The Wilhelmy Plate Method

In this method [6] a thin plate made from glass (e.g., a microscope cover slide) or platinum foil is either detached from the interface (nonequiHbrium condition) or its weight is measured statically, using an accurate microbalance. In the detachment method, the total force F is given by the weight of the plate W and the interfacial tension force  

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Table 1. Air/water/surface contact angles measured using the Wilhelmy plate method on surfaces incubated with deionised water for 10 minutes.

The LB film depositions were performed using a Joyce-Loebl Langmuir Trough IV equipped with a microbalance for measurement of the surface pressure by the Wilhelmy plate method. Filtered deionized water with a pH of 7 was used for the subphase. For the electron beam lithography study, PMMA was spread on the water surface from a dilute benzene solution ( 10 mg PMMA in 20 ml benzene). The novolac/PAC mixtures were spread from solutions ( 20 mg solids in 10 ml solvent) of isopropyl acetate. For the fluorescence studies, the PMMA/PDA mixture was spread on fee water surface from a dilute benzene solution (1.75 mg PDA and 8.33 mg PMMA in 20 ml benzene). Prior to compression, a 20 min interval was allowed for solvent evaporation. The Langmuir film was compressed to the desired transfer pressure at a rate of 50 cm2/min, followed by a 20 minute equilibration period. The Cr-coated silicon wafers and quartz wafers were immersed into fee subphase before... 

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 ... 

Figure 2.14 Diagram of the Wilhelmy plate method for measuring the surface tension of liquids.

What advantages does this (RIFS) technique have over the Wilhelmy plate method ... 

FIG. 6.3 Surface tension and capillary rise (a) the Wilhelmy plate method for measuring 7 (b) schematic illustration of capillary rise in a cylindrical tube of radius Rc. 

For a plate of rectangular cross section having length f and thickness t, P = 2(f + t) these dimensions can be accurately measured. By suspending the plate from a sensitive balance, we can also measure w with considerable accuracy. The apparatus is called a Wilhelmy balance, and the technique the Wilhelmy plate method. Thus, if the contact angle is known from an independent determination by, say, the tilted-plate method, then 7 can be evaluated by Equation (2). 

Strictly speaking, Equation (2) allows the vertical component of surface tension to be measured. Since this equals 7 cos 0, we are actually making a single measurement that involves two parameters. If 7 were independently known, the Wilhelmy plate method could also be used to determine 0. Whether we seek to evaluate 7, 0, or both, two experiments are needed, and these may not both involve the factor cos 0. In Section 6.8a we discuss a second type of measurement that can be made with the Wilhelmy apparatus that supplies a complementary observation so both 7 and 0 can be determined on a single instrument. 

An extensive discussion of the Langmuir balance technique and a comparison with the Wilhelmy plate method are given by MacRitchie (1990). This book also discusses modifications that are possible to the techniques and other experimental details. 

Interfaciai Tension Procedure. IFT measurements were made by the Wilhelmy plate method. The apparatus was the same as that described previously (2). A standard protocol was followed for all IFT determinations. The desired interface was formed at a specified temperature by partially filling a thermostatted sample holder with the desired aqueous phase. This phase, distilled water (mono triple) or a supernatant aqueous phase isolated from a complex coacervate system, completely covered the Wilhelmy plate (roughened platinum). The desired citrus oil was carefully layered onto the aqueous phase. It had been preheated (or cooled) to the same temperature as the aqueous phase. Once the citrus oil/aqueous phase interface was formed, the Wilhelmy plate was drawn completely through the interface and into the oil phase where it was zeroed. 

A number of the interfaces studied gave force measurements too low to detect. This corresponds to an IFT value of zero. However, the interfacial forces in such cases are not believed to be zero, but simply too low to measure by the Wilhelmy plate method. 

Because this method is not sensitive enough to reliably measure IFT values below approximately 0.2mj/m, all IFT values too low to measure by the Wilhelmy plate method are designated as "0" values. 

Figure 7 shows how temperature affects IFT of the G/A supernatant phase against lemon oil 2. All of the IFT curves shown decrease with time. The rate of aging increases as the temperature increases from 30 to 45°C, but a further temperature increase to 50 C had no added effect. The rate of IFT aging increases dramatically between 40 and 45°C. At 45 and 50 C, aging is so pronounced that IFT becomes too low to measure by the Wilhelmy plate method within 4 hrs. At 30 to 40 C, the rate of aging is reduced to such an extent that a finite IFT exists after 21 hrs. of aging. 

