Thermodynamics of Surface Phenomena

This section is based on concepts developed by Josiah Willard Gibbs in 1874-1878 [see also Guggenheim (1933) and Delahay (1965)]. 

Surface excesses are usually referred to the unit surface area of the dividing plane S (surface excess densities). 

The number of moles of component j in the real interphase is given by n, = 

We thus obtain an equation for the surface excess, F = substance per unit surface area  

FIGURE 10.4 Coordinates of interphase boundaries and Gibbs surface. 

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Special reference electrodes are introduced when considering the thermodynamics of surface phenomena on electrodes [5 5], That is, when we use a virtual reference electrode potential, which is referred to the electrode reversible with respect to cation or anion in the same solution, changes by a value of (Rr/F)ln a (a is a mean activity coefficient), then the thermodynamic relationships can be generalized for a solution of an arbitrary concentration without introducing any notions on the activity of a separate ion (Sect. 5). 

For thermodynamic treatmeat of surface phenomena, the thickness of the bouadary regioas can often be ignored or their effect eliminated by selection of a convenient location for the iaterface The Hquid—Hquid iaterface, (Fig- lb) is similarly associated with iaterfacial regioas, R and Rg,... 

Surface effects are negligible in many cases. However, when the surface-to-volume ratio of the system is large, surface effects may become appreciable. Moreover, there are phenomena associated with surfaces that are important in themselves. Only an introduction to the thermodynamics of surfaces can be given here, and the discussion is limited to fluid phases and the surfaces between such phases. Thus, consideration of solid-fluid interfaces are omitted, although the basic equations that are developed are applicable to such interfaces provided that the specific face of the crystal is designated. Also, the thermodynamic properties of films are omitted. 

Capillarity — (a) as a branch of science, it concerns the thermodynamics of surfaces and - interfaces. It is of utmost importance for - electrochemistry, e.g., treating the electrode solution interface (- electrode, - solution), and it extends to several other branches of physics, chemistry, and technical sciences [i]. The thermodynamic theory of capillarity goes back to the work of Gibbs, (b) In a practical sense capillarity means the rise or fall of a liquid column in a capillary caused by the interplay of gravity and -> interfacial tension and also phenomena like capillary condensation [ii]. 

Finally it is important to note that the variation of heats of adsorption tends to be compensated for by a simultaneous variation in entropies of adsorption, as already noted above in connection with the relation discovered by Everett. Consequently the adsorption equilibrium constant will not drift with coverage as much as expected from a consideration of heats of adsorption alone. This remark again supports the view that emphasis on heats of adsorption has distorted the complete thermodynamic picture of surface phenomena. 

If a small spherical particle, liquid or solid, is in equilibrium with its vapor, the pressure of the vapor must be greater than that in equilibrium with a planar surface of the same material as the particle. The vapor may be one component in an ideal gas mixture. An expression for the vapor pressure increase can be derived from classical thermodynamics, taking surface phenomena into account as follows. 

Krotov, V. V., The structure, syneresis, and kinetics of destruction of polyhedral disperse systems. In Problems of Thermodynamics of Heterogeneous Systems and the Theory of Surface Phenomena. Vol. 6, pp. 110-191 Izd. Leningrad. Univ., Leningrad, 1982 [in Russian]. 

The treatment of surface phases presented here is essentially that of Gibbs. (The reader is referred to the book J. Willard Gibbs, Collected Works, vol. I, pp. 219-328, Yale University Press, New Haven, Conn., 1948, for more details.) In Sec. 10-1, we derive the fundamental equations for the thermodynamic treatment of surface phenomena. In Sec. 10-2, we consider the dependence of the various surface properties on the position of the dividing surface. Section 10-3 is devoted to a study of the temperature and component derivatives of the surface tension. 

B.A. Noskov, in Thermodynamic of Heterogeneous Systems and Theory of Surface Phenomena, N. P. Markusin (Ed.), Publishing house of St. Petersburg State University, 1996, NIO, P. 178. 

The mechanisms of surface chemical reactions represent a problem in coordination chemistry, which is the study of complexes, molecular units comprising a central group surrounded by other atoms in close association. This book is principally an introduction to the interpretation of surface phenomena in soils from the point of view of coordination chemistry. Therefore the basic concept to be discussed is the surface functional group, the central moiety in surface complexes, whose formation provides the most important mechanism of adsorption by the solid phases in soils. No detailed consideration of adsorption isotherm equations or the thermodynamic theory of ion exchange is presented, except insofar as their tenuous relation with surface coordination chemistry is to be illustrated. The discussion in this book is intended to be self-contained, but a previous exposure to soil physical chemistry, soil mineralogy, and the fundamentals of inorganic chemistry will prove helpful. 

Looking back one can easily notice that before mid 1960s the thermodynamics of adsorption phenomena on platinum was considered mostly in terms of temperature dependence. This traditional approach was not specific for electrochemical thermodynamics, but there was no serious basis to involve other parameters. Another remarkable point is discussion exclusively in terms of hydrogen adsorption, imder more or less transparently formulated assumption of complete charge transfer with formation of uncharged adatom. It is shghtly strange future surface thermodynamics was outlined already in 1936, and its principal point was the interplay of ionic and atomic adsorption, but even 30 years later ionic contribution was still accounted only as very formal subtraction (double layer correction). When the idea of this interplay was first presented by Frumkin in more comprehensive form, it met immediately Breiter s support. ... 

Adsorption equilibria are normally considered in connection with processes occurring in porous media filters, catalysts, adsorbents, chromatographs, and rock of petroleum reservoirs. In macroporous and mesoporous media, adsorption is normally accompanied by another surface phenomenon, the capillary condensation. These two types of surface phenomena are closely connected because they are both produced by surface forces. On the other hand, these phenomena are relatively independent and may, to some extent, be discussed separately [3]. Moreover, the description of the coexistence of the adsorbed films and capillary condensate in the same capillary is a nontrivial problem. We present capillary condensation and adsorption separately, although their eommon roots are discussed in Section II. The (more or less) comprehensive description of the thermodynamics of multicomponent capillary condensation... 

The physical chemist is very interested in kinetics—in the mechanisms of chemical reactions, the rates of adsorption, dissolution or evaporation, and generally, in time as a variable. As may be imagined, there is a wide spectrum of rate phenomena and in the sophistication achieved in dealing wifli them. In some cases changes in area or in amounts of phases are involved, as in rates of evaporation, condensation, dissolution, precipitation, flocculation, and adsorption and desorption. In other cases surface composition is changing as with reaction in monolayers. The field of catalysis is focused largely on the study of surface reaction mechanisms. Thus, throughout this book, the kinetic aspects of interfacial phenomena are discussed in concert with the associated thermodynamic properties. 

The formation of ordered two- and three-dimensional microstructuies in dispersions and in liquid systems has an influence on a broad range of products and processes. For example, microcapsules, vesicles, and liposomes can be used for controlled drug dehvery, for the contaimnent of inks and adhesives, and for the isolation of toxic wastes. In addition, surfactants continue to be important for enhanced oil recovery, ore beneficiation, and lubrication. Ceramic processing and sol-gel techniques for the fabrication of amorphous or ordered materials with special properties involve a rich variety of colloidal phenomena, ranging from the production of monodispersed particles with controlled surface chemistry to the thermodynamics and dynamics of formation of aggregates and microciystallites. 

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