Colloidal Dispersion, Its Types and Method of Preparation

Colloidal Dispersions
Ms. Chitralekha G. Therkar
Assistant Professor
Dept. of Pharmaceutics,
Siddhivinayak College of Pharmacy, Warora

Dispersed Systems
Dispersed systems consists of particulate matter (API) i.e. dispersed phase which is distributed
throughout a continuous phase i.e. dispersion medium.
These systems are classified according to the mean particle diameter of the dispersed material,
1. Colloidal Dispersion (0.5 µm to 1 µm or 500 nm to 1000 nm)
 Particles which cannot resolved by simple microscope but can be detected by an electron
microscope.
 The particles of this systems can pass through filter paper but cannot pass through the
Semi Permeable Membrane due to small pore size.
 The particles of this systems are made to settle by the centrifugation method as they
cannot settled down directly by gravity.
 They diffuse very slowly.
 These types of colloids shows light scattering when passes a beam of light.
 They are generally of turbid appearance.
 The colloids shows Brownian Motion (shows random and irregular movement)
 Egs, Colloidal silver sols, natural and synthetic polymers.

2. Coarse Dispersion (10 - 1000 µm)
 The particles of this systems are visible by ordinary light microscope.
 They do not pass through the filter paper or the SPM.
 The particles are settled down by gravity.
 The particles do not diffuse and shows very negligible brownian motion.
 These dispersed systems are opaque in appearance.
 The coarse dispersions shows light scattering when subjected in front of beam of light.
 Egs, Emulsions, Suspension and RBC’s.
3. Molecular Dispersion (LT 0.01 µm)
 The particles of this system are invisible in electron microscope.
 They can pass through the filter papers as well as SPM.
 The particles of this system do not settle down on standing.
 They undergo rapid diffusion.
 These systems have very clear appearance.
 The dispersed systems shows brownian and kinetic motions.
 They do not show light scattering.
 Egs, Ordinary ions, Glucose, Urea, Oxygen.

Colloidal
dispersions
 The term colloid is derived from two words i.e. ‘kolla’ meaning Glue and ‘eoids’ meaning Like
which means glue like substances.
 Colloidal dispersions are the heterogeneous dispersed systems, where an internal phase
(dispersed particles) are distributed uniformly in a continuous or external phase (dispersion
medium) and the system have particles size range from 0.5 – 1 micron.
 The fluid colloidal systems which are composed of two or more components within them, they
are called as sols.
 In colloidal systems, particles pass through filter paper but cannot pass through semipermeable
membrane and they diffuse very slowly.
 Egs, Gums (acacia, tragacanth), Natural rubbers etc.
 Some colloids encountered in pharmacy includes surfactant micelles, suspensions, emulsions,
inhalation aerosols etc.

Sizes and shapes of
colloids
 Particles existing in colloidal dispersion have large surface area when compared with the surface
area of an equal volume of larger particles which means once a normal particle is incorporated as
a colloid its surface area gets increased. This enhanced surface is called as Specific surface.
 Specific surface – the surface area of that system per unit weight or volume of material.
And this possession of large specific surface results in,
1. Additional effect – Eg, platinum is a normal metal until it is incorporated as colloid. Once it get
dispersed as colloid, it acts as an effective catalyst which have quite large surface area than
normal platinum and which can adsorb reactants on their surface.
2. Coloration after increase in size – the colour of colloidal dispersion is related to the size of
particle, as the size of colloidal particles increases, they starts imparting a colour within system.
Eg, Red gold sol takes a blue colour when the particles increases in size.
1. Size

2. Shapes
 The shape adopted by colloidal particles in dispersion is important because the more extended the
particle, the greater is its specific surface and the greater is the attractive forces between the
particles of the dispersed phase and dispersion medium.
 In a friendly environment, a colloidal particle unrolls and exposes maximum surface area. Under
adverse conditions, it rolls up and reduces its exposed area.
 Shape of colloidal particles can affect the flow and sedimentation of that colloidal system and
sometimes it may influence the pharmacological action.
 The shapes that can be assumed for colloidal particles are,
1. Spheres and globules
2. Short rods and prolate ellipsoids (rugby ball shaped/elongated)
3. Oblate ellipsoids (disc shaped/flattened) and flakes.
4. Long rods and threads.
5. Loosely coiled.
6. Branched threads.
 The following properties are affected by changes in the shape of colloidal particles,
1. Flowability
2. Sedimentation
3. Pharmacological action.

