B.Sc. Second year Undergraduate degree course (CBCS Pattern)
Semester Third
Physical Chemistry (CHE-312)
Chapter – Surface Chemistry
Introduction to surface chemistry
Surface chemistry is a branch of
chemistry that deals with the study of chemical reactions, physical processes,
and phenomena that occur at the interface of two phases, usually involving a
solid surface and a gas or liquid phase. The surface in question is the
boundary between these phases, where molecules interact with each other in
distinct ways compared to their behavior in bulk.
In surface chemistry, scientists
investigate the properties and behaviors of surfaces, such as adsorption,
desorption, catalysis, corrosion, and surface reactions. The reactions and
processes that take place at the surface are critical in various fields,
including materials science, catalysis, electrochemistry, environmental
science, and nanotechnology.
Understanding surface chemistry is
essential for developing and improving various applications and technologies,
such as catalyst design for industrial processes, drug delivery systems,
corrosion prevention, and the development of high-performance materials.
Researchers in this field use techniques like spectroscopy, microscopy, and
computational modeling to gain insights into surface phenomena and reactions.
Some basic terms related to surface
chemistry
Adsorption:
The process of molecules or ions adhering to the surface of a solid or liquid.
Fig.- Adsorption process
Adsorbate:
Substance that is deposited on the surface of another substance. For example,
H2, N2 and O2 gases.
Adsorbent:
Surface of a substance on which adsorbate adsorbs. For example, Charcoal,
Silica gel, and Alumina.
Positive adsorption:
When the concentration of solute adsorbed on the solid adsorbent surface is
greater than in the bulk it is called positive adsorption.
Ex. Concentrated solution of KCl is shaken with blood charcoal, it shows positive Adsorption
Negative adsorption:
When the solvent from the solution may be absorbed by the adsorbent so that the
concentration of the solute decreases and the concentration of solution
increases than the initial concentration and it is called negative adsorption.
Ex. Dilute solution of KCl is shaken
with blood charcoal it shows negative adsorption.
Absorption: Absorption
is a separate mechanism from adsorption because molecules undergoing absorption
are soaked up by the length, not by the air. Adsorption is based on the surface
where a film of adsorbate is developed on the surface, and absorption includes
the complete volume of the absorbing agent.
Fig.- Absorption process
Difference between Adsorption and
absorption
|
Sr. No. |
ABSORPTION |
ADSORPTION |
|
1. |
It is a bulk phenomenon |
It is a surface phenomenon |
|
2. |
It is a slow process |
It is a fast process |
|
3. |
Substance uniformly distributed throughout the surface |
Higher concentration of molecular
species in the surface than
in the bulk |
|
4. |
Attainment of equilibrium takes time |
Equilibrium attained easily |
|
5. |
Eg.:Ammonia adsorbed in charcoal |
Eg.: Ammonia adsorbed in water |
Desorption:
The reverse process of adsorption, where adsorbed molecules are released from
the surface.
Fig.- Adsorption and desorption
Physisorption:
A type of adsorption where weak van der Waals forces are involved in binding
the adsorbate to the surface.
Chemisorption:
A type of adsorption where chemical bonds are formed between the adsorbate and
the surface.
Monolayer:
A single layer of adsorbed molecules on the surface.
Multilayer:
Multiple layers of adsorbed molecules on the surface.
Surface area:
The total area of the surface available for adsorption.
Surface reaction:
A chemical reaction that occurs at the surface of a solid.
Surface science:
The study of physical and chemical phenomena that occur at surfaces and
interfaces.
Surface modification:
The intentional alteration of a surface's properties through various chemical
or physical methods.
Surface energy:
The excess energy associated with atoms or molecules at the surface compared to
those in the bulk material.
Adsorption
materials
An adsorption material is a substance
that has a high surface area and is used to adsorb other substances. Adsorption
materials are used in a variety of applications, including water purification,
air purification, and separation processes.
Some common adsorption materials
include:
1.
