COLLOIDS what is colloids what is the uses of colloids

Dispersed Systems:
 Dispersed systems consist of particulate matter (dispersed
phase), distributed throughout a continuous phase (dispersion
medium).
 They are classified according to the particle diameter of the
dispersed material:
1- Molecular dispersions (less than 1 nm)
- Particles invisible in electron microscope
- Pass through semipermeable membranes and filter paper
- Particles do not settle down on standing
- Undergo rapid diffusion
- E.g. ordinary ions, glucose

Dispersed Systems:
2- Colloidal dispersions (1 nm - o.5 um)
- Particles not resolved by ordinary microscope, can be detected
by electron microscope.
- Pass through filter paper but not pass through semipermeable
membrane.
- Particles made to settle by centrifugation
- Diffuse very slowly
- E.g. colloidal silver sols, naural and synthetic polymers
3- Coarse dispersions (> 0.5 um)
- Particles are visible under ordinary microscope
- Do not pass through filter paper or semipermeable membrane.
- Particles settle down under gravity
- Do not diffuse
- E.g. emulsions, suspensions, red blood cells

Size and shape of colloids:
 Particles lying in the colloidal size have large surface
area when compared with the surface area of an equal
volume of larger particles.
 Specific surface: the surface area per unit weight or
volume of material.
 The possession of large specific surface results in:
1- platinium is effective as catalyst only when found in colloidal form
due to large surface area which adsorb reactant on their surface.
2- The colour of colloidal dispersion is related to the size of the
paticles
e.g. red gold sol takes a blue colour when the particles increase in
size

Size and shape of colloids:
- The shape of colloidal particles in dispersion is important:
The more extended the particle the greater its specific
surface the greater the attractive force between the
particles of the dispersed phase and the dispersion
medium.
 Flow, sedimentation and osmotic pressure of the
colloidal system affected by the shape of colloidal
particles.
 Particle shape may also influence the pharmacologic
action.

Purification of colloidal
solutions:
 When a colloidal solution is prepared is often
contains certain electrolytes which tend to
destabilize it. The following methods are used for
purification:
1- Dialysis:
- Semipermeable cellophane
membrane prevent the
passage of colloidal particles,
yet allow the passage of
small molecules or electrolytes.

Purification of colloidal
solutions:
2- Electrodialysis:
- In the dialysis unit, the movement of ions across the
membrane can be speeded up by applying an electric
current through the electrodes induced in the solution.
- The most important use of dialysis is the purification of
blood in artificial kidney machines.
- The dialysis membrane allows small particles (ions) to
pass through but the colloidal size particles
(haemoglobin) do not pass through the membrane.

Applications of colloidal solutions:
1- Therapy--- Colloidal system are used as therapeutic
agents in different areas.
e.g- Silver colloid-germicidal
Copper colloid-anticancer
Mercury colloid-Antisyphilis
2- Stability---e.g. 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 of the atmosphere.

4- 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- sulphur colloid gives a large quantity of sulphur
and this often leads to sulphur toxicity
5-Targeted Drug Delivery--- Liposomes are of colloidal
dimensions and are preferentially taken up by the
liver and spleen.

6- Photography:
A colloidal solution of silver bromide in gelatine is
applied on glass plates or celluloid films to form
sensitive plates in photography.
7- 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 Fe+3
ions neutralize the ion
charges on the colloidal particles.

Types of colloids
Colloids are usually classified according to:
1- The original states of their constituent parts

Types of colloids:
2-The nature of interaction between dispersed phase
and dispersion medium.
A-Lyophilic colloids (solvent attracting) (solvent
loving) – The particles in a lyophilic system have a
great affinity for 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

Types of colloids:
 Examples of lyophilic sols include sols of gum, gelatin, starch, proteins
and certain polymers (rubber) in organic solvents.
 the dispersed phase does not 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
 If the dispersion medium is separated from the dispersed phase, the sol
can be reconstituted by simply remixing with the dispersion medium.
Hence, these sols are called reversible sols.
 Prepared simply by dissolving the material in the solvent being used e.g.
dissolution of acacia in water.

Types of colloids:
B-lyophobic (solvent repelling) (solvent hating) - The particles
resist solvation and dispersion in the solvent.
- The concentration of particles is usually relatively low.
- Less viscid
- These colloids are easily precipitated on the addition of small
amounts of electrolytes, by heating or by shaking
- Less stable as the particles surrounded only with a layer of
positive or negative charge
- Once precipitated, it is not easy to reconstitute the sol by simple
mixing with the dispersion medium. Hence, these sols are called
irreversible sols.
- Examples of lyophobic sols include sols of metals and their
insoluble compounds like sulphides and oxides.
e.g. gold in water
charge

Types of colloids:
Prepared by:
I. Physical method (Bridge‘s arc method)
- This method is employed for obtaining colloidal solutions of
metals e.g. silver, gold, platinum
ice
Dispersion medium
(Water + kOH)

I. Physical method (Bridge‘s arc method)
 An electric current is struck between two metallic
electrodes placed in a container of water.
 The intense heat of the arc converts the metal into
vapours which condensed immediately in the cold
water bath.
 This results in the formation of particles of colloidal
size.

