Du Noiiy method

Just as it is possible to float objects heavier than water on the surface of water, it is possible to pull the surface upward (increase the surface area) by the application of a suitable force, and thereby calculate the surface tension. The du Noiiy method makes use of a clean platinum ring of radius r that is placed on the liquid surface. The liquid, which wets platinum, tends to adhere to the ring, which is slowly raised by the application of a force, which is previously calibrated, until the net force pulling the ring upward exceeds the surface tension and the ring breaks from the surface. At that point the surface tension force, F, is 2ly, where 1 is the circumference of the ring, and since F = mg. 

Fig. C.5 Measurement of surface tension by du Noiiy method. The platinum ring of radius r is allowed to touch and be wetted by the liquid surface. It is then raised by a force F (effected by a torsion type of balance) until the ring breaks away from the liquid...

The Wilhelmy plate method procedurally is identical to the du Noiiy method described earlier. Because an infinitely long plate has the same plane of symmetry as the suspended liquid, a correction factor is not necessary. The equation to measure the sinface tension is trivial and becomes... 

A method that has been rather widely used involves the determination of the force to detach a ring or loop of wire from the surface of a liquid. It is generally attributed to du Noiiy [42]. As with all detachment methods, one supposes that a first approximation to the detachment force is given by the surface tension multiplied by the periphery of the surface detached. Thus, for a ring, as illustrated in Fig. II-ll,... 

Interface by the du Noiiy Ring Method Basic Protocol 2 Dynamic Interfacial Tension Determination by the Drop D3.6.5... 

This unit will introduce two fundamental protocols—the Wilhelmy plate method (see Basic Protocol 1 and Alternate Protocol 1) and the du Noiiy ring method (see Alternate Protocol 2)—that can be used to determine static interfacial tension (Dukhin et al., 1995). Since the two methods use the same experimental setup, they will be discussed together. Two advanced protocols that have the capability to determine dynamic interfacial tension—the drop volume technique (see Basic Protocol 2) and the drop shape method (see Alternate Protocol 3)—will also be presented. The basic principles of each of these techniques will be briefly outlined in the Background Information. Critical Parameters as well as Time Considerations for the different tests will be discussed. References and Internet Resources are listed to provide a more in-depth understanding of each of these techniques and allow the reader to contact commercial vendors to obtain information about costs and availability of surface science instrumentation. 

The choice between the static methods (Wilhelmy plate method and the du Noiiy ring method) should primarily be based on the properties of the system being studied, in particular, the surfactant. As mentioned in UNITD3.5, the transport of surfactant molecules from the bulk to the surface requires a finite amount of time. Since static interfacial tension measurements do not yield information about the true age of the interface, it is conceivable that the measured interfacial tension values may not correspond to equilibrium interfacial tension values (i.e., the exchange of molecules between the bulk and the interface has not yet reached full equilibrium and the interfacial tension values are therefore not static). If the surfactant used in the experiment adsorbs within a few seconds, which is the case for small-molecule surfactants, then both the Wilhelmy plate method and the du Noiiy ring method are adequate. If the adsorption of a surfactant requires more time to reach full equilibrium, then the measurement should not be conducted until the interfacial tension values have stabilized. Since interfacial tension values are continuously displayed with... 

The main advantage of the static methods is cost. The equipment needed to conduct the dynamic measurements is approximately five times as expensive as the equipment required for static measurements (- 25,000 for a drop shape and drop volume analyzer versus - 5,000 for du Noiiy and Wilhelmy instruments). This is due to the additional capability of the former instruments to determine not only interfacial tension values but also the corresponding age of the interface. For more information on equipment, costs, and suppliers, see Internet Resources. 

The basic setup to determine static interfacial tension based on either the Wilhelmy plate method or the du Noiiy ring method (see Alternate Protocol 2) is shown in Figure D3.6.1. It consists of a force (or pressure) transducer mounted in the top of the tensiometer. A small platinum (Wilhelmy) plate or (du Noiiy) ring can be hooked into the force transducer. The sample container, which in most cases is a simple glass beaker, is located on a pedestal beneath the plate/ring setup. The height of the pedestal can be manually or automatically increased or decreased so that the location of the interface of the fluid sample relative to the ring or plate can be adjusted. The tensiometer should preferably rest on vibration dampers so that external vibrations do not affect the sensitive force transducer. The force transducer and motor are connected to an input/output control box that can be used to transmit the recorded interfacial tension data to an external input device such as a monitor, printer, or computer. The steps outlined below describe measurement at a liquid/gas interface. For a liquid/liquid interface, see the modifications outlined in Alternate Protocol 1. Other variations of the standard Wilhelmy plate method exist (e.g., the inclined plate method), which can also be used to determine static interfacial tension values (see Table D3.6.1). 

