Rings are for Fingers – Plates are for Surface Tension
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Technical Note Force tensiometry with ring and plate Technical note: Industry section: Method: Keywords: DuNoüy-ring method, Wilhelmy plate method, surfactant, surface tension, interfacial tension Rings are for Fingers – Plates are for Surface Tension This note explains some of the pitfalls of using ring tensiometry on surfactant solutions, without understanding the dynamics of ring situation, relative to the dynamics of surfaces. It answers, in some detail, the classic question of “When do I use a ring and when do I use a plate?”, and gives a prelude to another note in this issue on non-equilibrium tensiometry. Ring method The classic question from tensiometer users, the one surely heard thousands of times at KRÜSS, is, “Should I use the ring or the plate?” I refer, of course, to the Du Noüy ring method for surface or interfacial tension measurement versus the Wilhelmy plate method. Historically, the ring method has been more widely used for both types of measurements (measurements of air/liquid interfaces, commonly referred to as surfaces, and measurements of liquid/liquid interfaces, commonly referred to as interfaces). There are also several standards, perhaps most notably ASTM standards, which call for the ring method. These standards also exist for historical reasons. More old style manual ring surface tension instruments still exist in the world than any other type of tensiometer which has been developed since. The ring method has three main issues which make it a less than good option for measuring exact surface tensions. This is particularly true for surfactant based solutions, wherein the rate of surfactant diffusion to newly formed surfaces is particularly slow as is the case especially for large molecule surfactants, amphoterics, and fluorosurfactants. However, if you want to make the most use of your ring, you will put it in a jewelry box while you are measuring surface tension and only remove it for measurements of interfacial tension. 1) Most importantly, the ring method itself is designed to keep the surface in a non-equilibrium state during the measurement of surface tension. The ring is pulled through the surface to make the measurement (or in today’s more sophisticated tensiometers, at minimum, the ring expands and contracts the surface during the measurement – looking for the maximum force of the liquid meniscus). So the measurement of surface tension is really made on a surface which is in a non-equilibrium state. This does not matter to the measurement of surface tension if you are measuring a pure liquid, because then the surface tension is at all times the same. However, in surfactant solutions, wherein the surface tension is dependent on the presence and orientation of KRÜSS GmbH | Borsteler Chaussee 85 | 22453 Hamburg | Germany | ww

the surfactants at the surface, having the surface in a state of expansion during the measurement can make a huge difference in the measured surface tension. As an example, witness the data below, all performed with a properly shaped Du Nouy ring and with the necessary Harkins and Jordan correction factors for the mass of liquid trapped under the ring not due to surface tension (two other issues we will discuss shortly). The only difference between these measurements of surface tension is the speed at which the ring is being pulled through the surface - as noted. However, two...

These differences make the plate method much more accurate for determining the surface tensions of both pure liquids and surfactant solutions. Not having to stretch a meniscus during the experiment means that the surface area does not change during measurement. The plate simply touches the surface; it is typically dipped into the liquid and brought back and held within 1.0 microns of the surface position using high-end tensiometers. The the surface is allowed to relax and reach equilibrium with the plate present. Thus, if surfactants are present, they are given as long as they need to reach...