Colloids Explained: Types, Properties, and Applications

Colloids
Md Abu Hasnat Emon
Department of Chemistry
Begum Rokeya University,Rangpur.
Third year,second semester.
Course code:CHEM 3213
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2016-
1.a)What do you mean by dispersed system?Classify it with particle diameter and give example of each. 5.0

2.a)Discuss the kinetic/mechanical properties of colloids. 5

2014
1.(a)What are colloids?How can the colloidal solutions be purified?Describe two methods for purification. 5.0
Colloids:When the diameter of the particles of a substance dispersed in a solvent ranges from about 10A® to 2000A®,the system
is termed a colloidal solution,colloidal dispersion or simply colloid.
In the method of preparation of colloids the resulting sol frequently contains besides colloidal particles appreciable amounts of
electrolytes.To obtain the pure sol,these electrolytes have to be removed.This purification of sols can be accomplished by three
methods:
i.Dialysis ii.Electrodialysis iii.Ultrafiltration (2016)
i.Dialysis:Animal membranes(bladder) or those made of parchment paper and cellophane sheet,have very fine pores.These
pores permit ions or small molecules to pass through but not the large colloidal particles.When a sol containing dissolved ions or
molecules is placed in a bag of permeable membrane dipping in pure water,the ions diffuse through the membrane.By using a
continuous flow of fresh water,the concentration of the electrolyte outside the membrane tends to be zero.Thus diffusion of the
ions into pure water remains brisk all the time.In this way,practically ll the electrolyte present in the sol can be removed easily.
The process of removing ions or molecules from a sol by diffusion through a permeable membrane is called Dialysis and the
appuratus used for dialysis is called a Dialyser.

ii.Electrodialysis:In this process,dialysis is carried under the influence of electric field.Potential is applied between the metal screens
supporting the membranes.This speeds up the migration of ions to the opposite electrode.Hence dialysis is greatly accelerated.Evidently
electrodialysis is not meant for nonelectrolyte impurities like sugar and urea.
(b).Write down the applications of colloidal solutions. 4.0
i.Therapy-colloidal systems are used as therapeutic agents in different areas.e.g; silver colloid-germicidal,copper colloid-
anticancer,Mercury colloid-Antisyphillis
ii.Stability-lyophobic colloids prevent flocculation in suspensions.e.g; colloidal dispersion of gelatin is used in coating over tablets and
granules which upon drying leaves a uniform dry film over them and protect them from adverse conditions or the atmosphere.
iii.Absorption-As colloidal dimensions are small enough,they have a huge surface area.Hence,the drug constituted colloidal form is
released in large amount.e.g; sulpher colloid gives a large quantity of sulpher and this often leads to sulpher toxicity.
iv.Targeted drug delivery-Liposomes are of colloidal dimensions and are preferentially taken up by the liver and spleen.
v.Photography-A colloidal solution of silver bromide in gelatin is applied on glass plates or celluloid films to form sensitive plates in
photography.
vi.Clotting of blood-Blood is a colloidal solution and is negatively charged.On applying a solution of Fecl3 bleeding stops and blood
clotting occurs as Fe3+ ions neutralize the ion charges on the colloidal particles.

Dispersed phase Dispersion medium Common names Examples
Solid Solid Solid sol Coloured gemstone,coloured
glasses
Solid Liquid Sol Paints,muddy water,gold
sol,starch sol,arsenous
sulphide sol
Solid Gas Aerosol Smoke,dust
Liquid Solid Gel Gellies,Cheese
Liquid Liquid Emulsion Milk,cod liver oil
Liquid Gas Liquid alcosol Mist,fog,cloud
Gas Solid Solid foam Foam rubber,pumice stone
Gas Liquid Foam Froath,whipped cream
(c)Tabulate the types of colloidal solutions with example on the basis of original states of constituent parts. 3.5

