Surface/Interfacial Tension

Two common methods to measure the surface tension and interfacial tension are through the Wilhelmy Plate method or the du Noüy Ring method. A high-resolution balance measures the force required to remove the probe from the liquid. Both methods provide a means to determine the interfacial tension of a liquid without knowing the density of the sample. This allows for easy measurement of changing concentrations of solutions.

Typical Experimental Results

Interfacial Tension Results between Vegetable Oil and Changing Detergent Concentrations in Water

Interfacial tension between vegetable oil and changing concentrations of dish detergent in water.

Surface Tension Over Time for a Soap Water Solution.

Applications

For more information please read our application notes:
Interfacial Tension between Soap Water and Vegetable Oil, PDF
Surface and Interfacial Tension of Liquids, PDF

Instrument: Kyowa DyneMaster DY-700 Surface Tensiometer

Instrument Key Specifications

Interfacial Tension between Soap Water and Vegetable Oil

Inside a bulk liquid, intermolecular forces act on all sides of a molecule. Under static conditions, the net force acting on this molecule is zero; the forces are balanced out in all directions. At the interface of the liquid and immiscible gas, liquid or solid, the intermolecular forces are not balanced in all directions. This imbalance of forces result in excessive cohesive force and/or energy for the molecules at the interface and it is called interfacial tension. In the case of a liquid-air interface, the interfacial tension is called surface tension. In some applications, a high interfacial tension is not desirable. For these applications, adding surfactants to the solution often the best of choice. Surfactants are composed of two parts: a hydrophilic head and a hydrophobic tail. This combination of hydrophilic and hydrophobic components causes the surfactants to gather at the liquid interface to reduce the interfacial tension.

One method to measure the interfacial tension between two immiscible liquids is through the Wilhelmy Plate technique. The Wilhelmy Plate operates similar to the du Noüy Ring, but differs in geometry and is preferred for ease of use and increased accuracy. For interfacial tension measurements with the Wilhelmy Plate, the plate starts in the top liquid after being wetted with the bottom liquid. The plate then goes through the top liquid and approaches the bottom liquid. When the base of the plate reaches the liquid interface, the interfacial force acting on the plate is recorded.

With the force measured, the interfacial tension is determined through the following equation:

Where F is the measured force, L is the perimeter around the base of the plate and θ is the contact angle between the plate and liquid. Normally, the process of pre-wetting the plate surface with the bottom liquid and the plate design satisfactorily bring the contact angle to zero. With a contact angle of zero, the interfacial tension is dependant only on the measured force and the known perimeter of the plate.

Two common immiscible liquids are vegetable oil and water. In this application study, the effectiveness of surfactant/detergent in reducing the interfacial tension of vegetable oil and water is demonstrated. The interfacial tension between vegetable oil and water solutions of different concentrations of dishwashing detergent were measured with a Wilhelmy Plate technique on a DY-700 Surface Tensiometer (Kyowa Interface Science Co. Ltd., Japan). The DY-700 has the capability to utilize either the Wilhelmy Plate or du Noüy Ring technique for surface tension or interfacial tension measurements, but as mentioned before, the Wilhelmy plate is preferred.

As can be seen from Figure 1, surfactant/detergent is effective in reducing the interfacial tension between the vegetable oil and water. Even small amounts have a considerable impact on the interfacial tension between vegetable oil and water, which is really desired for cleaning dishes after use.

Surface and Interfacial Tension of Liquids

The surface tension and interfacial tension are the result of imbalanced inter-molecular forces at the surface of a liquid or at the interface between liquids. Inside a liquid, any molecule has an equal number of neighboring molecules. Therefore, the net force acts on any inside molecule is zeroed out. On the other hand, the surface molecule has reduced number of neighboring molecules and this arrangement results in an inward cohesive force acting on the surface molecules. Liquid surface tension and solid surface free energy are from the same origin and could be used interchangeably in some situations. Existence of surface and interfacial tension may be evidenced by phenomena like a small part made of a denser material floating on a liquid surface or morning dew beading up on leaves. In general, low surface tension liquids have better surface wetting properties. High surface tension liquids have a higher tendency to form droplets. Surface and interfacial tension have infinite numbers of industrial applications where liquids, liquid to liquid interface, or liquid to solid interface are of interest. One of the most known applications could be in surfactant. Surfactant is a surface active agent to reduce surface tension of a liquid in order to increase the solution’s wettability to surfaces or increase cleaning efficiency.

Kyowa’s Contact Angle Meters all have the ability to measure the surface and interfacial tension of a liquid. For conducting the measurement, the largest possible droplet is created on the end of the needle of liquid dispenser. Using either the ds/de or the Young-Laplace analysis routine, the surface and interfacial tension of the liquid is found. Both analysis routines are part of the FAMAS software available on the contact angle meters. This method is also referred to as the Pendant Drop method.

Any material that can be expelled through the dispenser tip can be measured by the Pendant Drop method. These materials include aqueous solutions, beverages, chemicals, cosmetic creams, food pastes, inks, oils, paints, surfactant solutions, tooth pastes, etc. To accurately measure the surface tension, the droplet has to reach equilibrium. Most liquids will reach an equilibrium point quickly. Thicker fluids may take a bit longer time to reach equilibrium. As demonstrated in Figure 1, the surface tension of a pure epoxy was found to be 44.2mN/m using the Young-Laplace routine.

The Pendant Drop method is not limited to surface tension measurements in air. A liquid can be used as the surrounding medium provided sufficient light is allowed to pass through. As shown in Figure 1, the interfacial tension of water in toluene could be easily determined. To ensure accurate measurement, the droplet was left suspended in toluene to reach equilibrium. Once equilibrium was reached, the interfacial tension of water in toluene was found to be 18.0mN/m using the Young-Laplace routine.