Surface tension Wilhelmy plate method

Figure 2 shows the equilibrium surface tensions (Wilhelmy plate method) of aqueous solutions (0.1 M U2SO4, pH 2) of surfactants I-IV in their reduced and oxidized states [2,4]. Below we discuss features of these plots that demonstrate the existence of three different mechanisms that lead to changes in surface tension upon oxidation of ferrocene to ferrocenium. 

Dynamic surface tension has also been measured by quasielastic light scattering (QELS) from interfacial capillary waves [30]. It was shown that QELS gives the same result for the surface tension as the traditional Wilhelmy plate method down to the molecular area of 70 A. QELS has recently utilized in the study of adsorption dynamics of phospholipids on water-1,2-DCE, water-nitrobenzene and water-tetrachloromethane interfaces [31]. This technique is still in its infancy in liquid-liquid systems and its true power is to be shown in the near future. 

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 ... 

If one considers a system consisting of water (with or without added electrolyte) + oil + surfactant (with or without a cosurfactant) at equilibrium, there will most likely be present more than two phases (due to the formation of emulsion or microemulsion). The determination of the interfacial tension, Yij> between the two liquid phases is, therefore, of much importance, in order to understand the forces which stabilize these emulsions or microemulsions. The interfacial tension can be measured by using a variety of methods, as described in detail in surface chemistry text-books (1-3). If the magnitude of yij is of the order of few mN/m (=dyne/ cm), then the methods generally used are Wilhelmy plate method or the drop volume (or weight) method (1-4). However, in certain systems ultra-low (or low) interfacial tensions have been reported. Since these low values are reported to be essential in order to mo-... 

The most useful method of measuring surface tension is by the well-known Wilhelmy plate method. If a plate-shaped metal is dipped in a liquid, the surface tension forces will be found to produce a tangential force (Figure 2.13). This is because a new contact phase is created between the plate and the liquid. 

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

The surface tension measurement was done at 25 C by a modified Wilhelmy plate method(Shimadzu ST-1). 

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. 

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. 

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... 

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. 

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. 

The independent measurements of surface tension were obtained by the tedious Wilhelmy plate method. Figure 3 illustrates such a calibration curve for one set of orifices and for five types of test fluids (methanol-water, ethanol-water, acetone-water, sodium lauryl sulfate in water saturated with methyl methacrylate, and polymethylmethacrylate latices). This is a "universal" calibration curve independent of the fluid being monitored. For the 63 data points shown in Figure 3, the least squares regression line is given by... 

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 ... 

Fig. 3.31. Equilibrium surface tension <7 vs. copolymer concentration C, (Wilhelmy plate method,...

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. 

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. 

Measurement of the surface or interfacial tension of liquid systems is accomplished readily by a number of methods of which the most useful and precise for solutions of surfactants are probably the drop-weight and Wilhelmy plate methods. An excellent discussion of the various methods for determining surface and interfacial tension is included in the monograph on emulsions by Becher (1965). 

Ue = 70 — 7 where 70 is the surface tension of water, and 7 is the surface tension of the saturated lipid solution. Ue values recorded by the Wilhelmy plate method 23), precision is 0.1 dynes/cm. 

Figure 6.5 Liquid surface tension determination by the Wilhelmy plate method. A rectangular plate of length, /, width, w, and thickness, d, of material having a density of ps, is immersed to a depth of h in a liquid of density pL 6 is the contact angle forming between the liquid and the plate.

Later, Neumann developed the static Wilhelmy plate method which depends on capillary rise on a vertical wall, to measure 6 precisely. A Wilhelmy plate whose surface is coated with the solid substrate is partially immersed in the testing liquid, and the height of the meniscus due to the capillary rise at the wall of the vertical plate is measured precisely by means of a traveling microscope or cathetometer. If the surface tension or the capillary constant of the testing liquid is known, then the contact angle is calculated from the equation, which is derived from the Young-Laplace equation... 

The static methods are based on studies of stable equilibrium spontaneously reached by the system. These techniques yield truly equilibrium values of the surface tension, essential for the investigation of properties of solutions. Examples of the static methods include the capillary rise method, the pendant and sessile drop (or bubble) methods, the spinning (rotating) drop method, and the Wilhelmy plate method. 

The force F required to detach a well-wetted thin ring of radius rr, from the liquid surface is measured in the duNotiy ring detachment method [6,26]. Within the first approximation one can assume that the equation relating the surface tension, a, to the detachment force, F, is analogous to that used in the Wilhelmy plate method, with the exception that the perimeter of the ring is used in place of the plate width, i.e. F = 4nrro. In reality, however, the curvature of the liquid surface at points of contact with a ring causes the surface tension vectors to be somewhat off the vertical (Fig. 1-19). 

Dynamic surface tensions of an aqueous l.5510" mol/cm Triton X-100 solution measured with the dynamic capillary (0), inclined plate (A,A), drop volume ( ), strip ( ) and Wilhelmy plate ( ) methods according to Rillaerts Joos (I9S2)... 

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. 

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