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Surface Free Energy and Measurement of Surface & Interfacial Tensions

Surface free energy, Measurement of surface tension, Force Tensiometers, Optical Tensiometers, Extreme Conditions, Measurement standards etc.

Surface free energy

Surface tension describes the energy present in a surface, sometimes called surface free energy. The unit of surface free energy is mN/m, which is equivalent to dynes/cm. An air-solid interface provides molecular interactions with surface free energy, which is a molecule's energy.

This type of energy is important in numerous applications. It determines how a solid behaves when exposed to liquids. An individual's body or the way a coating spreads on a solid is determined by its surface-free energy. It is important for adhesion between solids and liquids in a large variety of processes that solids and liquids interact. The surface tension and surface free energy play an essential role in the interactions of solids and liquids. Generally, any liquid that will encapsulate a solid with a high surface free energy will be easily absorbed by the solid.

Free energy at the surface cannot be directly measured. While using the contact angle measurement as a measure of wetting, surface free energy can also be regarded as a measure of intermolecular forces at the surface which does not depend on the liquid. One can easily predict how a liquid will behave if he or she knows the surface free energy of a solid.

Measurement of surface tension

Force Tensiometers for Surface Tension Measurement

The force tensiometer is used to measure topically expressed tension. Liquid-gas and liquid-liquid interfaces are measured by the forces exerted on probes at their interfaces. An intensely sensitive balance is used to connect the probe to the liquid interface of interest. Surface tension can be calculated by measuring the force exerted by the probe as it interacts with the liquid surface. The force and surface tension of the liquid depends on several factors, including the shape, size, and contact angle of the probe. The dimensions and shape of the probe can be easily adjusted. Platinum is a typical material for probes, which ensures that the probe is in direct contact with the liquid to be studied. A typical probe consists of either a du Noüy ring or a Wilhelmy plate. When sample volume is limited, the metal rod can be substituted for the Wilhelmy plate.

Surface Tensions Measured with Optical Tensiometers

Surface tension can be measured with optical tensiometers either statically or semi-dynamically. Several components are needed to make up an optical tensiometer: a camera, a dispenser for dispensing the drop, a stage for holding the sample, and a light source for illuminating the drop. From fully automated systems to completely manual instruments, optical tensiometers can be found in all types of applications.

Measuring Surface Tension in Extreme Conditions

The majority of surface tension measurements are performed at ambient temperature and pressure, but in some cases, temperature and pressure need to be controlled. Temperature can be controlled with optical as well as force tensiometers. With a force tensiometer, temperature control can be achieved with cooled or heated measuring cups. A temperature range of -20°C to 200°C is observed.

A measurement is performed using an optical tensiometer which measures the fluid in a cuvette that may either be cooled or heated. It is then determined based on the rising bubble method.

Optical tensiometers work well for measuring surface tension under pressure as well. It is especially important in applications that require high temperatures and pressures to evaluate phenomena at reservoir conditions, such as enhanced oil recovery. It is also necessary to control pressure when aqueous solutions are heated between and above 100 °C, to prevent bubble formation.

Measurement standards for surface tension

For surface tension measurements, several standards are available. A standard for measuring surface tension is the pendant drop method; however, most of the standards use force tensiometers, using wheels of du Noüy rings or Wilhelmy plates.

Measurement of interfacial tension

A liquid or a solid can develop interfacial tension between two incompatible phases (e.g., liquid-liquid, gas-solid, solid-liquid, or liquid-solid). Surface tension is used for the gas-liquid interface and surface free energy is used for the gas-solid interface, respectively.

In optical tensiometers, a value of interfacial tension is measured both statically and semi-dynamically. In addition to a camera, a dispenser for dispensing drops, a sample stage, and a light source for illuminating the drop on the sample stage, the optical tensiometer has a lighting system as well. Optical tensiometers run the gamut from entirely manual instruments to fully automated ones.

An interfacial tension measurement is carried out by making a drop of liquid on the needle tip, which is then measured to see if there is enough fluid to form two immiscible liquids.

When air is used to surround the needle tip, we measure interfacial tension by immersing it in another substance other than air. To measure interfacial tension, the droplet is typically held on top of the needle rather than hanging there, thereby causing it to rise. To measure interfacial tension, the droplet is typically held on top of the needle rather than hanging there, thereby causing it to rise. Since the densities and the transparencies of the liquids have to be accounted for when choosing a measurement setup, we need to consider both. Most usually, measurements are conducted on liquids that are either water-based or oil-based, of which oil is typically less transparent.

When oil drops are submerged in liquid, the light must be impeded from passing through the liquid, thus eluding the oil drop. Furthermore, oil generally has a lower density than water, so hanging oil drops cannot be formed in water. Any measurement can be performed by modern software regardless of how it is performed. A water-based liquid is commonly deposited into a small rectangular cuvette. Drops are created by injecting oil into a syringe. Interestingly, the only requirement for the method is knowing or measuring the densities of both liquids before measuring the interfacial tension. The software utilizes this information to calculate the interfacial tension throughout the calculation process. Ideally, the droplet should be as large as possible so that it can be shaped correctly. It is a rule of thumb that the smaller the density difference between two phases, the larger the droplet we need.

Measurement of tension at an interface is done with a force tensiometer

A force tensiometer can be used for measuring the interaction between surfaces. Instruments that measure the forces exerted on a probe placed at the interface between liquids and gases are based on this concept. Using a probe attached to an extremely sensitive balance, the probe is brought into contact with the liquid interface of interest. In the case of liquid-liquid interactions between the probe and the balance, the balance can measure the forces applied at the interface and calculate the interfacial tension. Various factors influence the force exerted by the probe, including its size, shape, the contact angle with the liquid, and interfacial tension in the liquids. Probes can be controlled with ease by changing their shape and size. Platinum probes are typically used for testing, as they help ensure the probe is in contact with liquids at zero degrees. Du Noüy rings and Wilhelmy plates are the two most commonly used probe configurations.

Extreme conditions for measuring interfacial tension

When processes take place at high temperatures or even under high pressures, interfacial tension plays a vital role. Optically based and force-based tensiometers can both control temperature. An electronic force tensiometer can be used to alter the temperature of a measuring cup to achieve the desired reading. -20 °C to 200 °C is the temperature range. With an optical tensiometer, measurement is done by having a liquid in a cuvette that can be either heated or cooled. To determine the interfacial tension value, a rising bubble method is applied. With the optical tensiometer, interfacial tension under pressure can also be measured. It is especially important in applications that require high temperatures and pressures to evaluate phenomena at reservoir conditions, such as enhanced oil recovery. A water solution needs to be controlled when heated up to and above 100 degrees C to prevent bubbles from forming.

Standards for measuring interfacial tension

Several surface tension standards can be used for surface tension measurements. In many of the standards, force tensiometers are used with either du Noüy rings or Wilhelmy plates, but a pendant drop method is also used for surface tension measurements. In many of the standards, force tensiometers are used with either du Noüy rings or Wilhelmy plates, but a pendant drop method is also used for surface tension measurements. In this regard, it follows ASTM D971 and IEC 62961, two standards designed specifically for this purpose.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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