The forces involved in vapor and gas adsorption by solids can be non-specific and non-directional bonds (van der Waals-London), which are bonds similar to those of liquefaction. This process is called Physisorption. Physisorption can end in chemical bonds between the substrate and the gas – in this case the process is called Chemisorption.
Physical adsorption of gases is usually quick. Since no additional energy is needed for this process to proceed, the physical adsorption of gases is almost always reversible. Heating the substrate, for example, causes desorption of the gas. Physical adsorption forms multiple layers in the substrate and, when saturated with vapor, physical adsorption occurs concurrently with liquefaction.
In Chemisorption only monolayers are formed. Chemisorption is a specific process that requires activation energy to start. Initially the molecule X2 should be physically stuck in the substrate. The following step is chemisorption, that needs energy and results in the disassociation of the molecule into two atoms (2X) that now connect to the substrate. The process varies from system to system. Figure 2.3.10 shows a qualitative diagram of potential interaction energy involved in chemisorption and physisorption as a function of distance of the molecules X2 to the substrate. The molecules don’t have any attraction in distances much larger than the size of the molecule, this is why the F curve touches the abscissa for larger distances between X2 and the substrate. When the distance between the molecule and the substrate approaches the atomic diameter, van der Waals-London forces begin to function. After this minimum distance, smaller distances would imply repulsion between electron clouds. This attraction is of small magnitude (it’s called Heat of Physisorption) when compared to the Heat of Chemisorption (Curve Q). The necessary energy for dissociation of X2 is presented for long distances between molecules and substrate. When the molecule nears the solid it is physically adsorbed. This reduces the positive potential of interaction until the point where the two curves cross, and starts the transition to dissociation of the molecule for two atoms X, both individually chemically bonded to the surface. For a given system, the Heat of Chemisorption is much higher that Physisorption. Also note that the distance between substrate and atoms is smaller in chemisorption than physisorption, because in the first case the molecule was disassembled into individual atoms that created covalent or ionic bonds with the substrate.
An important technological problem, for example, is the chemisorption of gas by metals.
