Summary of the Instruments Used for Measuring Surface and Interfacial Tensions
Maximum bubble pressure Oscillating jet Drop volume Surface waves
Du Nouy ring Wilhelmy plate Pendant drop Sessile drop Spinning drop
Drop volume Pendant drop Spinning drop
upward and downward in a controlled manner while the position of the ring is kept constant. Initially, the vessel is positioned so that the ring is submerged just below the surface of the liquid being analyzed. It is then slowly lowered and the force exerted on the ring is recorded. As the surface of the liquid moves downward, some of the liquid "clings" to the ring because of its surface tension (Figure 5.13). The weight of the liquid that clings to the ring is recorded by the force-measuring device and is related to the force that results from the surface tension.
The Du Nouy ring method is usually used in a "detachment" mode. The vessel is lowered until the liquid clinging to the ring ruptures and the ring becomes detached from the liquid. The force exerted on the ring at detachment is approximately equal to the surface tension multiplied by the length of the ring perimeter: F = 4nRf, where R is the radius of the ring. In practice, this force has to be corrected because the surface tension does not completely act in the vertical direction and because some of the liquid remains clinging to the ring after it has become detached.
where P is a correction factor which depends on the dimensions of the ring and the density of the liquid(s) involved. Values of P have been tabulated in the literature or can be calculated using semiempirical equations (Couper 1993). One of the major problems associated with the Du Nouy ring method, as well as any other detachment method, is that serious errors may occur when measuring the surface tension of emulsifier solutions rather than pure liquids. When a ring detaches from a liquid, it leaves behind some fresh surface which is initially devoid of emulsifier. The measured surface tension therefore depends on the speed at which the emulsifier molecules diffuse from the bulk liquid to the fresh surface during the detachment process. If an emulsifier adsorbs rapidly compared to the detachment process, the surface tension measured by the detachment method will be the same as the equilibrium value, but if the emulsifier adsorbs relatively slowly, the surface tension will be greater than the equilibrium value because the surface has a lower emulsifier concentration than expected.
The Du Nouy ring method can also be used to determine the interfacial tension between two liquids (Couper 1993). In this case, the ring is initially placed below the surface of the most dense liquid (usually water). The force acting on the ring is then measured as it is pulled up through the interface and into the oil phase. A similar equation can be used to determine the interfacial tension from the force exerted on the ring, but one has to take into account the densities of the oil and water phases and use a different correction factor. For two liquids which are partially immiscible with each other, the interfacial tension may take an appreciable time to reach equilibrium because of the diffusion of water molecules into the oil phase and vice versa (Hunter 1993).
The Du Nouy ring method can also be used to determine surface or interfacial tensions by continuously monitoring the force acting on the ring as the vessel containing the liquid is lowered, rather than just measuring the detachment force. As the liquid is lowered, the force initially increases, but at a certain position it reaches a maximum (when the surface tension acts vertically), before decreasing slightly prior to detachment. In this case, the maximum force, rather than the detachment force, is used in the equations to calculate the surface or interfacial tension (Couper 1993). The advantage of this method is that it does not involve the rupture of the liquid, and therefore there are fewer problems associated with the kinetics of emulsifier adsorption during the detachment process.
For accurate measurements, it is important that the bottom edge of the ring be kept parallel to the surface of the fluid and that the contact angle between the liquid and the ring is close to zero. Rings are usually manufactured from platinum or platinum-iridium because these give contact angles that are approximately equal to zero. The Du Nouy ring method can be used to determine surface tensions to an accuracy of about 0.1 mN m-1 (Couper 1993).
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