Lecomte du Noiiy (1919) stated that there were large discrepancies in interfactial tension results obtained with previously available methods, which illustrated the need to develop better measurement methods. He therefore developed a new method in which a platinum ring... 

The Wilhelmy plate and du Noiiy ring methods provide a single value for surface tension of a given surfactant at a given concentration. Examples are provided in unitd3.5 (see Figure D3.5.5). Examples of dynamic surface tension values are also provided in unitd3.5. 

In this method the force required to detach a ring from a surface or interface is measured either by suspending the ring from the arm of a balance or by using a torsion-wire arrangement (du Noiiy tensiometer). 

For emulsions, the interfacial tension is usually of most interest. Here, the du Noiiy ring, Wilhelmy plate, drop volume, pendant, or sessile drop methods are the most commonly used. The spinning drop or captive drop techniques are applicable to the very low interfacial tensions encountered in the enhanced oil recovery and microemulsion fields. The maximum droplet pressure technique can be used when there is little or no density contrast between the phases, such as in bitumen-water systems at elevated temperature. 

For foams, it is the surface tension of the foaming solution that is usually of most interest. For this, the most commonly used methods are the du Noiiy ring, Wilhelmy plate, drop weight or volume, pendant drop, and the maximum bubble pressure method. For suspensions it is again usually the surface tension of the continuous phase that is of most interest, with the same methods being used in most cases. Some work has also been done on the surface tension of the overall suspension itself using, for example, the du Noiiy ring and maximum bubble pressure methods (see Section 3.2.4). 

The du Noiiy ring surface tension method involves slowly raising a platinum ring through a liquid until it detaches from the surface (Figure 3.8, Lower). The force at the point of detachment is measured using a balance or torsion wire. If F is the force on the ring,... 

In all cases it should be appreciated that, when solutions rather than pure liquids are involved, there can be appreciable changes taking place with time at the surfaces and interfaces. Many of the methods which involve making new surfaces or interfaces are difficult or impossible to apply accurately to solutions. Also, where interfacial tensions are desired the second liquid competes with the first for attachment to whatever solid support is involved in the measurement method. This can cause hysteresis, and metastable menisci can result (eg., in du Noiiy, Wilhelmy and capillary methods). 

Ring method — Method to determine the - interfacial tension in liquid-gas systems introduced by Lecomte du Noiiy [i]. It is based on measuring the force to detach a ring or loop of a wire from the surface of a liquid. The method is similar to the -> Wilhelmyplate method when used in the detachment mode [ii]. See also -> electrocapillarity, -r electrocapillary curve, -> Gibbs-Lippmann equation, - Wilhelmy plate (slide) method, - drop weight method, - Lippmann capillary electrometer. 

In some cases the surface tension also was measured by the ring method using a du Noiiy tensiometer. 

Fig. 7. The increase in surface pressure of phospholipid monolayers as a function of signal-peptide concentration for the various E. coli LamB synthetic signal sequences (from Briggs, 1986). A monolayer of egg phosphatidylethanolamine and egg phosphatidylgly-cerol (65 35) was spread from a benzene solution onto 5 mM Tris buffer, pH 7.3, yielding a hnal surface pressure of 20 dyn/cm after evaporation of the benzene. The peptide was added by injecting a concentrated solution below the lipid-water interface. The surface pressure was measured by the du Noiiy ring method with a Fisher Autotensiomat equipped with a platinum-iridium ring. The plateau values are plotted as a function of the peptide concentration for the wild-type (O), Pro— Leu pseudorevertant (A), and deletion-mutant ( ) peptides.

The threshold of the equilibrium state can generally be reached slowly, and thus the surface tension values obtained by semi-static methods closely resemble those obtained by static ones. The rate of approaching the equilibrium state should be optimized in each system, in order to avoid lengthy measurements and to obtain surface tension values as close to the equilibrium ones as possible. Among the most common semi-static methods are the method of maximum pressure, the du Noiiy ring method and the drop-weight method. 

Fig. 1-19. Measurement of the surface tension by the ring detachment method (du Noiiy)...

Also, where interfacial tensions are desired, the second liquid competes with the first for attachment to whatever solid support is involved in the measurement method. This can cause hysteresis, and metastable menisci can result (e.g. in du Noiiy, Wilhelmy and capillary methods). 

The ring technique, and its many variations, is widely used in industrial laboratories. Several kinds of commerical apparatus incorporating a torsion balance are available under the name du Noiiy tensometer. The method is simple and rapid, and is capable of measuring the surface tension of a pure liquid to a precision of 0.3% or better. 

Also attempts to measure this interfacial tension in the complex coacervate gelatin (positive) + gum arabic (negative) with the Du NoiiY tensiometer remained without result the platinum ring was pulled out of the boundary surface even with the slightest force. Success has only recently been achieved with the aid of a microscopic capillary tube method of measuring this interfacial tension, whereby extraordinarily small values were indeed obtained. ... 

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