INTERFACIAL TENSION: DEFINITION, EQUATION, UNITS AND MEASUREMENT - PHYSICAL - 2024

The interfacial tension (γ) is the net force per unit length exerted on the contact surface between a phase (solid or liquid) and another (solid, liquid or gaseous). The net force is vertical to the contact surface and is directed towards the interior of the phases.

When one of the phases is a gas it is usually called surface tension. The phases in contact are immiscible, that is, they cannot dissolve together to form a solution. The region of contact between the phases is a geometric surface of separation called the interface. Interfacial tension is due to intermolecular forces present at the interface.

Forces between molecules of a liquid in contact with air

Interfacial tension plays an important role in many interfacial phenomena and processes such as emulsion production and oil production.

Definition

The properties of the interface are not the same as the properties inside the phases in contact, because different molecular interactions are manifested because in that region there are molecules that belong to both one phase and the other.

Molecules within a phase interact with neighboring molecules, which have similar properties. Consequently, the net interior force is zero because the attractive and repulsive interactions are the same in all possible directions.

The molecules that are on the surface between the two phases are surrounded by molecules from the same phase but also by neighboring molecules from the other phase.

In this case, the net force is not zero, and it is directed towards the interior of the phase in which there is greater interaction. The result is that the energy state of the molecules on the surface is greater than the energy state within the phase.

The net force acting inward per unit length along the interface is the interfacial tension. Due to this force, molecules spontaneously tend to minimize energy, minimizing the surface area for each unit of volume.

Definition based on work and energy

To attract a molecule from the inside to the surface it is necessary that the forces acting on the molecule exceed the net force. In other words, work is required to increase the interfacial surface.

Force necessary to increase the interfacial region. (https://commons.wikimedia.org/wiki/File:Surface_growing.png)

The greater the net intermolecular force, the greater the work to be done and the greater the energy input. For this reason, interfacial tension is also defined as a function of work or as a function of energy, as mentioned below:

Interfacial tension is the work required to create a unit area at the interface. Likewise, interfacial tension is defined as the free energy required per unit area created.

Equation and units of interfacial tension

The equation of the interfacial tension as a function of the net intermolecular force is:

γ = F / 2l

The reason why the interfacial tension decreases is that as the temperature increases, the kinetic energy increases due to the increase in the thermal movement of the molecules.

Measurement of interfacial tension

There are different methods of experimental measurement of the interfacial tension, among which the most suitable can be chosen according to the characteristic properties of the phases in contact and the experimental conditions.

These methods include the Wilhelmy plate method, the Du Nouy ring method, the pendant drop method, and the rotating drop method.

Wilhelmy plate method

It consists of measuring the downward force exerted by the surface of a liquid phase on an aluminum or glass plate. The net force exerted on the plate equals the weight plus the tensile force. The weight of the plate is obtained by a torsion sensitive microbalance attached to the plate by a device.

Du Nouy ring method

In this method, the force to separate the surface of a metal ring from a liquid surface is measured ensuring that before measuring, the ring is completely submerged in the liquid. The separation force is equal to the interfacial tension and is measured using a high precision balance.

Drop pendant method

This method is based on measuring the deformation of a drop hanging from a capillary. The drop is kept in balance while it is hanging because the tensile force equals the weight of the drop.

The elongation of the drop is proportional to the weight of the drop. The method is based on determining the elongation length of the drop due to its weight.

Drop pendant method

Rotating drop method

The rotating drop method is very useful for measuring very low interfacial tensions that are applied to the emulsion and microemulsion production process.

It consists of placing a drop of a less dense liquid inside a capillary tube filled with another liquid. The drop is subjected to a centrifugal force due to a rotating movement, with great speed, which elongates the drop on the axis and opposes the tensile force.

The interfacial tension is obtained from the dimensions of the geometric shape of the drop, being deformed, and the speed of rotation.

References

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