MODULE: 1- COLLOIDAL CHEMISTRY

True solution:
 Homogeneous system in which particles of the solute are of molecular size.
 Diameter range of particles is 0.1-1nm.
 Particle are not visible to the naked eye or even under powerful microscope.
 Diffuse rapidly.
 Do not settle down on standing.
 Particles cannot be retained on filter paper.
 Eg: sugar solution, NaCl solution
Suspension:
 Heterogenous system containing large particles.
 Diameter range greater than 1000nm.
 Particles are visible to the naked eye or under microscope.
 Do not show diffusion.
 Particles settle readily under gravity.
 Can be retained on filter paper.
 Eg: sand in water. BaSO₄ in water.
Colloidal solutions:
 Intermediate between true solutions and suspensions.
 Heterogeneous system in which dispersed particles are larger than molecules but not
large enough to seen under microscope.
 Diameter range is 1-1000nm.
 Have lower rate of diffusion.
 They do not settle down on standing.
 Cannot be retained on filter paper.
 Eg: Dispersion of starch, ferric hydroxide in water.
 Colloidal solution consist of two phases-dispersed phase and dispersion medium.
 Dispersed phase consist of colloidal particles having diameters in the range 1-1000nm.
Dispersion medium consist of the solvent or medium in which colloidal particles are
dispersed.
Classification of colloidal systems
a) Classification based on the states of aggregation of the two phases

b) Classification based on affinity between the two phases

 Classified into two- lyophilic and lyophobic colloids.
 If the dispersed phase is solid and dispersion medium is liquid, the colloidal system
is called sol. Sols are classified into lyophilic and lyophobic sols.
 c) Classification into macromolecular, multimolecular and associated colloids
Macromolecular colloids:
 Colloidal dispersions of macromolecular substances are called macromolecular
colloids.
 Strong chemical bonds are present between macromolecular particles.
 Most of these systems show lyophilic behaviour.
Eg: Sols of starch, gelatin , proteins,rubber etc
Multimolecular colloids:
 Colloidal particles are aggregate of atoms or molecules having diameter less
than 1 nm.
 Weak vandewaal’s force are present between particles.
 These systems generally show lyophobic behaviour.
 Eg: Aqueous sulphur sol, Aqueous Gold sol
Associated colloids:
 Certain substance behave as normal electrolytes at low concentrations and
show colloidal properties at high concentration due to the formation of
aggregated particles called micelles. Such colloidal systems are called
associated colloids.
 Eg: soaps and detergents
In soaps, micelles are formed above critical micelle concentration(CMC).

Purification of sols
 The excess amount of electrolytes or other soluble impurities destabilize the sols. It is
therefore, necessary to purify them. Purification methods generally adopted are
i) Electrodialysis:
 Dialysis is the process of removing soluble impurities from a colloidal system by
allowing them through a porous membrane such as parchment or cellophane. These
membranes are permeable to solvent molecules and impurities but not to colloidal
particles.
  The apparatus used for dialysis is called dialyser. A bag made from parchment
membrane is filled with impure sol. This is suspended in water taken in a large dish.
The soluble impurities diffuse leaving the pure sol behind. To speed up the process,
electric field is applied. Two electrodes are dipped in water outside the bag and electric
field applied. The ions present in the colloidal solution migrate out to oppositely
charged electrodes. This modified form of dialysis is called electrodialysis.