Types of colloids:
II. Chemical method :by oxidation
- Sulphur solution is obtained by bubbling H2S gas
through the solution of an oxidizing agent like HNO3
or Br2 in water , according to the following equations:
- Br2 + H2S S + 2 HBr
- HNO3 + H2S H2O + NO2 + S

Types of colloids:
C- Association / amphiphilic colloids
- Certain molecules termed amphiphiles or surface
active agents, characterized by two regions of
opposing solution affinities within the same
molecule.

Types of colloids:
- At low concentration: amphiphiles exist separately
(subcolloidal size)
- At high concentration: form aggregates or micelles
(50 or more monomers) (colloidal size)

Types of colloids:
Critical micelle concentration (C.M.C) : the
concentration at which micelle form
- The phenomenon of micelle formation can be
explained:
1- below C.M.C: amphiphiles are adsorbed at the
air/water interface
2- As amphiphile concentration is raised: both the
interphase and bulk phase become saturated with
monomers (C.M.C)
3- any further amphiphile added in excess:
amphiphiles aggregate to form micelles

Types of colloids:
- In water: the hydrocarbon chains face inwards into
the micelle forming hydrocarbon core and
surrounded by the polar portions of the
amphiphile associated with water molecules.
- In non-polar liquid: the polar heads facing inward
and the hydrocarbon chains are associated with
non-polar liquid.
- At concentrations close to C.M.C spherical
micelles
- At higher concentrations lamellar
micelles

COLLOIDS what is colloids what is the uses of colloids

Types of colloids:
 The formation of association colloids is
spontaneous, provided the concentration of
amphiphile in solution exceed C.M.C.

Comparison of colloidal sols
Lyophilic Associated Lyophobic
Dispersed phase
(large organic mole.
With colloidal size)
Dispersed phase
(micelles of organic
molec. Or ion –size
below the colloidal
range)
Dispersed phase
(Inorganic particles as
gold)
Molec. of dispersed
phase are solvated
Formed
spontaneously
Hydrophilic and
lyophilic portion are
solvated ,
Formed at conc. above
CMC
Not formed
spontaneously
The viscosity ↑ with ↑
the dispersed phase
conc.
The viscosity ↑ with ↑
the micelles conc.
Not greatly increase
Stable dispersion in
presence of
electrolytes
CMC↓ with electrolytes Unstable dispersion in
presence of
electrolytes

Optical Properties of
Colloids
1-Faraday-Tyndall effect
– when a strong beam of light
is passed through a
colloidal sol, the path of
light is illuminated (a visible
cone formed).
- This phenomenon resulting
from the scattering of light
by the colloidal particles.

Colloids
 The same effect is noticed when a beam of sunlight
enters a dark room through a slit when the beam of
light becomes visible through the room.
 This happens due to the scattering of light by
particles of dust in the air.

Colloids
2- Electron microscope
- Ultra-microscope has declined in recent years as it does
not able to resolve lyophilic colloids.
- so electron microscope is capable of yielding pictures of
actual particles size, shape and structure of colloidal
particles.
- Electron microscope has high resolving power, as its
radiation source is a beam of high energy electrons, while
that of optical microscope is visible light.

Colloids
3- Light Scattering
- depend on tyndall effect.
- used to give information about particle size and shape and for
determination of molecular weight of colloids.
- Used to study proteins, association colloids and lyophobic sols.
- Scattering described in terms of turbidity, T
- Turbidity: the fractional decrease in intensity due to scattering
as the incident light passes through 1 cm of solution.
- Turbidity is proportional to the molecular weight of lyophilic
colloid

Colloids
Hc / T = 1/M + 2Bc
T: turbidity
C: conc of solute in gm / cc of solution
M: molecular weight
B: interaction constant
H: constant for a particular system

Kinetic Properties of Colloids
1-Brownian motion
- The zig-zag movement of colloidal
particles continuously and randomly.
 This brownian motion arises due to the
uneven distribution of the collisions
between colloid particle and the solvent
molecules.
- Brownian movement was more rapid
for smaller particles.
- It decrease with increase the viscosity of
the medium.

2- Diffusion
- Particles diffuse spontaneously from a region of
higher conc. To one of lower conc. Until the conc. of
the system is uniform throughout.
- Diffusion is a direct result of Brownian motion.
- Fick's first law used to describe the diffusion:
(The amount of Dq of substance diffusing in time dt
across a plane of area A is directly proportional to the
change of concentration dc with distance traveled
dq = -DA (dc / dx) dt

D  diffusion coefficient
the amount of the material diffused per unit time
across a unit area when dc/dx (conc. gradient) is
unity.
- The measured diffusion coeffecient can be used to
determine the radius of particles or molecular
weight.