Classification of Dispersed
systems
1. Based on the physical state of two phases.
SN Dispersed phase Dispersion Medium Colloid Type Examples
01 Liquid Gas Liquid aerosol Cloud, mist, fog
02 Solid Gas Solid aerosol Dust, smoke
03 Gas Liquid Foam Whipped cream
04 Liquid Liquid Emulsion Milk, Mayonnaise
05 Solid Liquid Suspension Jelly, paint
06 Gas Solid Solid foam Pumice, marshmallow
07 Liquid Solid Solid emulsion Cheese, butter
08 Solid Solid Solid sols Pearls, opals

2. Based on the interaction between colloidal phases
1. Lyophilic colloids
2. Lyophobic colloids
3. Association colloids

1. Lyophilic colloids
 In this system, the dispersed particles have a greater affinity towards the dispersion medium
used for formulation (solvent loving).
 These systems are called as Hydrophilic when solvent is water and Lipophilic when solvent is
Oil.
 They are called as Oleophilic when solvents are Non Aqueous vehicles.
 The dispersion medium forms a sheath around the colloidal particle and solutes. This makes
the dispersion thermodynamically stable.
 As these systems are thermodynamically stable they are reversible, they can be reconstituted
after the solvent is removed from the system.
 Lyophilic colloids are usually obtained simply by dissolving the material in solvent being used
for formulation and for this reason, preparation of lyophilic colloids is relatively easy.

 Generally organic material is used to dispersed in the dispersion medium.
 Various properties of this class of colloids are due to the attraction between the dispersed
phase and dispersion medium, which leads to solvation, the attachment of solvent molecules
of the dispersed phase to solutes (if water is solvent or dispersion medium it is termed as
Hydration)
 Eg, the dissolution of acacia or gelatine in water leads to formation of a solution.
Preparation of Lyophilic Colloids –
 The simple dispersion of lyophilic material in a solvent leads to the formation of lyophilic
colloids.
 Firstly the colloidal particles to be constituted are crushed with solute or DP to get uniform
size.
 Then the required solvent or DM is added slowly and with continuous stirring until the
dispersion is formed.

2. Lyophobic
colloids
 Colloids are composed of materials that have a little attraction between the dispersed phase
and dispersion medium (solvent hating) or sometimes no attraction at all.
 These systems are stable because of presence of charge on particles, colloids are generally
composed of inorganic particles dispersed in water such as salts of gold, silver, silver iodide
etc.
 These lyophobic colloids and their properties differs from the lyophilic colloids by not having
a solvent sheath around the particles and it is necessary to use special method to prepare
lyophobic colloids which are given as follows,
 Preparation of lyophobic colloids –
1. Dispersion method – This method involves the breakdown of larger particles into
particles of colloidal dimensions. The breakdown of coarse material may be affected by the
use of the colloid mills, ultrasonic treatment in presence of stabilizing agents like, surface
active agent.

2. Condensation method – In this method, the colloidal particles are formed by the aggregation
of smaller particles such as molecules. These involves a high degree of initial supersaturation
followed by the formation and growth of nuclei.
Supersaturation can be brought about by,
I. Change in solvent
 For example, if sulfur is dissolved in alcohol and then this concentrated solution is poured into
an excess of water, many small nuclei form in the supersaturated solution. These grow rapidly
to form a colloidal sol.
 If a saturated solution of sulphur in acetone is poured slowly into hot water the acetone
vaporizes, leaving a colloidal dispersion of sulphur.
II. Chemical reaction
 For example, colloidal silver iodide may be obtained by reacting together dilute solutions of
silver nitrate and potassium iodide.
 If a solution of ferric chloride is boiled with an excess of water produces red sol of hydrated
ferric oxide by hydrolysis.
3. Milling and grinding method – generally mortar and pestle, sieves etc are used in small scale.
4. Super saturation method - Eg, sulfur is dissolved in alcohol and then the concentrated
solution is transfer to water, small nuclei will produced these grow rapidly to form colloidal solution

