Surfaces Chemistries

The surface area associated with a given mass of material subdivided into equal-size particles increases in inverse proportion to the linear dimensions of the particles. Thus the area exposed by unit mass (the specific surface area, as) is given by 6/pd, where p is the density of the material and d is the edge length in the case of cubic particles or the diameter in the case of spheres. If the material is made up of molecules of linear dimension h and molecular volume h3, then the fraction of molecules in the surface layer is given approximately by 6 h/d). Thus for a substance of molar volume 30cm3moP or of molecular volume 0.05 nm3 (e.g. silver bromide) h = 0.37 nm. For a 1 cm cube only 

It should be noted, however, that some typically colloidal phenomena, such as light scattering, are exhibited (though very weakly) by systems in which the microheterogeneity arises from random kinetic fluctuations in density in an otherwise uniform system of small molecules such as a gas or a liquid, while in some cases (e.g. suspensions of relatively coarse solid particles) certain colloid-like properties may persist to particle sizes much larger than the above maximum. 

The surface energy and surface tension both have units of and are equivalent (see Fig. C.l). 

When a drop of liquid is placed on a solid, a definite angle of contact exists at the point where the liquid meets the solid. This is shown in Fig. C.2 where the angle 6 is called the contact angle. 

Roussak and H.D. Gesser, Applied Chemistry A Textbook for Engineers and Technologists, DOI 10.1007/978-1-4614-4262-2, Springer Science-rBusiness Media New York 2013 

Standard surface tension in dynes per centimeter, versus air at 20°C 

The contact angle can be measured by direct observation with the use of magnification of the liquid drop on a flat surface of the solid material, or by inclination of a slide of the solid in the liquid until the meniscus flattens out. The latter method is shown in Fig. C.3. 

Water molecules will interact strongly with surfaces containing hydrophilic surface groups, such as hydroxyl, carboxyl, phosphate, and other hydrogen-bonding molecules. Simulation studies investigating the water structure within a few nanometres of a hydrophilic 

A large number of processes in the microelectronics and chemical industries depend on the chemical reactions occurring on solid surfaces. The STM provides an unparalleled opportunity to study those chemical reactions at the atomic level. In this section, we will describe two reactions on silicon surfaces that are directly related to the understanding of the processes of manufacturing silicon devices. 

The first one is the reaction of oxygen with a clean Si surface, or the initial stage of oxidation of the Si surface. On the Si(lll)-7 X 7 surface, the reaction activity and the local reaction mechanism are now understood at the atom-by-atom level (Avouris and Lyo, 1990 Avouris, Lyo, and Bozso, 1991 Pelz and Koch, 1991). Two different early products of oxidation and their site selectivity are identified with STM and STS. 

The topographs of the Si(lll)-7 X 7 surface after oxygen exposure are studied with a bias of +2 V, that is, probing the unoccupied states. As shown in Fig. 16.9, after an exposure of —0.2 L of oxygen to the clean Si(lll)-7X7 surface, two new sites are observed by STM a dark site such as those 

Prior to the STM study, the structure of the H-saturated Si(lOO) surface has been in dispute for many years (Boland, 1990, and references therein). The STM study of clean Si(lOO) has been discussed previously see Fig. 1.15. After hydrogen exposure, there are several possible structures, as shown in 

The materials chemist has to deal with three different types of chemistry solid state (in homogeneous solids), molecular (in homogeneous liquids or gases), and surface. Of these the last is the most important for materials chemistry because  

Solids react with fluids on their surface catalysis and corrosion are surface reactions of solids. 

When solid products are made, they grow by surface reactions. Syntheses of crystalline and amorphous solids and of nanostructured materials are surface processes. 

Many material properties are surface properties, which are easier to tune by modifying the surface than by changing the solid. 

molecular, and surface chemistry have much in common they share most types of bonding and the driving forces are the thermodynamic potentials. Heterogeneous reactions differ from homogeneous processes in kinetics, in the morphology-dependence of the reaction rates, and in the possibility of electrical control. 

