Why does the surface energy decrease during adsorption

Surface tension

Why educate Waterdrop? Why can Water strider walk on the water and not go under? These Phenomena are based on the so-called Surface tension. In this article we will explain what a Surface tension is exactly which unit it has and how you calculate it. In addition, we will show you various Tries, like you Measure surface tension can.

Would you like to have the topic explained in an animated instructional video? Then check out ours Video to do this!

Surface tension simply explained

You have probably already noticed that Waterdrop forms. The reason for such Drop formation lies in the Molecular forces of the respective liquid.

Through these forces occurs at the border between the liquid and the gas the so-called surface tension. She will therefore too Interfacial tension called.

The Molecular forces cause it energetically cheaper is the Surface small to keep. Since in three-dimensional space a Bullet the smallest surface has, form spherical drops. The exact physical explanation you will find further below.

Surface tension consequences and meaning

thanks to the Surface tension can be certain insectson the Water surface move, such as the Water strider. However, they pierce the surface, and to destroy thereby the Surface tension, could you also go under. You can also do others light objects, like arazor blade put on water, without that they goes down.

The consequence of Surface tension manifests itself in the fact that liquids try yours surface and thus the Interface between the liquid and one gas so small as possible too hold. There one Bullet the smallest surface has, educates himself neglect other forces, such as weight spherical drop. In this drops is the print compared to the surroundings elevated.

Physical explanation of surface tension

On the Molecules within oneliquid Act attractive and repulsive forces. This behavior of the molecules can be caused by the Lennard-Jones Potential to be discribed. At short distances the Molecules and at greater distances works a attractive force. It is important to point out that the repulsive forces as Contact forces can be understood and therefore direction-independent, so isotropic are. On the other hand, they are attractive forces depending on the direction so anisotropic distributed. Is the liquid in the Balance, so to lift the Powers to a Liquid molecule in the Interior the liquid on average over time on.

At the Interface between liquid and gas is the Symmetry not more given, because the Liquid molecules have in vertical direction no neighboring moleculesmore. This leads to the Molecules at the Interface just one resulting power ins Inner the liquid given is.

Would you like to get out of the Interior the liquid a Liquid molecule to the Move surface, so you have to go against this force a Doing work. That's why a molecule at the surface one for this amount of work higher energy, as a molecule inside the liquid. So it is energetically cheaper, if the liquid your surface respectively Interface minimized. Since in three-dimensional space for a body the smallest surface a Spherical surface is, forms that Water spherical drops.

Calculate surface tension

The Surface tension can on two different, but consistent types calculated become. On the one hand with one of the mechanics derived formula and on the other hand with a formula from the Thermodynamics.

First we go to that mechanical definition a. For this one poses the following experiment in front.

One takes Hanger, the widthin which one Liquid film clamped. Pulls one now With one force the hanger by a distance to the right, like that enlarged the surface of Liquid film around


The factor comes from considering the front and back of the liquid film. In the enlargement the surface you have to do the job

perform. We have an extra video for you Work here linked. This allows the Surface tensionto calculate. It's just defined by that relationship the work expended to enlarged surface.

The following is the thermodynamic version presented. If you go from constant temperature and constant pressure off, so can the Surface tension about the relationship the Change in free enthalpy and the modification the surface be calculated

In our extra contribution to free enthalpy do you get this explained in detail!

Surface tension unit

The unit the Surface tension simply results from the above formulas and is therefore measured in


Surface tension table

In the following table values ​​for various substances are listed, in particular the surface tension of water:

liquidSurface tension in mN / m
Ethylene glycol31,40
Polyether ketones46,00
Water (80 degrees Celcius)62,60
Water (50 degrees Celcius)67,90
Water (20 degrees Celcius)72,75
Mercury (18 degrees Celcius)471,00
Mercury (20 degrees Celcius)476,00

As you can see it is Surface tension of water is relatively high compared to other liquids.

Measure surface tension

In this section we will explain which Methods you the Measure surface tension can.

Ironing method

In the Ironing method if you use one hanger with a Wire. This wire is immersed in the liquid and then increased with a Precision spring balance little by little the traction on the Hanger. This moves the Wire out the liquid and a Liquid film between wire and the surface of the liquid is created. At some point rips the Liquid film from. To the Wire out the water so you have to lift Work against the Perform surface tension. If you make a note of the maximum pulling force, in which the liquid film tears, one can get out of the length of Wire and the density the liquid the Calculate surface tension.

Measurement with capillary effect

In thin tubes it happens with certain liquids to the so-called Capillary effect. He causes the liquid in thin tubes, against the weight force, up increases. This one effect can be used to the Surface tension to to calculate. All you need is a Vessel with one liquid and a thin capillary. If you put the Capillary in the Liquid, so increases the Water level in the capillary at. By then reading the Rise height, can be done with the help of the density the liquid and the diameter the capillary the Calculate surface tension.

Other methods

There are still many other methods, with which one Measure surface tension can. A few of them are listed below:

  • Du Noüy ring method
  • Wilhelmy plate method
  • Contact angle measurement
  • Spinning-drop method
  • Pendant drop method
  • Bladder pressure method
  • Drop volume method
  • Test ink method
  • Sessile drop method.