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Liquid Interface, Surface & Interfacial Tensions

Liquids show a significant cohesive force between adjacent molecules because of the water's surface tension. These forces act as a cohesive force.

Liquid interface

Liquids show a significant cohesive force between adjacent molecules because of the water's surface tension. Within a liquid, these forces act as a cohesive force. Molecules surround themselves with other molecules that are equally attracted to them in every direction. Molecules at the liquid/air interface (at the surface of liquids) are affected by this phenomenon. Liquid molecules which are below or adjacent to each other have only attractive cohesive forces. A molecule in one phase may become bound to a molecule in another phase via adhesive forces. Therefore, molecules on the liquid's surface are forced inward towards the bulk of the liquid, causing them to contract and pull with a force F.

Keeping equilibrium requires counteracting the inward tension in the surface with an equal force. A NONETENSION [γ] may also be defined as the force that should be applied per unit length perpendicular to the surface to counterbalance the net inward pull. The density is expressed in dynamometers per centimeter. INTERFACIAL TENSION is a measure of the force per unit length at the interface between immiscible liquid phases in the form of dynes/cm. The interfacial tensions between two liquid phases are less than those between two liquid surfaces because of the adhesive force between them. This is because liquid and gas phases have separate adhesive forces. There can be no interfacial tension between completely miscible liquids. ST is higher when there is more intermolecular force of attraction, so increases in the hydrogen bond or the molecular weight directly correlate to ST

Surface tension

Think about the boundary between a liquid and a gas. Both phases contain a majority of molecules in bulk, but some molecules are located at the interface with the other phase. Each of the molecules in the bulk interacts similarly with all the molecules surrounding it - the pull in all directions is similar, resulting in a zero net force. Molecular molecules at the interface, on the other hand, are more prone to gravitate towards the bulk of the phase they are in. Using surface tension as an example, we can define it as the force that serves to keep the liquid together.

Liquid molecules form surface tension because they are in a state of cohesiveness. Molecules in the bulk interact with each other equally in all directions. A molecule's neighbors at the surface, however, are not always the same. In this way, molecules are drawn toward the bulk by a net inward force. Surface tension arises from this phenomenon. There are different types of molecular interactions that affect surface tension. As an example, water has a high surface tension because its molecules interact with each other through relatively strong hydrogen bonds.

Interfacial and surface tension can be measured in several different units, most commonly in mN/m (or dyes/cm). Among other things, surface tension measurements are used to characterize surfactant solutions and have a direct impact on the spreading of coating formulations.

Interfacial tension

A liquid-liquid or a liquid-solid interface experiences interfacial tension, which arises from the force between dissimilar phases. Surface tension is used for the gas-liquid interface and surface free energy is used for the gas-solid interface, respectively. Regardless of the phase pair involved, the force is caused by the same mechanism. Molecules in bulk materials (liquids or solids) share the same kind of molecules with their neighbors, so all are pulled equally back and forth so that there is no net force. When molecules are brought together at an interface, the situation is different. Similar molecules will be seen on either side and below, where they will interact with each other through cohesive forces.

Surface tension and surface free energy values can be extremely high in air-liquid and air-solid systems, respectively, which is the cause of strong cohesive forces. The adhesive forces between the molecules of immiscible liquids or liquid and solids, however, also balance out the forces at the interface when two immiscible liquids or liquid and solids are in contact. In terms of interfacial tension (IFT), dynes/cm and mN/m are the common units. Both units are the same. Measurement of the tension between immiscible liquids is used in numerous industrial applications.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of, 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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