Capillary plate method

From the beginning of the 1980s, some effective experimental approaches based on new principles have been invented for the study of interfacial reactions in solvent extraction chemistry. Recently, some methods were developed from our laboratory, the highspeed stirring (HSS) method [4,5], the two-phase stopped flow method [6], the capillary plate method [7], the reflection spectrometry [8], and the centrifugal liquid membrane (CLM) method [9]. 

With some acids (e.g., succinic acid and sulplianilic acid) more satisfactory results are obtained by reversing the order of mixing, i.e., by adding the solution of the so um salt of the acid to the reagent. It should be pointed out that the melting points of the derivatives as determined on the electric hot plate (Fig. II, 11, 1) may differ by 2-3° from those obtained by the capillary tube method. In view of the proximity of the melting points of the derivatives of many acids, the mixed m.p. test (Section 1,17) should be applied. 

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. 

It is important to note that a number of different techniques are based on the Wilhelmy method. Here, we will describe only the capillary rise method (based on the Wilhelmy gravimetric plate technique) presented by Lim and Wang [101] and Wang [192]. For more detail on the other Wilhelmy methods, please refer to Mathias et al. [9]. 

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. 

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. 

There are static and dynamic methods. The static methods measure the tension of practically stationary surfaces which have been formed for an appreciable time, and depend on one of two principles. The most accurate depend on the pressure difference set up on the two sides of a curved surface possessing surface tension (Chap. I, 10), and are often only devices for the determination of hydrostatic pressure at a prescribed curvature of the liquid these include the capillary height method, with its numerous variants, the maximum bubble pressure method, the drop-weight method, and the method of sessile drops. The second principle, less accurate, but very often convenient because of its rapidity, is the formation of a film of the liquid and its extension by means of a support caused to adhere to the liquid temporarily methods in this class include the detachment of a ring or plate from the surface of any liquid, and the measurement of the tension of soap solutions by extending a film. 

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 experimental methods for the determination of liquid viscosity are similar to those used for gases ( 8.VII F) (i) transpiration, through capillaries, (ii) torque on rotating cylinders, or the damping of oscillating solid discs or spheres, in the liquid, (iii) fall of solid spheres through the liquid, (iv) flow of liquid through an aperture in a plate, (v) capillary waves. Methods (i) and (ii) are mostly used for absolute, the others for comparative, measurements. 

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. 

The detachment of a ring or a plate (a Wilhelmy plate) from the surface of a liquid or solution is a static surface tension measurement method, which gives the detachment force of a film of the liquid and its extension from the liquid surface. These methods are less accurate than the capillary rise method, but they are normally employed in most surface laboratories because of their ease and rapidity. 

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... 

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. 

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)... 

Liquids have a surface tension with respect to the gaseous phase and an interfacial tension with respect to other liquids. Such interfacial and surface tensions may be measured by a whole series of different methods in the case of low-molar-mass liquids. But polymer liquids have very high viscosities, and so, only a few methods of measurement are suitable. The capillary method and the wire ring method are not suitable since the measured surface tension depends on the speed at which the test is carried out. All of what are known as static methods are suitable, i.e., the suspended drop method and the Wilhelmy plate method. 

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. 

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