ii) Ultrafiltration:
 Ultrafiltration is the process of removing soluble impurities from the colloidal system
by allowing them to diffuse through an ultrafilter.
 The ultrafilter will allow solvent and impurities to pass , but not the colloidal particles.
 Ultrafilter are made from cellophane. It can also be prepared from ordinary filterpaper
by impregnating them with gelatin or collodion.
 To speed up the process, pressure or suction is used to force the impurity solution out
of the sol.
PROPERTIES OF SOLS
Kinetic property
a) Brownian movement:
 The ceaseless,erratic, zig-zag motion exhibited by the colloidal particles is called
Brownian movement.
 It is due to the unequal bombardment of colloidal particles by constantly moving
particles of dispersion medium. The molecules of the medium impart momentum
to the colloidal particles.
 Brownian movement counteracts the force of gravity acting on colloidal particles,
it is responsible for the stability of colloids.
Optical property
b) Tyndall effect
 When a beam of light is passed through a colloidal solution and is then viewed at
right angles to it, the path of beam becomes visible as a bright streak. This is called
Tyndall effect. The path is called Tyndall beam or Tyndall cone.
 Waves of shorter wavelength will be more intense in the scattered light .Hence
colourless solutions have bluish Tyndall beam.
 Tyndall effect arises due to scattering of light by colloidal particles.
 Intense Tyndall effect is observed only when (i) the diameter of the dispersed phase
is not much smaller than the wavelength of light used. (ii) there is a large difference
in the refractive indices of the dispersed phase and dispersion medium
 Lyophilic sols generally show weak Tyndall effect as the difference between
refractive indices of dispersed phase and dispersion medium is small. This is due to
the presence of heavy sheath of solvent around sol particles.
 Tyndall effect is not observed for true solutions because the solute particles are too
small to scatter light. Thus, it can be used as a method to distinguish between true
solutions and colloidal solutions.
 Tyndall effect is made use of in ultramicroscope to study about colloidal solutions.
The light from the source is condensed by a series of slits and lenses and passed
through the sol. The scattered light (Tyndall beam) is viewed through a microscope
placed at right angles to the beam. The colloidal particles are as bright spots of light
moving irregularly against dark background.
ELECTRICAL PROPERTIES
 Colloidal particles carry electrical charge, either positive or negative.
 Charge on colloidal particles is due to preferential adsorption of ions on the
particles.
Ferric hydroxide [(Fe(OH)₃] sol contains positively charged particles due to the
selective adsorption of Fe³⁺ ions.
Arsenious sulphide (AS₂S₃) sol contains negatively charged particles due to the
selective adsorption of S²ˉ produced from the dissociation of H₂S.
 Charge also arises due to self-dissociation and formation of ionic micelles.
In aq. Dispersion of soap (RCOONa), RCOOˉ aggregate together to form
negatively charged ionic micelles.
 Sol particles also become charged by electron capture during electro-dispersion or
by frictional electrification.
Electrical double layer and zeta potential
 The stability and electrical properties of colloids can be explained by the existence
of an electrical double layer at the interface between dispersed phase and dispersion
medium.
 The electrical double layer consist of a fixed layer and a diffuse (mobile) layer of
equal and opposite charges.
 Fixed layer consist of either positive or negative ions held firmly on solid. The
diffuse layer consist of mobile layer of ions extending to some distance into the
liquid phase. It contains both positive and negative ions . Their distribution is such
that the net charge is equal and opposite to that on fixed layer. The ion which is
found excess in mobile layer is called counter ions.
 The distribution of ions is not uniform in the diffuse layer. Close to the fixed layer,
there will more ions of opposite sign due to preferential attraction.
  The difference in potential between the position of closest approach of the ions to
the fixed layer at the surface and the electroneutral region of bulk liquid across the
diffuse layer is called zeta potential.
Zeta potential,ζ=4πed/D
Electrophoresis
 When a colloidal solution is placed in an electrical field, the electrically charged
colloidal particles migrate towards the oppositely charged electrode. This
phenomenon is called electrophoresis.
 Electrophoresis occurs due to the formation of electrical double layer. When an emf
is applied, the oppositely charged fixed and diffuse layers are displaced with respect
to each other. The charged sol particles are now free to move, they move towards
oppositely charged electrode.
Isoelectric point
 The pH at which the sol particles of an ampholytic substance become electrically
neutral and exhibit no movement in an electric field is called isoelectric point.
Eg: The isoelectric point of albumin is 4.9
Stability of sols
 In lyophobic and lyophilic colloids, the sol particles carry similar electrical charges.
Repulsion between them holds the particles apart. Hence, they do not coalesce into
non colloidal aggregates.
 In lyophilic colloids, a heavy sheath of solvent, resulting from extensive solvation,
acts as a barrier to their aggregation. Hence they are more stable than lyophobic
colloids.