• 2.(a)What are lyophilic colloids?why are they called reversible colloids?Why Mgcl2 is better coagulant than Kcl for As2S3 sol? 5.0
• Lyophillic colloids: Lyophillic coloids are those in which the dispersed phase exhibits a definite affinity for the medium or the solvent.
• *If water is the dispersing medium,it is often known as a hydrosol or hydrophilic.
• *readily solvated(combined chemically or physically with the solvent) and dispersed,even at high concentrations.
• *more viscid
• *the dispersed phase doesn’t precipitate easily
• *the sols are quite stable as the solute particle surrounded by two stability factors a)negative or positive charge b)layer of solvent
• Examples:sols of gum,gelatin,starch,proteins and certain polymers in organic solvents.
• The dispersed phase of lyophilic sols when separated by coagulation or evaporation of the medium,can be reconverted into the colloidal form just
on mixing with the dispersion medium.Therefore this type of sols are also called reversible sols.
• From a study of the precipitating action of various electrolytes on particular sol,Hardy and Schulze gave a general rule which states that-
• The coagulation capacity of an electrolyte depends upon the valence of ion responsible for causing coagulation.The ion responsible for causing
coagulation is the one which carries charge opposite on colloidal particles.
• The greater is the valence of the oppositely charged ion of the electrolyte added to a colloidal solution,the faster is the coagulation of the colloidal
solution.
• *Negatively charged sol of As2S3
• Al3+>Ba2+,Mg2+>Li+,Na+
• *Positively charged sol Fe(OH)3
• [Fe(CN)6]4-> PO4
3->SO4
2->Cl-
• As Mg2+ has valence=2 is greater than K+ valence=1 so MgCl2 is better coagulant than KCl.
• The minimum concentrations(in millimoles per litre) of an electrolyte required to cause the coagulation of a sol is called the flocculation value of
the electrolyte.

• (b)Explain the stability of colloids.(2017) 4.0
A true colloidal solution is stable.Its particles do not ever coalesce and separate out.The stability depeds upon two factors-
i)presence of like charge on sol particles:
The dispersed particlse of a hydrophobic sol posses a like electrical charge(all positive or all negative) on their surface.Since like charges repel
one another,the particles push away from one another and resist joining together.However,when an electrolyte is added to a hydrophobic sol,the
particles are discharged and precipitated.
ii)presence of solvent layer around sol particle:
The lyophilic sols are stable for two reasons.Their particles posses a charge and in addition have a layer of the solvent bound on the surface.For
example, a sol particle of gelatin has a negative charge and a water layer envelopes it.When sodium chloride is added to colloidal solution of
gelatin,its particles are not precipitated.The water layer around the gelatin particle does not allow the Na+ ions to penetrate it and discharge the
particle.The gelatin sol is not precipitated by addition of NaCl solution.Evidently lyophilic sols are more stable than lyophobic sols.
• Two main mechanism for colloid stabilization are-
*Steric stabilization-surrounding each particle with a protective solvent sheath which prevent adherence due to Brownian movement.
*Electrostatic stabilization-providing the particles with electric charge. Ebn Din Hasnat Emon

(c)Define gold number.The gold numbers of A,B,C and D are 0.005,0.05,0.5,5.0 respectively.which of these has the greatest protective action?explain. 3.5
The number of milligrams of a hydrophilic colloid that will just prevent the precipitation of 10 ml of a gold sol on the addition of 1 ml of 10 per cent sodium
chloride solution is defined as Gold number.
The onset of precipitation of the gold sol is indicated by a colour change from red to blue when the particle size is just increases.
The gold numbers of hydrophilic colloids given as A=0.005,B=0.05,C=0.5,D=5
The smaller the gold number of a hydrophilic colloid,the greater its protective power.so A has the highest protective power.
Gelatin has a small gold number and is an effective protective colloid,starch has a very high value which shows that it is an ineffective protective colloid.
The use of protective colloids to stabilize colloidal system is widespread.In the preparation of ice cream,gelatin is added to act as a protecting agent to the
colloidal particles of ice.If the ice particles coagulate,the smooth texture of ice cream is lost.Argyrol,used in eye drops,is a sol of silver protected by organic
material.
6.(a)Associated colloids:The molecules of substances as soaps and artificial detergents are smaller than the colloidal particles.However,in concentrated solution
these molecules form aggregates of colloidal size.Substances whose molecules aggregate spontaneously in a given solvent to form particles of colloidal
dimensions are called Associated or Association colloids.Sometimes they are also called amphiphilic colloids because their two regions have opposing solution
affinities within the same molecule.At low concentration amphiphiles exist separately(subcolloidal size) but at high conc. They form aggregates or miscelles(50 or
more monomers)(colloidal size)

