The Surface Area of Solids

The specific surface area of a solid is one of the first things that must be determined if any detailed physical chemical interpretation of its behavior as an adsorbent is to be possible. Such a determination can be made through adsorption studies themselves, and this aspect is taken up in the next chapter there are a number of other methods, however, that are summarized in the following material. Space does not permit a full discussion, and, in particular, the methods that really amount to a particle or pore size determination, such as optical and electron microscopy, x-ray or neutron diffraction, and permeability studies are largely omitted. 

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There are various methods for the determination of the surface area of solids based on the adsorption of a mono-, or polymolecular layer on the surface of the solid. These methods do not measure the particle diameter or projected area as such, but measure the available surface per gram or milliliter of powder. The surface measured is usually greater than that determined by permeability methods as the latter are effectively concerned with the fluid taking the path of least resistance thru the bed, whereas the adsorbate will penetrate thru the whole of the bed as well as pores in the powder particles. These methods appear to be more accurate than surface areas calculated from weight averages or number averages of particle size because cracks, pores, and capillaries of the particles are included and are independent of particle shape and size... 

Temperature. A rough rule is that the value of k doubles for every rise in temperature of 10 C. Particle size. Increasing the surface area of solids by pulverization increases the reaction rate. Catalysts and inhibitors. A catalyst is a substance that increases the rate of a reaction but is recovered unchanged at the end of the reaction. Inhibitors decrease the rate. 

As noted earlier, the phenomenon of adsorption is encountered in such diverse applications as the separation of components in chromatography, die removal of toxic gases by activated charcoal, heterogeneous catalytic reactions and die dyeing of fabrics. The surface area of solids is most... 

The Brunauer-Emmett-Teller (or BET) adsorption isotherm applies only to the physisorption of vapours but it is important to heterogeneous catalysis because of its use for the determination of the surface areas of solids. The isotherm is given by the following equation,... 

Among places where condensates accreted into significant solid bodies, such as planets, habitable realms have always been rarer than places that were either too cold or too hot for life to exist. Much of our Solar System s mass is still far too hot for life. Most of the deep interiors of the gas giants and rocky planets are too hot, as is, of course, the Sun itself. Most of the surface area of solid bodies in the Solar System are too cold - the icy satellites of the outer planets and the myriad comets and Kuiper Belt Objects on the far outer fringes of the Solar System. In this sense, places like the surfaces of Earth and Mars and Europa s subsurface ocean are indeed very rare places. 

A liquid does not have a fixed shape, so the surface area of a liquid can be easily changed. (The surface area of solids can also be changed by processes such as grinding. However, this requires a considerable amount of energy.) In condensed phases, molecules on the surface have a different environment from molecules in the bulk therefore, a measure of the surface area is necessary to completely define the state of the system. In Chapter 11, we will discuss surface effect in liquids by use of the surface tension, y, which is the extra energy per unit... 

The surface area of particles is related to particles size, as discussed previously. The surface area of powders affects the drug dissolution rate, powder flow, cohesiveness, and adsorption. Furthermore, the surface area of solid materials may also... 

BET. Tins model (33) estimates the coverage corresponding to one monolayer of adsorbate and is used to measure the surface areas of solids ... 

In the rheological phase system there are many advantages the surface area of solid particles can be efficiently utilized, the contact between solid particles and fluid is close and uniform, heat change is very efficient, local overheating can be avoided, and the reaction temperature can be easily controlled. In addition, many substances have greatly increased solubility and new reaction behavior in this state. 

Despite the lack of consistency of the theory, however, the model has persisted and the BET equation continues to be one of those most commonly used. Moreover, the BET method for determining the surface area of solids is internationally accepted and is routinely used in the determination of that parameter. This is because the method is able to provide fairly acceptable results concerning the surface area of many solids, despite the defects inherent to the theory. This and other paradoxes of the BET theory remain to be elucidated and it is because of this that we were prompted to make a review of the BET plot, proposing other ways to check the model that serve to clarify some aspects of the theory. 

For practical purposes all adsorptions can be classified as one of two types. It can involve merely the van der Waals interaction between the substrate and the catalyst, a process that is termed physical adsorption or physisorption. Alternately, it can involve the formation of catalyst-substrate bonds as discussed above. This is termed chemical adsorption or chemisorption. While the latter is the basis for the chemistry of catalysts, physisorption is the basis for the BET procedure which is commonly used to measure the surface area of solids. ... 

Particle size. Increasing the surface area of solids by pulverization increases the reaction rate. 

The BET equation has been used to determine the surface area of solids from gas adsotption data. The equation not only predicts the shape of the adsorption isotherm, but also gives the volume of gas Vm required to form a monolayer. The BET equation has the form... 

By using porous carbon materials with a veiy high surface area in both positive and negative electrodes, a large amount of electrical charge was found to be stored, and electric double-layer capacitors (EDLCs) were developed [81,82], The fundamental concept of this capacitor is the formation of electric double-layers on the surface of electrodes, as illustrated in Fig. 28. The total amount of electeic charges aligned in double layers on both electrodes increases by the application of potential difference and it is easily understood to depend on the area of this interface, i.e. the surface area of solid electrodes. 

The distinguished career of Professor Paul H. Emmett has spanned six decades, beginning with his Ph.D. research under A.F. Benton at the California Institute of Technology in 1922. His pioneering contributions to the field of catalysis have provided the foundation for much of the present-day work in the field. Among his most notable contributions is the BET method for determining the surface area of solids, done in collaboration with Stephen Brunauer and Edward Teller. Surface area measurement by the BET method is probably the most widely used characterization method in catalysis today. 

FIGURE 3 AFM images of the surface area of solid phase obtained after laser ablation of colloidal solution of [Zn+ 0.01 M SDS]. Basic colloid (a) and colloid after laser irradiation UV range (b). 

Gas-phase reactions are easily visualized this way, but reacting liquids and solids must be looked at differently. A large piece of solid contains a large number of molecules, but, as described before, only those on the surface can react with the molecules of another substance. For this reason, the total amount of solid in a sample is not as important as the surface area of solid in contact with other reactants. The effective concentration of a solid therefore depends on its surface area and state of division. A 100-pound sack of flour is difficult to bum when it is in a single pile. The same 100 pounds, dispersed in the air as a fine dusf, bums very rapidly, and a dusf explosion results (see I Figure 8.10). The effective concentrations of reacting liquids must be thought of in a similar way unless the reactants are completely miscible. 

The combustion rate of a solid in air will in most cases be limited by the surface area of solid presented to the air. Even if the combustible solid is in particulate... 

To make our combustion fundamentals considered above applicable to dust explosions we need only to add in the influence of particle size on reaction rate. Assuming the combustion reaction rate is now determined by the surface area of solid fuel particles (assumed spherical) exposed to the air, the heat release term (2) in Equation (15.4) becomes ... 

The surface area of solids has long been important in physical sciences, applied sciences, and environmental sciences, because this feature is capable of affecting the quality and utility of many materials. For this reason it is important to determine and control surface areas accurately. Likewise, knowledge of porosity and surface area are frequently important keys in understanding the structure, formation, and potential applications of different natural materials. 

There are five factors that affect the rate, or speed, with which a chemical reaction takes place the nature of the reactants, the surface area of solids, the concentration of reactants, temperature, and the presence of catalysts. 

The Rates of Reaction Depends on the Surface Area of Solids... 

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