Surface Tension (ST), γ

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Definition:

Surface Tension (ST) is the measurement of the cohesive (excess) energy present at a gas/liquid interface.

Summary/Description:

The molecules of a liquid attract each other. The interactions of a molecule in the bulk of a liquid are balanced by an equally attractive force in all directions. Molecules on the surface of a liquid experience an imbalance of forces as indicated below.

The net effect of this situation is the presence of free energy at the surface. The excess energy is called surface free energy and can be quantified as a measurement of energy/area. It is also possible to describe this situation as having a line tension or surface tension, which is quantified as a force/length measurement. The common units for surface tension are dynes/cm or mN/m. These units are equivalent.

Polar liquids, such as water, have strong intermolecular interactions and thus high surface tensions. Any factor, which decreases the strength of this interaction will lower surface tension. Thus an increase in the temperature of this system will lower surface tension. Any contamination, especially by surfactants, will lower surface tension. Therefore researchers should be very cautious about the issue of contamination. Some surface tension values of common liquids and solvents are shown in the table below.