3- Osmotic pressure
- van 't hoff equation:
 = cRT
- Can be used to determine the molecular weight of
colloid in dilute solution.
- Replacing c by C / M (where C = the grams of solute /
liter of solution, M = molecular weight)
/C = RT/M

 = osmotic pressure
R= molar gas constant
4- Sedimentation
- The velocity of sedimentation is given by Stokes‘ Law:
v = d2 (i-e)g/18η
V = rate of sedimentation
D = diameter of particles
 = density of internal phase and external phase
g = gravitational constant
η = viscosity of medium

5- Viscosity:
- It is the resistance to flow of system under an applied stress.
The more viscous a liquid, the greater the applied force
required to make it flow at a particular rate.
- The viscosity of colloidal dispersion is affected by the shape of
particles of the disperse phase:
Spherocolloids dispersions of low viscosity
Linear particles more viscous dispersions

Electric Properties Of Colloids
 The particles of a colloidal solution are electrically charged and
carry the same type of charge, either negative or positive.
 The colloidal particles therefore repel each other and do not
cluster together to settle down.
 The charge on colloidal particles arises because of the
dissociation of the molecular electrolyte on the surface.

E.g. As2S3 has a negative charge
During preparation of colloidal As2S3 , H2S is absorbed
on the surface and dissociate to H+
(lost to the
medium) and S-2
remain on the surface of colloid
.

Electric Properties Of Colloids
 Fe(OH)3 is positively charged
Due to self dissociation and loss of OH-
to the
medium,so they become [Fe(OH)3] Fe+3

Electrophoresis
 Electrophoresis is the most known electrokinetic
phenomena. It refers to the motion of charged
particles related to the fluid under the influence of an
applied electric field.
 If an electric potential is applied to a colloid, the
charged colloidal particles move toward the
oppositely charged electrode.

Electro-osmosis
 It is the opposite in principal to that of electrophoresis.
 When electrodes are placed across a clay mass and a
direct current is applied, water in the clay pore space is
transported to the cathodically charged electrode by
electro-osmosis.
 Electro-osmotic transport of water through a clay is a
result of diffuse double layer cations in the clay pores
being attracted to a negatively charged electrode or
cathode. As these cations move toward the cathode,
they bring with them water molecules that clump
around the cations as a consequence of their dipolar
nature.

Sedimentation Potential
 The sedimentation potential also called the
(Donnan effect).
 It is the potential induced by the fall of a charged
particle under an external force field.
 It is analogous to electrophoresis in the sense that a
local electric field is induced as a result of its motion.
 if a colloidal suspension has a gradient of concentration
(such as is produced in sedimentation or centrifugation),
then a macroscopic electric field is generated by the
charge imbalance appearing at the top and bottom of
the sample column.

Streaming Potential
 Differs from electro-osmosis in that the potential
is created by forcing a liquid to flow through a bed
or plug of particles.

Stability of colloids
 Stabilization serves to prevent colloids from
aggregation.
 The presence and magnitude, or absence of a
charge on a colloidal particle is an important factor
in the stability of colloids.
 Two main mechanisms for colloid stabilization:
1-Steric stabilization i.e. surrounding each particle with
a protective solvent sheath which prevent
adherence due to Brownian movement
2-electrostatic stabilization i.e. providing the particles
with electric charge

A- Lyophobic sols:
- Unstable.
- The particles stabilized only by the presence of electrical charges
on their surfaces through the addition of small amount of
electrolytes.
- The like charges produce repulsion which prevent coagulation of
the particles and subsequent settling.
- Addition of electrolytes beyond necessary for maximum stability
results in accumulation of opposite ions and decrease
zeta potential coagulation precipitation of colloids.

- Coagulation also result from mixing of oppositely
charged colloids.
B- Lyophilic sols and association colloids:
- Stable
- Present as true solution
- Addition of moderate amounts of electrolytes not
cause coagulation (opposite lyophobic)
** Salting out:
Definition: agglomeration and precipitation of lyophilic
colloids.

 This is obtained by:
1- Addition of large amounts of electrolytes
- Anions arranged in a decreasing order of
precipitating power: citrate > tartrate > sulfate >
acetate > chloride> nitrate > bromide > iodide
- The precipitation power is directly related to the
hydration of the ion and its ability to separate water
molecules from colloidal particles
2- addition of less polar solvent
- e.g. alcohol, acetone

- The addition of less polar solvent renders the
solvent mixture unfavourable for the colloids
solubility
** Coacervation:
Definition: the process of mixing negatively and
positively charged hydrophilic colloids, and hence
the particles separate from the dispersion to form a
layer rich in the colloidal aggregates (coacervate)

Sensitization and protective
colloidal action:
 Sensitization: the addition of small amount of
hydrophilic or hydrophobic colloid to a
hydrophobic colloid of opposite charge tend to
sensitize (coagulate) the particles.
 Polymer flocculants can bridge individual colloidal
particles by attractive electrostatic interactions.
 For example, negatively-charged colloidal silica
particles can be flocculated by the addition of a
positively-charged polymer.

Sensitization and protective
colloidal action:
 Protection: the addition of large amount of
hydrophilic colloid (protective colloid) to a
hydrophobic colloid tend to stabilize the system.
 This may be due to:
The hydrophile is adsorbed as a monomolecular layer
on the hydrophobic particles.

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