Wetting of solid surfaces

Just as at fluid interfaces, the spreading tendency of a liquid at a solid/gas interface is given by Equation 7.4. If a is the solid, P is the gas and y is the liquid, the spreading coefficient 5 defined in Equation 7.4 is now written as 

The subscripts S, L and V correspond to solid, liquid and vapour respectively. This is a classical result which is valid on a macroscopic scale. On a microscopic scale various other factors arise in the region of the junction of the three phases. This microscopic behaviour is treated by de Gennes [26] but will not be examined further here. 

There is a hysteresis effect associated with the wetting process which arises, at least in part, from the presence of defects and impurities in the surface under study. Thus advancing and retracting contact angles are slightly different. It is conventional to use the advancing contact angle in the Zisman plot. 

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The wetting of solid surfaces is very apparent when considering the difference between Teflon and metal surfaces. To understand the degree of wetting between the liquid, L, and the solid, S, it is convenient to rewrite Equation 5.3 as follows ... 

Zettlemoyer, A.C., 1965. Immersional wetting of solid surfaces. Industrial and Engng. Chemistry, 57 27-36. 

The wetting of solid surfaces by liquids has been studied for many years, both theoretically and experimentally. Useful reviews of this work include those by de Gennes [1], Dietrich [2], Adamson (Chapters 10, 13, and 16 in [3]), and Sullivan and Telo de Gama [4]. Before beginning a discussion of this phenomenon in the case of vapors condensed onto solid carbons, it is helpful to summarize some general features of wetting. Thus, the characteristics of the wetting of solids by liquids will be briefly described in this introductory section, to be followed by a discussion of some of the experimental and simulation studies of vapors on carbon surfaces. 

According to Rehbinder, the ratio of the heats of wetting of solid surfaces with water (Hf and hydrocarbon (H0f represents a criterion for the hydrophilicity of the surface for hydrophilic surfaces p = Hw/Hoii > 1, while for hydrophobic ones p < 1. For instance, for activated carbon (hydrophobic surface) p 0.4, for quartz (hydrophilic surface) p = 2, for starch (strongly hydrophilic surface) p 20. In the case of contact with either water or hydrocarbon, the heat of wetting can be expressed per gram of powder (adsorbent), and hence one does not need to measure the surface area of the powder studied. 

The lowering of the interfacial tension due to surfactant adsorption allows one to finely control the wetting of solid surfaces by liquids. Let us now demonstrate how such controlled wetting is done by discussing the most important and typical use of organic surfactants for the hydrophilization and hydrophobization of surfaces. 

In the decade after the war, Zisman and coworkers at the U.S. Naval Research Laboratory investigated the surface chemistry of the spreading of oils and other liquids on solids and by 1958 had published an essentially complete picture of the various mechanisms involved. In another paper Zisman [29] has reviewed our present knowledge of the molecular mechanisms and theory relating to the wetting of solid surfaces. 

The processes discussed in this chapter demonstrate the great variety of phase equilibrium that can arise beyond the basic vapor-liquid problems discussed in most of the previous chapters. Many other systems could be included The adsorption of gases onto solids (used in the removal of pollutants from air), the distribution of detergents in water/oil systems, the wetting of solid surface by a liquid, the formation of an electrochemical cell when two metals make contact are all examples of multiphase/multicomponent equilibrium. They all share one important common element their equilibrium state is determined by the requirement that the chemical potential of any species must be the same in any phase where the species can be found. These problems are beyond the scope of this book. The important point is this The mathematical development of equilibrium (Chapter 10) is extremely powerful and encompasses any system whose behavior is dominated by equilibrium. 

Cahn, J. W. (1982). Transitions and Phase Equilibria Among Grain Boimdaty Structures. Proceedings of Conference on the Structure of Grain Boundaries, Caen, France. Journal de Physique, Vol. 43, pp. C6199-213, ISSN 0449-1947 Chernov, A. A. Mikheev, L. V. (1988). Wetting of Solid Surfaces by a Structured Simple Liquid Effects of Fluctuations. Physical Review Letters, Vol. 60, No 24, p. 2488-2491, ISSN 0031-9007... 

Explain briefly the Zisman plot and how it can be used for estimating the wetting of solid surfaces. 

Many interesting aspects are associated with the static and dynamic properties of liquid surfaces and interfaces and with the wetting of solid surfaces. 

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