Surface tension is a physical phenomenon of liquid matter, which is the result of the gravitational force of molecules. Molecules on the surface of the liquid (also called interface molecules) are attracted by substances with the same chemical properties outside the liquid and by molecules inside the liquid, but the gravitational force of the internal molecules is greater than the gravitational force of the external molecules of the liquid. In this way, the molecules on the liquid surface shrink inward. The force of contraction is called "surface tension", also known as cohesion. Taking water as an example, the surface tension of water is the inward force of the surface molecules of water, which is the cohesive force of water. Let us illustrate with the occurrence of a drop of water.
On the leaves of a drop of water, some water molecules contact the air, which are called surface molecules of water; some water molecules do not contact air, which are called internal molecules of water. The internal molecules of water are attracted by the surrounding water molecules symmetrically and evenly; but the situation of the surface molecules of water is different. On the one hand, it is attracted by the internal molecules of water, and on the other hand, it is attracted by water molecules in the air. The gravitational force it receives is unbalanced, because the air is a gas-based mixture, and the water molecules it contains are less than the internal molecules of water, and the gravitational force is smaller, so the surface molecules of the water shrink inward. , resulting in the surface tension of water. This is how a drop of water falls on a leaf to form a small ball.
From the above analysis, it can be seen that the surface tension is the inward contraction force of the liquid surface. The greater the inward contraction force, the greater the surface tension, that is, the greater the cohesion, the greater the surface tension, and the consumed "work" (measured The unit, lerg=lOJ) is also bigger. According to the measurement, the surface tension of water is 73erg/cm2, kerosene is 24erg/cm2, alcohol is 22erg/cm2, and water is 472erg/cm2. The surface tension of the sample is high, and the wettability is very poor. No matter what substance is put into the sample liquid, it will not be wetted, or if the sample liquid is placed on any substance, it will appear in the shape of a ball. Kerosene and alcohol have low surface tension and are excellent wetting agents. Water has a relatively high surface tension and is also an excellent wetting agent. But from the point of view of washing, this surface tension is unsatisfactory, and trying to reduce the surface tension is very important for washing. Why is the wettability poor when the surface tension is high? This is because the affinity (attractive force) between the surface molecules and the substance to be wetted is smaller than that of the internal molecules, so the surface molecules shrink inward and do not combine with the substance to be wetted. If the affinity between the surface molecules and the substance to be wetted is enhanced, enhanced to be greater than the molecular attraction of the internal molecules, so that it is more
The wetting effect is brought into play, that is, the surface tension is reduced. The reason why the surface tension of water can be reduced is based on this.
