Measurement of surface tension

Surface tension is measured with tensiometers and several techniques are described in the literature. The most common methods are listed below. Inherent to the most precise of these techniques is to measure the weight of the meniscus. Accordingly, the quality of the balance determines the quality of the measured data and this is also why Kibron outperforms all competing tensiometers, without exception. Yet, this is only one the secrets behind Kibron's success: our products have revolutionized the measurement of surface tension also because of utilizing the best measurement technique.

Wilhelmy plate

A thin plate (perimeter about 40 mm) is lowered to the surface of a liquid and the downward force to the plate is measured. Surface tension is the force divided by the perimeter of the plate.

The long perimeter is used so as to compensate for the insensitivity of the balance. For example, for a Wilhelmy plate with a wetted length of 40 mm the meniscus of water weighs 492 milligrams.

The plate must be completely wetted before the measurement to ensure that the contact angle between the plate and the liquid is zero. If this is not true the Wilhelmy method is not valid. For reproducible results the position of the plate must be maintained constant, meaning that the lower end of the plate is exactly on the same level than the surface of the liquid. Nevertheless, for accurate results the values measured should be corrected for buoyancy.

Word of warning: using filter paper as a Wilhelmy plate

Occasionally one still sees a filter paper used instead of a Pt-plate. The benefit of this is that wetting is very efficient. However, the downside is a significant loss of material and "the chromatography effect". The easiest way for you to see this is to include a small amount of a fluorescent tracer (such as fluorescent lipid) into the monolayer and record the surface pressure. First, you will see that the time needed to reach equilibrium is long because of adsorption of material to the filter paper. Second, if you inspect the filter paper fluorescence upon UV-illumination, you will see a significant amount of the fluorescent tracer accumulated at the top of the filter paper. This is essentially chromatography on the filter paper of your lipids and other film materials. We NEVER recommend using filter paper.

DuNouy ring

A ring (standard perimeter about 60 mm) is pulled through the liquid/air interface and the maximum downward force to the ring is measured.

This method is not direct. One must use correction factors which take account the dimensions of the ring (the perimeter, ring wire thickness and the effect of the opposite inner sides of the ring to the measurement), as well as correction for buoyancy. Another factor making this technique prone for errors is that the ring geometry must not be distorted or dented. This is the reason why companies sell ring repair tools. Yet, this repair seldom works and to be on the on the safe side it is best to purchase a new Pt-ring. Unfortunately, these are rather expensive.

Maximum pull on a rod - the method of choice

Padday et al., J Chem Soc Far Trans I, 1974, 71: 1919-31

This method is a combination of the above two methods. A thin rod is immersed into the sample and then pulled out and the maximum force is measured. The diameter of the probe is irrelevant and only if the diameter changes there is a need to recalibrate. This is automated and fast in all Kibron instruments. When calibration is done using a liquid with a known surface tension there is no need for correction. The weight of the meniscus depends only on surface tension, rod diameter, and the density of the liquid (see Padday et al., 1974).

Importantly, the geometry of the DyneProbes developed by Kibron makes correction for buoyancy unnecessary. The unmatched sensitivity, excellent resolution (better than 2 micrograms) and low noise of Kibron's microbalance allow to use very small diameter rods, with pure water giving a meniscus of only 11.8 mg. After years of experience we can confirm that this is without doubt the absolutely most accurate method for recording surface tension, used in Kibron's high precision tensiometers. Importantly, the same approach can also be used for continuous monitoring of surface pressure, such as recording of compression isotherms. In this case maximum pull force is not recorded, but the rod touching the surface is used as a miniature Wilhelmy probe. Again, because of the inherent high sensitivity of Kibron balance long perimeter for wetting is not necessary, allowing sample volumes as low as 45 microliters. None of the instruments sold by our competitors come even close! Further, one measurement takes only approx. 20 seconds and in Kibron's Delta-8, with 8 microbalances, 96 surface tension measurements are done in less than three minutes.

Pendant drop

The liquid is injected from a needle so that it forms a drop on the tip of the needle. The drop is then optically observed and the surface tension is calculated from the shape of the drop.

For tensiometers, which use this method, a computer controlled instrument and a sophisticated software is mandatory.

Spinning drop

This method is used when the surface tension values are so low that tensiometers, utilizing the above methods are not able to reach the measurement range.

A small drop of a sample is injected inside a thin tube with another liquid. The tube is then rotated at a high speed and the interfacial surface tension is calculated from the angular speed of the tube and the shape of the drop.

Bubble pressure

In this method a capillary tube is first immersed in the liquid sample. subsequently constant flow of gas is purged through the tube forming small bubbles into the liquid.

The pressure needed to form a bubble is measured and the surface tension of the sample is calculated from the pressure difference between inside and outside the bubble and the radius of the bubble. This kind of tensiometers are used in many "on-line" measurements. However, the range of surface tensions accessible to this method is limited and cleaning of the system is difficult, resulting in deviation.