The colloidal aggregates of soap or detergent molecules formed in the solvent are referred to as micelles.For example-
Explanation:Soap or detergent molecules ionizes in water to form an anion and sodium ion.Thus sodium stearate a typical soap
furnishes stearate anion and sodium ion in aqueous solution.
As many as 70 stearate ions aggregates to form a micelle of colloidal size.The stearate ion has a long hydrocarbon chain with a
polar –COO- group at one end.The zigzag hydrocarbon tail is shown by a wavy line and the polar head by a hollow circle.In the
micelle formation,the tails being insoluble in water and directed toward the centre,while the soluble polar heads are on the surface
in contact with water.The charge on the micelle due to the polar heads accounts for the stability of the particles.
In non polar liquid the polar heads facing inward and the hydrocarbon chains are associated with non polar liquid.
The concentration at which micelle form is known as critical micelle concentration.At conc.close to C.M.C spherical micelles are
formed and at higher conc. Lamellar micelles are formed.

2014+2015+2016-Tyndal effect:When a strong beam of light is passed through a sol and viewed at right angles,the path of light shows up a
hazy beam of cone.This is due to the fact that sol particles absorb light energy and then emit it in all directions in space.This scattering of
light,illuminates the path of the beam in the colloidal dispersion.The phenomena of the scattering of light by the sol particles is called tyndal
effect.
The illuminated beam of cone formed by the scattering of light by the sol particles is often reffered as tyndal beam or tyndal cone.The hazy
illumination of the light from the film projector in a smoke filled theatre or the light beams from the headlights of car on a dusty road are
familiar examples of the tyndal effect.If the sol particles are large enough,the sol may even appear turbid in ordinary light as a result of tyndal
scattering.
True solution does not shows tyndal effect.Since ions or solute molecules are too small to scatter light,the beam of light passing through a
true solution is not visible when viewed from the side.Thus tyndal effect can be used to distinguish a colloidal solution from a true solution.

2014+2016+2017-Brownian motion:When a sol is examined with an ultra microscope,the suspended particles are seen as a shining
specks of light.By following an individual particle it is observed that the particle is undergoing a constant rapid motion.It moves in a series of
short straight line paths in medium,changing directions abruptly.The continuous rapid zig zag movement executed by a colloidal particle in the
dispersion medium is called Brownian movement or motion.This phenomena is so named after Sir Robert Brown who discovered it in 1827.
Suspension and true solutions do not exhibit this property.
Explanation or Contribution of Einstein on Brownian movement:The explanation was advanced by Albert Einstein around 1955 by
mathematical consideration based on the kinetic molecular theory.According to him,at any instant a colloidal particle was being struck by
several molecules of the dispersion medium.The movement of the particle was caused by unequal number of molecules of the medium
striking it from opposite directions.When more molecules struck the particles on one side than on other,the direction of movement changed.
In a suspension,the suspended particles being very large the probability of unequal bombardments diminishes.The force of the molecules
hitting the particle on one side is cancelled by the force of collisions occurring on the other side.Hence they do not show Brownian
movement.The phenomena of Brownian movement is an excellent proof of the existence of molecules and their ceaseless motion in liquids.It
also explains how the action of gravity,which would ordinarily cause the settling of colloidal particles,is counteracted.The constant pushing of
the particles by the molecules of the dispersion medium has a stirring effect which does not permit the particles to settle.

2014+2017--Electronic properties of colloids:
i.The sol particles carry an electric charge-The most important property of of colloidal dispersion is that all the suspended particles posseses
either a positive or negative charge.The mutual forces of repulsion between similarly charged particles prevent them from agreegating and
settling under the action of gravity.This gives stability to the sol.The sol particles acquire positive or negative charge by preferential adsorption
of positive or negative ions from the dispersion medium.e.g; a ferric hydroxide sol particles are positively charged because these adsorb Fe3+
ions from ferric chloride used in the preparation of the sol.Since the sol as a whole is neutal,the charge on the particle is counterbalanced by
oppositely charged ions termed counterions(in this case Cl-) furnished by the electrolyte in medium.
Electrical double layer theory: In this theory, charge is imparted to the particles by placing ions which are adsorbed preferentially at
immovable points which for the first layer. The second layer consists of diffused mobile ions. The charge present on both the layers is equal.
This two-layer arrangement leads to a development of potential called zeta or Electrokinetic potential. As a result of this potential developed
across the particles, under the influence of electric field these particles move.
ii. Electrophoresis(2016,2017)
The phenomenon of movement of colloidal particles under an applied electric field is called electrophoresis.
If the particles accumulate near the negative electrode, the charge on the particles is positive. On the other hand, if the sol particles
accumulate near the positive electrode, the charge on the particles is negative.
The apparatus consists of a U-tube with two Pt-electrodes in each limb. When electrophoresis of a sol is carried out with out stirring, the
bottom layer gradually becomes more concentrated while the top layer which contain pure and concentrated colloidal solution may be
decanted. This is called electro decanation and is used for the purification as well as for concentrating the sol. The reverse of electrophoresis
is called Sedimentation potential or Dorn effect. The sedimentation potential is setup when a particle is forced to move in a resting liquid.
This phenomenon was discovered by Dornand is also called Dorn effect.

