8/8/2019 Adsorption 12.10.9

1/34

UNIT - 4. SURFACE CHEMISTRY

ADSORPTION

Introduction

Adsorption: Concentration or assimilation of a gas (or liquid) at the surface

of a solid (or liquid)

Occlusion. The adsorption of gases on the surface of solids

Adsorbent: The material providing the surface upon which adsorption

occurs

Adsorbate: The substance get adsorbed or attached on the surface ofadsorbent

Ex: Charcoal, Silica gel, Alumina gel, Clay, etc.

Desorption: The removal of adsorbed substance from the surface

Positive adsorption: Concentration of the adsorbate is more on the surface

of the adsorbent than in the solution (bulk).

e.g., In the concentrated solution of KCl, charcoal adsorbs KCl rather than

water and this leads decrease in concentration of KCl in solution.

Negative adsorption: Concentration of the adsorbate is less on the surface

of the adsorbent than in the solution (bulk)

e.g. In the dilute solutions of KCl, charcoal adsorbs water, thereby the salt

concentration is increased.

Exothermic nature of adsorption

Occurs spontaneously as the unbalanced or residual forces acting along the

surface

The adsorbent has a tendency to attract and retain molecules of other

species

8/8/2019 Adsorption 12.10.9

2/34

Results in decrease in the residual forces, thereby resulting decrease of

surface energy, which in turn appears in the form of heat.

The amount of heat evolved when 1 mole of any gas (or vapour) is adsorbed

on a solid adsorbent surface, is called enthalpy (or heat) of

adsorption.

Adsorption and Absorption:

Absorption: Substance distributed throughout the body of the solid or liquid

Adsorption Absorption

1. Concentration or assimilation of a

gas (or a liquid) at the surface of a

solid (or

liquid)

2. A surface phenomenon

3. A fast process

4. Equilibrium is attained easily

1. The substance assimilated is

uniformly distributed throughout the

body of the solid

or liquid.

2. A bulk phenomenon

3. A slow process

4. Attainment of equilibrium takes

some time

8/8/2019 Adsorption 12.10.9

3/34

8/8/2019 Adsorption 12.10.9

4/34

5. Forces responsible for such

adsorption are very weak

6. The rate of adsorption increases

with the increase of pressure or

concentration of the adsorbate. Near

saturation pressure, multilayers are

formed

7. The amount of adsorption on a

surface is more function of the

adsorbate than the adsorbent

8. Such adsorption involves very

small or little activation energy

9. The equilibrium is established

rapidly

10. No surface compound formation

takesplace

11. It is not very specific in nature

Such adsorption, generally, involves

appreciable activation energy

responsible for such adsorption are

quite strong

The rate of adsorption decreases with

the increase of pressure or

concentration of the adsorbate. Near

saturation pressure, adsorption rate

decreases, since the adsorption is

confined only to upper surface layer

of adsorbent

The amount of adsorption is

characteristicof both adsorbate and

adsorbent.

Activation energy is involved in

chemical bond formation

Establishment of equilibrium

requires time

Actual surface compound formation

between the adsorbent and adsorbate

takes place.

It is highly specific in nature

8/8/2019 Adsorption 12.10.9

5/34

ADSORPTION OF GASES ON SOLIDS

Factors affecting the adsorption of gases on solid surfaces

(1) Nature of the gas:

Easily liquefiable gases (like HCl, NH3, Cl2, etc.) are adsorbed more easily

than the permanent gases (like H2, N2, O2, etc.).

The ease of liquefaction of a gas depends upon its critical temperature

The higher the critical temperature (Tc), the more easily the gas is liquefied

and consequently, more readily it is adsorbed.

The critical temperature increases the adsorption also increases

Adsorption of various gases on 1 g of activated carban

Gases SO2 NH3 CO2 N2 H2

C.T (K) 430 406 304 126 33

Amount of gas

adsorbed (ml)

380 180 48 8.0 4.5

(2) Nature of adsorbent:

Greater the surface area of the adsorbent, greater is its adsorption capacity.

Activated charcoal and silica gel are excellent adsorbents, since their

structure is highly porous and hence, possess large surface areas.

Activated charcoal and finely divide solid substances are better adsorbents.

Activation of adsorbent:

Activation leads to increase in the surface area i.e., adsorping power of the

adsorbent

i)Creation of roughness

Mechanical rubbing of metallic adsorbents,

Subjecting to some chemical reactions of metallic adsorbents

8/8/2019 Adsorption 12.10.9

6/34

ii) Increasing effective area

Sub-dividing the solid adsorbents into finer particles and hence, increasing

surface area

Strong heating in superheated steam of some adsorbents, e.g., when charcoal

is subjected to the action of superheated steam, its pores are opened, thereby

adsorption increases.