Activated carbon: Activated
carbon is a highly porous material that is made from carbon-rich materials such
as wood, coconut shells, and coal. Activated carbon has a very high surface
area, which makes it an excellent adsorbent for a variety of substances.
Activated carbon is used in water purification to remove impurities such as
dissolved metals, organic compounds, and bacteria. It is also used in air
purification to remove pollutants such as dust, pollen, and smoke.
2.
Zeolites:
Zeolites are microporous, crystalline aluminosilicates. Zeolites have a high
surface area and a well-defined pore structure, which makes them effective adsorbents
for a variety of substances. Zeolites are used in water purification to remove
impurities such as heavy metals and organic compounds. They are also used in
air purification to remove pollutants such as volatile organic compounds
(VOCs).
3.
Silica gel: Silica
gel is a porous, amorphous form of silicon dioxide. Silica gel has a high
surface area and is used to adsorb a variety of substances, including water,
alcohols, and hydrocarbons. Silica gel is used in water purification to remove
moisture. It is also used in air purification to remove moisture and VOCs.
4.
Graphene: Graphene
is a two-dimensional material made of carbon atoms arranged in a hexagonal
lattice. Graphene has a very high surface area and is a very efficient
adsorbent for a variety of substances. Graphene is used in water purification
to remove impurities such as heavy metals and organic compounds. It is also
used in air purification to remove pollutants such as VOCs and particulate
matter.
The choice of adsorption material
depends on the specific application. For example, activated carbon is a good
choice for removing organic compounds from water, while zeolites are a good
choice for removing heavy metals from water. Silica gel is a good choice for
removing moisture from air, while graphene is a good choice for removing VOCs
from air.
Factors affecting adsorption
1.
Nature of Adsorbent
The adsorption depends on the type of
adsorbents used. When the adsorbent is highly porous the rate of adsorption
increases. Activated carbon, metal oxides like aluminum oxide, silica gel and
clay are commonly used adsorbents. The rate of adsorption can be increased by
activation process. It helps in enhancing the pores in the adsorbent
Eg. Charcoal adsorbs 0.011 gms of CCl4
at 24°C and activated charcoal adsorbs 1.48 gm of at 24°C.
2.
Surface area of adsorbent
Increase in surface area of the
adsorbent increases the adsorption of gases and the extent of adsorption
depends on two factors
i)
Greater the surface area greater the
adsorption-Increase in surface area increases the number of adsorbing sites.
ii)
Larger the porosity greater the
adsorption-Finely divided and highly porous materials acts as good adsorbents.
Eg. Charcoal and silica gel (excellent adsorbents).
3.
Nature of Gases
The amount of gas adsorbed by a solid depends
on the nature of the gas. Easily liquefiable gases like HI, NH3, CI2,
SO2 etc., are adsorbed more easily then the permanent gases like H2,
N2, and O2 etc. Physical adsorption is non-specific in
nature, so any gas will be adsorbed on the surface under any given conditions
of temperature and pressure. Chemisorption is specific in nature so only those
gases which forms chemical bonds will be adsorbed.
4.
Exothermic Nature
Heat of adsorption is defined as the
energy liberated when 1 g mol of a gas is adsorbed on a solid surface. Increase
in temperature increases the kinetic energy of the gas molecules and it results
in more number of collisions of gas molecules over the adsorbent surface.
5.
Pressure
When pressure is increased then the
rate of adsorption increases initially. The extent of adsorption is expressed
as x/m where ‘x’ is amount of adsorbate ; ‘m’ is mass of adsorbent when the
dynamic equilibrium is established between free gas and the adsorbed gas. But
after some time it reaches appoint where no more adsorption occurs and at this
point adsorption is independent of pressure.
Surface Chemistry: Introduction,some basic terms, adsorption
materials, factors affecting adsorption
Surface Chemistry:characteristics of adsorption, types of
adsorption, classification of adsorption isotherms, Langmuir adsorption
isotherm
Surface Chemistry:Freundlich's adsorption isotherm, BET
theory , application of adsorption
Surface Chemistry: Multiple Choice questions