Those involved in the characterization and application of activated carbons mnst realize that no single analytical chemistry discipline can successfully explain all surface chemical properties. A broad and comprehensive view can be obtained only when a battery of methods, often based on different physicochemical principles, are used. Owing to space limitations, only a brief review of the techniques is presented in this section. For more information the reader is directed to ref. 

Naval Research Laboratory. Center for Computational Materials Science. More information. Crystal stracmre of YBajCujOy.. Available from http //cst-www.nrl. navy.mil/lattice/struk.picts/ . 

Wolfgang Pauli once stated that the surface was invented by the devil, illustrating the complexity and difficulty of studying the surfaces of materials. This prompts a fundamental question What is the surface of a material The simplest definition is that the surface is the boundary at which the atoms that make up one material terminate and interface with the atoms of a new material. If the surface is considered to be just the outermost layer of atoms of a material, then it comprises on average only 10 atoms per square centimeter (1 square centimeter equals 0.155 square inch), as compared to the bulk of the material, which consists of approximately 10 atoms per cubic centimeter. Surface chemistry is important in many critical chemical processes, such as enzymatic reactions at biological interfaces found in cell walls and membranes, in electronics at the surfaces and interfaces of microchips used in computers, and the heterogeneous catalysts found in the catalytic converter used for cleaning emissions in automobile exhausts. 

The STM utifizes the quantum mechanical phenomenon of tunneling to visuahze the positions of atoms on surfaces. A sharp metal tip is attached to a piezoelectric translator, which can position the tip with angstrom (1 A = 1 x 10 meters) precision. As the tip is scanned over the surface, electrons move between the tip and sample and a tunneling current is produced. This current is very sensitive to (i.e., exponentially dependent upon) the 

Tunneling is the process in which electrons can pass from one metal to another, even though they are not in contact. This process occurs by coupling of the electronic states between the two surfaces. 

A piezoeiectric is a ceramic materiai (typicaiiy a mixture of Pb, Zr, Ti, and 0) that changes size with applied voltage. Quartz is an example of a naturally occurring piezoelectric. Piezoelectric materials are used to control the tip position in scanned probe microscopes because the changes in the piezoelectric dimensions can be controlled with sub-angstrom precision. 

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R. Aveyard and D. A. Haydon, An Introduction to the Principles of Surface Chemistry, Cambridge University Press, Cambridge, UK, 1973. 

P. C. Hiemenz, Principles of Colloid and Surface Chemistry, 2nd ed., Marcel Dekker, New York, 1986. 

L. I. Osipow, Surface Chemistry, Theory and Industrial Applications, Krieger, New York, 1977. 

D. J. Shaw, Introduction to Colloid arul Surface Chemistry, Butterworths, London, 1966. 

With the preceding introduction to the handling of surface excess quantities, we now proceed to the derivation of the third fundamental equation of surface chemistry (the Laplace and Kelvin equations, Eqs. II-7 and III-18, are the other two), known as the Gibbs equation. 

Although Gibbs published his monumental treatise on heterogeneous equilibrium in 187S, his work was not generally appreciated until the turn of the century, and it was not until many years later that the field of surface chemistry developed to the point that experimental applications of the Gibbs equation became important. 

H. Van Olphen and K. J. Mysels, Physical Chemistry Enriching Topics from Colloid arul Surface Chemistry, Theorex (8327 La Jolla Scenic Drive), La Jolla, CA, 1975. 

G. L. Gaines, Jr., in Surface Chemistry and Colloids (MTP International Review of Science), Vol. 7, M. Kerker, ed.. University Park Press, Baltimore, 1972. 

In recent years, advances in experimental capabilities have fueled a great deal of activity in the study of the electrified solid-liquid interface. This has been the subject of a recent workshop and review article [145] discussing structural characterization, interfacial dynamics and electrode materials. The field of surface chemistry has also received significant attention due to many surface-sensitive means to interrogate the molecular processes occurring at the electrode surface. Reviews by Hubbard [146, 147] and others [148] detail the progress. In this and the following section, we present only a brief summary of selected aspects of this field. 

While the confirmation of the predicted long-range dispersion attraction between surfaces in air has been a major experimental triumph, the forces between particles in solution are of more general interest in colloid and surface chemistry. The presence of a condensed medium between the surfaces... 