Lyophilic colloids Lyophobic colloids
Colloidal particles have greater affinity for the
dispersion medium.
Colloidal particles have little affinity for the
dispersion medium.
Owing to their affinity for the dispersion
medium, the molecules disperse
spontaneously to form colloidal solution.
Material does not disperse spontaneously, and
hence lyophobic sols are prepared by dispersion
or condensation methods.
These colloids form “Reversible sols” These colloids form “Irreversible sols”.
Viscosity of the dispersion medium is increased
greatly by the presence of colloidal particles.
Viscosity of the dispersion medium is not
increased by the presence of colloidal particles.
Dispersions are stable, generally in the presence
of electrolytes, they may be salted out by high
concentrations of very soluble electrolytes.
Lyophobic dispersions are unstable in the
presence of even small concentrations of
electrolytes because they already have charges.
Dispersed phase consists generally of large
organic molecules such as gelatin, acacia lying
within colloidal size range.
Dispersed phase ordinarily consists of
inorganic particles, such as gold or silver.
Comparison of Lyophilic and Lyophobic
colloids

3. Association
colloids
 Surface active agents have two distinct regions of opposing solution affinities within the same
molecule or ion and are known as Amphiphiles.
 When present in liquid medium at low concentration, the amphiphiles exist separately and in
subcolloidal size. As the concentration of them is increases, the aggregation occurs over a narrow
concentration range. These aggregates which may contain 50 or more separate monomers, are
called as Micelles.
 Because the diameter of each micelle is of the order of 50Å, micelles lie within the colloidal Size
range. The concentration of monomer at which micelles are formed is termed as Critical
Micelle Concentration (CMC). The number of monomers that aggregate to form a micelle is
known as the Aggregation number of the micelle.
 In water, the hydrocarbon chains of amphiphiles, faces inward into the micelle to form their
own hydrocarbon environment. Surrounding this hydrocarbon core are the polar portions of
the amphiphiles associated with the water molecules of the continuous phase.

 The association colloid can be classified as anionic, cationic, nonionic and ampholytic (zwitter ionic)
depending upon the charges on the amphiphiles. The opposite ions bound to the surface of charged
micelles are termed counter ions or gegenions, which reduces the overall charge on the micelles.
 The viscosity of the system increases as the concentration of the amphiphile increases because
micelles increase in number and become asymmetric.

Optical properties of colloids
 When a strong beam of light is passed perpendicularly through two
solutions, i.e. true solution and colloidal solution, which are placed
against a dark background,
1. The path of light beam is not visible in case of true solution.
2. The path of light beam is visible (scattered) in case of colloidal
solution and further it is forming a shadow (beam or cone) at the
dark background.
 In case of colloidal sols, some of the light gets absorbed, some gets
scattered and remaining gets transmitted through the sample. And
due to the light scattering, the sol appears turbid, and this
phenomenon is called as Faraday – Tyndall Effect.
 The illuminated beam or cone formed by the sol particles is called
Tyndall beam or Tyndall cone.
1. Faraday Tyndall Effect

2. Light Scattering effect
This is based on Faraday - Tyndall effect. It is widely used in the determination of size, shape and
interactions of the colloids. As the turbidity depends upon the size of particles dispersed, it is used
in determining the molecular weights of colloids.
Principle –
Scattering is expressed in terms of turbidity τ, the fractional decrease in the intensity due to
scattering as the incident light passes through 1 cm of solution.
The turbidity can be calculated as follows,
τ - turbidity in cm-1, c - concentration of the solute in gm/cm3
M=weight of the average molecular weight in g/mol
B – Interaction constant, H – Optical constant depending upon Refractive Index

3. Ordinary light microscope – The light microscope which uses light as its source of
radiation resolves particles separated by a distance of 1800 A or may be MT that. But the limitation of
this light microscope is that it used to resolve only particles ranging in above particle size range.
4. Electron microscope – the electron microscope uses a beam of accelerated electrons as a
source of light. These microscopes have a higher resolving power than light microscope and can be use
to reveal the structure of smaller objects.
There are Two types of electron microscopes,
 Transmission Electron Microscope – it uses high voltage electron beam which is produced by
an electron gun fitted with tungsten filament.
 Scanning Electron Microscope – this method produces image by probing the specimen with a
focused electron beam. When this beam of electrons interacts with the specimen, it loses its
energy and the image is displayed by the SEM Maps.
Applications –
 Particles size analysis
 Particle detection
 Material qualification
 Research and Development