iii. Electro-osmosis(2016,2017)
In it the movement of the dispersed particles are prevented from moving by semipermeable membrane.
Electro-osmosis is a phenomenon in which dispersion medium is allowed to move under the influence of an electrical field, whereas colloidal
particles are not allowed to move.
The existence of electro-osmosis has suggested that when liquid forced through a porous material or a capillary tube, a potential difference is
setup between the two sides called as streaming potential. So the reverse of electro-osmosis is called streaming potential.

iv.Coagulation or flocculation(2016):The stability of a sol is due to the charge present on the colloidal particles.However,if the charge on
colloidal particles is destroyed,they are free to come nearer and grow in size and get precipitated.This phenomena is termed as coagulation or
flocculation.The coagulation of colloidal solution can be achieved by the addition of an electrolyte.It is necessary for the stability of a sol
because the ions of the electrolyte get adsorbed on colloidal particles and impart them some charge.
2013
1.(a)What are colloids?How are they classified on the basis of interaction between the dispersed phase and dispersed medium?compare
amongst them. 5.0 (2017)

1(b).Discuss two methods for preparing colloidal solution. 4 (2017)
Lyophilic sols may be prepared by simply warming the solid with the liquid dispersion medium e.g; starch with water.On the other
hand,lyophobic sols have to be prepared by special methods.These methods fall into two categories.
A-Dispersion methods in which large macro sized particles are broken down to colloidal size.
B-Aggregation method in which colloidal size particles are built up by aggregating single ions or molecules.
*Dispersion methods- i)Mechanical dispersion using colloid mill ,II)Peptization , iii)Bredig’s arc method
Bredig’s Arc method(Physical method):It is used for preparing hydrosols of metals e.g; silver,gold and platinum.An arc is struck between the
two metal electrodes held close together beneath de-ionized water.The water is kept cold by immersing the container in ice/water bath and a
trace of alkali is added.The intense haet of the sparks across the electrodes vapourises some of the metal and the vapour condenses under
water.Thus the atoms of the metal present in the vapour aggregates to form colloidal particles in water.Since the metal has been ultimately
converted into sol particles(via metal vapour),this method has been treated as dispersion.Now metal sols can be made by suspending coarse
particles of the substance in the dispersion medium and striking an arc between two electrodes.

*Aggregation method:These methods consist of chemical reactions or change of solvent whereby the atoms or molecules of the dispersed
phase appearing first,coalesce or aggregate to form colloidal particles.
Chemical-i)Double decomposition:
ii)Reduction:
iii)Oxidation:
iv)Hydrolysis:
Change of solvent:When a solution of sulpher or resin in ethanol is added to an excess of water,the sulpher or resin sol is formed owing to
decrease in solubility.The substance is present in molecular state in ethanol but on transference to water,the molecules precipitates out to
form colloidal particles.

2(b).Discuss the origin and structure composition of the electrical double layer formed in a colloidal system. 4
Electric Double Layer
Electric Double Layer is the phenomenon playing a fundamental role in the mechanism of the electrostatic stabilization of colloids.
Colloidal particles gain negative electric charge when negatively charged ions of the dispersion medium are adsorbed on the particles surface.
A negatively charged particle attracts the positive counterions surrounding the particle.
Electric Double Layer is the layer surrounding a particle of the dispersed phase and including the ions adsorbed on the particle surface and a
film of the countercharged dispersion medium.
The Electric Double Layer is electrically neutral.
An Electric Double Layer consists of three parts:
Surface charge - charged ions (commonly negative) adsorbed on the particle surface.
Stern layer - counterions (charged opposite to the surface charge) attracted to the particle surface and closely attached to it by the
electrostatic force.
Diffuse layer - a film of the dispersion medium (solvent) adjacent to the particle. Diffuse layer contains free ions with a higher concentration of
the counterions. The ions of the diffuse layer are affected by the electrostatic force of the charged particle.
The electrical potential within the Electric Double Layer has the maximum value on the particle surface (Stern layer). The potential drops with
the increase of distance from the surface and reaches 0 at the boundary of the Electric Double Layer.
When a colloidal particle moves in the dispersion medium, a layer of the surrounding liquid remains attached to the particle. The boundary of
this layer is called slipping plane (shear plane).
The value of the electric potential at the slipping plane is called Zeta potential, which is very important parameter in the theory of interaction
of colloidal particles.