Dislocation theory as a portion of the subject of solid-state physics is somewhat beyond the scope of this book, but it is desirable to examine the subject briefly in terms of its implications in surface chemistry. Perhaps the most elementary type of defect is that of an extra or interstitial atom—Frenkel defect [110]—or a missing atom or vacancy—Schottky defect [111]. Such point defects play an important role in the treatment of diffusion and electrical conductivities in solids and the solubility of a salt in the host lattice of another or different valence type [112]. Point defects have a thermodynamic basis for their existence in terms of the energy and entropy of their formation, the situation is similar to the formation of isolated holes and erratic atoms on a surface. Dislocations, on the other hand, may be viewed as an organized concentration of point defects they are lattice defects and play an important role in the mechanism of the plastic deformation of solids. Lattice defects or dislocations are not thermodynamic in the sense of the point defects their formation is intimately connected with the mechanism of nucleation and crystal growth (see Section IX-4), and they constitute an important source of surface imperfection. 

R. S. Gould and S. J. Gregg, The Surface Chemistry of Solids, Reinhold, New York, 1951. 

G. A. Somoijai, Principles of Surface Chemistry, Prentice-Hall, Englewood Cliffs, NJ, 1972. 

G. A. Somoijai, Introduction to Surface Chemistry and Catalysis, Wiley, New York,... 

There is a large volume of contemporary literature dealing with the structure and chemical properties of species adsorbed at the solid-solution interface, making use of various spectroscopic and laser excitation techniques. Much of it is phenomenologically oriented and does not contribute in any clear way to the surface chemistry of the system included are many studies aimed at the eventual achievement of solar energy conversion. What follows here is a summary of a small fraction of this literature, consisting of references which are representative and which also yield some specific information about the adsorbed state. 

This chapter and the two that follow are introduced at this time to illustrate some of the many extensive areas in which there are important applications of surface chemistry. Friction and lubrication as topics properly deserve mention in a textbook on surface chemistiy, partly because these subjects do involve surfaces directly and partly because many aspects of lubrication depend on the properties of surface films. The subject of adhesion is treated briefly in this chapter mainly because it, too, depends greatly on the behavior of surface films at a solid interface and also because friction and adhesion have some interrelations. Studies of the interaction between two solid surfaces, with or without an intervening liquid phase, have been stimulated in recent years by the development of equipment capable of the direct measurement of the forces between macroscopic bodies. 

A very important but rather complex application of surface chemistry is to the separation of various types of solid particles from each other by what is known as flotation. The general method is of enormous importance to the mining industry it permits large-scale and economic processing of crushed ores whereby the desired mineral is separated from the gangue or non-mineral-containing material. Originally applied only to certain sulfide and oxide ores. 

This chapter concludes our discussion of applications of surface chemistry with the possible exception of some of the materials on heterogeneous catalysis in Chapter XVIII. The subjects touched on here are a continuation of Chapter IV on surface films on liquid substrates. There has been an explosion of research in this subject area, and, again, we are limited to providing just an overview of the more fundamental topics. 

L. H. Little, Infrared Spectra of Adsorbed Molecules, Academic, New York, 1966. 68a. M. L. Hair, Infrared Spectroscopy in Surface Chemistry, Marcel Dekker, New... 

G. D. Halsey, see Twenty Years of Colloid and Surface Chemistry The Kendall Award Adresses, Amer. Chem. Soc., Washington, DC, 1973. 

The molecular emphasis of modem chemisorption studies has benefited the field of catalysis by giving depth and scope to the surface chemistry of catalytic processes. To paraphrase King [1], quantitative answers have become possible to the following questions ... 

G. Ertl and J. Kuppers, Low Energy Electrons and Surface Chemistry, Verlag Chemie, Berlin, 1985. 

G. A. Somorjai, Principles of Surface Chemistry, Prentice-Hall, Englewood Cliffs, NJ, 1972 G. A. Somoijai and L. L. Kesmodel, MTP International Review of Science, Butterworths, London, 1975. 

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