Surface Chemistry-introduction, Adsorption, types, and application

LEARNING OBJECTIVES

In this article, the author has explained about surface chemistry, adsorption and types of adsorption.

What is surface chemistry?

It is that branch of chemistry, which deals with the interaction of a certain substance at the surface of a solid substance.

When a solid surface is exposed to a gas or a liquid, the molecules from the gas or a solution phase concentrate at the surface of solid. We can also say that molecules of gas are accumulated on the surface of the solid.

image showing surface chemistry example

Surface chemistry is the science of the interactions between substances and surfaces. These include solid-liquid interactions, such as the formation of films and other deposits, as well as the interactions between solids themselves, such as the formation of solid-solid aggregates.

Other surface phenomena include adsorption and reaction, in which the adsorbed species undergo chemical reactions and undergo changes in their structure. In this sense, surface chemistry differs from surface science.

While surface science studies isolated surfaces in a vacuum, and the effects of other elements, surface chemistry includes these as well as the interactions between the surface and surrounding media.

Surfaces are the outermost layer of a material, or an element. They can be considered as an interface, where two or more elements come together to form a heterogeneous system.

Surface chemistry studies the chemical properties of surfaces and the behavior of molecules at surfaces, such as surface diffusion, adsorption, and reaction.

Surface chemistry also covers the chemical modification of surfaces, including the creation of new functionalities, or the immobilization of molecules, as well as the removal of undesired species.

Surface chemistry has its own branch of knowledge. A chemist who studies surface chemistry is called a surface chemist.

What Are Types Of Surfaces?

There are three basic types of surfaces:

  • Gaseous
  • Liquid
  • Solid

Each of these three surface types has its own characteristics, and these differences cause surface chemistry to be complex. Each type of surface is defined by the way in which the molecules of the liquid phase interact with the molecules of the solid phase.

1: Gases

A gas is a collection of molecules that is neither liquid nor solid. Gases are composed of atoms that are held in random arrangement by the intermolecular forces of attraction and repulsion. There are two types of gases:

  • Energetic gas
  • Non-energetic gas

An energetic gas is one that contains the potential energy of the molecules, which is released when the gas is heated. The molecules of energetic gases have a strong force of attraction. Non-energetic gases contain no potential energy and are therefore very rare.

Non-gases

These are the gases that are liquid at room temperature. Examples of non-gases are air, nitrogen, and oxygen. The molecules of these gases are held together by the strong force of attraction, and are easily separated.

What is adsorption?

This phenomenon of accumulation of the molecules of a gas or a liquid at a solid surface is called adsorption.

Adsorbate and Adsorbent

That substance which concentrates upon the surface of a solid is called adsorbate and the solid is called adsorbent.

Examples:

1: Methylene blue is on organic dye. When finally divided charcoal is stirred into the dilute solution of methylene blue, then the molecules of dye are adsorbed by the particles of charcoal. The process of adsorption is noted by the effect, that intensity of the colour of solution decreases,

2: It has also been observed that the pressure of the gases like SO2 Cl2 and NH3 are decreased, when powdered charcoal is placed in the vessels of these gases. Actually, the molecules of these gases concentrate on the charcoal surface and we say that gas has been adsorbed on the surface of charcoal.

Difference between adsorption and absorption

As we have discussed above, that in case of adsorption the concentration is present on the surface of solid. In the case of absorption, the outer substance penetrates into the body of absorbent.

Anyhow, absorption and adsorption take place side by side. For this purpose, a new term has been introduced which is called sorption. This phenomenon includes both adsorption and absorption.

Why absorption takes place?

The atoms and molecules of solid substance present in the bulk of a solid are satisfied due to surrounding atoms and molecules. Their valencies are fulfilled.

The atoms and molecules at the surface of solid are unbalanced .There are residual attractive forces on the solid surface: These residual forces are responsible to hold the molecules of adsorbate.

Types of adsorption

When a gas is adsorbed on the surface of a solid, then two types of adsorptions can be thought of

  • Physical adsorption
  • Chemical adsorption

Physical adsorption

ix that type of adsorption, which is due to presence of Van der Wal attractive forces between the gas molecules and the solid surface The adsorption of H, and O, on the surface of charcoal is physical adsorption. This is also called Van der Waals adsorption.

Chemical adsorption

When the molecules of a gas are held by the solid substance by chemical bonds, then it is called chemical adsorption. It is also called chemisorption. Hydrogen is chemisorbed on nickel.

Hydrogen has physical adsorption on nickel and the after dissociation of H, it gets chemisorption on the surface of nickel. It means that process of adsorption is a combination of two types of adsorptions i.c. physical and chemical.

Difference of physical and chemical adsorption

When the gases are adsorbed on the solid surface then, there can be physical or chemical adsorption. These two types of adsorptions differ in many respects. Let us discuss some of them.

1: Surface area

The extent of adsorption depends upon surface area. Greater the surface area of adsorbent, greater the amount of the gas adsorbed. If nickel and platinum metals are finally divided, then they adsorb the hydrogen gas to greater extent just like charcoal and silica gel.

2: Nature of gas

Greater the critical temperature of gas, greater the possibility of adsorption. Critical temperature of SO2 is 157°C and 1 g of activated charcoal adsorbs 380 cm³ of SO2 but adsorption of CH4 and H2 is less.

3. Heat of adsorption

It is the amount of heat evolved when Ig mole of a gas is adsorbed on the solid surface. This heat of adsorption is less for physical adsorption and is in the range of 5 kcal mol-1 It varies from 20-100 kcal mol-1 for chemisorption

4: Reversible process

The gas which is adsorbed on the solid surface, can be desorbed under reverse conditions of temperature and pressure. Anyhow, chemisorption is not reversible process because a compound is produced upon the surface of the solid.

5: Effect of temperature

Low temperature favours the physical adsorption while chemisorption generally increases with temperature For example, nitrogen shows physical adsorption on the surface of iron at 190°C, but at 900°C chemisorption takes place to form nitride

6. Effect of pressure

It is a known fact that there is a dynamic equilibrium between the adsorbed gas and that which is present in the bulk of gas. It is found that the increase of pressure leads to the increase of adsorption. When the pressure is decreased, then it causes desorption.

7: Thickness of adsorbed layer

It was determined by Langmuir that the layer of the adsorbed gas has a thickness of one molecule at low pressure in the case of physical adsorption. If the pressure is high, then multi-molecular thick layer is formed.

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