Kinetic properties of colloidal
system
Kinetic properties of colloidal systems relate to the motion of particles with respect to the
dispersion medium. The motion may be thermally induced (Brownian movement, diffusion,
osmosis) or gravitational force induced (sedimentation) or applied externally (viscosity).
The kinetic properties are,
1. Brownian motion 2. Diffusion 3. Osmotic pressure 4. Sedimentation 5. Viscosity
1. Brownian motion
 Colloidal particles undergo random collisions with the molecules of dispersion medium and
follows an irregular zigzag path. If the particles up to 0.5 µm diameter are observed under the
microscope or the light scattered by colloidal particles is viewed using an ultra microscope, an
erratic motion is seen. This movement is referred to as Brownian motion.
 The motion of molecules cannot be observed.
 The velocity of the particles increases with decreasing the size.
 If added the viscosity agent brownian motion stops.
 It arises as the consequences of Kinetic Theory.

Colloidal Dispersion, Its Types and Method of Preparation

2. Diffusion - As a result of brownian motion, colloidal particles spontaneously diffuse from a
region of higher concentration to lower concentration. The rate of diffusion is expressed by Fick's first
law,
𝑑𝑀
𝑑𝑡
= - DS
𝑑𝑐
𝑑𝑙
 According to the law, dM is the amount of substance diffusing in per unit time dt across a plane of
area (S) is directly proportional to the change of concentration dc, with distance traveled dl.
 D is diffusion coefficient and has dimension of area per unit time, dc/dl is concentration gradient. The
negative sign denotes that the diffusion takes place in a direction of decreasing concentration. The
diffusion coefficient of a dispersed material is given by Stokes - Einstein equation,
D =
𝑅𝑇
6πη𝑟𝑁
Where,
N - Avogadro’s number (6.023×1023
molecules per mole), R - molar gas constant and r is the radius
of spherical particle at η viscosity.
 The analysis of above equations allows us to formulate three main rules of diffusion,
1. The velocity of molecules increase with reduction of particle size.
2. The velocity of molecules increase with increasing temperature.
3. The velocity of molecules decrease with increasing viscosity of the medium.

3. Osmotic Pressure
 Osmosis is the spontaneous movement of solvent molecules through SPM into a region of higher
solute concentration in a direction that tends to equalize the solute concentration on both sides.
 The external pressure required to be applied so that there is no net movement of solvent across the
membrane is called Osmotic Pressure.
 The osmotic pressure 𝜋, of a dilute colloidal solution is described by the Van't Hoff equation,
𝜋 = 𝑐𝑅𝑇
Where,
c - molar concentration of solute, R – gas constant and T – temperature.
This equation can be used to calculate the molecular weight of a colloid in a dilute solution.
Replacing c with cg/M in above equation, in which ‘cg’ is the grams of solute per liter of solution and ‘M’
is the molecular weight, then we obtain,
𝜋 =
𝐶𝑔
𝑀
RT
𝜋
𝑐𝑔
=
𝑅𝑇
𝑀
 The above equation is true when the concentration of colloids is low (ideal system).
 For linear lyophilic molecules or high molecular weight polymers, following equation is valid.

𝜋
Cg
= RT
1
𝑀
B Cg
Where, B is an interaction constant for any solvent/solute system. And the plot of
𝜋
Cg
vs Cg is linear.
𝜋
Cg
Cg
1. The extrapolation of line to the vertical axis where Cg = 0 gives RT/M and if the temperature at which
the determination was carried out is known, the molecular weight of solute can be determined.
2. From the slope of the line, the value of interaction constant (B) can be determined.
3. Line II indicates linear colloid in which the solvent have high affinity for the dispersed particles.
4. Line I is observed for the same colloid if it is present in a relatively poor solvent having a reduced
affinity for the dispersed material.

