Question

Explain the procedure for defining the hysteresis loop class for adsorption and
desorption isotherms?
Minimum word limit 600

Plagiarism 10%max

Answer 1

The adsorption hysteresis is much similar to all other hysteresis like Magnetic, yield stress hysteresis in material. The whole hysteresis deals with that the the effect applied on a system will not retrace the original path but will come to the initial state along a different path. This is what happening in the adsorption hysteresis too. So let us consider an adsorption process taking place, actually we are conducting an experiment, let it be adsorption on macroporous material.

We are increasing the pressure during  the adsorption , we know that as pressure increases adsorption rate also increases.Therefore the amount of material adsorbed will be increasing. We make plot of pressure VS amount of material adsorbed per gram of adsorbent ( P vs X). We can see the trend that the curve keeps on moving upward with a positive slope since amount getting adsorbed is increasing.

Please note that we cannot use non-porous material since using non porous material will not give such hysteresis.For mesoporous and macroporous we see this hysteresis.After reaching a certain pressure and rate now what we do is reduce the pressure and we can see the amount of material getting adsorbing decreasing and desorption starts. Plot this as before as Pressure vs amount of material adsorbed per gram of adsorbent ( P vs X). We can see a trend of the plot coming down with a negative slope.

During the desorption we can see that it does not follow the same path that of the path taken by adsorption.This is hysteresis here. Please refer the image. In any case, if we observe the hysteresis curve we can see that the desorption equlibrium pressure is always less than the pressure during adsorption.

In the image of the plot P vs X we can see that after adsorption when desorption takes place the curve takes different pathway when coming down. Also we can see that at the same value for X that is for same amount of material adsorbed on the adsorbent , the pressure for adsorption and desorption are different. Always the desorption pressure will be lesser than the adsorption pressure for same amount of adsorbate adsorbed on the adsorbent. On the other way for the same pressure the amount of adsorbate on adsorbent (X) will be different during adsorbtion and desorption.It is clear from the graph. From the plot we can conclude that for same pressure the X value will be greater for desorption that means more adsorbate on the adsorbent.

At the end when it is about to reache the initial state both curves coincide and reaches the initial state.

The reason for such adsorption hysteresis is because;

Several explanations have been proposed to account for such hysteresis. The assumption made was that during adsorption the vapour does not completely fill and wet the walls of the adsorbent capillaries because an impurity, such as air, may be permanently adsorbed on the walls.Normally many impurites will be present in the adsorbent which reduces the contact area for adsorbtion. As we said earlier we keep the pressure increasing which will displaces any impurities until, at the saturated vapour pressure of the adsorbate, almost complete coverage takes place. On desorption, the angle of contact in the Kelvin equation is thus zero. Hence, for a given volume adsorbed, the pressure Pa on adsorption is greater than that on desorption Pd

Kelvin equation is where P = pressure ,sigma = surface tension , theta= angle of contact

L= length.

As angle of contact changes during desorption (Zero) ,the pressure value also changes. This is the reason behind the hysteresis.