Importance of Surface Tension between Surfactant and Water

Dec 24, 2020

surface tension of water
The surface tension of water is 72 nm / cm at room temperature and has little change with temperature. The surface tension of these materials can be determined by the surface tension of the materials, such as water. If the water drops on the surface of plastic or lotus leaf, we call it non wetting, or poor wetting. On the contrary, if the water drops spread evenly on the surface, it shows good wetting effect. In the process of our life and production, the surface behavior effect due to the difference of surface tension is recognized and utilized by human beings. For example, in the process of painting, printing, surface treatment, cleaning, etc., it is necessary to keep water well wetted on the target surface; on the contrary, in the process of water proofing treatment, it is hoped that the target surface can not be wetted by water, showing hydrophobicity.

function of surfactant
In order to change the surface tension of water to achieve good wetting effect on other surfaces, it is usually necessary to add surfactants. Only a small amount of these substances can greatly reduce the surface tension of water. With the continuous development of water-based products, the demand for surface active agents will increase greatly.

Surface tension is a characterization of the distribution of surfactant molecules on the surface of water. Because surfactants are amphoteric molecules containing hydrophilic and hydrophobic groups, they will be distributed on the surface of water in water. Hydrophilic groups face water and form hydrogen bonds with water, while hydrophobic groups face air. In this way, a monolayer composed of surfactant hydrophobic groups will be formed on the surface of water, thus forming a new surface, and this surface tension will replace the surface tension of water, showing a new surface behavior. If the new surface tension is lower than the contact target surface, the water-based system will achieve good wetting of the target surface.

The word surfactant comes from English surface. It is actually a condensed word of the phrase surface active agent. It also has a name called tenside. All the substances which can significantly reduce the surface tension of liquid by adding a small amount of them are collectively referred to as surfactants. Their surface activity is for a particular liquid, usually water. One end of the surfactant is a non-polar hydrocarbon chain (hydrocarbon radical), which has very little affinity with water, and is often called a hydrophobic group; the other end is a polar group (such as - Oh, - COOH, - NH Ψ, - so ₃ h, etc.), which has a great affinity with water, so it is called hydrophilic group, which is called "amphiphilic molecule" (lipophilic hydrophilic molecule). In order to achieve stability, surfactant can be dissolved in water in two ways

1. A monolayer is formed on the liquid surface.
The hydrophilic group is left in the water and the hydrophobic group is extended to the air to reduce the repulsion. The repulsive force between the hydrophobic group and the water molecule is equivalent to making the water molecule on the surface subject to an outward thrust, counteracting the original inward pull of the water molecule on the surface, even if the surface tension of the water decreases. This is the basic principle of foaming, emulsifying and wetting of surfactants. In the oil-water system, surfactant molecules will be adsorbed on the oil-water interface, and the polar groups will be inserted into the water, and the non-polar parts will enter the oil and arrange in the interface. This creates tension between the oil and water phases, which reduces the interfacial tension between the oil and water. This property has an important influence on the wide application of surfactants.
2. The formation of "micelles".
The micelles can be spherical or layered, and the hydrophobic groups are hidden in the micelles as much as possible and the hydrophilic groups are exposed. If polar group is represented by sphere and hydrophobic nonpolar group is represented by column, monolayer and micelle are formed. If there is oil insoluble in water in the solution, it can enter the center of spherical micelles and the interlayer of layered micelles. This is called solubilization of surfactants.

Surfactants can be used for washing, emulsifying, foaming, wetting, soaking and dispersing, and the amount of surfactants is small (generally a few percent to a few thousandths), easy to operate, non-toxic and non-corrosive. It is an ideal chemical product, so it has important applications in production and scientific research. At the same concentration, the non-polar component of surfactant is large, and its surface activity is strong. That is to say, in the homologues, the surface activity with more carbon atoms is greater. However, if the carbon chain is too long, the solubility in water is too low and has no practical value.