Wilhelmy slide method

The methods so far discussed have required correction factors to the respective ideal equations. Yet there is one method, attributed to Wilhelmy [50] in 1863, that entails no such corrections and is very simple to use. 

The basic observation is that a thin plate, such as a microscope cover glass or piece of platinum foil, will support a meniscus whose weight both as measured statically or by detachment is given very accurately by the ideal equation (assuming zero contact angle)  

It should be noted that here, as with capillary rise, there is an adsorbed film of vapor (see Section X-6D) with which the meniscus merges smoothly. The meniscus is not hanging from the plate but rather fiom a liquidlike film [53]. The correction for the weight of such film should be negligible, however. 

An alternative and probably now more widely used procedure is to raise the liquid level gradually until it just touches the hanging plate suspended from a balance. The increase in weight is then noted. A general equation is 

As an example of the application of the method, Neumann and Tanner [54] followed the variation with time of the surface tension of aqueous sodium dode-cyl sulfate solutions. Their results are shown in Fig. 11-15, and it is seen that a slow but considerable change occurred. 

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The film pressure is defined as the difference between the surface tension of the pure fluid and that of the film-covered surface. While any method of surface tension measurement can be used, most of the methods of capillarity are, for one reason or another, ill-suited for work with film-covered surfaces with the principal exceptions of the Wilhelmy slide method (Section II-6) and the pendant drop experiment (Section II-7). Both approaches work very well with fluid films and are capable of measuring low values of pressure with similar precision of 0.01 dyn/cm. In addition, the film balance, considerably updated since Langmuir s design (see Section III-7) is a popular approach to measurement of V. 

In the Wilhelmy slide method, it seems best to partly immerse the slide and determine the change in height with constant upward pull or the change in pull at constant position of the slide. If the latter procedure is used, then... 

This method suffers from two disadvantages. Since it measures 7 or changes in 7 rather than t directly, temperature drifts or adventitious impurities can alter 7 and be mistakenly attributed to changes in film pressure. Second, while ensuring that zero contact angle is seldom a problem in the case of pure liquids, it may be with film-covered surfaces as film material may adsorb on the slide. This problem can be a serious one roughening the plate may help, and some of the literature on techniques is summarized by Gaines [69]. On the other hand, the equipment for the Wilhelmy slide method is simple and inexpensive and can be just as accurate as the film balance described below. 

This is one of the many detachment methods of which the drop weight and the Wilhelmy slide methods are also examples. As with all detachment methods, one supposes that, within an accuracy of a few percent, the detachment force is given by the surface tension multiplied by the periphery of the surface (liquid surface) detached (from a solid surface of a tubing or ring or plate). This assumption is also found to be acceptable for most experimental purposes. Thus, for a ring, as illustrated in Figure 2.12,... 

Measuring the force (the weight) needed to break away a body from the liquid surface is the basis for many variations of the detachment method. The body may be in the form of a rod with a circular base (Pin detachment method), a rod with a rectangular base (Wilhelmy slide method), a horizontal circular ring (Ring method), etc. The different forms of the detachment method have been used for surface tension measurement of liquids from room temperature up to approximately 1000°C. It is evident that the detachment methods can be used only in cases when the liquid wets the measuring body. 

Figure 6.13. Wilhelmy slide method using plate immersion.

Several methods for measurement of contact angle are summarized by Adamson (28). Neumann and Renzow (31) describe the Wilhelmy slide method which offers significantly higher precision than other techniques. ... 

Wilhelmy Plate Method A method for determining surface or interfadal tension on the basis of measuring the force needed to pull an inert plate, held vertically, through an interface. Also termed the Wilhelmy slide method. See also du Nouy Ring Method. 

The Wilhelmy slide method is somewhat similar to the ring-pull method. A very thin plate, such as a microscope cover glass or a sheet of mica, is hung from one arm of a balance and allowed to dip in the solution (Fig. 18.3). If p is the perimeter of the slide, the downward pull on the slide due to surface tension is yp. If F and F are the forces acting downward when the slide is touching the surface and when it is suspended freely in air respectively, then... 

The Wilhelmy slide method requires no correction, in contrast with the previous techniques, and is simple to use. It is based on the principle that a thin plate immersed vertically in the liquid supports a meniscus whose weight, measured statically or by detachment, is given very accurately by the equation... 

The Wilhelmy plate method provides an extremely simple approach that, unlike the ring detachment method, permits the measurement of continuously varying or dynamic surface tensions. If a thin plate (e.g., a microscope slide, a strip of platinum foil, or even a slip of filter paper) is attached to a microbalance and suspended so that its lower edge is just immersed in a liquid, the measured apparent weight Wj, is related to the actual weight of the plate Wp and the surface tension y by the following simple equation ... 

In addition to the sessile drop method which measures the contact angle directly, Neumann and Renzow (1969) have developed the Wilhelmy slide technique to measure it to 0.1° precision. As shown in Fig. 2.20, the meniscus at a partially immersed plate rises to a finite length, h, if the contact angle, 0, is finite. 6 is calculated from... 

The Wilhelmy plate method, as shown in Figure 4.10, is similar to du Nouy s ring method, but it uses a thin mica plate or microscope slide. The plate is suspended from a balance and dips into the liquid. The force, F, required to detach the liquid meniscus surrounding the plate depends on the surface tension or interfacial tension by ... 

See also - electrocapillarity, - electrocapillary curve, -r Gibbs-Lippmann equation, - Wilhelmy plate (slide) method, - ring method, - Lippmann capillary electrometer. 

For measurement of interfacial tension see also -> Wilhelmy plate (slide) method, -> drop weight method, -> ring method. There are also a number of other static and dynamic methods for the determination the interfacial tension [viii]. 

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. 

Wilhelmy plate (slide) method, drop weight method,... 

It has been observed the formation of continuous liquids on high-energy solid surfaces, such as mica, quartz, and silica, when those surfaces are subjected to immersion and emersion in water [13,14]. A continuous film of water on a hydrophilic (i.e., high energy) plasma polymer-coated glass slide moments after it was immersed to a depth of 3 cm in a beaker of DDI water is shown in Figure 26.21A. The water film remained continuous as it receded to the bottom of the plate. After about 2 min the water film front receded to approximately 1 cm from the bottom of the plate, as shown in Figure 26.21B. The presence and stability of a continuous water film can be detected and quantified by the Wilhelmy method [15]. 

The measurement of surface pressure (jr) was accomplished using the Wilhelmy method in which a glass slide was partially immersed in the water subphase and the effective change in weight of the slide was recorded after a monomolecular film was spread and compressed at the air-water interface. The equation relating ir to change in weight of the slide is ... 

This difference is, of course, independent of the buoyancy correcUon and, accordingly, when used in this way the buoyancy correction is unnecessary. One absolute requirement of the Wilhelmy method is that the substrate solution must wet the slide, i.e., the angle of contact between the slide and solution must be zero. The Wilhelmy method has been employed by Dervichian (25), by Abribat and Dognon (26), by Harkins and Anderson (27) and by others. The reviewer has found tUn microscope cover glasses which are 6X6 serve as excellent Wilhelmy plates. A battery of these plates fixed in a parallel position and properly spaced from each other can be suspended from an arm of a chmn-o-matio balance and rather extreme sensitivity can be achieved. With the Wilhelmy balance thoe is no question of leakage arotmd barriers as the q>read film is compressed. 

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