External and internal surface

In discussions of the surface properties of solids having a large specific surface, it is convenient to distinguish between the external and the internal surface. The walls of pores such as those denoted by heavy lines in Fig. 1.8 and 1.11 clearly comprise an internal surface and equally obviously the surface indicated by lightly drawn lines is external in nature. In many cases, however, the distinction is not so clear, for the surfaces of the primary particles themselves suffer from imperfections in the forms of cracks and fissures those that penetrate deeply into the interior will contribute to the internal surface, whereas the superficial cracks and indentations will make up part of the external surface. The line of demarcation between the two kinds of surface necessarily has to be drawn in an arbitrary way, but the external surface may perhaps be taken to include all the prominences and all of those cracks which are wider than they are deep.,The internal surface will 

An interesting example of a large specific surface which is wholly external in nature is provided by a dispersed aerosol composed of fine particles free of cracks and fissures. As soon as the aerosol settles out, of course, its particles come into contact with one another and form aggregates but if the particles are spherical, more particularly if the material is hard, the particle-to-particle contacts will be very small in area the interparticulate junctions will then be so weak that many of them will become broken apart during mechanical handling, or be prized open by the film of adsorbate during an adsorption experiment. In favourable cases the flocculated specimen may have so open a structure that it behaves, as far as its adsorptive properties are concerned, as a completely non-porous material. Solids of this kind are of importance because of their relevance to standard adsorption isotherms (cf. Section 2.12) which play a fundamental role in procedures for the evaluation of specific surface area and pore size distribution by adsorption methods. 

8 Classification of pore sizes micropores, mesopores and macropores 

The pore systems of solids are of many different kinds. The individual pores may vary greatly both in size and in shape within a given solid, and between one solid and another. A feature of especial interest for many purposes is the width w of the pores, e.g. the diameter of a cylindrical pore, or the distance between the sides of a slit-shaped pore. A convenient classification of pores according to their average width originally proposed by Dubinin and now officially adopted by the International Union of Pure and Applied Chemistry is summarized in Table 1.4. 

The basis of the classification is that each of the size ranges corresponds to characteristic adsorption effects as manifested in the isotherm. In micropores, the interaction potential is significantly higher than in wider pores owing to the proximity of the walls, and the amount adsorbed (at a given relative pressure) is correspondingly enhanced. In mesopores, capillary condensation, with its characteristic hysteresis loop, takes place. In the macropore range the pores are so wide that it is virtually impossible to map out the isotherm in detail because the relative pressures are so close to unity. 

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Diffusion alurninide and sihcide coatings on external and internal surfaces for high temperature corrosion protection in parts such as gas-turbine blades is estimated at 40 x 10 /yr in North America and about 50 x 10 worldwide. 

Severe wastage was evident on both external and internal surfaces of the tube section. There were two round holes on one side. Deep pockets of internal surface corrosion penetrated to the external surface (Fig. 13.11A). The depressions contained sulfides, mainly concentrated near external surfaces. Below were large amounts of porous elemental copper (Fig. 13.1LB). 

A wide variety of protective coating types and systems is available for corrosion control on external and internal surfaces of structural and process plant in marine and offshore engineering. These are discussed in detail elsewhere in this text, and the purpose here is to highlight the critical importance of certain design and related operational aspects which affect both the selection and performance of protective coating systems. The following design considerations should be made ... 

Fig. 4.28 Cate-controlled molecular recognition and fluorescence detection by selective functionalization of external and internal surfaces of mesoporous silicates. Reprinted with permission from [225], R. D. Radu et al.,J. Am. Chem. Soc. 2004, 126, 1640. 2004, American Chemical Society.

Two-dimensional (planar) defects—external and internal surfaces... 

Heterogeneously catalyzed hydrogenation is a three-phase gas-liquid-solid reaction. Hydrogen from the gas phase dissolves in the liquid phase and reacts with the substrate on the external and internal surfaces of the solid catalyst Mass transfer can influence the observed reaction rate, depending on the rate of the surface reaction [15]. Three mass transfer resistances may be present in this system (Fig. 42.1) ... 

Because of the difference between the 2 1 and 1 1 structures, their external and internal surfaces are fundamentally different. 

Reflection with an intensity Ir from the external and internal surfaces. 

In aU these examples the external and internal surface areas of the sohd increases... 

Practical solid-catalyzed rate processes also may be influenced by rates of diffusion to the external and internal surfaces. In the latter case the rate equation is modified by inclusion of a catalyst effectiveness to become... 

The spraying device works in the following way. The prepared liquid mixture of reactive components flows to channel 3 through pipeline 4, and is distributed in a circular direction by the rotation of core 2. This movement simultaneously reduces the apparent viscosity of the liquid. Then the liquid goes to the ring nozzle 5. Porous rings 6 and 7 are placed on the external and internal surfaces of nozzle 5 at a distance of (1 - 20)h from the exit (where h is the distance between the... 

The lipid bilayer is such that the polar heads (often phosphatidylcholine or phosphati-dylethanolamine) of the phospholipids are juxtaposed on the external and internal surfaces of the membrane, causing the ends of the hydrophobic (i.e., long-chained alkyl) portions of the phospholipids to extend inside the membrane. Also contained within the lipid bilayer are cholesterol and other sterols. 

