as carboxylic acid, sulfonic acid, sulfuric acid, amino or amine group, hydroxyl group, amide groups, ether bonds, etc. can also be used as polar hydrophilic groups; While the hydrophobic group is often a non-polar hydrocarbon chain, such as a hydrocarbon chain with more than 8 carbon atoms.

Two types of molecular groups with diametrically opposite structures and properties, the hydrophilic group and the hydrophobic group, are located at both ends of the same molecule and are connected by chemical bonds, forming an asymmetric and polar structure, thus giving this type of special molecule both hydrophilic and lipophilic properties, but not overall hydrophilic or lipophilic properties. This unique structure of surfactants is often called the amphiphilic structure, and the surfactant molecules are therefore often called amphiphilic molecules.

Surfactants are divided into ionic surfactants (including cationic surfactants, anionic surfactants, amphoteric surfactants), nonionic surfactants, compound surfactants, other surfactants, etc. Sodium lauryl sulfate surfactant is a very typical anionic surfactant. It achieves the overall cleaning effect through emulsification, wetting, solubilization, dispersion and foaming.

1)Emulsification: Due to the high surface tension of oil in water, when oil is dropped into the water and stirred vigorously, the oil is crushed into fine beads and mixed with each other to form an emulsion, but the stirring stops and the layers are re-stratified. If you add sodium lauryl sulfate surfactant and stir vigorously, it will not be easy to separate for a long time after stopping. This is emulsification. The reason is that the hydrophobicity of the oil is surrounded by the hydrophilic groups of the surfactant, forming a directional attraction, which reduces the ability of the oil to disperse in water, making the oil well emulsified.

2) Wetting: There is often a layer of wax, grease or scaly substances adhered to the surface of parts, which are hydrophobic. Due to the contamination of these substances, the surface of the parts is not easily wetted by water. When sodium lauryl sulfate surfactant is added to the aqueous solution, the water droplets on the parts will be easily dispersed, greatly reducing the surface tension of the parts and achieving the purpose of wetting.

3) Solubilization: Oil substances can be dissolved after adding surfactant, but this dissolution can only occur when the concentration of surfactant reaches the critical concentration of colloid. The solubility is determined according to the solubilization object and properties. In terms of solubilization, long hydrophobic hydrocarbon chains are stronger than short hydrocarbon chains, and saturated hydrocarbon chains are stronger than unsaturated hydrocarbon chains. The solubilization effect of nonionic surfactants is generally more significant.

4) Dispersion: Solid particles such as dust and dirt particles are easier to gather together and settle in water. The molecules of sodium lauryl sulfate surfactant can divide the solid particle aggregates into fine particles, allowing them to be dispersed and suspended in the solution. It plays a role in promoting the uniform dispersion of solid particles.

5) Foaming: The formation of foam is mainly due to the directional adsorption of active agents, which is caused by the reduction of the surface tension between the gas and liquid phases. Generally, low molecular active agents are easy to foam, while high molecular surfactant have less foam. Myristate yellow has the highest foaming property, and sodium stearate has the worst foaming property. Anionic surfactant have better foaming properties and foam stability than non-ionic surfactant. For example, sodium lauryl sulfate surfactant has strong foaming properties.