Surface area determination by gas adsorption

When a solid is exposed to a gas, the gas molecules impinging on the surface may not be elastically reflected, but may remain for a finite time. This is designated as adsorption as opposed to absorption, which refers to penetration into the solid body. 

The graph of the amount adsorbed (V), at constant temperature, against the adsorption pressure (P), is called the adsorption isotherm. For a gas at a pressure lower than the critical pressure, i.e. a vapor, the relative pressure x = P/Pq, where Pq is the saturation vapor pressure, is preferred. 

The amount adsorbed depends upon the nature of the solid (adsorbent), and the pressure at which adsorption takes place. The amount of gas (adsorbate) adsorbed can be found by determining the increase in weight of the solid (gravimetric method) or determining the amount of gas removed from the system due to adsorption by application of the gas laws (volumetric method). 

A commonly used method of determining the specific surface of a solid is to deduce the monolayer capacity V from the isotherm. This is defined as the quantity of adsorbate required to cover the adsorbent with a monolayer of molecules. Usually a second layer may be forming before the monolayer is complete, but V is deduced from the isotherm equations irrespective of diis. There are also other gas adsorption methods in which die surface area is determined without determining the monolayer capacity. 

Adsorption processes may be classified as physicid or chemical, depending on the nature of the forces involved. Physical adsorption, also termed van der Waals adsorption, is caused by molecular interaction forces the formation of a physically adsorbed layer may be likened to the condensation of a vapor to form a liquid. Ihis type of adsorption is only of importance at temperatures below the critical temperature for tbie gas. Not only is the heat of physical adsorption of the same order of magnitude as that of liquefaction, but physic ly adsorbed layers behave in many respects like two dimensional liquids. 

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Adsorption from the gas phase is commonly applied in determining the specific surface area of finely dispersed materials. For that purpose, assumptions have to be made concerning the dimensions of the gas molecules and the structure of the adsorbed layer under saturation conditions (fully packed monolayer, multilayer, etc.). Small gas molecules may enter pores and capillaries in porous materials. Hence, by comparing the surface area determined by gas adsorption with the outer surface area obtained from, for example, electron microscopy, the porosity of the material can be estimated. Moreover, by using different types of gas having different molecular dimensions, an impression of the pore size distribution may be obtained. 

Adsorption of ionic, nonionic and polymeric surfactant on the agrochemical solid gives valuable information on the magnitude and strength of the interaction between the molecules and the substrate as well as the orientation of the molecules. The latter is important in determining colloid stability. Adsorption isotherms are fairly simple to determine, but require careful experimental techniques. A representative sample of the solid with known surface area A per unit mass must be available. The surface area is usually determined using gas adsorption. N2 is usually used as the adsorbate, but for materials with relativdy low surface area, such as those encountered with most agrochemical solids, it is preferable to use Kr as the adsorbate. The surface area is obtained from the amount of gas adsorbed at various relative pressures by application of the BET equation [96]. However, the surface area determined by gas adsorption may not represent the true surface area of the solid in suspension (the so-called wet surface). In this case it is preferable to use dye adsorption to measure the surface area [99]. 

Surface area determination by gas adsorption 47 2.4 BET equation for multilayer adsorption... 

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