Liquids static surface tension measurement methods

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. 

Table 2.1 Static Surface Tension Measurement Methods for Liquids...

In summary, the advantages of this technique are that (i) it is relatively quick, easy, and inexpensive to set up, (ii) it is a static method (the interfacial area is not changing as the measurement occurs (see Section 1)), and (iii) although most commonly used for liquid/vapour surface tension measurements, it can also be used to measure liquid/liquid interfacial tensions. The disadvantages are that (i) a relatively large amount of the solution of interest is required, (ii) the results depend on a contact angle that is usually difficult to measure (and thus one usually must trust that it is equal to zero), and (iii) for improved accuracy, theoretical corrections to the ideal case are needed. 

A number of methods are available for the measurement of surface and interfacial tension of liquid systems. Surface tension of liquids is determined by static and dynamic surface tension methods. Static surface tension characterises the surface tension of the liquid in equilibrium and the commonly used measurement methods are Du Notiy ring, Wilhelmy plate, spinning drop and pendant drop. Dynamic surface tension determines the surface tension as a function of time and the bubble pressure method is the most common method used for its determination. 

The dynamic methods depend on the fact that certain vibrations of a liquid cause periodic extensions and contractions of its surface, which are resisted or assisted by the surface tension. Surface tension therefore forms an important part, or the whole, of the restoring force which is concerned in these vibrations, and may be calculated from observations of their periodicity. Dynamic methods include determination of the wave-length of ripples, of the oscillations of jets issuing from non-circular orifices, and of the oscillations of hanging drops. Dynamic methods may measure a different quantity from the static methods, in the case of solutions, as the surface is constantly being renewed in some of these methods, and may not be old enough for adsorption to have reached equilibrium. In the formation of ripples there is so little interchange of material between the surface and interior, and so little renewal of the surface, that the surface tension measured is the static tension ( 12. ... 

Surface tension measurement techniques are divided into methods for solids and liquids. There are two modes for measuring the surface tension of liquids static and dynamic. Values reported in the literature are often static surface tensions of liquids. Tables 2.1-2.3 present a brief description of the common techniques for surface tension measurement of liquid and solid materials. Some of these methods have been described in further detail. 

With the use of a modern electro-balance, very precise surface tension measurements can be obtained without the use of any theoretical corrections. This fact, along with the fact that this is a static measurement technique, makes the plate method a popular choice for precise equilibrium surface tension measurements. Note also that instrumentation which makes use of this method is readily available commercially (see Table 11.1). The disadvantages of this method are that a relatively large amount of liquid is needed. 

An almost overwhelmingly large number of different techniques for measuring dynamic and static interfacial tension at liquid interfaces is available. Since many of the commercially available instruments are fairly expensive to purchase (see Internet Resources), the appropriate selection of a suitable technique for the desired application is essential. Dukhin et al. (1995) provides a comprehensive overview of currently available measurement methods (also see Table D3.6.1). An important aspect to consider is the time range over which the adsorption kinetics of surface-active substances can be measured (Fig. D3.6.5). For applications in which small surfactant molecules are primarily used, the maximum bubble pressure (MBP) method is ideally suited, since it is the only... 

Provides measuring techniques of contact angle, surface tension, interfacial tension, and bubble pressure. Suitable methods for both static and dynamic inteifacial tension of liquids include du Nous ring, Wilhelmy plate, spinning drop, pendant drop, bubble pressure, and drop volume techniques. Methods for solids include sessile drop, dynamic Wilhelmy, single fiber, and powder contact angle techniques. 

Viscosity and density of the component phases can be measured with confidence by conventional methods, as can the interfacial tension between a pure liquid and a gas. The interfacial tension of a system involving a solution or micellar dispersion becomes less satisfactory, because the interfacial free energy depends on the concentration of solute at the interface. Dynamic methods and even some of the so-called static methods involve the creation of new surfaces. Since the establishment of equilibrium between this surface and the solute in the body of the solution requires a finite amount of time, the value measured will be in error if the measurement is made more rapidly than the solute can diffuse to the fresh surface. Eckenfelder and Barnhart (Am. Inst. Chem. Engrs., 42d national meeting, Repr. 30, Atlanta, 1960) found that measurements of the surface tension of sodium lauryl sulfate solutions by maximum bubble pressure were higher than those by DuNuoy tensiometer by 40 to 90 percent, the larger factor corresponding to a concentration of about 100 ppm, and the smaller to a concentration of 2500 ppm of sulfate. 

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 various dynamic methods give the surface tension of more or less recently formed surfaces, and may yield results different from the static methods, if adsorption occurs, and is incomplete at the moment when the tension is actually measured. One factor in dynamic measurements, which cannot be satisfactorily measured at present, is the time which has elapsed between the formation of the surface from the homogeneous interior liquid, and the actual measurement of the surface tension. If this could be varied, and measured with an accuracy of say 10 4 second, a valuable new weapon would be available for investigating the progress of adsorption. Bohr s work on oscillating jets is probably the best on any dynamic method. 

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... 

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. 

Surface tension of liquids can be measured by either of the two methods static and dynamic. The static methods are based on the assumption that the liquid has attained surface equilibrium. For pure liquids and solutions of crystalloids the process of attainment of equilibrium is very fast and the static methods are best suitable. But for colloidal solutions a considerable time is required to reach the equilibrium state and therefore the dynamic methods of measuring surfacf tension are preferred. The dynanJc methods measure the tension of a liquid before the surface film has had time to form. TTiere are other methods too which fall between the static and the dynamic methods. Among the static methods, the most commonly used ones are (0 the capillary rise method, (ip the du Nouy ring method, (Up the Wilhelmy balance method, and (iv) the drop-weight method.,... 

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. 

The advantages of this method include the following (i) it is relatively easy and inexpensive to set up (and commercial systems are available), thanks to the advent of inexpensive electronic pressure transducers (ii) it can be used in extreme environments, such as to measure the surface tension of molten metals, where it would be difficult to image the bubble interface, needed for use in the shape methods and where it would also be difficult to obtain a plate, ring or capillary tube made of suitable materials (iii) the results do not depend on a difficult-to-measure contact angle (iv) a relatively small amount of material is needed (v) in theory, this technique can be applied to liquid/liquid interfaces. The main disadvantage to this technique is that it is not truly a static measurement, as discussed above in Section 1. 

Whether we assume, however, the existence of a fixed angle of contact between a given liquid and a given solid, or derive it from a static molecular model, as above, we can proceed at once to explain all the common capillary phenomena. Tlie three ideas of tension at a surface, internal pressure, and angle of contact suffice with these concepts and the key expression (1.14) for the internal pressure inside a curved surface we can solve all common equilibrium capillary problems by the methods of dassical statics. This is not a field we wish to explore in detail but show here only how the rise in a capillary tube is treated, since this forms the basis of the commonest method of measuring surface tension. 

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