# Interfacial tension: definition, equation, units and measure

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

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

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

## Definition of

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

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

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

In this case, the net force is not zero and is directed to the phase with the greatest interaction. The result is that the energy state of molecules on the surface is greater than the energy state within the phase.

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

### Definition based on work and energy

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

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

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

## Interfacial tension equation and units

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

γ = F / 2l 

F = net force

l = interface length

The number 2 that appears in equation  means that there are two surfaces, one for each side of the interface.

The interfacial tension as a function of the work required to generate a unit of surface area is expressed by the following equation:

γ = W / ΔA 

W = Work

ΔA = Increased surface area

The creation of the interfacial area is accompanied by an increase in free training energy.

γ = ΔE / ΔA 

ΔE = interface formation energy

The units of interfacial tension in the international system are N / m or Joules / m 2 . Dinas / cm or mN / m is also commonly used.

## temperature dependence

One of the main factors affecting interfacial tension is temperature. As the temperature increases, the interaction forces decrease, as a consequence, the net force that contracts the surface also decreases, causing a decrease in the interfacial tension.

If the temperature continues to rise, there will come a time when the interfacial tension will be canceled and there will be no more separation surface between the phases. The temperature at which the interfacial tension is canceled is called the critical temperature ( c ) .

The reason the interfacial tension decreases is that increasing temperature increases kinetic energy by increasing the thermal motion of molecules.

## Interfacial tension measurement

There are different methods of experimental measurement of interfacial tension, among which you can choose the one that suits you best, 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 drip method, and the rotary drop method.

### Wilhelmy’s 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 is equal to the weight plus the tensile force. The weight of the plate is obtained by a torsion sensitive microbalance connected to the plate by means of 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, prior to measurement, 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 Drop Method

This method is based on measuring the deformation of a drop hanging in a capillary. The fall remains in balance while suspended because the tensile force is equal to the weight of the fall.

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

### rotary drop method

The rotary 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 into a capillary tube filled with another liquid. The fall is subjected to a centrifugal force due to a rotational movement, with great speed, which prolongs the fall on the axle and opposes the pulling force.

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

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