(3) Effect of pressure:

The extent of adsorption (x/m) or (S) (where x is the mass of adsorbate, gas,

and m is the mass of the adsorbent) depends upon the pressure.

Adsorption isotherm is a graph plotted between magnitude of adsorption

and pressure, at constant temperature.

Graph: refer class notes

The extent of adsorption (x/m) increases with increasing pressure (P) and

becomes maximum at Ps, called the saturation pressure.

(4) Effect of temperature:

Adsorption isobar is a graph plotted between magnitude of adsorption and

temperature, at constant pressure.

Since adsorption is an exothermic reaction, so with an increase in

temperature, the amount adsorbed (x/m) should decrease.

However, in case of chemisorption, the amount adsorbed (x/m) initially

increases and then decreases, because chemisorption (like an ordinary

chemical reaction) requires some activation energy.

Adsorption isobars can be used to distinguish between physical and

chemical adsorptions.

8/8/2019 Adsorption 12.10.9

7/34

Thus, in physical adsorption, there is a regular decrease in extent of

adsorption as temperature increases; whereas in chemisorption, there is

initial increase and then decrease in extent of adsorption as temperature

increases. (Ref. graph)

ADSORPTION OF SOLUTES FROM SOLUTIONS

Solid surfaces adsorb solutes from solutions in two ways

1) Solid substances adsorb dissolved substances (solutes) from solutions

Activated animal charcoal adsorbs Colouring matter present in sugar

solution, thereby making the latter colourless.

Activated charcoal adsorbs certain acids like acetic and oxalic present in

water, thereby acid concentration in water decreases.

Ammonia from solutions of NH4OH and phenolphthalein from solution of

acids or bases

2) An adsorbent adsorbs certain solute from solution in preference to

other solutes.

Charcoal adsorbs non-electrolytes more readily than electrolytes from a

solution.

Alumina adsorbs electrolytes in preference to non-electrolytes.

Factors influencing adsorption of solutes from solution

1) Effect of temperature and concentration

Adsorption from solution increases with rise of temperature and increase in

concentration of solution.

Freundlich adsorption equation is found applicable

Adsorption from solution decreases with rise of temperature and decrease in

concentration of solution.

8/8/2019 Adsorption 12.10.9

8/34

Positive adsorption: when conc. Of the solution is high, the adsorption of

solute is high and the conc. of solute is more on adsorbent surface than in the

solution (bulk)

Nagative adsorption: when conc. Of the solution is low, the adsorption of

solute will be low and the conc. of solute is less on the adsorbent surface

than in the solution (bulk)

2) Effect of surface area

Adsorption increases with increase in surface area of the adsorbent

8/8/2019 Adsorption 12.10.9

9/34

ADSORPTION ISOTHERMS

A graph plotted between the magnitude of adsorption and pressure at

constant temperature

Types of adsorption Isotherms

Type I: Monomolecular layer adsorption

Postulated by Langmuir

The rate of adsorption increases with the increase of pressure or

concentration of the adsorbate until it reaches the saturation pressure

Furthur increase in pressure will not increase the amt of adsorption

E.g., Adsorption of N2 or H2 on charcoal

Type II Multimolecular layer adsorption

Show large deviations from Langmuir model

The amt of adsorption increases with increase in pressure

Additional layer formation due to the extension of vander waals force

e.g., Adsorption of N2 on Pt at -195 C

e.g., Adsorption of Br2 on silica at 80C

Type III: Capillary condensation with multimolecular layer formation

Condensation of gases in the minute capillary pores of adsorbent

Multimolecular layer formation

e.g.: Adsorption of benzene on silica gel at 50o

C

e.g.: Adsorption of H2O vapour on activated Carbon at 100oC

REFER THE GRAPHS FROM NOTES

8/8/2019 Adsorption 12.10.9

10/34

FREEUNDLICHS ADSORPTION ISOTHERM

The Freundlich equation or Freundlich adsorption isotherm is an

adsorptionisotherm, which is a curve relating the concentration of a solute

on the surface of an adsorbent, to the concentration of the solute in the liquid

with which it is in contact.

For Adsorption of gases on solids

x/m = KP1/n

For Adsorption of solutes on solid in solutions

x/m = KC1/n

Where

x/m= extent of adsorption

where

x = mass of adsorbate

m = mass of adsorbent

p = Equilibrium pressure of adsorbate

c = Equilibrium concentration of adsorbate in solution.

