attension Theta, User Manual

Page: 66 / 74

63
6.2 Theory of surface tension measurements
Definition
Surface tension is a measurement of the cohesive energy present at an interface. All of the molecules of
a liquid attract each other. These interactions of molecules in the bulk of a liquid are balanced so that
equal forces attract them in all directions. Molecules on the surface of a liquid however experience an
imbalance of forces as indicated below.
Figure 6.2 . Surface tension is caused by intermolecular forces.
The excess energy present at the surface 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 a
surface tension which is quantified as a force / length measurement. The most common unit for surface
tension are dynes / cm or mN / m. These units are equivalent.
The surface is the interface of two fluids. The experiment is referred to as a surface tension
measurement if it involves a gaseous phase. If two liquids are tested then the experiment is referred to
as an interfacial tension measurement. In either case the denser fluid is referred to as the heavy phase
and the less dense fluid is referred to as the light phase. Solids also may be described to have a surface
free energy at their interfaces but direct measurement of its value is not possible through the techniques
used for liquids.
Polar liquids like water have strong intermolecular interactions and thus have high surface tension. Any
factor which decreases the strength of the interaction will lower the surface tension, and so an increase
to the temperature of the system will lower surface tension. Any contamination, especially by surfactants,
will lower surface tension which is why researchers should be very concerned about cleanliness and
preventing contaminants.