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...

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 time required to conduct an interfacial tension experiment depends largely on the properties of the surfactants and less on the chosen measurement method. A notable exception is the drop volume technique, which, due to the measurement principle, requires substantial ly more time than the drop shape analysis method. Regardless of the method used, 1 day or more may be required to accurately determine, e.g., the adsorption isotherm (unit D3.s) of a protein. This is because, at low protein concentrations, it can take several hours to reach full equilibrium between proteins in the bulk phase and those at the surface due to structural rearrangement processes. This is especially important for static interfacial tension measurements (see Basic Protocol 1 and Alternate Protocols 1 and 2). If the interfacial tension is measured before the exchange of molecules... 

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. ... 

As compared to static methods, the semi-static methods for surface tension measurement are based on achieving a metastable equilibrium, and focused mainly on investigating the conditions under which the system loses that equilibrium. 

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. 

Measurements of static surface tension (SST) and DST using the Wilhelmy plate and overflowing circular weir methods have been described in detail in a previous publication [4]. The set of conditions specified for DST measurements yields a surface age of 0.1 + 0.05 s with low-viscosity solutions. 

After having gained some experimental experience, Heyrovsky simplified KuCera s method by measuring the drop-time of several drops under constant mercury pressure instead of collecting and weighing each time 80 drops of 2-second duration. However, even after 3 years of tedious work, he could not reconcile the results of the dynamic surface tension measurements with those of the static method. 

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. 

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. 

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. 

The ring method enables the static surface tension of the dispersion to be determined. When polymer dispersions are applied on large-scale coating machines, it is also important how fast the surface tension of a freshly generated surface is able to decrease. A device which permits this dynamic surface tension to be measured is the maximum bubble pressure tensiometer [5]. In this method, gas bubbles are blown... 

What are some of the dynamic methods for measuring surface tension What are the differences between these and the static methods ... 

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. 

The many methods available for the measurement of surface and interfacial tensions can be classified as static, detachment and dynamic, the last of these being used to study relatively short time effects. Static methods usually offer a greater potential for accurate measurement than detachment methods (especially when solutions of surface-active agents are involved)43, but detachment methods tend to be the more convenient to operate. With careful experimentation and exclusion of contaminants (especially surfactants), it is usually possible to measure surface tensions to an accuracy of 0.01 to 0.1 mN m-1. It is unwise to use water which has been in contact with ion-exchange resins. 

The plate method can also be used as a static method (Figure 4.5b) for measuring changes in surface tension (see page 99). The change in the force required to maintain the plate at constant immersion as the surface tension alters is measured. 

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. 

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