colloids - wikispaces ppt- •aka colloidal dispersion •heterogeneous mixture which has suspended...
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Prof. Odyssa Natividad RM. Molo
Thomas Graham (1861) while doing his classical researches
on liquid diffusion observed two classes of substances
Crystalloid: readily pass through animal & vegetable membrances
Colloids: diffused very slowly & could not pass through membrane
Came from the Greek word kola which means glue-like
aka colloidal dispersion
Heterogeneous mixture which has suspended
Particles size: solution < colloid < suspension
Can pass through filters but are too large to
pass through membranes.
Distinguished from a true solution through
The scattering of light.
Although undetected in
normal lightning, presence
of the small particles that
remain suspended can be
demonstrated by shining a
beam of intense light. The
beam is visible from the side
because light is scattered by
the suspended particles.
What stabilizes a colloid?
Why do the particles remain
suspended rather than form larger
aggregates and precipitate out?
The answer is complicated, but the
main factor seem to be electrostatic
What stabilizes a colloid?
A colloid, like all other macroscopic substances, is electrically neutral.
However, when a colloid is placed in an electric field, the dispersed particles all migrate to the same electrode and thus must all have the same charge (electrophoresis).
How is this possible?
The center of a colloidal particles (a tiny ionic crystal, a group of molecules or a single large molecule) attracts from the medium a layer of ions, all of the same charge. This group of ions, in turn, attracts another layer of oppositely charged ions. Because the colloidal particles all have an outer layer of ions with the same charge, they repel each other and do not easily aggregate to form particles that are large enough to precipitate
Components of Colloids
Colloidal particles/Dispersed phase/substance
Similar to solute in solution
Similar to solvent in solution
Classification of Colloids (based on states of dispersed phase & dispersion medium)
COLLOID TYPE DISPERSE
Aerosol (liq aerosol) Liquid Gas Mist, fog, clouds, spray
Aerosol (solid aerosol) Solid Gas Dust, smoke, airborne bacteria,
Foam Gas Liquid Whipped cream, soap suds, shaving
Emulsion Liquid Liquid Milk, mayonnaise, hand lotion
Sol Solid Liquid Paint, clay, blood plasma, gelatin,
starch paste, glue
Solid foam Gas Solid Marshmallow, styrofoam, pumice
stone, bread dough
Solid emulsion or gel Liquid Solid Butter, cheese, gelatin, curd
Solid sol Solid Solid Ruby glass, gemstone, cement
Classification of Colloids (based on affinity for solvent)
There is a strong affinity between the dispersed
phase & the dispersion medium
There is no apparent affinity
If solvent if water:
Hydrophilic (water-loving)& hydrophobic
Characteristics of lyophilic & lyophobicLYOPHILIC LYOPHOBIC
The particles may or may not migrate in an
The particles are charged and hence move
towards electrode in an electrical field
Viscosity is much higher than that of the
Viscosity is same as that of the medium
Self-stabilized due to strong attraction
between 2 phases
Stabilizer is required
Not easily precipitated by the addition of
Coagulated easily by adding electrolytes
Not visible under ultramicroscope Visible under ultramicroscope
Prepared by directly dissolving Prepared by indirect method
Some Properties of Colloids
Tyndall effect Discovered in the course of his study into light beams by British
Exhibited by colloid: both beneficial & harmful
Brilliant colors at sunset and the blue color of the sky and the
sea are caused by thick layer of small particles and molecules in
the atmosphere and in the sea.
Smoke, fog, mist and smog are colloids which pollute the
Is the rapid, haphazard zigzag motion of colloidal
particles caused by the collision of colloidal particles
with the molecules of the dispersion medium.
Colloidal particles do not settle
because of this.
First observed under a special microscope by Robert
Brown, a Scottish botanist, when he suspended
pollen grains in water.
Is the binding in thin layer of molecules to
activated carbon to remove unpleasant
odors and flavors,
cigarette filters to remove carcinogenic
compounds in the smoke, and
gas masks to provide protection against
toxic or irritating gases.
Colloidal particles adsorb charged particles (ions)
from their surrounding medium.
Colloids are either (+) or (-) ions, but within a
system, all particles have the same electrical charged
so they repel each other.
Prevents colloids from forming aggregates or
precipitates, making the system stable.
When colloids attract other colloids of opposite
charge, precipitation or coagulation occurs.
Electrical charge application
In industries, (electrostatic/Cottrell precipitator) used to
remove soot particles and other suspended solids from gases
in smokestacks before they are released to the atmosphere.
Charged plates are placed inside the smokestack
where they attract the charged colloidal particles
which are held or precipitated on the plates. When
current is turned off, particles fall from the plates are
How electrostatic precipitator works
1) Smoke particles pick
up a negative charge.
2) Smoke particles are
attracted to the
3) Collecting plates are
knocked down to
remove the smoke
How photocopier/laser printer works
Preparation of Colloids
Large particles are broken down to colloidal
dimension & dispersing them to colloidal
Particles of molecular size are allowed to
coalesce or condense to form bigger
particles of colloidal size
Breaking down of big particles by grinding
or by ultrasonics (use of high-intensity
Usually leads to extensive subdivision that
causes smaller particles to reunite (1) due
to the mechanical forces involved and (2) by
the attractive force
Grinding or wet-milling in
the presence of surface-
Adding an emulsifying
Soap in an oil-water
mixture to produce an
Egg yolk in mayonnaise
made of oil and
calamansi juice or
Peptizing agent: reduces big particles to colloidal size
Enzymes that break down food particles during the
Homogenizer: machine that emulsifies or blends a substance
Milk is homogenized to
prevent fat from rising
to the surface
Process of combining molecules in supersaturated solutions
to precipitate or coagulate them in colloidal size
By exchange of solvent
By changing physical state
By controlled condensation
By Chemical methods: reduction, hydrolysis, oxidation
Purification of Colloidal Solutions
(ordinary filter paper
impregnated with a
solution to make pores
smaller so collloidal
particles cannot pass
Purification of Colloidal Solutions
Process of separation of crystalloids from colloids by
diffusion through the animal or vegetable membrane
carried out in a
direct current field
to accelerate process
Application of Colloids
Purification of water
Impure water is treated with alum, Al3+ ions of alum precipitate
negatively charged clay particles, bacteria, colloidal particles and the
Purification of polluted air
Smoked is negatively charged colloidal suspension consisting of C
particles. Air is passed through precipitator where the charge is
neutralized & C settles down while