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, it is really the quality of the balance which determines the quality of the measured data. This is also why Kibron outperforms all competing tensiometers, with no exceptions. Yet, this is only one the secrets behind Kibron's unparalleled performance: our products have revolutionized the measurement of surface tension also because of utilizing the most precise measurement technique.
This high precision variant of the Du Noüy method was developed by Padday et al. (J Chem Soc Far Trans I 71: 1919-1931, 1974). The technique is very simple: a thin rod (instead of a ring or a plate) 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 the above article by Padday et al.).
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. Good demonstration of the accuracy of the Kibron technology is when measuring small changes in surface tension (see pdf for test of the sensitivity of our portable, low cost tensiometer, AquaPi). Notably, the low cost AquaPi actually beats the performance of the top of the line, expensive tensiometers of our competitors! Last but not least, unlike the ring and Wilhelmy plate, the rod method also works for highly viscous liquids: oils, polymers, paints and the like, as used in Kibron's most advanced new tensiometer, EZ-PI Plus.
See how it works: AquaPi in action
Rod vs ring?
Importantly, the geometry of the precision manufactured DyneProbes developed by Kibron makes correction for buoyancy unnecessary, unlike when using the ring method. This is simply due to the fact that at the point of maximal pull no part of the probe is immersed into the liquid.
The unmatched sensitivity, excellent resolution (better than 0.2 micrograms), and the very low noise of Kibron's microbalance allow to use very small diameter rods, with pure water giving a meniscus of only 11.8 mg. Notably, some of our competitors offer platinum rods for 'small sample volume measurements' and show data measured with their rods and tensiometers, revealing large scatter. However, the poor quality of their data merely reveals the noise of their balance and the poor quality of their Pt-rods, further reflecting the overall poor quality of their manufacturing.
It is also stated by companies making ring tensiometers that it is the Du Noüy-ring which should be used as a reference. This does not make any sense. After all, as expected the ring and rod do yield the same surface tension values (see comparison here). Ring was simply used by Du Noüy because of the insensitivity of the balances available to him. In essence, the only difference between the Padday et al. and Du Noüy methods is the probe geometry. High sensitivity balance does not require large meniscus and allows to use a miniature rod.
To summarize, compared to ring the rod has the following advantages:
Importantly, the rod 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 small as 45 microliters to be measured. 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.
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 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, particularly in the low cost Langmuir-troughs and tensiometers. The benefit of the filter paper is that wetting is very efficient. However, the downside is a significant loss of material and the well known "chromatography effect". The easiest way to see this is to include a small amount of a fluorescent tracer (such as a 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.
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.
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.
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.
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.