Liquid surface energy definition

There exists as we have noted a separate phase at the interface between a liquid and a gas. The magnitude of the vapour-liquid interfacial energy is markedly dependent on the composition of the liquid and although experimental data are somewhat scanty, the surface energy is also affected by the nature of the gas in contact with it. It is to be anticipated that at the interface between two immiscible liquids a similar new interfacial phase will come into existence possessing a definite surface energy dependent on the composition of the two homogeneous liquid phases. 

Consider now that some adsorption of liquid vapours on the solid surface occurs (Figure 1,34.b), leading to a reduction of its surface energy by a quantity Aliquid surface). For the sake of clarity, we denote by cr< v and W the solid surface energy and the work of adhesion in the absence of adsorption, while pure solid/pure liquid/vapour system held at constant temperature, the solid surface is in equilibrium with a saturated vapour of the liquid at a partial pressure of Psat, the equilibrium values of [Pg.45]

In this introduction it may be useful to give a brief definition of surface tension and surface free energy. The dimension of the surface tension is related to unit length. Lenard s classical experiment (1924) is one of the best demonstrations of the surface force of a liquid acting on an extended wire in contact with a liquid surface. By carefully lifting the wire from the level of the surface, a force can be measured for as long as the pendent lamella remains in contact with the liquid bulk. The force measured in this way, divided by the length of the wire, leads to a well-... 

Interfadal tension between two fluid phases is a definite and accurately measurable property depending on the properties of both phases. Also, the contact angle, depending now on the properties of the three phases, is an accurately measurable property. Experimental approaches are described, e.g., in Refs. 8,60, and 63 and in Ref. 62, where especially detailed discussion of the Wilhehny technique is presented. Theories such as harmonic mean theory, geometric mean theory, and acid base theory (reviewed, e.g., in Refs. 8, 20, and 64) allow calculation of the soHd surface energy (because it is difficult to directly measure) from the contact angle measurements with selected test liquids with known surface tension values. These theories require introduction of polar and dispersive components of the surface free energy. 

Equation (13) shows that, at constant temperature (in the case of an isotherm), n is equal to the Iree energy of the surface (J m ) covered by a definite amount of the adsorbed layer. Despite this fact there are some experts and scientists who regard n as the tangential pressure or tangential tension of the adsorbed layer. This statement is valid for totally mobile layers only, which may be the layers adsorbed on liquid surfaces. However, on solid surfaces, especially on oxide surfaces, where the attractive interactions between the adsorbent and adsorptive molecules are great, the concept of a totally mobile layer is unacceptable and the only exact definition of H is Eq. (13). 

By measuring the contact angle between a solid surface and a free-standing liquid drop, the relative energetics between the two can be evaluated. For example, if a drop of water beads up on a substrate, then the surface likely has nonpolar or hydrophobic characteristics. Conversely, if water wets the substrate, then the surface is more likely polar or hydrophilic. Figure 2.8-1 illustrates the definition of the contact angle (0) and the relevant surface energy values. 

In Chapter III, surface free energy and surface stress were treated as equivalent, and both were discussed in terms of the energy to form unit additional surface. It is now desirable to consider an independent, more mechanical definition of surface stress. If a surface is cut by a plane normal to it, then, in order that the atoms on either side of the cut remain in equilibrium, it will be necessary to apply some external force to them. The total such force per unit length is the surface stress, and half the sum of the two surface stresses along mutually perpendicular cuts is equal to the surface tension. (Similarly, one-third of the sum of the three principal stresses in the body of a liquid is equal to its hydrostatic pressure.) In the case of a liquid or isotropic solid the two surface stresses are equal, but for a nonisotropic solid or crystal, this will not be true. In such a case the partial surface stresses or stretching tensions may be denoted as Ti and T2-... 

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