Solid surface energy immiscible liquids

For a compound to be qualified as a surfactant, it should also exhibit surface activity. It means that when the compound is added to a liquid at low concentration, it should be able to adsorb on the surface or interface of the system and reduce the surface or interfacial excess free energy. The surface is a boundary between air and liquid and the interface is a boundary between two immiscible phases (liquid-liquid, liquid-solid and solid-solid). Surface activity is achieved when the number of carbon atoms in the hydrophobic tail is higher than 8 [3]. Surfactant activities are at a maximum if the carbon atoms are between 10 and 18 at which level a surfactant has good but limited solubility in water. If the carbon number is less than 8 or more than 18, surfactant properties become minimal. Below 8, a surfactant is very soluble and above 18, it is insoluble. Thus, the solubility and practical surfactant properties are somewhat related [1]. 

Energy requirements for stable spreading. When a drop of a liquid is placed on the surface either of another immiscible liquid, or of a solid, it may spread to a film, or may remain as a drop without spreading. The surface tensions of the two liquids, and the interfacial tension between them, determine whether or no the liquid spreads and the same holds if the lower phase is a solid. 

Consider a system consisting of a solid with a cross-section of unit area and a liquid of a different species, immiscible with the solid, held at constant temperature. Both the solid and liquid surfaces are assumed free of any adsorbed species. When the solid surface is lowered towards the liquid until contact is established (Figure 1.34.a), the change of interfacial free energy of the system, or the work gained, is given by ... 

In the case of high surface energy solids, virtually all liquids spread spontaneously on the surface. Therefore, a two phase liquid method was used (4,2)-In this method, a drop of a polar liquid, L, is placed on the solid surface, S, the surrounding medium being a non-polar hydrocarbon H, immiscible with the contact liquid (Figure 1). 

Surface tensions arise from the unbalance of molecular attractive (and repulsive) forces that result at an interface from the different nature of the materials that come in contact. Surface tensions are more readily observed in liquids—between immiscible liquids or between a liquid and a gas. Surface tensions are also present on the surface of solids, but the rigidity of the solid structure prevents their manifestation in an observable way, except in combination with the surface of a liquid in contact with the solid surface. Thus, while the surface tension and the surface free energy in a liquid are the same, this is not the case on solid surfaces. 

Spreading The tendency of a liquid to flow and form a film coating an interface, usually a solid or immiscible liquid surface, in an attempt to minimize interfacial free energy. Such a liquid forms a zero contact angle as measured through itself. 

When three different phases make contact with each other, where three surfaces intersect at a triple point, we obtain three contact angles and three interfacial tension values, as can be seen in Figure 3.7. We can obtain contact angle equilibria when we place an immiscible drop on a liquid or solid in air or a vapor phase there are many applications of contact angle measurement in industry and surface science (see Chapter 9). In these conditions, the total excess surface internal energy can be written from Equation (201) so that... 

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