Water molecule cluster size is a relative indication of surface tension. Smaller water clusters have relatively less cohesion as a result of a fewer number of hydrogen bonds between the individual water molecules. The surface tension is decreased because the water clusters are fractionated.

Surface tension is a measurement of the cohesive energy present at an interface. The molecules of a liquid attract each other. The interactions of a molecule in the bulk of a liquid are balanced by an equal 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 thissituation 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. This excess energy exists at the interface of two fluids. If one of the fluids is the vapor phase of a liquid being tested the measurement is referred to as surface tension. If the surface investigated is the interface of two liquids the measurement is referred to as interfacial tension. In either case the more dense fluid is referred to herein 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 techniques used for liquids. 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.

Using Tensiometry to Measure Surface Tension:

The measurement of surface and interfacial tension as performed by a tensiometer is based on force measurements of the interaction of a probe with the surface of interface of two fluids. With any of the techniques described herein you may perform interfacial tension measurements just like surface tension measurements by insuring that the bulk of the probe is submersed in the light phase prior to beginning the experiment.

In these experiments a probe is hung on a balance and brought into contact with the liquid interface tested. The forces experienced by the balance as the probe interacts with the surface of the liquid can be used to calculate surface tension. The forces present in this situation depend on the following factors; size and shape of the probe, contact angle of the liquid/solid interaction and surface tension of the liquid.

The size and shape of the probe are easily controlled. The contact angle is controlled to be zero (complete wetting). This is achieved by using probes with high energy surfaces. KSV probes are made of a platinum/iridium alloy which insures complete wetting and easy and reliable cleaning.
The mathematical interpretation of the force measurements depends on the shape of the probe used. Two types of probes are commonly used, the DuNouy Ring and the Wilhelmy Plate.

The more popular method involves the DuNouy Ring. This method utilizes the interaction of a platinum ring with the surface being tested. The ring is submerged below the interface and subsequently raised upwards. As the ring moves upwards it raises a meniscus of the liquid. Eventually this meniscus tears from the ring and returns to it’s original position. Prior to this event, the volume, and thus the force exerted , of the meniscus passes through a maximum value and begins to diminish prior to the actually tearing event.

Here is the general procedure for determination and calculation of this surface tension in dynes/cm.

1)The ring is above the surface and the force is zeroed.
2)The ring hits the surface and there is a slight positive force because of the adhesive force between the ring and the surface.
3)The ring must be pushed through the surface (due to the surface tension) which causes a small negative force.
4)The ring breaks through the surface and a small positive force is measured due to the supporting wires of the ring.
5)When lifted through the surface the measured force starts to increase.
6)The force keeps increasing until
7)The maximum force is reached         

8)After the maximum there is a small decrease of in the force until the lamella breaks.
The calculation of surface or interfacial tension by this technique is based on the measurement of this maximum force. The depth of immersion of the ring and the level to which it is raised when it experiences the maximum pull are irrelevant to this technique. The original calculations based on the ring technique were based on theories which apply to rings of infinite diameter and do not consider an additional volume of liquid which is raised due to the proximity of one side of the ring to the other.