4. Sedimentation
The velocity, v, of sedimentation of spherical particles having a density ρ, in a medium of density ρ0
and a viscosity η0 is given by Stoke's law,
𝑣 =
2‫ݎ‬2(ߩ − ߩ0)݃
9η0
Where ‘g’ is the acceleration due to gravity.
 If the particles are subjected only to the force of gravity, then the lower size limit of particles obeying
Stokes's equation is about 0.5 µm. This is because Brownian movement becomes significant and
tends to offset sedimentation due to gravity and promotes mixing instead.
 Consequently, a stronger force must be applied to bring about the sedimentation of colloidal
particles.
 In a centrifuge, g is replaced by ω2
x, ω - angular velocity or angular acceleration 2 times the speed
of rotor in revolution per second, x is the distance of particle from the center of rotation then the
equation becomes,
𝑣 =
2‫ݎ‬2 (ߩ − ߩ0) ω2‫ݔ‬
9η0

5. Viscosity
Einstein equation of flow for the colloidal dispersions of spherical particles is given by,
η = η0(1 + 2.5ϕ)
Where,
‘η0’ is the viscosity of dispersion medium, ‘η’ is the viscosity of dispersion when volume fraction of
colloid particles is ϕ.
The volume fraction is defined as the volume of the particles divided by the total volume of dispersion.
Relative viscosity (ηrel) =
η
η0
= 1 + 2.5 ϕ
Specific viscosity (ηSp ) =
η
η0
- 1 = 2.5 ϕ
By determining η at various concentration and knowing η0 the specific viscosity can be calculated.

Electrical properties of
Colloids
1. Electrical double layer - The theory of the electric double layer deals with the distribution of
ions and the magnitude of an electric potentials that occur in the locality of the charged surface.
Consider, a solid charged surface in contact with an aqueous solution of an electrolyte.
 Suppose, some of the cations are adsorbed onto the solid surface (aa’) giving it a positive charge
and these are called potential determining ions.
 Then the counter anions are attracted to the positively charged surface by electric forces and
forms a region called tightly bound layer.
 In this layer there are fewer anions than cations adsorbed onto the solid surface and hence the
potential at bb’ is still positive.

 At a particular distance from the surface, the concentration of anions and cations are equal and
form an electrically neutral region.
 The whole system is electrically neutral even though there are regions of unequal distributions of
anions and cations and the region bounded by the bb’ and cc’ interface is called diffuse second
layer where an excess of anions are present.
 Beyond cc’, the distribution of ions is uniform and an electrically neutral region exists and the
potential at the solid surface is called Nernst potential and is defined as the difference in potential
between actual solid surface and the electrically neutral region of the solution.
 The potential at plane bb’ is called Zeta potential and is defined as the difference in potential
between surface of tightly bound layer and the electrically neutral region of the solution.
 Conditions –
1. If zeta potential falls below a particular value (+30mV or -30mV), the attractive force exceed the
repulsive force and results in aggregation of colloidal particles.
2. Zeta potential decreases more rapidly when the concentration of electrolytes is increased or the
valency of counter ions is higher.

2. Electrophoresis – The movement of particles inside a liquid or a gel medium where they are
suspended under the influence of an electric field is called as Electrophoresis.
The movement is expressed in terms of velocity, and the velocity of particles is dependent on
following factors,
 Strength of electric field
 The viscosity of medium
The velocity of a particle in an electric field is commonly referred as Electrophoretic Mobility (UE),
Zeta Potential (ξ) of the particle is calculated by using Henry’s Equation,
UE =
2εξf(Ka)
3η
Where,
𝛆 is dielectric constant
η is Viscosity
F(Ka) is Henry’s function.

3. Electro osmosis
 The movement of a liquid relative to the stationary charged surface under the influence of an
electric field is called as Electro – Osmosis.
 If the sol is enclosed by compact diaphragms, then the motion of the sol particles be
mechanically prevented.
 Two electrodes are inserted in the dispersion medium outside the diaphragms, the dispersion
medium moves through the diaphragm towards one of the electrodes.

4. Donnan Membrane Equilibrium – It is the behavior of charged particles near a SPM that
sometimes fails to distribute evenly across the two sides of the membrane. The usual cause is the
presence of different charged substances that is unable to pass through the SPM and thus creates an
uneven electric charge.
Eg, The large anionic proteins in blood are not permeable to capillary walls due to small cations
attached to proteins. But the cations are not compulsorily attached to all proteins so, some of the
proteins are passable from SPM.

