Physisorption

Physisorption, also called physical adsorption, is a process in which the electronic structure of the atom or molecule is barely perturbed upon adsorption.

Overview The fundamental interacting force of physisorption is Van der Waals force. Even though the interaction energy is very weak (~10–100 meV), physisorption plays an important role in nature. For instance, the van der Waals attraction between surfaces and foot-hairs of geckos (see Synthetic setae) provides the remarkable ability to climb up vertical walls. Van der Waals forces originate from the interactions between induced, permanent or transient electric dipoles. In comparison with chemisorption, in which the electronic structure of bonding atoms or molecules is changed and covalent or ionic bonds form, physisorption does not result in changes to the chemical bonding structure. In practice, the categorisation of a particular adsorption as physisorption or chemisorption depends principally on the binding energy of the adsorbate to the substrate, with physisorption being far weaker on a per-atom basis than any type of connection involving a chemical bond.

Modeling by image charge

To give a simple illustration of physisorption, we can first consider an adsorbed hydrogen atom in front of a perfect conductor, as shown in Fig. 1. A nucleus with positive charge is located at R = (0, 0, Z), and the position coordinate of its electron, r = (x, y, z) is given with respect to the nucleus. The adsorption process can be viewed as the interaction between this hydrogen atom and its image charges of both the nucleus and electron in the conductor. As a result, the total electrostatic energy is the sum of attraction and repulsion terms:

V = e 2 4 π ε 0 ( − 1 | 2 R | + − 1 | 2 R + r − r ′ | + 1 | 2 R − r ′ | + 1 | 2 R + r | ) . {\displaystyle V={e^{2} \over 4\pi \varepsilon _{0}}\left({\frac {-1}{|2\mathbf {R} |}}+{\frac {-1}{|2\mathbf {R} +\mathbf {r} -\mathbf {r} '|}}+{\frac {1}{|2\mathbf {R} -\mathbf {r} '|}}+{\frac {1}{|2\mathbf {R} +\mathbf {r} |}}\right).}

The first term is the attractive interaction of the nucleus and its image charge, and the second term is due to the interaction of the electron and its image charge. The repulsive interaction is shown in the third and fourth terms arising from the interaction between the nucleus and the image electron, and, the interaction between the electron and the image nucleus, respectively. By Taylor expansion in powers of |r| / |R|, this interaction energy can be further expressed as:

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