K and 1/n are constants for a given adsorbate and adsorbent at a particulartemperature

From the adsorption isotherm, the following observations can easily be

made:

(i) At low pressure, the graph is almost straight line, 1/n = slop =

tan 45o

= 1

Log x/m = log K + 1 log P

thereby indicating x/m P or x/m = KP

(ii) At high pressure, the graph becomes almost parallel to X-axis,

thereby indicating 1/n = slop= tan 0 = 0

Log x/m = log K + 0 log P

8/8/2019 Adsorption 12.10.9

11/34

x/m = constant or x/m = K

(iii) At intermediate pressure, x/m depends on 0 to 1 power of pressure (i.e.,

fractional power of pressure). 1/n = 1 to 0

x/m P1/n

x/m = KP1/n

On taking the logarithm of both sides, the above expression assumes the

form:

Log x/m = log K + 1/n log P or

Log x/m = log K + 1/n log C

Thus, if log x/m is plotted against log P or log C, a straight line would be

obtained. The slope of the curve will give 1/n; while the intercepts on log

x/m axis at P = 0 or C=0 would give K

Limitations:

(i) Purely empirical basis

(ii) Valid upto certain pressure and invalid at high pressure

(iii) Fails when concentration of adsorbate is very high

(iv) K and n are temperature dependents, vary with temperature

8/8/2019 Adsorption 12.10.9

12/34

LANGMUIRS THEORY OF ADSORPTION

Assumptions or Postulates

1. Valencies at the surface of adsorbent atoms are not fully satisfied. Each

solid surface contains a fixed no. of active adsorption sites and each siter can

adsorb only one molecular or atomic species of the adsorbate

2. The residual valency force on the surface of adsorbent is effective only up

to a small distance (about 2 10-8

cm) and hence, the adsorbed gas layer is

only one molecule thick.

3. The phenomenon of adsorption consists of two opposing processes,

namely, condensation of the molecules of the adsorbate on the surface of the

adsorbent and evaporation or desorption of the adsorbed molecules from the

surface of the adsorbent.

4. A dynamic equilibrium is set up, when the rate of condensation becomes

equal to the rate of evaporation.

5. There is no interaction between the adjacent adsorbed molecules

6. The adsorbed gas molecule does not move around on the surface.

7. Each solid adsorbent surface has a fixed number of adsorption sites and

each site can adsorb only one atomic or molecular species

8. The heat of adsorption for all the adsorption sites are same irrespective of

the fraction of the surface covered with adsorbed molecule

Derivation Langmuirs adsorption equation.

Refer notes

= aP/1+aP (1)

x= amt of the adsorbate per gm of adsorbent

P = Pressure

a is adsorption co-efficient

8/8/2019 Adsorption 12.10.9

13/34

x

x =k (aP/1+aP)

x = ka P/ 1+ aP

x = bP / 1+ aP

1/b + aP / b = P/x

The equation (1) may be re-written as:

P/x = aP /b +1/b

Thus, if we plot P/x against P, we should get a straight line.

Case I: At very low pressures, aP becomes negligible in comparison with 1,

hence, equation (1) reduces to:

1/b>> aP/b (2)

P/x = 1/b

bP = x

x P

i.e., amount of adsorption per unit weight of adsorbent at a given

temperature is directly proportional to the pressure of the gas at low

pressures.

Case II: At high pressures, aP is very high as compared with 1 and,

therefore, (1) takes

the form

aP / b = P /x

x = (b/a) constant (3)

i.e., at high pressures, the extent of adsorption at a given temperature is

independent of

8/8/2019 Adsorption 12.10.9

14/34

pressure of the gas, because the surface becomes completely covered. Case

III: At intermediate pressure, equation 3 becomes

x/m = KP1/n

(4)

Where n lies between 0 and 1. Equation (4) is Freundlichs adsorption

isotherm

Limitations

1. This equation is also not valid at high pressure

2. Apparent saturation of a surface is observed even only a small

fraction of the area of the adsorbent is covered

3. The adsorption maximum is also variable on temperature and

prussure, This equation does not explain the multimolecular layer

formation

8/8/2019 Adsorption 12.10.9

15/34

APPLICATIONS OF ADSORPTION

(1) Activated charcoal

Gas masks in which all undesirable (toxic) gases are adsorbed selectively by

charcoal; while purified air passes through its pores.

Removing colouring matter of sugar solution and the decoloration of

vinegar.

Charcoal adsorption filters are used for removing organic matter from

drinking water and from industrial effleunts

Production of vacuum in Dewars flask.

