Surface Chemistry: Adsorption and its types

The branch chemistry which deals with the nature of surfaces and change occurring on the surfaces is called surface chemistry. Adsorption on solid or on solution surfaces and colloidal properties are important surface effects which are useful to understand many physical and chemical properties of the substances.

Adsorption is a surface phenomenon. It arises because of the unbalanced forces on the surface of the solids and liquids. ‘The phenomenon of higher concentration of molecular species (gases or liquids) on the surface of a solid than in the bulk is called adsorption’.

The solid substance on the surface of which adsorption occurs is known as adsorbent. The substances that get adsorbed on the solid surface due to intermolecular attractions are called adsorbate. The adsorbent may be a solid or a liquid and the adsorbate may be a gas or a solute in some solution. For examples, Activated charcoal has a tendency of adsorb a number of gases like ammonia, sulphur dioxide, chlorine, phosgene, etc. In this case, charcoal acts as an adsorbent while gas molecules act as adsorbate).

Difference between Adsorption and Absorption: The Important points of distinction between adsorption and absorption are-

Positive adsorption: When the concentration of adsorbate is more on the surface of adsorbent relative to its concentration, in the bulk, it is called positive adsorption.

Negative adsorption: When the concentration of the adsorbate is less on the surface relative to its concentration in the bulk, it is called negative adsorption. For example, in case of some liquid solutions, it is observed that the concentration of the solute is less on the surface than in the bulk.

Types of Adsorption: Depending upon the nature of forces existing between adsorbate molecules and adsorbent, the adsorption can be classified into two types-

1)  Physical Adsorption

2) Chemical Adsorption

1)  Physical Adsorption:

When the particles of the adsorbate are held to the surface of the adsorbent by the physical forces such as Van der Waals forces, the adsorption is called physical adsorption or physisorption. The attractive forces are weak and therefore, these can be easily overcome either by increasing the temperature or by decreasing the pressure. In other words, physical adsorption can be easily reversed.

2. Chemical adsorption:

When the molecules of the adsorbate are held to the surface of the adsorbent by the chemical forces, the adsorption is called chemical adsorption or chemisorption. In this case, a chemical reaction occurs between the adsorbed molecules and the adsorbent on the surface. This type of adsorption is irreversible.

Difference between Physical and chemical adsorption: Some of the important points of distinction between physical and chemical adsorption are given below in the table:

Factors affecting the adsorption of gas on solid: The extent of adsorption of a gas on a solid surface is affected by the following factors:

1. Nature of the gas 2. Nature of adsorbent 3. Effect of pressure
2. Effect of temperature 5. Activation of absorbent

1) Nature of the gas:

The adsorption depends upon the nature of the gas adsorbed. The easily liquefiable gases such as HCI, NH₃, Cl₂ etc. are adsorbed more than the permanent gases such as N₂ and O₂.

The ease of liquefication of a gas depends upon its critical temperature. The critical temperature of a gas is the minimum temperature above which a gas cannot be liquefied however high the pressure may be applied. The higher the critical temperature, the more easily a gas is liquefied and hence more readily it will be adsorbed.

2) Nature of adsorbent:

The extent of adsorption of a gas depends upon the nature of adsorbent. Activated charcoal can adsorb gases which are easily liquefied. Many poisonous gases are adsorbed by charcoal. Therefore, it is used in gas masks for adsorbing these poisonous gases. Gases such as H₂, N₂ and O₂ are generally adsorbed on finely divided transition metals e.g. Ni and Cu. The extent of adsorption depends on the available surface.

3) Effect of pressure:

The extent of adsorption of a gas per unit mass of adsorbent depends upon the pressure of the gas. The relation between the amounts of substance adsorbed by the adsorbent and the equilibrium gas pressure (or concentration for solutions) at constant temperature is called an adsorption isotherm. This may be expressed in the form of an equation or graph. The extent of adsorption is usually expressed as (x/m) where x is the mass of adsorbate and m is the mass of the adsorbent.

It is clear from the figure that extent of adsorption (x/m) increases with pressure and becomes maximum corresponding to pressure Ps, called equilibrium pressure. Since adsorption is a reversible process, desorption also takes place simultaneously. At this pressure (Ps), the amount of gas adsorbed becomes equal to the amount of gas desorbed so that the extent of adsorption becomes constant even though the pressure is increased. This state is also called saturation state and is called saturation pressure.

Adsorption isotherms of different shapes have been observed experimentally. There are two type of adsorption namely Freundlich adsorption and Langmuir adsorption isotherm.

Freundlich’s Adsorption Isotherm:

The variation of extent of adsorption (x/m) with pressure (P) was given mathematically by Freundlich. From the adsorption isotherm as shown in the figure above, the following observations can be easily made:

1. i) At low pressure:

The graph is almost straight line which indicates that (x/m) is directly proportional to the pressure. This may be expressed as:

(x/m) α P

Or,       (x/m) = k P

Where, k is a constant.

(ii) At high pressure:

The graph becomes almost constant which means that (x/m) becomes independent of pressure. It may be expressed as:

(x/m) = Constant

Or,       (x/m) α P°                                           (P° = 1)

Or,       (x/m) = k P°

iii) At intermediate pressure:

The extent of adsorption (x/m) will depend upon the power of pressure which lies in between 0 to 1. This may be expressed as-

(x/m) α P^1/n

Or,       (x/m) = k P^1/n

Where, n can take any whole number value which depends upon the nature of adsorbate and adsorbent. The above relationship is also called Freundlich’s adsorption isotherm.

The constant k and n can be determined by taking logarithm on both sides of above equation, we get,                                             log (x/m) = log k  + 1/n log p

Thus, if we plot a graph between log(x/m) and p, a straight line will be obtained. The slope of the line is equal to 1/n and the intercept is equal to log k.

4) Effect of temperature:

The process of adsorption is exothermic. Therefore, with the increase in temperature at constant pressure, the extent of adsorption (x/m) will decrease. The graph between extent of adsorption and temperature at constant pressure is called adsorption isobar. However, this is true for physical adsorption as shown in figure (a) below. In case of chemisorption, the adsorption initially increases with rise in temperature and then decreases as shown in the figure (b) below.

This behaviour is expected because like all chemical reactions, some activation energy is required for chemisorption. At low temperature, x/m is small. As temperature is increased, the molecules of the adsorbate gain energy and become equal to activation energy so that proper bonds are formed with the adsorbent molecules. Therefore, initially amount of gas adsorbed increases with rise in temperature.  Further increase of temperature will increase the energy of molecule which have already been adsorbed. This would increase the rate of desorption and therefore, decrease the extent of adsorption.