Interfacial tension measurement

Interfacial tension is the force between two dissimilar phases that can be gas-liquid, gas-solid, liquid-liquid, or liquid-solid interface. In the case of the gas-liquid interface and the gas-solid interface, terms of surface tension and surface free energy are used, respectively.

Regardless of the phase pair in question the mechanism that causes the force is the same. In the bulk of the material (whether liquid or solid), the molecules have the same kind of molecules as their neighbors and are thus pulled equally in all directions resulting in a net force of zero.

At the interface, the situation is different. There, the molecules will see similar molecules on their sides and below and interact with them through cohesive forces. In air-liquid and especially in air-solid systems these cohesive forces can be really strong which causes high surface tension and surface free energy values, respectively. However, when two immiscible liquids or liquid and solid are in contact, the adhesive forces between the dissimilar molecules also play a significant role and balance out the forces at the interface. The common units for interfacial tension (IFT) are dynes/cm or mN/m. These units are equivalent.

Interfacial tension measurements between two immiscible liquids are used in several industrial areas. Liquid-solid interfacial tension, on the other hand, cannot be directly measured and is not further discussed here.

Optical tensiometers are utilized to measure static and semi-dynamic interfacial tension values. The main components of the optical tensiometer are a camera, dispenser to dispense a drop, sample stage, and the light source to illuminate the drop on the sample stage. Optical tensiometers range from completely manual systems to fully automated instruments.

Interfacial tension measurement between two immiscible liquids is based on the pendant drop method where a drop of liquid is formed at the end of the needle tip.

In the interfacial tension measurement instead of having air as the surrounding phase, the droplet at the needle tip is immersed in another liquid. Interfacial tension is measured the measurement is typically done upside down, meaning that the droplet will rise rather than hang on the tip of the needle. Thus, the method is also sometimes called a rising bubble. The reason for this is that the densities and the transparencies of the liquids need to be considered when determining the measurement setup. Most typically the measurement is done between water-based and oil-based liquids, out of which oil is typically less transparent. Thus, the oil must be drop phase to allow the light to travel through the liquid where the oil drop is immersed. Also, oil is typically less dense than water, which means that it is not possible to form a hanging oil drop in water. Regardless of the way the measurement is done, modern software can perform the measurements with no problems.

In practice, the water-based liquid is typically placed in a small rectangular cuvette. A small amount of oil is taken into a syringe that is used to form the drop. The only requirement for the method is that the densities of both liquids need to be either known or measured before the interfacial tension measurement as the software utilizes this information in calculations. To achieve the correct shape of the drop, the droplet needs to be as large as possible. As a rule of thumb, the less the density difference between the phases, the bigger the droplet needed.

Force tensiometer can be used to measure interfacial tension. These instruments are based on measuring the forces exerted on a probe that is positioned at the liquid-liquid interface. The probe is connected to a very sensitive balance and the liquid interface of interest is brought into contact with the probe. The forces measured by the balance as the probe interacts at the liquid-liquid interface can be used to calculate the interfacial tension. The force depends on the following factors: size and shape of the probe, contact angle between the probe and the liquid, and the interfacial tension between the liquids. The size and shape of the probe are easily controlled. Probes are typically made of platinum which helps to ensure a zero degree contact angle between the probe and the liquid to be studied. Two configurations of probes are commonly used; du Noüy ring and Wilhelmy plate.

Interfacial tension measurement with Du Noüy ring and Wilhelmy plate

Interfacial tension measurement with the Du Noüy ring is based on the same principle as surface tension measurement.

However, in interfacial tension measurements, the ring is first immersed in the denser phase which is typically water. Then the less dense liquid, often oil, is poured on top. Care must be taken not to contaminate the ring when pouring the liquid. The measurement is then started and the sample stage with the liquid container is moved down. When the ring is at the interface, the software will detect that automatically and continue to move the stage down until the detected force starts to drop. This will indicate to the software that the maximum force is already passed, and the stage can move up again (i.e. ring moves down) and the measurement can be repeated.

The measurement should always be done from polar liquid to dispersive. Typically, this means the aforementioned situation where the ring is first immersed in water which is also the denser phase. In some rare situations, the water is a less dense liquid. This is, for example, the case when interfacial tension between water and fluorocarbons is being measured. Then the measurement needs to be started again with the ring in the water phase, but the stage will now start to move up as the water is the top phase. This method is called a push mode. There is a need to use an additional weight on the du Noüy ring for it to push through the interface.

For software to be able to calculate the interfacial tension, the densities of both phases are required as a correction needs to be done to measured results. This is done automatically by the software.

Wilhelmy plate can also be used to measure interfacial tension. The measurement principle is similar to Du Noüy ring but now a platinum plate is used as the measurement probe. However, when the Wilhelmy plate is used, the buoyancy caused by the light phase needs to be compensated. Due to slightly more complex measurement, the Du Noüy ring is more often used for interfacial tension measurements.

Interfacial tension is an important parameter in many processes that take place at high temperatures or even high pressures.

Temperature control is possible with both optical and force tensiometers. With a force tensiometer, the measuring cup can be cooled or heated to reach the desired temperature. Temperature ranges from -20 °C to 200 °C.

With the optical tensiometer, the measurement is done by having the liquid in a cuvette which can be cooled or heated. A rising bubble method is then used to define the interfacial tension value.

With the optical tensiometer, it is also possible to measure interfacial tension under pressure. This is important especially in enhanced oil recovery applications where high temperatures and pressures are needed to evaluate phenomena at reservoir conditions. Pressure control is also needed if aqueous solutions need to be heated close to and above 100 °C to prevent bubble formation. 

There are several standards available for surface tension measurements. Most of the standards utilize force tensiometer with either du Noüy ring or the Wilhelmy plate method but there is also a standard for surface tension measurement using the pendant drop method. 

Sigma 702ET is a special instrument available for interfacial tension measurements between transformer oil and water. It follows two special standards written for this purpose; ASTM D971 and IEC 62961.