There are several theoretical treatments of the solid-liquid interface. We will look at a few common ones, not so much from the position of needing to use them, but more
from the point of what they can tell us.
• A. Helmholtz Double Layer
• This theory is a simplest approximation that the surface charge is neutralized by opposite sign counterions placed at an increment of d away from the surface.
• The surface charge potential is linearly dissipated from the surface to the contertions satisfying the charge. The distance, d, will be that to the center of the countertions,
i.e. their radius. The Helmholtz theoretical treatment does not adequately explain all the features, since it hypothesizes rigid layers of opposite charges. This does not
occur in nature.
• B. Gouy-Chapman Double Layer
• Gouy suggested that interfacial potential at the charged surface could be attributed to the presence of a number of ions of given sign attached to its surface, and to an
equal number of ions of opposite charge in the solution. In other words, counter ions are not rigidly held, but tend to diffuse into the liquid phase until the counter
potential set up by their departure restricts this tendency. The kinetic energy of the counter ions will, in part, affect the thickness of the resulting diffuse double layer.

• Gouy and, independently, Chapman developed theories of this so called diffuse double layer in which the change in concentration of the
counter ions near a charged surface follows the Boltzman distribution
• n = noexp(-zeY/kT) ,where no = bulk concentration, z = charge on the ion,e = charge on a proton, k = Boltzmann constant
C. Stern Modification of the Diffuse double Layer
The Gouy-Chapman theory provides a better approximation of reality than does the Helmholtz theory, but it still has limited quantitative
application. It assumes that ions behave as point charges, which they cannot, and it assumes that there is no physical limits for the ions in
their approach to the surface, which is not true. Stern, therefore, modified the Gouy-Chapman diffuse double layer. His theory states that
ions do have finite size, so cannot approach the surface closer than a few nm. The first ions of the Gouy-Chapman Diffuse Double Layer are
not at the surface, but at some distance d away from the surface. This distance will usually be taken as the radius of the ion. As a result, the
potential and concentration of the diffuse part of the layer is low enough to justify treating the ions as point charges.Stern also assumed that it
is possible that some of the ions are specifically adsorbed by the surface in the plane d, and this layer has become known as the Stern
Layer. Therefore, the potential will drop by Yo - Yd over the "molecular condenser" (i.e., the Helmholtz Plane) and by Yd over the diffuse
layer. Yd has become known as the zeta (z) potential.
This diagram serves as a visual comparison of the amount of counterions
in each the Stern Layer and the Diffuse Layer.

Thus, the double layer is formed in order to neutralize the charged surface and, in turn, causes an electrokinetic potential between the surface
and any point in the mass of the suspending liquid. This voltage difference is on the order of millivolts and is referred to as the surface potential.
The magnitude of the surface potential is related to the surface charge and the thickness of the double layer. As we leave the surface, the
potential drops off roughly linearly in the Stern layer and then exponentially through the diffuse layer, approaching zero at the imaginary
boundary of the double layer. The potential curve is useful because it indicates the strength of the electrical force between particles and the
distance at which this force comes into play. A charged particle will move with a fixed velocity in a voltage field. This phenomenon is called
electrophoresis. The particle’s mobility is related to the dielectric constant and viscosity of the suspending liquid and to the electrical potential at
the boundary between the moving particle and the liquid. This boundary is called the slip plane and is usually defined as the point where the
Stern layer and the diffuse layer meet. The relationship between zeta potential and surface potential depends on the level of ions in the
solution.The figure above represents the change in charge density through the diffuse layer. One shows considered to be rigidly attached to the
colloid, while the diffuse layer is not. As a result, the electrical potential at this junction is related to the mobility of the particle and is called the
zeta potential. Although zeta potential is an intermediate value, it is sometimes considered to be more significant than surface potential as far as
electrostatic repusion is concerned.
*Electrophoresis:Electrophoresis is defined as the migration of charged ions in an electric field. In metal conductors, electric current is carried by
the movement of electrons, largely along the surface of the metal. In solutions, the electric current flows between electrodes and is carried by
ions. The ions that migrate towards the anode, because of their anodic migration, are called “anions”. The ions which will migrate to the cathode
are called “cations”.
*History of electrophoresis development
The concept of electrophoresis was discovered a couple of centuries ago by Ruess [1]. In 1807, he noted the migration of particles towards the
anode when applying an electrical current through a suspension of clay in water. However, it was not until the year 1942 that electrophoresis
became a major scientific method or procedure. Coleman and Miller [2] conducted an experiment and discovered the migration of neutral
hexose toward the anode in a borax (sodium borate) solution. Subsequently, the usage and limitations of the electrophoresis for other
compounds was tested. These compounds included those containing adjacent “-OH” groups and also high concentration of neutral sugars [3, 4,
5]. In 1952, Consden and Stanier [6] successfully separated sugar using electrophoresis and only then was electrophoresis started being used for
DNA and RNA separation.
.

