Causes of adsorption of activated alumina
The reason why it has adsorption capacity is that its surface has excess energy - called "surface free energy" (also known as "surface free force field or surface gravitational field"). The properties of molecules in the surface layer of an object are different from those in the interior. In Figure 1, XY is the liquid-gas interface, a is a molecule in the liquid, and B is a molecule on the surface layer. It can be seen that a is attracted by the surrounding molecules, and the force is equal in all directions, and the resultant force is zero. On the one hand, it is attracted by the molecules inside the liquid; On the other hand, it is attracted by gas molecules outside the liquid. Since the gas density is much smaller than the liquid density, its gravitational attraction to B molecules can be omitted. Therefore, it can be considered that the molecules in the surface layer are only attracted by the molecules in the liquid, and the resultant force direction is perpendicular to the liquid surface and points to the interior of the liquid, so that the liquid has the trend of reducing the surface area and potential energy as much as possible. The reason why this trend can not continue is that there is a force on the liquid surface against the reduction of its surface area, which is "surface tension". The existence of surface tension makes the surface layer molecules have more energy than the internal molecules. This excess energy is called "surface free energy".
In fact, the reason for the adsorption of activated alumina is not only the surface tension on the liquid-gas interface, but also the "interfacial tension" with the same properties in any system that can form the interface (such as solid-gas interface and solid-liquid interface). The decrease of surface tension is the thermodynamic reason for the adsorption.