Measuring Surface Tension

There is an excess energy at a surface, measurable as a surface tension(8.). This causes the surface layer of a curing polymer to react farther and faster than the bulk. In the case of polyimide, it also promotes some surface layer structural rearrangement (-8%) to isoimide( ), causing changes in both mechanical(10) and electrical ( ll I properties. Further, a recent IR study (12) demonstrated that the polyimide structure obtained on curing depended on the cure schedule. 

When a vertical foam film is illuminated, part of the light is scattered by thermal fluctuational microwaves in the film surface. Measuring the intensity of the scattered light makes possible the calculation of the film tension and the energy of molecular interactions in the film [89,90]. 

The quantity 11/ is a measure of the so-called disjoining action , introduced by Derjaguin in 1936 [12]. The disjoining pressure n [8] is determined by the long-range interaction forces between the surfaces of the film (normal to the both surfaces of tension there) and tends to zero when the film thickness is sufficiently large [5]. Eq. (3.15) proposes a more general definition of IT than that for the equilibrium case (Eq. (3.10))... 

Solutions of Electrolytes. In solutions of certain electrolytes, among them NaCl and KCl, we find that the surface tension increases with concentration, indicating a negative surface concentration. This is a result of interionic electrostatic attraction, which tends to make the ions draw together and away from the surface. Measurements have shown that in dilute solution the surface tension increases linearly with concentration thus we see from Eq. (13) that the surface concentration must be directly proportional to the bulk concentration. In practice the surface tension often drops initially at very low concentrations and then rises linearly this effect is small and will not influence the present results. 

It is also clear from Equation (5.2) that surface or interfacial tension - that is, the force per unit length tangential to the surface, measured in units of miUinewtons per metre - is dimensionally equivalent to an energy per unit area measured in millijoules per square metre. Eor this reason, it has been stated that the excess surface free energy is identical to the surface tension, but this is tme only for a single-component system - that is, a pure liquid (where the total adsorption is zero). 

Some workers have interpreted the emulsification of fountain solution in an ink from the point of view of surface energetics and colloidal behavior. Surface measurements in the form of contact angles, spreading coefficients, interfacial tensions and surface tensions have been widely used to explain the interactive behavior of inks and fountain solutions. 

Surface Tension - The surface tension is the cohesive force at a liquid surface measured as a force per unit length along the surface or the work which must be done to extend the area of a surface by a unit area, e.g., by a square centimeter. Also called free surface energy. 

In this chapter, we will discuss how the chemical and physical properties of substances at interfaces differ from those in the bulk. For quantitative description, quantities like surface tension and surface energy have to be introduced. With the help of these quantities, phenomena known from everyday life like the lotus effect can be explained. However, perhaps you are more interested to learn how detergents clean Then have a look at Sect. 16.3 which deals with the adsorption on liquid surfaces. The next section covers the adsorption on solid surfaces and the variation of the extent of coverage with pressure or concentration of the substance to be adsorbed. Langmuir s isotherm, the simplest description of such an adsorptiOTi process, is deduced by kinetic interpretation of the adsorption equilibrium. Alternatively, it can be derived by introducing the chemical potential of free and occupied sites and cmisideiing the equilibrium condition. In the last part of the chapter, some important applications such as surface measurement and adsorption chromatography are discussed. 

Techniques for Measuring Contact Angle and Surface/Interfacial Tension... 

A solid is defined as a material that is rigid and resists stress. A solid surface may be characterized by its surface free energy and surface energy. The surface energy (tension) of a solid caimot be measured in a similar manner to that of a liquid, due to the difficulty caused by the reversible formation of its surface. The methods for the determination of surface energy of solids are described in this chapter. 

There are many methods available for the measurement of surface and interfacial tensions. Details of these experimental techniques and their limitations are available in several good reviews [101-104]. Table 5 shows some of the methods that are used in petroleum recovery process research. A particular requirement of reservoir oil recovery process research is that measurements be made under actual reservoir conditions of temperature and pressure. The pendant and sessile drop methods are the most commonly nsed where high temperatur pressure conditions are required. Examples are discussed by McCaffery [i05] and DePhUippis et al. [J06]. These standard techniques can be difficult to apply to the measurement of extremely low interfacial tensions (< 1 to 10 mN/m). For ultra-low tensions two approaches are being used. For moderate temperatures and low pressures the most common method is that of the spinning drop, especially for microemulsion research [107], For elevated temperatures and pressures a captive drop method has been developed by Schramm et al. [JOS], which can measure tensions as low as 0.001 mN/m at up to 200 °C and 10,000 psi. In aU surface and interfacial tension work it should be appreciated that when solutions, rather than pure liquids, are involved appreciable changes can occur with time at the surfaces and interfaces, so that techniques capable of dynamic measurements tend to be the most useful. 

The interplay between these various factors is complex and often requires experimental measurement under as realistic conditions as possible to appropriately determine the impact of surfactant on wettability. It is the migration to, and the adsorption of, the surfactant at the fluid and solid interfaces along with the orientation and density of the adsorbed surfactant molecules that modifies the fluid-surface interfacial tension/ wettability. Surfactant adsorption at an interface is a necessary, but not a sufficient condition for wettability alteration. Although details of adsorption will be covered in Chapter 4, this section includes a brief treatise on it with the other known variables that can affect wettability modification with surfactants. 

The Wilhelmy Method This is a technique that can be used for both surface/interfacial tension and contact angle measurements (Fig. 7b). To measure the surface tension, a plate with known perimeter P is attached to a balance. 

For pure liquids with a constant surface tension, the measured tension depends on the distance from the orifice, and is higher than the static value. Since a dynamic surface tension for pure liquids does not really make sense( ), this is certainly an artifact. It is only at longer distances from the orifice that the surface tension becomes equal to the static value, and therefore the shorter adsorption times must be discarded and cannot be used for adsorption dynamics studies of solutions containing surfactants. 

We recall the lUPAC definition The mechanic properties of the interfacial layer between two fluids, including the equilibrium shape of the surface, may be calculated by applying the standard mathematical techniques of mechanics to the forces associated with the surface of tension. The resulting equations— which comprise the subject of capillarity— form the basis of experimental methods of measuring surface tension. lUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units, App. II, Part I,... 

The firat is a generalization of l.aplace s equation for a surface tension tr measured at, or referred to, an arbitrary dividing surface. Since Ap is invariant with respect to the choice of R, it follows that, in general, a must formally be a function of R, which we write a[li]. Moreover if we define the surface of tension as the radius at which the tension acts, in conformity with the model described in the opening paragraph of 2.2, then a comparison of (2.1) with (2.57) shows that the second term vanishes at this surface. That is,... 

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