Principle: When charged molecules are placed in an electric field, they migrate toward either the positive or negative pole according to their
charge. In contrast to proteins, which can have either a net positive or net negative charge, nucleic acids have a consistent negative charge
imparted by their phosphate backbone, and migrate toward the anode.
An ion placed in such an electric field will experience a force:
Where,
F = electrophoretic force
K = a constant
q = net charge on the protein (atomic charges/protein molecule)
This force will cause the protein to accelerate towards either the cathode or the anode, depending on the sign of its charge. Of
course there are other forces such as frictional force when ions move in the electric field. The influence of them cannot be
understood easily by a formula, so we omit it.
Electrophoresis exploits the fact that different ions have different mobility in an electric field and so can be separated by this way.
Proteins and nucleic acids are electrophoresed within a matrix or "gel". Most commonly, the gel is cast in the shape of a thin slab,
with wells for loading the sample. The gel is immersed within an electrophoresis buffer that provides ions to carry a current and
some type of buffer to maintain the pH at a relatively constant value.The gel itself is composed of either agarose or polyacrylamide,
each of which has attributes suitable to particular tasks.
Factors influencing Electrophoresis:1) The buffer pH
It will influence the direction and rapid of the protein migration.
Movement of proteins depends on various aspects; one of them is the charges on the proteins. Proteins are sequence of amino acids that
can be ionized depend on their acid or basic character. The protein’s net electric charge is the sum of the electric charges found on the
surface of the molecule as a function of the environment. The rate of migration will depend on the strength of their net surface charges: The
protein that carries more +ve charges will move towards the cathode at a faster rate. On the contrary, the protein that carries more -ve
charges will move towards the anode at a faster rate. In this regard, proteins can be separated based on their electric charges.
Depending on the pH of the buffer, proteins in a sample will carry different charges. At the pI (isoelectric point) of a specific protein, the
protein molecule carries no net charge and does not migrate in an electric field.

At pH above the pI, the protein has a net negative charge and migrates towards the anode. At pH below the pI the protein obtains a net
positive charge on its surface and migrates towards the cathode.
2) The voltage gradient
The rate of migration will depend on the voltage gradient: There is more voltage gradient in the electric field, protein will move towards the
anode (or the cathode) at a faster rate.
3) Electo-osmosis
Liquid’s relative move upon solid medium in an electric field is called electo-osmosis. In applied electric field, electo-osmosis distorts the
sample stream and limits the separation. For example, Paper electrophoresis has poor resolution because of electo-osmosis. The surface of
paper has -e, so the buffer has +e derived from hydrogen ions because of electrostatic induction.
Then +e drive buffer to cathode in electric field,these flows distort the electrophoretic migration of sample by causing a varying residence
time. Thus, sample will move more or less than normal.

