interfacial tension
Broadly speaking, interfacial tension is defined as the force that must be expended to increase the size of an interface between two adjacent phases that are not completely mixed. In a narrow sense, the term refers to liquid/liquid and liquid/solid phase boundaries, while for liquid/air interfaces we refer to the surface tension, and for solid/air interfaces we refer to the free energy of the solid surface.
background：
As a measure of work per unit area or force per wetted length, interfacial tension is expressed in units of N/m or mN/m and is represented by the symbol σ or γ.
At a phase boundary, the sum of interactions with molecules of the same phase is greater than the sum of interactions with molecules of the other phase. Consequently, molecules at the interface have fewer attractive interaction partners than in the bulk phase. Therefore, these phases form the smallest possible interface without the action of external forces. Some work had to be done to increase the size of the interface.
The interfacial tension caused by the directional adsorption of surfactants at the interface between two immiscible liquids is significantly lower than the surface tension of the two liquids. Systems exhibiting interfacial tension generally consist of water, oil, surfactants or mixtures of surfactants and salts. Usually the value of interfacial tension is 10-2~10-1mN/m, which is called low interfacial tension. The value of interfacial tension below 10-3mN/m is called ultra-low interfacial tension.
Molecular environment at bulk phases and interfaces
Like surface tension, there is a difference between static interfacial tension (measured in equilibrium with a mechanically unchanged interface) and dynamic interfacial tension (measured when the interface is changing).
significance:
Interfacial tension plays an important role in many processes and phenomena in which different phases come into contact with each other:
Emulsions and emulsifiability: Interfacial tension affects emulsification and the tendency to phase separate.
Oil flooding (e.g. tertiary crude oil production): If the interfacial tension is lowered by surfactants, it is possible to move the organic phase after flooding with water.
Quality testing of hydrophobic liquids: The aging of hydrophobic liquids is often closely related to the reduction of water interfacial tension. In this case, the measurement of interfacial tension is an important quality test, such as transformer oil.
In the case of solid-liquid phase boundaries, interfacial tension affects the long-term stability of interfacial contacts, for example during gluing and coating processes.
Measurement methods
Liquid-liquid interface:
Ring method according to Du Noüy: In this method the force acting on the wetting ring due to the tension of the drawn-off thin layer of liquid is measured when moving the ring from one phase to another.
Plate method according to Wilhelmy: In this method the force acting on a wettable plate vertically immersed in the lower phase is measured.
Rod method: Same as the plate method, but uses a cylindrical rod with a smaller wetted length to measure smaller liquid volumes.
Drop volume method: Measures the volume of a drop of liquid when it is separated from another liquid at a vertical capillary.
Spinning drop method: A horizontal capillary filled with a bulk phase and a droplet phase is set to spin. The droplet diameter elongated by centrifugal force is related to the interfacial tension.
Pendant drop method: In a large liquid phase, the shape of the drop on the needle is determined by factors such as interfacial tension. Interfacial tension can be determined from droplet images using droplet shape analysis.
Liquid-solid interface:
Measurement of contact angle: In addition to its surface free energy, the interfacial tension of a solid with a specific liquid can also be calculated from the contact angles of various liquids with solids.