 Brownian movement is also responsible for the stability of sols.
Coagulation of sols
 The precipitation of the dispersed phase of a colloidal solution through the induced
aggregation of its particles is known as coagulation or flocculation.
 Coagulation can be done by neutralizing the charges on the sol particles. This can
be done
1. By causing persistent electrophoresis
2. By the addition of an oppositely charged colloid.
3. By persistent dialysis to remove the adsorbed ions.
4. By the addition of electrolytes.
On addition of electrolytes, the colloidal particles take up oppositely charged
ions from the electrolyte and gets neutralized.
Eg: The negatively charged As₂S₃ sol can be coagulated on adding some BaCl₂
solution.
Hardy- schulze rule
  The law states that “greater the valency of the ion bearing a charge opposite to that
on sol particles, the greater is its power to cause coagulation”.
Eg: The power of Na⁺, Ba²⁺, and Al³⁺ to cause coagulation of negatively charged sol
like As₂S₃ sol is in the order Na⁺< Ba²⁺<Al³⁺.
The coagulating power of some ions in coagulating a positively charged sol like
Fe(OH)₃ sol is Clˉ< SO₄²ˉ< PO₄³ˉ<[ Fe(CN)₆]⁴ˉ.
Flocculation value
 The minimum concentration of an electrolyte required to cause coagulation of a sol is
called flocculation value.
 It is expressed in terms of millimoles of electrolyte per litre of the sol.
 The flocculation values of NaCl, BaCl₂ and AlCl₃ for the negatively charged As₂S₃ sol
are respectively 52, 0.69 and 0.093.
Protective colloids
 A lyophilic substance that offers protection to a lyophobic sol from the precipitating
effect of electrolytes are called protective colloids.
 Eg: Gelatin can protect a gold sol.
 The lyophilic substances differ in their protective powers. It is usually expressed in
terms of Gold numbers.
 Gold number of a protective colloid is the weight in milligrams of dry protective colloid
that just prevents the coagulation of 10 ml of a standard gold sol on adding 1ml of 10%
sodium chloride solution.
 Smaller the gold number of a lyophilic colloid, greater is its protective action.
Applications of coagulation
 Rubber latex is a colloidal solution of negatively charged rubber particles dispersed in
water. It is coagulated using formic acid or acetic acid to yield rubber.
 Smoke, which is a colloidal solution of carbon particles in air, is precipitated and
removed by electrophoresis in Cottrel smoke precipitator.
 Sewage water contains particles of dust, dirt, mud and garbage of colloidal size. They
are removed by electrophoresis. The colloidal particles move towards the oppositely
charged electrodes, get coagulated and settle down.
Emulsions
 A colloidal system in which both the dispersed phase and dispersion medium are liquids
is known as emulsion.
 One of the phase is water and the other is a liquid immiscible with it, usually known as
oil.
 There are two types of emulsions-
i) Oil-in-water emulsions: oil is the dispersed phase and water is the dispersion
medium. Eg: milk, vanishing cream,emulsified drugs etc.
 ii) Water-in- oil emulsions: water is the dispersed phase and oil is the dispersion
medium. Eg: Cod liver oil, butter, ice-cream etc.
 Emulsification-The process of making an emulsion is termed as emulsification. It is
made by shaking together the two immiscible component liquids. Such emulsions have
properties similar to lyophobic colloids and are unstable due to large interfacial tension
between dispersed phase and dispersion medium.
 Stable emulsions are prepared by adding emulsifiers or emulsifying agents. They are
capable of lowering the interfacial tension between oil and water and thereby
preventing the aggregation of droplets. They have properties similar to lyophilic
colloids. Eg of emulsifiers- soaps and detergents, proteins, gums etc.
 The breaking up of an emulsion is called demulsification. This can be brought by adding
electrolytes, by destroying the emulsifier or by physical methods such as freezing,
boiling, centrifugation.
Applications of emulsions and emulsification
 Milk and cream which are emulsions of fat in water are used as constituents of our diet.
 Large number of drugs in the form of emulsions are used in the body.
 An emulsion of asphalt is used in making roads.
 Several Cosmetics are used in the form of emulsions.
 Several high quality paints are emulsions.
 Froth floatation process used in the concentration of sulphide ores make use of
emulsification.
 Cleansing action of soaps and detergents is based on their emulsifier properties. Their
cleansing action is ascribed to their ability to emulsify grease with water. The
hydrocarbon chain of the soap or detergent gets attached to the oil (grease) and its polar
head gets attached to water. It forms an interfacial film, lowers the interfacial tension
between grease and water and promotes emulsification. Most of the dirt sticks to the
grease and is released from the cloth during rubbing.
 A number of medicines are used in the form of sols, emulsions and gels for easy and
effective absorption and assimilation by body tissues.
Eg: Colloidal sulphur is applied as germ killer.
Oozing of blood from minor cuts is stopped using coagulants like alum.
Sols of gold, calcium and iron are taken orally and through injections for raising the
vitality of human system.
Argyrol and protargol are sols of silver used in ophthalmic lotions.
Several antacids, antibiotic syrups, cough syrups, antiseptic lotions, antifungal creams
etc are colloidal in nature.