2015
1.a)What are colloids?How are they classified?Classify the following sols according to charge- Gold,Fe(OH)3,Gelatin,As2S3
On the basis of the original states of their constituent parts-8 types(Solid sol,sol,Aerosol,Gel,Emulsion,Liquid alcosol,Solid foam,Foam)
On the basis of the interaction between dispersed phase and dispersion medium- 3 types(Lyophilic,Lyophobic,Amphiphilic)
On the basis of the charge-3 types(positive sol,negative sol,amphoteric sol)
The sol that adsorb positive ion from the medium is known as positive sols-Ferric hydroxide sol particles are positive because they adsorb the common ion Fe3+
from the aqueous medium. Fe(OH)3 + Fe3+ --- Fe(OH)3/Fe3+ sol
The sol that adsorb negative ion from the medium is known as negative sol-Arsenic sulphide sol particles aquire a negative charge since they adsorb the common
ion S2- from the medium.similarly gold platinum are also negative sol.
The sols which contain both positive and negative charge are called amphoteric sols-Gelatin is an amphoteric protein with isoelectric point between 5 & 9
depending on raw material and method of manufacture.
1.b)Explain when a colloidal solution is treated as follows-
i)A beam of light is passed through it. Ans:Tyndall effect(discussed)
ii)An electrolyte is added. Ans:Coagulation-When excess of an electrolyte is added to a sol,the dispersed particles are precipitated.The electrolyte furnishes both
positive and negative ions in the medium.The sol particles adsorb the oppositely charged ions and get discharged.The electrically neutral particles then aggregate
and settle down as precipitate.A negative ion causes the precipitation of a positively charged sol,and vice versa.The effectiveness of an anion or cation to
precipitate a sol,will naturally depend on the magnitude of the charge or valence of the effective ion.The precipitating effect of an ion can be given by Hardy-
Schulze rule.(Related with the topic-Flocculation value)

1.c)Describe a method for purifying colloidal solutions. 3.5 Ans:Dialysis or Electrodialysis or Ultrafiltration.
2.a)What are emulsions?How are they classified?How are they prepared? 5.0
An emulsion can be defined as a colloid consisting of two or more non-homogenous type of liquids wherein one of the liquid contains the
dispersion of the different form of liquids.
Emulsion Examples-Emulsions basically consist of a dispersion of two liquids that are immiscible with each other. One of the liquids act as the
dispersion medium and the other will act as the dispersed phase. In simple words, emulsions are colloids in which both the dispersed phase
and dispersion medium are liquids. Oil and the mixtures of water are the emulsions when are shaken together. The oil forms drop and then
disperses throughout the water.
The term emulsion is also applied to a group of mixed systems called as solutions, or gels or suspensions. Take, For example, the photographic
emulsion is a gelatin gel consisting of tiny crystals dispersed in it. Some other examples of emulsions include butter which is an emulsion of
water in fat and egg yolk containing lecithin.
• Properties Of Emulsions
Emulsions contain both a continuous and the dispersed with the boundary coming between the phases that are called “interface”.
Emulsions have a cloudy appearance due to many phase interfaces scattering light passing through the emulsions.
Emulsions appear in white colour when the light is dispersed in equal proportions.
If the emulsion is dilute, then higher-frequency and the low-wavelength type of light will be scattered in more fractions, and this kind of
emulsion will appear in blue in colour. This is also referred to as the Tyndall effect.
Emulsions can be classified on the basis of the properties of the dispersed phase and the dispersion medium.
1) Oil in water (O/W):In this type of emulsion, the oil will be the dispersed phase and water will be the dispersion medium. The best example
for o/w emulsion is milk. In milk, the fat globules (which act as the dispersed phase) are suspended in water (which acts as the dispersion
medium).
2) Water in oil (w/o):In this type, water will be the dispersed phase and oil will be the dispersion medium. Margarine (a spread used for
flavouring, baking and working) is an example of water in oil emulsion.

• Emulsifier / Emulsifying agent
These are substances which are added to emulsions for stabilization purpose. The various characteristics of emulsifiers are given below:
They are substances which have a hydrophilic end (polar) as well as a hydrophobic end (non-polar).
They are soluble in both water and oil.
Emulsifiers form a layer between the dispersed phase and the dispersion medium, thereby preventing the dispersed phase particles to come
together to form larger particles and separate out.
Emulsifiers can be cationic, anionic or even non-polar.
It is not just the percentage of water and oil which decides whether it is oil-in-water or a water-in-oil emulsion. On the other hand, it depends
on which among water and oil can solvate the emulsifier to a larger extent.
If the emulsifier is more soluble in water then the water becomes the dispersion medium and oil becomes the dispersed phase and hence we
get oil in water emulsion.
On the other hand, if the emulsifier is more soluble in oil, then oil becomes the dispersion medium and water becomes the dispersed phase.
The commonly used emulsifiers for o/w emulsions are proteins, gums, soaps etc.
The commonly used emulsifiers for w/o emulsions are heavy metal salts of fatty acids, long-chain alcohols etc.
• How do Emulsifiers Work?
To understand this, firstly we need to understand the process of coalescing. Coalescing is the process in which the similar particles in the
emulsions come together to form larger and bulkier particles leading to the separation of the dispersed phase and dispersion medium.
Emulsifiers help in preventing coalescing by forming a physical barrier between the dispersed phase and dispersion medium. As we
have seen before emulsifiers, like soap, has both a hydrophilic end and a hydrophobic end. Hence they can attach to both polar and
non-polar substances. Let us take the example of sodium stearate. C17H35COO–Na can be represented as;