Physical stability of colloidal
systems
 The presence or absence of a charge on a colloidal particle is an important factor in the stability
of colloidal systems.
 Stabilization is accomplished essentially by two means,
1. Providing the dispersed particles with an electric charge.
2. Surrounding each particle with protective solvent sheath that prevents mutual adherence
when the particles collide as a result of brownian movement.
 This second effect is significant only in the case of lyophilic sols.

 The addition of an electrolyte to lyophilic colloid in moderate amount does not result in
coagulation.
 If sufficient salt is added, agglomeration and sedimentation of the particles may result. This
phenomenon referred as “Salting out”
 In general, lyophilic colloids are stable because of the solvent sheath around the particles.
 At high electrolyte concentration, ions get hydrated and water is no more available for hydration
of particles. This results in flocculation or salting out of colloidal particles.
1. Effect of an electrolyte on Lyophilic
colloids
1. Effect of an
electrolyte

2. Effect of an electrolyte on Lyophobic
colloids
 A lyophobic sol is thermodynamically unstable. The particles in such sols are stabilized only by
the presence of electric charges on their surfaces.
 The like charges produce a repulsion that prevents coagulation of the particles. Hence, addition
of a small amount of electrolyte to a lyophobic sol tends to stabilize the system by imparting a
charge to the particles.
 In colloidal dispersions frequent encounters between the particles occurs as a result of brownian
movement. Such interactions are mainly responsible for the stability of colloids.
 There are two types of interactions, vanderwaal’s attraction and electrostatic repulsions.
 When attractions predominate, the particles adhere after collisions and aggregate is formed and
when repulsions predominate, the particles rebound after collisions and remain individually
dispersed.

2.
Peptization
The stability of colloids is due to following reasons,
1. Development of electric charge on surface of colloidal particles.
2. Formation of solvent sheath by the process of salvation.
3. Brownian motion
Origin of surface charge – the surface charges originates from following,
1. Ionization of surface groups.
2. Differential loss of ions from the crystal lattice.
3. Adsorption of charged species.

 When negatively and positively charged hydrophilic colloids are mixed, the particles may
get separated from the dispersion to form a layer which is rich in the colloidal aggregates.
 These colloid rich layer is known as a “Coacervate”, and the phenomenon in which
macromolecular solutions separate into two layers is referred to as “Coacervation”
 As an example, consider the mixing of gelatin and acacia. Gelatin at a pH below 4.7
(Isoelectric point) is positively charged and acacia carries a negative charge, that is
relatively unaffected by pH in the acid range, When solutions of these colloids are mixed in
a certain proportion, coacervation results.
3.
Coacervation

4. Protective Colloid
Action
 When a large amount of hydrophilic colloid carrying opposite charge, is added to hydrophobic
colloids, these gets adsorbed on the hydrophobic particles and form a protective layer around it.
 This adsorbed layer prevents the precipitating ions reaching the sol particles.
 Therefore, the further coagulation is prevented and the system becomes stabilized.
 The entire colloid behaves like a hydrophilic colloid.
 The colloid that helps to stabilize the other colloid is known as Protective Colloid.

 The ‘protective property’ is expressed most frequently in terms of the “Gold number”.
 The gold number is the minimum weight in milligrams of the protective colloid (dry weight of
dispersed phase) required to prevent a color change from red to violet in 10 mL of a gold sol on
the addition of 1 mL of a 10% solution of sodium chloride.
 The gold numbers for some common protective colloids are as follows,
1. Gelatin (gold number 0.005-0.01)
2. Albumin (gold number 0.1)
3. Acacia (gold number 0.1-0.2)
 Basically, gold sol is a hydrophobic colloid and has red color. When an electrolyte like NaCl is
added, coagulation of colloid is observed indicating violet color.
 When protective colloids are added, these stabilize the gold sol and prevent the change to
violet color.
 Lower the gold number, greater the protective action.

1. Brownian motion
2. Electrostatic forces of repulsion
3. Van der Waals forces of attraction.
4. Solvent on approach of neighboring particles (repulsion or attraction)
General factors affecting the Stability of
colloids

Colloidal Dispersion, Its Types and Method of Preparation

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