The conventional model developed to explain cell membrane characteristics influencing drug permeability is routinely referred to as the fluid-mosaic model (Figures 2.1 and 2.2). In this model the main components, for our purposes, are a phospholipid (e.g., sphingomyelin and phosphatidylcholine) bilayer (8 nm), with polar moieties at both the external and internal surfaces, and with proteins periodically traversing the phospholipid plane perpendicularly. 

Many catalysts are porous solids of high surface area and with such materials it is often useful to distinquish between the external and internal surface. The external surface is usually regarded as the envelope surrounding discrete particles or agglomerates, but is difficult to define precisely because solid surfaces are rarely smooth on an atomic scale. It can be taken to include all the prominences plus the surface of those cracks, pores and cavities which are wider than they are deep. The internal surface comprises the walls of the rest of the pores, cavities and cracks. In practice, the demarcation is likely to depend on the methods of assessment and the nature of the pore size distribution. The total surface area (As) equals the sum of the external and internal surface areas. The roughness of a solid surface may be characterized by a roughness factor, i.e. the ration of the external surface to the chosen geometric surface. 

Particle and Pore Sizes, External and Internal Surfaces... 

As mentioned previously, the total surface of the rock and soils is the sum of the external and internal surfaces. Both external and internal surfaces have charges, and so, when the internal surface is significant (such as in the case of humic substances and expandable clay minerals), that extent of the interfacial layer is quite great. All system has external surfaces, while internal surfaces are significant only in the case of certain minerals and organic matter therefore, we will discuss first the properties of external surfaces. 

The ratio of the external and internal surfaces as well as the charges on the different surfaces is determined by the crystal structure and particle size. For clay minerals, the internal surface area is about 80%-95% of the total surface area. 

Interfacial Processes Related to External and Internal Surfaces 1.3.3.1 Adsorption... 

Besides water and ions, other substances can also be adsorbed on both the external and internal surfaces. Since the surface of rocks and soils is strongly hydrophilic on a macroscopic scale, hydrophilic substances are more strongly adsorbed than hydrophobic substances. Some authors, however, say that the hydrophilic-hydrophobic character strongly varies on molecular scale. So the oxygen atoms next to the isomorphic substitutions are hydrophilic, while the oxygen atoms far from them are hydrophobic. The hydrophobic oxygen atoms are, for example, responsible for the adsorption of hydrophobic pesticides (Laird 2004). 

Besides metal ions and hydrogen ions, other cationic substances (e.g., cationic organic molecules or cationic tensides) can also be exchanged in the external and internal surfaces. Cationic tensides can make the surface hydrophobic (Lagaly and Dekany 2005). Furthermore, the distance between the layers is widened, enabling polymer chains to move in. This is an important condition in the preparation of nanocomposites containing layer silicates. Recently, the production and application of nanocomposites has become widespread, and many papers have been presented in this field (e.g., Paiva et al. 2008 Pavlidoua and Papaspyrides 2008). 

Montmorillonite has some important characteristics that justify its use as a model substance for the study of the interfacial processes of rocks and soils. It is a dioctahedral three-layer clay (2 1 clays, TOT) an A10(0H) octahedral sheet is between two tetrahedral Si04 layers (Chapter 1, Table 1.2). The distance between the layers is not fixed (—O—O-bonds) the layers can be expanded. Because of the layered structure, it has two surface types external and internal surfaces. The external surface is the surface of the particles (edge surface), and its size depends on particle size distribution. Its area can be measured by the BET method, usually by the adsorption of nitrogen gas at the temperature of liquid nitrogen (Chapter 1, Section 1.1.3). The internal surface is the surface between the layers (interlayer surface), and its size can be determined by introducing substances into the interlayer space (e.g., water) (Chapter 1, Section 1.1.3). The internal surface area is independent of particle size distribution. 

The morphology of the particles in rocks and soils means the size or size distribution, shape of the particles and pores, and the specific surface area of the external and internal surfaces. 

In Eq. (3). is the flux when intracrystalline transport is rate controlling, Af, is the real flux. 8 AE is the difference between the activation energy for escape from within the crystal to the externally adsorbed layer and the activation energy for diffusion. It is generally a positive quantity [35]. K and K represent the Langmuir parameters for adsorption on the external and internal surfaces, respectively. When internal and external adsorption isotherms are taken to be identical or are within Henry s law range, Eq. (3) becomes... 

If a difference in adsorption on the external and internal surfaces of the membrane is taken into account, the effects become more eomplicated. There are two possibilities. First, adsorption on the external surface is weaker than adsorption within the crystals. In this case interfacial processes become less important the higher is the adsorption strength on the outer surface. This is because the concentration in the pores is always higher than on the external layer. On the other hand, when adsorption on the external surface is strong compared to adsorption in the pores, the flux becomes restricted by the interfacial processes. The reason for this is that escape from the crystal becomes more difficult when the external surface is highly occupied. The effects of adsorption on the ratio between the ideal and the measured flux are summarized in Table 1. 

Note K and K are the Langmuir adsorption parameters on the external and internal surfaces, respectively. (From Ref- 35.)... 

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