Chapter 2a - Colloid Properties

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PROPERTIES OF COLLOID SOLUTION


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Colloid Properties

1. Heterogenous in nature

2.Tyndall Effect

3.Brownian Movement

4.Electrophoresis

5.Coagulation

6.Adsorption


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HETEROGENEITY

A colloidal solution is heterogenous system

consisting of two phases of

dispersed phase colloidal particles of a solid)

the aqueous dispersion medium.

Often a colloidal sol appears to be

homogeneous as the particles are small in

size

and not visible to the naked eye.

However, this is disproved when it is viewed

under electron microscope iluminated by a

strong beam of light).


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Optical Properties (TYNDALL EFFECT) If a beam of light illuminates a rough surface, or a cloud

of small particles, some of the light is removed from the

beam and redistributed in all directions. This angular redistribution is called scattering.The

scattered rays go off in many directions different fromthat of the incident light.

A light wave is an electromagnetic wave that travelsthrough the vacuum of outer space.

Light waves are produced by vibrating electric charges. Alight wave is an electromagnetic wave that travelsthrough the vacuum of outer space. Light waves are

produced by vibrating electric charges. This scattering of light can be due to simple reflection

because the size of the particles is smaller thanwavelength of visible light which are, therefore, unable

to reflect light waves.


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The colloidal particles become self luminous due to absorption oflight energy which is then scattered from their surface.

In case of lyophobic the difference is more so Tyndall effect is wellobserved; while in Lyophilic it is less so Tyndall effect is very poor.

The phenomenon of scattering of light by sol particles to form

illuminated beam or cone is called Tyndall effect or

Tyndall beam or Tyndall cone.

Examples:

Aerosol Scattering- because the aerosol particles areusually a little bigger than the wavelength of light, theyscatter all wavelengths about equally well. The scatteredlight is, on the average, white. That's why clouds arewhite, and a smoggy sky is whitish or grayish.

Molecular Scattering- Rayleigh Scattering. Because the

molecules are smaller than the wavelength, they scatter shortwavelengths (which are more nearly comparable to the size of themolecules) better than long wavelengths.


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Crepuscular Rays


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Tyndall scattering occurswhen the dimensions of theparticles that are causing thescattering are larger than thewavelength of the radiationthat is scattered.

It is caused by reflection ofthe incident radiation from

the surfaces of the particles,reflection from the interiorwalls of the particles, andrefraction and diffraction ofthe radiation as it passes

through the particles. Tyndall, John, Born 1820,

County Carlow, Ireland .,Died1893, Surrey, England


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Applications1. This phenomenon has been employed as the basic principle

for the construction of ultra-microscope. It has been used to

detect solid suspended impurities in solution.

2. On the basis of Tyndall effect we can explain that sky is blue in

day light. The reason is that dust particles along with water

are in the atmosphere. These dust particles scatter blue light

and other colours are absorbed therefore the colour of the

sky is blue. Blue color is the shortest wavelength of sunlight.

Since in night scattering of light is not taking place hence sky

is black in night.

3. Tail of comets.

4. Blue colour of sea water.

5. Blue tinge of smoke.


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Mechanical Effect (Brownian Movement) When colloidal solutions have been observed through ultra microscope, the colloidal particles are

seen in constant and rapid zig-zag called Brownian movement.

Sir Robert Brown first observed the phenomenon in 1827. Suspensions and true solutions do not

exhibit Brownian movement.

This brownian motion arises due to the uneven distribution of the collisions between colloid particleand the solvent molecules.

- Brownian movement was more rapid for smaller particles.

- It decrease with increase the viscosity of the medium.

- The Brownian movement is due to the bombardment of colloidal particles by molecules of dispersion

medium.

- The intensity of motion depends upon the size of the particles and the viscosity of the dispersionmedium.

- The smaller the particles and the less viscous the dispersion medium, the more vigorous is theBrownian movement and vice versa


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Cronology In 1827, when the British botanist Robert Brown was observing through a

microscope, placing pollen into water, he noticed that the particles coming out of the

pollen were moving constantly.

At first, Brown thought they were living creatures that moved by themselves,however, even those 'non-living' objects that looked like stone powder were moving

the same way.

Brown could not explain the reason for the movement, but the name of the

discovery is "the Brownian movement".

We cannot see atoms and molecules directly, but we can see movement of particles

under a microscope, and indirectly understand the existence of atoms and

molecules.

This 'Brownian movement' has a significant meaning in the history of science. As a

conclusion, it has proved the existence of atoms and molecules.

However, in 1905, Albert Einstein believed that the movement of the particles in gas

or liquid was the cause of the Brownian movement and analyzed it mathematically(incidentally, that same year Einstein published 3 papers "theory of Brownian

motion", "theory of photoelectric effect" and "special relativity").

Then in 1908, the French scientist Jean Perrin, observing the Brownian theory

explained that the theory was correct.


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Applications(a) Confirmation of Kinetic Theory of Gases:It offers a visible proof of a

rapid random kinetic motion of molecules in liquid as postulated by kinetictheory of gases.

(b) Determination of Avogadros number:With the help of ultra-microscopethe number of particles in a given mass of the colloidal solution can becounted and the Avogadros number being the number of molecules inone mole of any substance; can be calculated.

(c) Stability of Colloidal Solution:The constant rapid zig-zag motion of the

particles prevents them from setting due to gravity and thus helps instabilising colloidal solution to some extent.

