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| Emulsion, Adsorption, Adsorption Isotherm |
In general, the study of chemical interactions at
interfaces is referred to as surface chemistry. It has a lot in common with
surface engineering, which tries to alter the chemical composition of a surface
by adding particular elements or functional groups that have specific intended
effects or that enhance the qualities of the surface or interface. The
disciplines of heterogeneous catalysis, electrochemistry, and geochemistry all
benefit greatly from surface science.
You must have pondered how the particles in some types
of liquids stay continuously sustained not settling down. What precisely takes
place at the boundary between phases? Surface chemistry is the area of
chemistry that studies the interactions between different phases, particularly
those between liquid and gas. Surfaces actively participate in processes such
as chromatography, electrode reactions, colloid creation, and catalysis.
Adsorption
Adsorption is the word used to describe the
accumulation of species at higher concentrations on a substance's surface as a
result of intermolecular force. For instance, activated charcoal may absorb
gases like H2, O2, and N2.
Enthalpy of Adsorption is the quantity of heat
energy released when one mole of gas is adsorbed on the surface area unit of an
adsorbent.
Adsorption
Molecules attach on the surface of solid.
Absorption
Molecules moves into the surface of solids.
Desorption
Molecules moves out of the solid surface or removing
molecules from adsorbent.
Adsorbate
Substance that adsorbed on the solid surface is called
Adsorbate, for example gas adsorb on solid surface.
Adsorbent
It the
surface where adsorption occur, for example solid surface where gas adsorb is
called adsorbent.
Types of Adsorption
Adsorption in surface chemistry is divided into two
categories based on the strength of the interaction between the adsorbent and
the Adsorbate.
(A) Physical Adsorption
Physisorption, also known as physical adsorption. Between
adsorbent and adsorbate, a weak van der Waals force is present.
It has following Characteristics:
1. It has forces of
Van der Waals that are weak.
2. The procedure can
be undone. It lacks specificity in nature.
3. This procedure has
several layers.
4. Low enthalpy of
adsorption (between 20 and 40 KJ/mole).
5. Very little
activation energy
6. Desorption process
simply occur.
7. Affecting
variables include pressure, temperature, and the surface area of the adsorbent.
It is caused by the potent chemical interactions of
the adsorbent and adsorbate.
Characteristics:
1. Strong chemical
forces are the nature of the forces, and nature is quite particular.
2.
It cannot be
reversed
3. It is a
single-layered technique with high adsorption enthalpies (between 40 and 400
KJ/mole).
4.
Extremely high
activation energy
5. Desorption is very
challenging
6. Factors influencing depend on Adsorbent surface area and adsorbate type Temperature.
Pressure's impact
on a gas's adsorption on a solid
The ratio of gas adsorbed to the total gas is
proportional to the gas pressure. Adsorption increases with pressure until a
maximum is reached and then remains constant.
Temperature
effects on gas adsorption on solids
Gases always adsorb onto solids in an exothermic manner. Physical adsorption adheres to the Lechatlier Principle, which states that as temperature rises, less gas is adsorbed. As temperature rises, chemisorption increases. It reaches its peak before declining. Adsorption isobars are obtained by graphing the proportion of gas adsorbed and temperature at constant pressure.
Adsorption Isotherms
Isotherm means constant temperature, so at constant temperature
amount of gas adsorbed on adsorbent with pressure expressed by using curve this
is called adsorption isotherm.
Isotherm of Freundlich Adsorption
The quantity of gas a unit mass of solid adsorbent can
adsorb and pressure at a specific temperature can be empirically related,
according to German scientist Freundlich, in 1909.
x/m = k.P1/n (n > 1)
where, at pressure "P," "x" is the
mass of the gas adsorbed on the mass "m" of the adsorbent. The
constants "k" and "n" depend on the characteristics of the
gas and the adsorbent at a specific temperature.
To illustrate the relationship, a graph representing
the mass of the gas adsorbed per grams of the adsorbent is shown against
pressure. Physical adsorption here reduces as temperature rises at a fixed
pressure. When the pressure is
high, the curves saturate. Taking the log of the above equation now,
Log
x/m = log k + 1/n log P.
We can plot log x/m on the y-axis and log P on the
x-axis in order to examine the reliability of the Freundlich isotherm. The
Freundlich isotherm is true if the plot displays a straight line; otherwise, it
is false. The intercept on the y-axis provides the value of log k, whereas the
slope of the straight line provides the value of 1/n.
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| Freundlich Adsorption Isotherm |
Limitations
Value of 1/n ranges between o and 1 only so,
When x/m is constant and 1/n = 0, the adsorption is
free of pressure dependence. The amount of adsorption is directly proportional
to pressure when 1/n = 1, x/m = k P, or x/m P.
Results from experiments back up both of the
conditions mentioned. The experimental isotherms appear to always approach
saturation at high pressure. Freundlich isotherm fails at high pressure because
it cannot account for this observation.
The adsorption of gases on solids under higher pressure is not covered by it.
Isotherm of Langmuir Adsorption
At greater pressures, the Freundlich adsorption
isotherm fails, indicating that his theory is not totally accurate. Langmuir
presented a novel idea in 1916 that takes into account the influence of
chemical (intermolecular) interactions. The Langmuir adsorption isotherm is the
mathematical equation he developed for the adsorption phenomena based on the
kinetic molecular theory of gases.
θ = bP / (1 + bP)
where,
θ is of the surface covered by species that have been
absorbed
The adsorption coefficient is b.
P is the pressure being used.
Assumptions
The Langmuir adsorption isotherm requires the
following presumptions in order to function:
1.
The solid surface
must be uniform.
2.
It ought to
contain a set quantity of adsorbent surface.
3.
Only one molecule
can adsorb at each location (mono-layered adsorption).
4.
A monomolecular
layer is where molecules can only adsorb.
5.
In the vapour
phase, the adsorbed gas acts perfectly.
6.
The molecules that
are adsorbed do not interact.
7.
At their dynamic
equilibrium, the rates of adsorption and desorption become equal.
Emulsions
Emulsions are liquid-based colloidal solutions that
contain both the dispersed phase and the dispersion medium.
Paints, colours, milk, and vanishing cream, for
instance. Surface chemistry divides emulsions into two categories based on the
dispersion medium:
Water and Oil Emulsions
This involves dispersing oil or fat droplets in water.
Consider milk.
Oil Emulsions with Water
The water droplets in this are mixed with the oil. For
instance, vanishing cream.
Emulsions are prone to instability; hence emulsifiers
are typically added to improve stability. Example: Casein is a natural emulsion
found in milk.
Emulsion applications
They are utilised in syrups, Paints, Toothpaste, Fat
Digestion, Colorants and dyes
See more
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