The surface tension is a physical property having all fluids and is characterized by resistance to their surfaces oppose any increase in its area. This is the same as saying that said surface will seek the smallest possible area. This phenomenon intertwines several chemical concepts, such as cohesion, adhesion and intermolecular forces.
Surface tension is responsible for the formation of surface curvatures of liquids in tubular vessels (graduated cylinders, columns, test tubes, etc.). These can be concave (curved in the shape of a valley) or convex (curved in a dome shape). Many physical phenomena can be explained by considering the changes that the surface tension of a liquid undergoes.
One of these phenomena is the tendency of liquid molecules to agglomerate in the form of drops, when they rest on surfaces that repel them. For example, the drops of water that we see on top of the leaves, cannot wet it due to its waxy, hydrophobic surface.
However, there comes a time when gravity plays its role and the drop spills out like a column of water. Similar phenomenon occurs in spherical drops of mercury when spilled from a thermometer.
On the other hand, the surface tension of water is the most important of all, as it contributes to and organizes the state of microscopic bodies in aqueous media, such as cells and their lipid membranes. In addition, this tension is responsible for the fact that the water evaporates slowly, and some bodies denser than it can float on its surface.
Causes of surface tension
The explanation for the phenomenon of surface tension is at the molecular level. The molecules of a liquid interact with each other, in such a way that they are cohesive in their erratic movements. A molecule interacts with its neighbors next to it and with those above or below it.
However, this does not happen the same with the molecules on the surface of the liquid, which are in contact with air (or any other gas), or with a solid. The molecules of the surface cannot cohesion with those of the external environment.
As a result, they do not experience any forces that pull them upward; only downward, from its neighbors in the liquid medium. To counteract this imbalance, the molecules on the surface “squeeze”, because only in this way can they overcome the force that pushes them down.
A surface is then created where the molecules are in a more tense arrangement. If a particle wants to penetrate the liquid, it must first cross this molecular barrier proportional to the surface tension of said liquid. The same applies to a particle that wants to escape to the external environment from the depths of the liquid.
Therefore, its surface behaves as if it were an elastic film that shows resistance to deformation.
Surface tension is usually represented by the symbol γ, and is expressed in units of N / m, force times length. However, most of the time its unit is the dyn / cm. One can be converted into the other by the following conversion factor:
1 dyn / cm = 0.001 N / m
Surface tension of water
Water is the rarest and most amazing of all liquids. Its surface tension, as well as several of its properties, are unusually high: 72 dyn / cm at room temperature. This value can increase to 75.64 dyn / cm, at a temperature of 0ºC; or decrease to 58.85 ºC, at a temperature of 100 ºC.
These observations make sense when you consider that the molecular barrier tightens even more at temperatures close to freezing, or “loosens” a bit more around the boiling point .
Water has a high surface tension due to its hydrogen bonds. If these in themselves are noticeable within the liquid, they are even more so on the surface. The water molecules are strongly entangled, forming dipole-dipole interactions of the type H 2 O-HOH.
Such is the efficiency of their interactions that the aqueous molecular barrier can even support some bodies before they sink. In the applications and experiments sections we will return to this point.
All liquids present surface tensions, either to a lesser or greater degree than water, or whether they are pure substances or solutions. How strong and tense the molecular barriers of its surfaces are, will depend directly on their intermolecular interactions, as well as structural and energetic factors.
For example, the molecules of gases in the liquid state only interact with each other through London dispersive forces. This is consistent with the fact that their surface tensions have low values:
-Liquid helium, 0.37 dyn / cm at -273 ºC
-Liquid nitrogen, 8.85 dyn / cm at -196 ºC
-Liquid oxygen, 13.2 dyn / cm at -182 ºC
The surface tension of liquid oxygen is higher than that of helium because its molecules have greater mass.
Non-polar and organic liquids are expected to have higher surface tensions than these condensed gases. Among some of them we have the following:
-Dietilether, 17 dyn / cm at 20 ºC
– n- Hexane, 18.40 dyn / cm at 20 ° C
– n -Octane, 21.80 dyn / cm at 20 ° C
-Toluene, 27.73 dyn / cm at 25 ºC
A similar trend is observed for these liquids: surface tension increases as their molecular masses increase. However, the n -octane should, according to this, have the highest surface tension and not the toluene. Here molecular structures and geometries come into play.
Toluene molecules, flat and ring-shaped, have more effective interactions than n -octane. Therefore, the surface of toluene is “tighter” than the surface of n -octane.
As there are stronger dipole-dipole interactions between the molecules of a polar liquid, their tendency is to show higher surface tensions. But this is not always the case. Among some examples we have:
-Acetic acid, 27.60 dyn / cm at 20 ºC
-Acetone, 23.70 dyn / cm at 20 ºC
-Blood, 55.89 dyn / cm at 22 ºC
-Ethanol, 22.27 dyn / cm at 20 ºC
-Glycerol, 63 dyn / cm at 20 ºC
-Fused sodium chloride, 163 dyn / cm at 650 ºC
-Solution of 6 M NaCl, 82.55 dyn / cm at 20 ºC
Molten sodium chloride is expected to have enormous surface tension – it is a viscous, ionic liquid.
On the other hand, mercury is one of the liquids with the highest surface tension: 487 dyn / cm. In it, its surface is composed of strongly cohesive mercury atoms, much more than water molecules can be.
Surface tension alone has no applications. However, this does not mean that it is not involved in various daily phenomena, which if they did not exist, would not occur.
For example, mosquitoes and other insects are able to walk through water. This is because their hydrophobic legs repel water, while their low mass allows them to stay afloat on the molecular barrier without falling to the bottom of the river, lake, pond, etc.
Surface tension also plays a role in the wettability of liquids. The higher its surface tension, the less its tendency to seep through pores or cracks in a material. In addition to this, they are not very useful liquids for cleaning surfaces.
It is here where detergents act, reducing the surface tension of the water, and helping it to cover larger surfaces; while improving its degreasing action. By decreasing its surface tension, it makes room for air molecules, with which it forms bubbles.
On the other hand, lower higher tensions are linked to the stabilization of the emulsions, which are very important in the formulation of a different range of products.
Finally, some experiments that can be carried out in any domestic space will be cited.
A metal clip is placed on its surface in a glass of cold water. As seen in the image above, the clip will stay afloat thanks to the surface tension of the water. But if a little lava china is added to the glass, the surface tension will drop dramatically and the paper clip will suddenly sink.
If on the surface we have a paper boat or a wooden pallet, and if the dishwasher or detergent is added to the head of a swab, then an interesting phenomenon will occur: there will be a repulsion that will propagate them towards the edges of the glass. The paper boat and wooden pallet will move away from the detergent-smeared swab.
Another similar and more graphic experiment consists of repeating the same operation, but in a bucket of water sprinkled with black pepper. The black pepper particles will drift away and the surface will change from pepper covered to crystal clear, with the pepper on the edges.
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