(d) Size of Colloidal Particles:The colloidal particles remain under theinfluence of Brownian movement and gravitation force. Thus both theseeffects act on colloidal particles and these particles distribute themselvesin a vertical column in accordance with the following equation:

where = Avogadro number, R = Gas constant, T = Temperature (K), number ofparticles at depths respectively, densities of particles and of the liquidmedium respectively. The radius r, of the particle can be determinedeasily.


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Sols of basic dyestuffs, ferric hydroxide, aluminium hydroxide etc., are some

common examples of positive sols.

Colloidal solutions of gums, starch, soap solution, metals (Ag, Cu, Au, Pt etc.),

metal sulphides, and some acid dyestuffs are the examples of negative sols


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Electrophoresis

Cation= positively charged ion, it moves toward the

cathode (-)Anion= negatively charged ion, it moves toward theanode (+)

Amphoteric substance= can have apositive/negative/zero charge, it depends on

conditions

Principle:

Some substances have different net charges and canbe separated into several fractions in external electricfield.

But velocity of a particle also depends on the:

size, shape of the particle and given appliedvoltage


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APPLICATIONS(a) Determination of charge:The nature of the charge of a colloidal particle can be ascertained by its migration in an

electric field.

(b) Electrodeposition of rubber:The negatively charged particles of rubber suspended in the latex of rubber plant

can be deposited on another articles making them anode only as a result of electrophoresis.(c) Removal of carbon particles from smoke:The removal of negatively charged carbon particles from smoke can be

done by passing through a chamber provided with highly positive charged metallic knob.

(d) Purification of water:The sewage contains negatively charged particles suspended in water. They may be

removed by coagulating them on anode as a result of electrophoresis.

(ii) Electro-osmosis: When electrophoresis of dispersed particles in a colloidal system is prevented by some

suitable means, the dispersion medium itself begins to move in an electric field. This phenomenon isknown as electro-osmosis.

(iii) Coagulation:We know that the stability of the colloidal solution is due to mutual repulsion between

similarly charged colloidal particles. When the charge on the colloidal particles is neutralized by the

addition of an electrolyte or oppositely charged sol, the precipitation takes place. Thus the process of

precipitating a colloidal solution is known as coagulation or flocculation.

For example, the negatively charged sol is readily coagulated on addition of a solution of (due to ions). The

positively charged sol is readily coagulated on addition of a solution of NaOH (due to ions).

GEL ELECTROPHORESIS


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GEL ELECTROPHORESIS Gel electrophoresis separatesmolecules on the basis of their charge and

size.

The charged macromolecules migrate across a span of gel because theyare placed in an electrical field.

The gel acts as a sieve to retard the passage of molecules according totheir size and shape.

The gel is made from agar, DNA is a negative molecules. Agar comes fromsea weed

The gel is 1% agarous and has no electrical charge.

Molecules sort based on Charge Size

Shape

The most common types of gels are: Starch gels:seldom used nowadays

Agarose gels: for separation of nucleic acids and large proteins

Polyacrylamide gels:for separation of most proteins and small nucleicacids

DNA moves in an

DNA i t i t ll f t


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DNA moves in anelectrical field

so how does thathelp you compareDNA fragments?

size of DNA fragmentaffects how far ittravels

small piecestravel farther

large piecestravel slower &lag behind

+

DNA

swimming through Jello

A method ofseparating DNA in agelatin-like material

using an electricalfield

DNA is negativelycharged

when its in anelectrical field it

moves towardthe positive side

DNA is cut into smaller fragments.

Loading dye is used to indicate the

fragments of DNA are behind the dye

The negative DNA molecule is attracted to

the positive electrode.The smallest fragments move the greatest

distance.


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Particle Sizes Become Larger

Solutions Colloidal Dispersions Suspensions

All particles are onthe order of atoms,

ions, or small

molecules (0.1-1

nm)

Particles of at least onecomponent are large clusters

of atom, ions or small

molecules, or are very large

ions or molecules (1-1000

nm)

Particles of at least onecomponent may be

individual seen with a

low power microscope

(over 1000 nm)

Most stable to

gravity

Less stable to gravity Unstable to gravity

Most homogeneous Also homogeneous, but

borderline

Homogeneous if well

stirred only


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Transparent (but Often translucent or Often opaque but,

often colored) opaque, but may be may appear translucent

transparent

No Tyndall effect Tyndall effect Not applicable

(suspensions cannot betransparent)

No Brownian Brownian movement Particles separate unless

movement system is stirred

Cannot be separated Cannot be separated Can be separated by

by filtration by filtration filtration

Solutions Colloidal Dispersions Suspensions

Homogeneous to Heterogeneous >


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ADSORPTION


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COAGULATION The stability of a sol is due to the charge present on the colloidal particles.

Due to similar charges, colloidal particles repel one another and are unable to

combine together to form larger particles. However, if the charge on colloidal particles is destroyed, they are free to come

nearer and grow in size.

When the particles become sufficiently large, they get precipitated.

The aggregation of colloidal particles can be considered as involving twoseparate and distinct steps:

Particle transport to effect interparticle collision.

Particle destabilization to permit attachment when contact occurs.

This phenomenon is termed as coagulation or flocculation.

The coagulation of colloidal solution can be achieved by the addition of anelectrolyte.

It is to be noted that a small amount of electrolyte is necessary for the stability

of a sol because the ions of the electrolyte get adsorbed on colloidal particlesand impart them some charge.

However, when an electrolyte is added in substantial amount the positivelycharged ions of the electrolyte neutralize the charge on colloidal particles andcompel the sol to get coagulated.


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Chapter 2a - Colloid Properties