When this is added to an o/w emulsion, molecules of C17H35COO– surround the oil droplet with their non-polar tails/hydrophobic end (the
hydrocarbon chain) extending into the oil & their polar heads/hydrophilic end (the carboxylate ion) facing the water as given in the figure.
This arrangement brings a stronger adhesive force between the oil (dispersed phase) and water (dispersion medium). This new formed
adhesive force will be more than the cohesive force between oil – oil and water – water. Hence, oil particles will not have the tendency to
come together to form larger particles. This helps in preventing coalescing, thereby stabilizing the emulsion.
Note:For w/o emulsion, the orientation of the emulsifier would be the opposite as that of o/w. i.e. non-polar (hydrophobic end) tail extends
outside and polar (hydrophilic end) head faces inwards.

2.b)Explain the Schulze hardy rule for coagulation with examples. 4.0 (Discussed)
2.c)Define gold number.The lower the value of gold number of a lyophilic colloid the greater is its protection action.Justify. (2017) 3.5
The lower the gold number means less amount of lyophobic sol enough to prevent the precipitation of 10 ml of a gold sol on the addition of 1
ml of 10% NaCl solution,that is more protective action.e.g; gelatin has gold num 0.005-0.01 whereas starch has gold number 25 which
indicates Gelatin acts as an effective protective colloid whereas starch is an ineffective protective colloid.
2015+2016+2017-Origin of charge of colloids- All the dispersed particles of a particular sol carry a positive or negative charge.They acquire
this charge by
i)Adsorption of ions from the aqueous medium-In most cases the charge on the sol particles originates by the selective adsorption of ions
common to the particles from the dispersion medium.
Examples-The sol that adsorb positive ion from the medium is known as positive sols-Ferric hydroxide sol particles are positive because they
adsorb the common ion Fe3+ from the aqueous medium. Fe(OH)3 + Fe3+ --- Fe(OH)3/Fe3+ sol
The sol that adsorb negative ion from the medium is known as negative sol-Arsenic sulphide sol particles aquire a negative charge since they
adsorb the common ion S2- from the medium.similarly gold platinum are also negative sol.
The sols which contain both positive and negative charge are called amphoteric sols-Gelatin is an amphoteric protein with isoelectric point
between 5 & 9 depending on raw material and method of manufacture.
It is not necessary that a particular sol particles always adsorb the same kind of ions.In fact,the particles may adsorb the anions or cations
whichever are in excess and acquire the corresponding charge.e.g; AgCl sol produced by the addition of AgNO3 solution to NaCl solution,bears
a positive charge if Ag+ ions are in excess.On the other hand,if Cl- ions are in excess,the AgCl sol acquires a negative charge.

ii)Ionization of surface group: (a)Charge on soaps and detergents sols-Soaps and detergent sol particles are aggregates of many molecules.The
hydrocarbons tails of the molecules are directed to the centre,while the groups -COO-Na+(or –OSO3
-Na+) constitute the surface in contact with
water.As a result of ionization of the surface groups,the particle surface is now made of the anionic heads -COO-(or –OSO3
-).This makes the sol
particles negative.
(b)Charge on protein sols-Protein sol particles possess both acidic and basic functional groups.In aqueous solution at low pH, the –NH2
group(basic) acquires a proton to give NH3
+ while at high pH the –COOH group(acidic)transfers a proton to OH- to give –COO-.Thus the protein
sol particle has positive charge at low pH and negative charge at high Ph.At an intermediate pH called the isoelectric point,the particles will be
electrically neutral.The changes in the charge of the protein sol are shown by the direction of movement of the dispersed phase in
electrophoresis.

Colloids Explained: Types, Properties, and Applications

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