Adsorption at the Solid-Vapor Interface

L Energetic Considerations Physical Adsorption versns Chemisorption 

For solid-vapor adsorption, because of its exothermic nature, the amount of gas adsorbed onto a solid will decrease as the temperature of the system is increased. That is why clean solid surfaces are more easily prepared by heating to high temperatures. In addition, high vacuum reduces the vapor pressure of the adsorbed gas and reduces its tendency to adsorb. 

The heats of adsorption, AH, for gases onto a given sohd can, in principle, be measured in a variety of ways and will, in reversible systems, adhere to the Clausius-Clapeyron equation 

In the absence of complicating factors such as capillary condensation and competitive adsorption, the process of physical adsorption has no activation energy that is, it is diffusion-controlled and occurs essentially as rapidly as vapor molecules can arrive at the surface. The process will be reversible and equilibrium will be attained rapidly. Because the forces involved are the same as those involved in condensation, physical adsorption will generally be a multilayer process—that is, the amount of vapor that can be adsorbed onto a surface will not be limited simply by the available solid surface area, but molecules can stack up to a thickness of several molecules in a pseudoliquid assembly (Fig. 9.3). If the vapor pressure of the gas reaches saturation level, in fact, the condensation and adsorption processes overlap and become indistinguishable. The fact that physical adsorption can be a multilayer process is very important to the mathematical modeling and analysis of the process, as will be seen below. 

Unlike physical adsorption, chemisorption involves very specific interactions between the solid surface and the adsorbing molecules, as illustrated by the much higher heats of adsorption. Another important result of that specificity is that chemisorption is by nature limited to the formation of a monomolecu-lar adsorbed layer. Chemisorption processes will generally have some activation energy and may therefore be much slower than physical adsorption. They may also exhibit hysteresis that is, they may not be readily reversible. 

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A question that arises immediately, however, is that of the extent to which the solute component is also present atthe solid-vapor surface-i.e., the extent to which an equilibrium mixed adsorbed film is present. Qualitative observations by Bartell andFu tended to suggest that in their cases very little adsorption at the solid-vapor interface occurred, justifying their neglect of tt gyo. It would take the independent verification procedure suggested, however, to confirm the correctness of this assumption and of Equation 25. A complete study should include adsorption experiments with appropriate mixed vapors, so as to obtain tt g o independently. 

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