how to plot an adsorption isotherm using surface coverage (theta) for corrosion inhibitors?
@sumedha moharana
Whenever a gas phase is in contact with a solid phase, there will be an equilibrium established between the molecules in the gas phase and the corresponding adsorbed species which are bound to the surface of the solid. As with all chemical equilibria, the position of equilibrium will depend upon some factors. These factors include stability of the species, temperature and the pressure of the gas.
The temperature and pressure will then exerts effect opposite to concentration of the absorbed species. For this reason, the surface coverage will increase by rising the gas pressure but reduced with the same rising of temperature.
Adsorbtion isotherms are usually plotted with the amount of adsorbate in the Y-axis (e.g. mass of adsorbate/mass of adsorbent) and the concentration (for liquid phase solutes) or pressure (for a gaseous adsorbate) in the X-axis at constant temperature.
If you want to plot in terms of surface coverage (theta) you need to know how much adsorbate is needed to form a full monolayer on your surface.
Fernando Hernández Ramírez Thankyou for your response. If I want to plot in terms of surface coverage (theta), how can I proceed further. Do I have to do weightloss only to get the value of surface coverage ?
"If I want to plot in terms of surface coverage (theta), how can I proceed further" - that's not an easy question to answer. You may:
- If you have experimental evidence that your adsorption kinetics is of first-order or pseudofirst order, you may use Langmuir isotherm directly. There is a relationship between theta and concentration.
- Otherwise, you got to find a method to measure the surface area or number of active sites on your surface. There is no general answer to this question. In electrochemistry and corrosion many different techniques are used: cyclic voltametry, impedance measurements, capacitance measurements, etc.
- Or you may just *asume* that Langmuir isotherm is applicable and do some rough estimation of the coverage based on this model.
Remember: the fundamental question is how much adsorbate is needed to form a full monolayer, and you need experiments or a theoretical model to answer this question. Weight loss corrosion experiments only give information about the average corrosion rate on a surface. Other techniques are needed for surface characterization.
What is the minimum amount of corrosion inhibitor that you need in order to stop corrosion rate significantly? If known, is it safe to assume that this is a full monolayer of inhibitor? What else do you know about your system? Only you can answer these questions.
Fernando Hernández Ramírez
Thank you for your response. I shall look into more details through literature