Surface Chemistry

Surface Chemistry

Table of Contents


Surface Chemistry is an important branch of Chemistry that deals and talks about the various phenomena that occur at the surface level or interface level that is the boundary separating two phases. In order to find an accurate result, one needs to conduct studies on clean surfaces.

Adsorption and Absorption

Adsorption is the term used to define the attraction or the retention quality of the molecules present on the surface. Absorption on the other hand is the process in which the substance gets evenly distributed; be it through solid or liquid.

Factors that affect the adsorption of gases by solids

  • Nature of the gas in question
  • Temperature
  • Pressure applied
  • Nature and surface area of adsorbent

The simultaneous process of absorption and adsorption is known as sorption.

Difference between Adsorption and Absorption



It is a phenomenon pertaining to the surface

In this process, the substance does the penetration of the surface

The rate declines after it increases initially

Uniformity is maintained

Temperature is the affecting factor

Temperature doesn’t affect it

The process is exothermic in nature

Absorption is endothermic in nature


Difference between Adsorption and Absorption
Types of Adsorption

There are mainly two categories of adsorption. They are physical adsorption and chemical adsorption.

Physical Adsorption: It is also known as physisorption. This process takes place if the adsorption of gas happens on a solid surface by the forces of vander waals and doesn’t form any type of chemical bond between the two; adsorbent and adsorbate is known as physical adsorption.

Chemical Adsorption: Also known as Chemisorption, this happens when a gas is adsorbed on the solid surface by forces that form chemical bond. As a result of this, surface compounds are formed.

There are differences between the two adsorptions. They are as follows-

Physical Adsorption

Chemical adsorption

Multi-molecular layer is formed

It forms uni-molecular layer

No activation energy is required

Activation energy is required

Weak Vander Waal’s forces are in use

Chemical bonding forces are in use

With the increase in the area of the adsorbent, it increases

With the increase in the surface area of the adsorbent, it increases

Low temperature is required for it to take place

It increases with the increase in temperature

Adsorption Isotherm

It is the graphical depiction of the adsorbate amount that has been adsorbed on adsorbent surface along with pressure when the temperature is constant. One of the important isotherms to know is Freundlich Isotherm.

Freundlich Isotherm

As per this isotherm, the extent of the adsorption at a given surface increases with the gradual increase in the pressure applied and concentration while the temperature remains constant.

The expression of the extent of surface is x/m. Here x stands for adsorbate mass and m stands for adsorbent mass and that is applicable when the equilibrium of adsorption is reached. 




Freundlich IsothermAdsorption isotherm is the graph that shows the relation of the amount of gas adsorbed per gram of adsorbent with that of the equilibrium pressure of the adsorbate at a temperature that is constant.

There are certain limitations of the isotherm. They are as follows-

  • The values used for experimentation undergo changes when high pressure is applied
  • The relation is not accurate but an approximate one
  • Limited range of pressure holds more value in this isotherm

Factors that affect adsorption from solution as well as adsorption bars

There are various factors that affect adsorption. Let us know those factors.

  • Adsorbent nature and the nature of the adsorbate
  • The surface area of the adsorbent
  • Temperature

Colloidal Solutions

There are solutions in which the size of the solute particles falls between 10 to the power -9 and 10 to the power -6. These type of solutions is known as colloidal solutions.

Properties of Colloidal Solutions

  • Aggregates are formed bigger in size in comparison to the true solutions
  • Tyndall effect in colloidal solutions is scattering
  • Depending on the wavelength of the scattered light dispersed by the particles, colloidal solution gets its color
  • The Brownian movement is affected by the size and the viscosity of the solution
  • The particles of colloidal solutions carry electric charge along with them.


Micelle is formed as a result of the concentration called Critical Micelle Concentration.

Preparation and Purification of Colloidal Solution

Here is how you can prepare and purify the colloidal solution-

Mechanical disintegration: For this process to take place, there is the need of colloid mill, ball mill or ultrasonic disintegrator. Into the mill, suspension of substance in water mill is introduced. The suspension particle size is then reduced to the size of a colloid.

Electro disintegration: For getting the colloidal solutions of metals, this process is used. Intense heat is used and this heat breaks down the metals in smaller particles.

Peptization: The process of conversion of fresh precipitate into particles of colloidal solution by using dispersion medium in shaking it. This happens in the presence of electrolyte.

Purification methods

Purification: The process in which the unwanted particles are purified to get the required minimum level.

Dialysis: The separation of particles of colloids from that of crystalloids by the diffusion of the mixture through animal membrane.

Few things to remember about colloidal solutions are that they are extremely stable in nature when compared to other solutions and they have immense filterability.

Important Properties

Colligative properties: If the solutions depend on the number of molecules of solute particles in the solution are known as colligative properties.

Mechanical properties: This property can also be called as the Brownian movement. It is the process of the movement of the colloidal particles constantly in the zig-zag pattern.

Optical Properties: It is also known as the Tyndall effect and it is nothing but the scattering of the light by the colloidal particles.

Tyndell effect has certain conditions-

  • It can show its impact when the diameter of the dispersed particle is not extremely smaller than that of the wavelength of the light in use
  • The difference between refractive indices and dispersion medium is not too large


It is a colloidal dispersion in which the dispersed phase as well as the medium of dispersions is in liquid forms.

Types of Emulsion

  • Emulsion of oil in water
  • Emulsion of water in oil

Applications of Emulsion

  • For the building of roads, bridges etc.
  • Oily drugs formation
  • Ordinary soap, washing powder etc.
  • In the making of Dettol etc.


Substances that alters the rate of reaction process without any change is the mass and composition are known as catalysts.

Catalysis: The process that takes place in the decomposition with the help of catalyst is known as catalysis.

Types of catalysis: Catalysts as well as reactants can have the same form or in different forms. On the basis of it, catalysis can be divided as :-

Homogeneous catalyst: If they are in the same form/phase, for example, if the catalyst is solid, the reactant is also solid, it is known as homogenous catalysis.

Homogeneous catalyst
Heterogeneous catalysis
: On the other hand, if the catalyst and the reactant are in different phases, it is known as heterogeneous catalysis.

Heterogeneous Catalysis
Features of Catalyst

  • With the same reactants in consideration, the selectivity feature of catalysts is different
  • Mobilization of the catalyst depends on the adsorption strength. Stronger the adsorption, higher the immobility

Enzyme as a catalyst

Complex organic molecules that are produced by plants and animals are known as enzyme catalysts. They are responsible for various reactions to take place on the earth’s surface and even below the earth’s surface.

Properties of enzyme catalysts

  • They are efficient in nature
  • They are specific in higher order
  • They become active when they are kept under a specific temperature
  • Under optimum pH balance, they work actively
  • With activators, they increase their efficiency while inhibitors reduce their efficiency

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