Difference Between Interfacial Tension and Surface Tension

Molecules in a bulk experience an attractive force from all directions in a liquid. This type of force occurring between them was termed the cohesive force of attraction. All the molecules at the surface of the liquid experience a net attractive force towards the bulk of the liquid, that is, the center. This force is called the surface tension of the liquid. And the force experienced by the rest of the molecules in bulk was termed as the interfacial tension as it occurs at the interface of two layers of the liquid in the container.

Interfacial Tension vs Surface Tension

The main difference between interfacial tension and surface tension is the place where they both take place. Surface tension occurs on a single liquid surface, whereas interfacial tension is defined as the interface of two immiscible liquids or any two substances. In fact, surface tension is the derivation of interfacial tension in the case when the force from the second surface is zero or negligible.

Interfacial Tension and Surface Tension

Interfacial tension is the property between any two substances, but mostly between two immiscible liquids. The interface of the substances can be liquid-liquid, liquid-solid, or solid-air. Interfacial tension is the energy cost per unit area associated with creating an interface between two substances. It is a force of interaction between two, unlike molecules. It is mostly explained by the adhesion force between molecules.

On the other hand, surface tension is confined just to the surface molecules in the liquid. This type of attraction force is a liquid-air interface force of attraction. Surface tension is a force of interaction between like molecules, that is, the molecules that are like each other. This interaction force is generally called a cohesive force. The cohesive force between the molecules is, in fact, responsible for surface tension. This force prevents the liquid molecules from separating from each other.

Comparison Table Between Interfacial Tension and Surface Tension

Parameters of ComparisionInterfacial TensionSurface Tension
DefinitionProperty of liquid with gasProperty between any two substances
Place of occurrenceSingle liquid surfaceThe interface of two immiscible liquids
Type of interfaceLiquid-air interfaceLiquid-liquid, liquid-solid, solid-sir interface
Type of forceCohesive forceAdhesion force
Strength of forceGreater forceLesser force

What is Interfacial Tension?

Interfacial tension is the force of attraction between any two substances forming an interface. The substances can be anything such as liquid-liquid, liquid-solid, or even solid-air. It is the energy cost per unit of the area associated with creating an interface between two immiscible liquids. The S.I. unit of the interfacial tension is millinewton per meter (mN/m).

To understand interfacial tension, first, it is better to know about adhesion force. Adhesion force is the interaction between unlike molecules. When two immiscible liquids are brought in contact, the force of interaction between them is called the adhesion force. Adhesion forces also play an important role when a liquid is brought in contact with a solid.

Interfacial tension is very similar to surface tension but varies in some factors. It is also affected by temperature, pressure, and substrates, as same as surface tension. One of the most common examples of interfacial force is found at the interface of oil and water. The oil-water interface has high interfacial energy, whereas the water-soap interface has a much lower interfacial energy.

What is Surface Tension?

Surface tension is the force of attraction experienced by the molecules present on the surface of a liquid. The molecules experience a force towards the bulk of the liquid. This force is termed surface tension. By this, a “film” is formed that makes it more difficult to move any object through the surface or bulk of the liquid. The S.I. unit of surface tension is also millinewton per meter (mN/m).

Surface tension is basically the derivation of the interfacial tension itself when the force from the side of the second surface is zero or negligible in comparison to the other force. Surface tension and interfacial tension are quite similar as well. For surface tension, one surface is a liquid, and the other surface is a gas. For example, the transition from water to air forms a surface where surface tension acts in the liquid.

Opposite to interfacial tension, cohesive forces are responsible for surface tension occur. Cohesive forces are the force of interaction between molecules that are like each other. They are attraction forces that resist the separation of molecules from the surface. For example, solids have such strong, cohesive forces that they do not stick to any other substance. Whereas liquids also have adhesion forces that allow them to interact with other substances.

Main Differences Between Interfacial Tension and Surface Tension

  1. Surface tension is the force of interaction between molecules that are like each other, whereas interfacial tension is the force of interaction between unlike molecules.
  2. Surface tension occurs at the interface of liquid and air, whereas interfacial tension can occur between the interface of any two substances like liquid-liquid, liquid-solid, or solid-air.
  3. Surface tension is greater than the interfacial force because, in the gas phase, cohesive forces are smaller in magnitude than in the liquid phase.
  4. Surface tension is the force of attraction experienced by the surface molecules towards the bulk of the liquid, whereas interfacial tension is the force of attraction experienced by the molecules of the two substances on either side of the interface.
  5. Surface tension is caused by the cohesive force between the molecules, whereas interfacial tension is similar to the adhesion force between the bulk molecules.

Conclusion

The bottom line is that surface tension and interfacial tension is not as different as they are thought to be. They are quite similar in many factors but are still two different types of forces. Both surface tension and interfacial tension are important for industrial purposes and play an important role even in our everyday lives without us even knowing it. One of the important facts to remember is that the surface tension is greater than the interfacial tension, even though they are quite similar in some ways.

References

  1. https://pubs.acs.org/doi/abs/10.1021/je034062r
  2. https://www.sciencedirect.com/science/article/pii/S1359029411000276
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