Surface force Wilhelmy plate method

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. 

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 ... 

There are two modifications to the Wilhelmy plate method. In the first modification, the cup carrying the liquid is mobile and is lowered until the previously immersed plate becomes detached from the liquid surface, and the maximum vertical pull, / max on the balance is noted, similarly to the ring method. Then the capillary force, for the zero contact angle, can be given as... 

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). 

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-plate method consists in partially immersing a plate in a wetting liquid. The surface tension of the liquid 7/v acts downward on the plate. When the plate is wetted and its lower edge just resides on the liquid surface, the force acting on the plate is 7/v /per, where /per is the perimeter of the plate. By measuring the restraining force on the plate in air and in contact with... 

Figure 9 A graph of force against immersion depth for a solid plate in a liquid of surface tension %. This is known as the Wilhelmy plate method.

The surface tension was calculated from the maximum force measured by the Wilhelmy plate method using a 2.0-cm-wide and 0.2-mm-thick platinum plate. The purification of the solutions and the dilution were made in a cylindrical cell of 8-cm diameter. The measurements were performed at 25.00 0.05 °C. The concentrations of the solutions were determined from electrical conductivity measurements by means of a calibration curve. Five parallel measurements were made at each concentration and between the consecutive measurements the solution surface was sucked off to form a fresh surface and to eliminate the possible effect of contamination from the environment. The measurements were performed 1-2 min after the fresh surface had formed. Time dependence was not observed in the concentration range of the decyl sulfate solutions investigated. The reproducibility of the maximum force was 2x10 N (which corresponds to 0.05 mN/m surface tension). 

The methods so far discussed have required more or less tabular solutions, or else correction factors to the respective ideal equations. Further, if one needs to make continuous measurements, then it is not easy to use some of these methods (such as capillary rise or bubble method). The most useful method of measuring the surface tension is by the well-known Wilhelmy plate method. If a smooth and flat plate-shaped metal is dipped in a liquid, the surface tension forces will be found to give rise to a tangential force. Figure 1.20. This is because a new contact phase is created between the plate and the liquid. 

A stand-alone static method is the popular Wilhelmy plate method (Figure 1.18). In this method, a completely wetted platinum plate is brought into contact with a liquid surface, and a pull force is applied to the plate. Equilibrium is achieved when that force, corrected by the buoyancy force acting on the immersed part of the plate, is balanced by the surface tension, that is, F + dbHpg = 2(d + b)a. The force F is measured with a sensitive dynamometer, which typically forms the core of modem surface tension meters. 

Figure 3. Illustration of critical micelle concentration (CMC) measurement results using the modified Wilhelmy plate method. CMC is obtained as the intercept of the linear portion of two slopes relating wetting force (or surface tension) to analyte concentration.

The surface tension measurement techniques can be divided into the following three categories (i) Force Methods, which include the truly static methods of the capillary rise and Wilhelmy plate methods, as well as the dynamic detachment methods of the Du Nouy ring and drop weight, (ii) Shape Methods, which include the pendant or sessile drop or bubble, as well as the spinning drop methods, and (iii) Pressure Methods, which are represented by the maximum bubble pressure method. These techniques are summarized in the following sections of this chapter. 

The Wilhelmy plate method (Ludwig Ferdinand Wilhelmy [1812-1864]) can be used to measure surface or interfacial tension at an air-liquid or liquid-liquid interface. In this method, a thin plate is oriented perpendicular to the interface, and the force exerted on it is measured. The plate is often made from glass or platinum which may be roughened to ensure complete wetting. The plate is cleaned thoroughly and attached to a scale or balance via a thin metal wire. The force on the plate due to wetting is measured via a tensiometer or microbalance and used to calculate the surface tension (y) using the Wilhelmy equation ... 

The most appropriate method for determining the wettability of fibres is to use the tensiometric or Wilhelmy plate method. If a thin fibre is suspended below the pan of an electromicrobalance the force Fq acting on the fibre when it just touches the surface of a liquid (Figure 3a) is given by... 

The Wilhelmy plate method [323,324], the sessile drop method [328,340], and the capillary height method [325-328] measure equilibrium surface tension, if sufficient time is allowed for the adsorption of surfactant molecules at the surface to attain the state of equilibrium. The Wilhelmy plate method measures the force exerted on a vertical plate partially immersed in the liquid (Fig. 9.20). If wetting of the plate is complete, the force, F, is proportional to the surface tension, y, and the circumference, L. of the plate ... 

The du Nouy ring and Wilhelmy plate methods (Figure 2.4) are the two most frequently used techniques for measuring surface tension at a liquid air interface or interfacial tension at a liquid—liquid interface. Only the du Nouy method can be applied to measure interfacial tension. Both of these techniques are based on pulling an object with a well-defined geometry of the surface of liquids and measuring the puU force. These techniques are also known as pull-force methods. In the Wilhelmy method, a plate is the puU object, while in the du Nouy technique, a ring is used. These techniques are named after the two scientists who conducted some of the early research in the field of surface tension measurement. In 1863, Wilhelmy [16] described the measurement of capillary constants without a detailed calculation of... 

The total surface tension of liquids (7 ) is most easily determined by the Wilhelmy plate method (Adamson, 1990). Briefly, a thin rectangular plate (e.g., a chromic acid cleaned glass microscope cover slip, or an acid cleaned platinum plate) is suspended vertically from the arm of a microbalance, until it just touches the hanging plate. Upon contact with the liquid, a small additional force, or additional weight (AW), is exerted on the plate, so that (Adamson, 1990) ... 

Before equations such as Eqs. 6, 7 and 8 can be used, values for the surface energies have to be obtained. While surface energies of liquids may be measured relatively easily by methods such as the du Nouy ring and Wilhelmy plate, those of solids present more problems. Three approaches will be briefly described. Two involve probing the solid surface with a liquid or a gas, the third relies on very sensitive measurement of the force required to separate two surfaces of defined geometry. All involve applying judicious assumptions to the experimental results. 

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,... 

Another common method used to measure the surface tension of liquids is called the Wilhelmy plate . These methods use the force (or... 

Devise a method to accurately use two Wilhelmy plates (see Fig. 6) to measure the surface tension of a Langmuir film. Do not forget about the gravitational and buoyancy forces. Such a device is called a Wilhelmy balance. 

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. 

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