Figure 8 contains duplicate IFT aging curves for a G/A supernatant phase against orange oil 1 at 50°C as well as an IFT aging curve for this interface at 1.2 C. The duplicate 50 C aging curves approach linearity as the aging time increases. Both curves decay to a value too low to measure by the Wilhelmy plate method in 4.7-... 

Figure 11 compares IFT aging curves for a G/GA supernatant phase against lemon oil 1 at 40 and 45°C. In both cases, IFT decreases in a nonlinear manner to a value too low to measure by the Wilhelmy plate method. The 45°C aging curve decays to a value too low to measure in 2.1 hours, somewhat faster than the time needed for IFT of the lemon oil 1/water interface to decay to this value. 

Apparatus and Procedure. Surface Isotherms. The technique for determining the n-A and AV-A curves of the lipid films has been described (6). Briefly, the Wilhelmy plate method was used to measure surface tension, from which the surface pressure was calculated (n = 7h2o—yfiim) The surface potential was measured by means of a radioactive (226Ra) air electrode and a saturated calomel electrode connected to a high impedance model 610 B Keithley electrometer (Keithley Instruments, Cleveland, Ohio). 

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 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). 

The Wilhelmy plate method can also be used to determine interfacial tension at a liquid/liquid interface by layering a second, less dense fluid on top of the primary fluid after the proper meniscus between the plate and the primary interface has been formed. In this case, it is crucial that the second fluid be applied carefully to avoid disrupting the primary interface. In addition, a small correction is necessary to account for the buoyancy that the plate experiences due to the presence of the second fluid. 

Important techniques to measure the surface tension of liquids are the sessile drop method, the pendant or sessile bubble method, the Du-Notiy ring tensiometer, and the Wilhelmy-plate method. 

A widely used technique is the Wilhelmy plate method introduced in Section 2.4. If the contact angle is larger than zero, the force, with which the plate is pulled into the liquid, is 27pi cos 0. Here, l is the width of the plate. 

Contact angles are commonly measured by the sessile drop, the captive bubble, and the Wilhelmy plate method. To characterize the wetting properties of powders the capillary rise method is used. 

The film pressure is usually measured by the Wilhelmy plate method. Usually the Wilhelmy plate is a piece of absorbent paper hanging into the water subphase. The force acting on it is proportional to the surface tension. More rarely, the force on the barrier is determined directly. 

For the Wilhelmy plate method, a thin plate with a perimeter of about 4 cm is lowered to the surface of a liquid and the downward force directed on the plate is measured. Surface tension is the force divided by the perimeter of the plate. For this method to be valid, the liquid should completely wet the plate before the measurement, which means that the contact angle between the plate and the liquid is zero. Furthermore, the position of the plate should be correct, which means that the lower end of the plate is exactly on the same level as the surface of the liquid. Otherwise the buoyancy effect must be calculated separately. 

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 ... 

The contact angle of an electrolyte on a solid electrode changes when changing the potential. This effect was used by Morcos and Fischer, who measured the potential-dependent capillary rise of an electrolyte meniscus at the surface of a partially immersed metal plate 11551. The interfaeial energy at a solid elec-trode/solution interface can also be measured by the Wilhelmy plate method 1156,157],... 

The initial concentration (0.001 wt% active) was selected from the measurement of surface and interfacial tensions. It lies in the range of CMC value for these measurements, which were performed by using the Wilhelmy plate method in a Rosano surface tensiometer. The standard procedure described in the manual was carefully followed. Surface tension was measured against the air, whereas the interfacial tension was measured against isooctane, which was used to simulate dense CO2. It is well known fact that good foamers are most effective well above the CMC range and in subsequent tests the concentrations were increased well above that range. 

The Wilhelmy plate method may have its problems for surfactant solutions. Adsorption of these on the plate will generally affect the contact erngle, amd, hence, the extent of hysteresis. 

For some reason the method has lost most of its popularity, probably because sessile droplet methods can nowadays be carried out routinely, providing both the interfacial tension and contact angle. Simultaneously obtaining these two quantities is also feasible with a combination of the Wilhelmy plate method and that of the capillary rise at a stationary vertical plate (see sec. 5.4g). The former gives w = 2y( cos or if b and the latter gives = 2y(l - sin a] / App. see [ 1.3.16]. So we have two equations with two unknowns between which y can be eliminated and a obtained using sin a + cos a = 1. For details, especially in the presence of surfactants, see refs. 

Equilibrium surface pressure (TTg) values were calculated as TTg = ct-q — (Te, where (a-g) is the equilibrium surface tension and oq is the solvent surface tension. They were measured by the Wilhelmy plate method, using a platinum plate attached to a Sigma digital tensiometer. The range of concentrations studied were 5 X 10 to 2% wt. 

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