Langmuir Adsorption Model - Background and Experiments

Background and Experiments

In 1916, Irving Langmuir presented his model for the adsorption of species onto simple surfaces. Langmuir was awarded the Nobel Prize in 1932 for his work concerning surface chemistry. He hypothesized that a given surface has a certain number of equivalent sites that a species can “stick”, either by physisorption or chemisorption. His theory began when he postulated that gaseous molecules do not rebound elastically from a surface, but are held by it in a similar way to groups of molecules in solid bodies.

Langmuir published two papers that proved the assumption that adsorbed films do not exceed one molecule in thickness. The first experiment involved observing electron emission from heated filaments in gases. The second, a more direct proof, examined and measured the films of liquid on and adsorbent surface layer. He also noted that generally the attractive strength between the surface and the first layer of adsorbed substance is much greater than the strength between the first and second layer. However, there are instances where the subsequent layers may condense given the right combination of temperature and pressure.

The most important empirical data came from a set of experiments that Langmuir ran to test the adsorption of several gases on mica, glass and platinum. The experiments began at very low pressures (~100 bar) in order to more easily measure the change in quantities of free gas and also to avoid condensation. He then ran the experiments at different temperatures and pressures, which proved the pressure dependence demonstrated below.