Role of Adsorption in Surface Chemistry

We see a lot of materials around us.  We see that every solid material has a surface, although surface is also present in liquid materials but in surface chemistry, we deal with surfaces of solids. We often notice that one matter gets collected at the surface of another matter. We see dirt on clothes, we colour clothes, we see that ammonia gets collected on the surface of active charcoal etc. This gathering of one material over another is a very common phenomenon known as ‘adsorption’. It can be permanent and temporary according to the nature and property of the materials. 

Meaning of Adsorption-

The meaning of adsorption is defined by the forces of the residual surface whose action brings some change in the concentration of the surface. Adsorption is a very crucial phenomenon in surface chemistry. It occurs due to imbalance of the molecular forces present at the surfaces of solid or liquid materials. These molecular forces are in a balanced position inside the solids or liquids but not on the surfaces. This imbalance of the molecular force is responsible for the unsaturation of the surface. Due to this unsaturation, solids or liquids try to attract the ions, atoms or molecules on the surface and retain them, so that the surface could get saturated and balanced. This tendency of solids or liquids is responsible for the physical or chemical bonding. A time arrives when the concentration of these attracted particles surpasses the concentration of particles inside the solid, this alteration in the concentration of the surface is called the process of ‘adsorption’. 

Now, the surface of the material over which the particles from outside get accumulated, is called the ‘adsorbent’ and ions or atoms or molecules which get accumulated on adsorbent are called the ‘adsorbate’. In the accumulation of ammonia on the surface of active charcoal, charcoal is the adsorbent and ammonia, the adsorbate.

The process in which the adsorbate and adsorbent are placed very close together due to which the molecules of adsorbate intrude or trespass inside the adsorbent and get dispensed in all over the lattice, is called the ‘absorption’. Starch’s absorption of iodine, adsorption of bromine on the activated charcoal shows the difference between the adsorption and absorption.

If the process if adsorption and absorption take place at the same time which means that the particles do not cluster at the surface and go inside the surface, it becomes difficult to identify the exact quantity of adsorption and absorption, this is called ‘sorption’. For example- the process of sorption happens when palladium and hydrogen come in contact with each other.

At the surface of adsorbent, excreted energy in adsorption of one mole of adsorbate is known as the ‘enthalpy of adsorption’. 

Adsorption depends on various factors-

Adsorbent’s nature- There is a domain of field of residual forces, more the domain, more the adsorption.

Behaviour of Adsorbent- The gases at the high critical temperature are better at absorption than the gases at low critical temperature.

Pressure and Temperature- Generally, the increasing pressure also increases the process of adsorption at the surface of gases. If we do the same at low temperatures, the adsorption of gases increases rapidly in the beginning but later it gains a limiting value. This effect of pressure at adsorption is often represented through an adsorption isotherm.

At constant pressure, by a constant quantity of adsorbent, a relation in mathematical form is established between the quantity of adsorbate and equilibrium pressure, known as the adsorption isotherm.

Conclusion-

The adsorption is a process in which the adhesion of various molecules or ions or atoms at the surface of a matter takes place. If the concentration of adsorption is more on the surface in comparison to the concentration in the interior part, it is called positive adsorption and the opposite of it is called the negative adsorption. Many factors like temperature, pressure and nature of adsorbent affects the rate of adsorption. Adsorption usually takes place at the surface of solids but this concept is true for all kinds of surfaces also.