Adsorption Isotherms Type III, IV and V What is an adsorption isotherm? Adsorption is the process whereby molecules adhere to a surface by forming either physical (physisorption) or chemical (chemisorption) bonds. The process of Adsorption is usually studied through graphs know as adsorption isotherm. The graphs are a plot of Adsorption against pressure. Why are adsorption isotherms even important? Adsorption is a widely used phenomenon in industry. One of the major application of adsorption isotherms is in the selection of appropriate material for the catalyst in specific reactions. Adsorption isotherms are most commonly used to determine surface areas of materials and their surface properties. Types of Isotherms: There are five main types of adsorption isotherms. Monolayer formations are modelled using the Langmuir Equation and the multilayers using the BET theorem. Scope of this write up is to look at only the last three types. Monolayer formation:- Langmuir equation: θ=N_s/N=m/m_∞ =V/V_∞ =Kp/(1+Kp) Multilayer formation:- BET theorem: P/(n(P^0-P))=1/(n_m c)+(c-1)/(n_m c) (P/P^0 ) Adsorption Isotherm Type III: The adsorption Carbon tetrachloride on macroporous surface like Silica at 293K gives rise to this type of isotherm. This type of isotherm is used to explain a multilayer adsorption. It is obtained when the amount of gas adsorbed increases without limit as its relative pressure approaches unity. No flat portion on the gas shows that there is no mono-layer formation. At low gas pressure, the graph shows low adsorption indicating low gas-solid affinity. Adsorption Isotherm Type IV: Adsorption of Nitrogen gas on Mesoporous surfaces like porous glass gives rise to type IV isotherms. At lower pressure, the curve is quite similar to a type II isotherm. The flattish part depicts the formation of a monolayer and the rest of the graph shows capillary condensation. The absorbent gets saturated below the saturation vapour pressure due to capillary condensation. Adsorption Isotherm Type V: Water molecules adsorbing onto the surface of charcoal at 1273K gives rise to type V isotherms. The adsorption process of the molecules is similar to those that give rise to type IV isotherms. However there is no monolayer formation which is why the graph as no flat shape at low relative pressures. As adsorption is low at low gas pressures, it suggests weak gas-solid affinity. This type of adsorption also has capillary condensation. Why do many isotherms follow the same pattern as of type III, IV and V? Adsorption depends on the gas being adsorbed and the surface of the adsorbent. Type III IV and V are generally produced my Macroporous and Mesoporous surfaces. Several materials give rise to isotherms that are the similarly shaped as those types. This is because many materials have Macroporous and Mesoporous surfaces, hence they give rise to similar shaped isotherms.

• published: 06 Nov 2013
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