(2) Silica and alumina gels

Removing moisture and for controlling humidity of room.

Silica gel has been employed for drying air, used in blast furnaces.

(3) Adsorption chromatography.

Selective adsorption by alumina, magnesia, etc., has been used for

separating different pigments

(4) Arsenic poisoning

Colloidal ferric hydroxide is administered which adsorbs the arsenic poison

and retains it and can thus be removed from the body by vomiting.

(5) Fullers earth

Refining petroleum and vegetable oils from unwanted materials, due to its

good adsorption capacity.

(6) Heterogeneous catalysis

Adsorption is the key process in catalysis

The adsorption of reactants on the catalyst surface provide close proximity

to the reactants and form products and products desorbed from the surface.

Contact process

Habers process

8/8/2019 Adsorption 12.10.9

16/34

Hydrogenation of oils.

(8) Mordants (aldsorbent)

Dying cloth, Mordants adsorb the dye particles, which otherwise do not stick

to the cloth.

(9) Measurement of surface area

Surface area of powder and rough surface can be measured (BET method)

(10) Water Conservation

The adsorbed stearic acid on the surface of water minimizes evaporation of

water

(11) Ore Dressing

Froath floation process: Low grade sulphide ores are freed from earthy

impurities

Role in Activated carbon

Activated carbon has very high surface area and it adsorb odorous, gaseous,

and liquid contaminants forming a strong chemical bond or attraction. Its

adsorption property is put to use in following

1. In producing high vacua - For this partly evacuated apparatus is connected

to a vessel containing activated carbon, cooled in liquid air. At this

temperature, carbon adsorbs residual air very effectively.

2. In gas masks or respirators - it adsorbs poisonous or foul smelling and

other harmful gases and vapour more readily than it adsorbs air.. Thus the air

gets filtered on passing thro the gas mask before breathing.

3. Remove offensive odor from the air (deodorizer) in air-conditioning

process in large restaurants, auditoriums and in refrigerators, in shoe insole

4. In room air purifiers, the activated carbon is often combined with zeolite

and thus acts as a filter for odour control, toxin removal and as a chemical

8/8/2019 Adsorption 12.10.9

17/34

sieve. In some units it is impregnated with potassium iodide or blended with

impregnated alumina to increase the adsorbent qualities. These home air

purifiers are particularly helpful to people with Multiple Chemical

Sensitivity (MCS) as well as for asthma patients

5. To remove impurities from gases such as hydrogen, nitrogen, Helium,

acetylene etc.

6. In cigarette filters used either as granule or powder in filters to remove

some harmful elements of cigarette smoke, or taste and flavour control

7. During waste disposal containing domestic, clinical, chemical waste etc.

by high temperature incineration, the flue gas is made to pass carbon to

remove heavy metals, dioxins and other harmful substances prior to release

in the air.

8/8/2019 Adsorption 12.10.9

18/34

ROLE OF ADSORPENTS

In Ion-Exchange Adsorption

The process of releasing the charged ion and adsorbing another ion with

similar charge by an adsorbent

Application: Water Softening

In Ion-Exchange Adsorption can be used in water softening process

1. Deionization or Demineralization

Demineralisation of water is done in an ion exchanger. In This process

anions and cations present in the hard water can be exchanged with the same

charged ions in the ion exchanger.

Ion Exchange resins are insoluble cross linked long chain macro polymer

with micro porous structure and the functional groups attached to the chains

are responsible for the ion exchanging properties.

The ion-exchanger commonly used is Styrene Divinyl Benzene Copolymer:

CH CH2 CH CH2

CH CH2

CH CH2

CH CH2CH CH2

8/8/2019 Adsorption 12.10.9

19/34

Cation Exchanger: Resins containing acidic functional group ( COOH,

SO3H) are capable of exchanging their H+

ions with other cations of hard

water. Cation exchange resin is represented as RH

CH CH2 CH CH2

CH CH2

CH CH2

CH CH2CH CH2

SO3 H- + SO3 H

- +

SO3 H- +

SO3 H- +

Capable of exchanging their cations with other cations

Polymers containing acidic functional gps like sulphonic (-SO3H) and

carboxylic(-COOH) acids

Sulphonated is more acidic because its pKa = 1, so completely ionized when

in contact with waterGenerally represented as R

-H

+

Exchange H+

ions with other cations

2R-H

++ Ca

+===== R2Ca

++ 2H

+

Cation exchange reaction is reversible, the original resin can be

regenerated on treatment with acid

Anion Exchanger: Resins containing basic functional groups ( NH2) orquaternary ammonium groups are capable of exchanging their OH

-ions with

other anions of hard water. Anion exchange resin is represented as ROH

8/8/2019 Adsorption 12.10.9

20/34

CH CH2 CH CH2

CH CH2

CH CH2

CH CH2CH CH2

-+CH2 N(CH3)3 OH

-+CH2 N(CH3)3 OH

-+CH2 N(CH3)3 OH

-+CH2 N(CH3)3 OH

Capable of exchanging their anion OH- with other anions (Cl-, SO42-, etc,..).

Polymers containing basic functional gps like quaternary Ammonium

hydroxide and amino gps

Quaternary Ammonium hydroxide is more basic in nature, since its pKb =

14, so completely ionized when in contact with water

Generally represented as R+OH

-

Exchange OH-

ions with other anionsR

+OH

-+ Cl

-===== R

+Cl

-+ OH

-

Anion exchange reaction is reversible, the original resin can be

regenerated on treatment with base

Process:

The hard water is first passed through a cation exchange column, H+

ions can be exchanged with the cations like Calcium, Magnesium ions

in hard water.

RH+

+ M+ RM

++ H

+

CaCl2 + RH2 RCa + 2HCl

8/8/2019 Adsorption 12.10.9

21/34

NaCl + RH2 RNa + HCl

The cation free water is then passed through an anion exchange

column, in which all the anions like chlorides, sulphates etc are

exchanged with OH- ions.

ROH-+ X

- RX

-+ OH

-

H+

+ OH- H2O

ROH + HCl RCl+ H2O

The water coming out is completely free from cations and anions.

This water is known as DM water or deionised water.

Regeneration: when the cation exchange resin is exhausted, it can be

regenerated by passing a solution of dil. HCl or H2SO4.

RNa + HCl RH + NaCl

Similarly for the exhausted anion exchanger dil.NaOH can be used.

RCl + NaOH ROH + NaCl

Advantages

1. This process can be used to soften highly acidic or alkalinewaters

2. It produces water of very low hardness (2 ppm)

Disadvantages

1. This equipment is costly and more expensive chemicals areneeded.

2. Turbid water clogs the pores in the ion exchange bed, so turbidwater cannot be used. Turbidity should be lower than 10 ppm

8/8/2019 Adsorption 12.10.9

22/34

2. Zeolite process

REFER NOTES:

ZeNa2 + Ca2+

- ZeCa + 2Na+

8/8/2019 Adsorption 12.10.9

23/34

Adsorption Chromatography

Based on the differences in the adsorption coefficients of substances on

solid

An analytical technique for identification and separation of components of a

mixture based on their differences in their adsorption coefficients of

substances on solid.

Column chromatography that in which the various solutes of a solutionare allowed to travel down an absorptive column, the individual components

being absorbed by the stationary phase

The mobile phase is allowed pass through the stationary phase. The

stationary phase retains the components of mobile phase at different points

depends on their adsorption.

Stationary phase:

Adsorbent packed in the column. Ex: silica gel, alumina.

Mobile phase:

The mixture of components is allowed to flow slowly over the stationaryphase.

Process

StepI: Separation of various compounds

When the solution containing different solutes is poured down a column,

filled with finely divided adsorbent. partial separation takes place due to the

difference in adsorption coefficient.

The components with higher adsorption tend to be retained at the top, theother components are adsorbed successively at various distance depends on

their adsorption.

A number of horizontal bands or zones or rings of different colours are

produced in the column. The colored zones or banded column of the

adsorbed substance is called as Chromatogram.

8/8/2019 Adsorption 12.10.9

24/34

StepII: Identification of compounds

Colored compounds produce colored rings, zones or bands.

Colorless compounds are observed by either exposing UV light on the

column or by spraying suitable chemical reagent in order to make them

colored.

The chemical reagents are called as developers.

The process of visualization of a colorless chromatogram is called as

development of chromatogram.

StepIII: Separation and estimation

The separation is improved by passing suitable solvent (developer) slowly

through the column.

The various zones are dissolved separately in suitable solvents and estimated

The process of recovery of various substances is elutionand the solvent is

called eluent.

*The adsorbate (solute) should possess sufficiently high solubility in the

solvent

*The competition between the solute and solvent molecules for the binding

sites (adsorption) in the adsorbent should also be considered.

*The solvent should not elute the solute more quickly and also should not

take a long time to elute, because leads to long retention time and band

broadening effects

*Some of the commonly employed solid adsorbents in the increasing order

of adsorptive power are,

powdered cellulose < starch < sucrose < CaCO3 < magnesia < silica gel