Ch8. colloids system

Chapter 8
Colloidal systems
By: Aliyi Gerina [B.pharm]
April 5, 2022
Colloidal Systems by Aliyi G. Bule Hora University
1

Outline
 Introduction (definition, classification and
applications)
 Optical properties of colloids
 Kinetic properties of colloids
 Electrical properties of colloids
April 5, 2022
2

Learning Objectives
1.Differentiate between different types of colloidal systems
and their main characteristics.
2.Understand the main optical properties of colloids and
applications of these properties for the analysis of colloids.
3.Appreciate the major kinetic properties of colloids.
4.Understand the main electrical properties of colloids and
their application for the stability of colloids.
5.Understand the benefits of modern colloidal drug delivery
systems.
April 5, 2022
3

Introduction
 Dispersed systems
─ Are systems which consist of:
 Particulate matter (dispersed phase)
 It is the component present in small proportion
and is just like a solute in a solution
 Dispersion medium (continuous phase)
 A component present in excess and is just like a
solvent in a solution
April 5, 2022
4

Introduction,…
 Dispersed systems consist of a dispersed phase distributed
through out a continuous or dispersion medium.
 Based on the size of the dispersed phase, three types of
dispersed systems are generally considered:
(a)Molecular dispersions
(b)Colloidal dispersions
(c)Coarse dispersions
 Molecular dispersions
 are homogeneousin character and
 form true solutions.
 Colloidal and coarse dispersions
 are examples of heterogeneous systems.
April 5, 2022
Colloidal Systems by Aliyi G. Bule Hora Univ
ersity
5

Introduction,…
Class Particle
size
Characteristic of system Examples
Molecular
dispersion
< 1 nm o Invisible in electron microscope
o Pass through semi permeable
membrane
o Undergo rapid diffusion
Oxygen molecules
and glucose
Colloidal
dispersion
1–500 nm  Invisible by ordinary microscope
 Visible in electron microscope
 Pass through filter paper
 Do not pass semi permeable
membrane
 Diffuse very slowly
Colloidal silver
sols,
natural and
synthetic polyers
Coarse
dispersion
> 500 nm  Visible under microscope
 Do not pass through normal filter
paper
 Do not pass semi permeable
membrane
 Do not diffuse
RBCs,
Most
Pharmaceutical
suspensions and
emulsions
April 5, 2022
6

Colloids Systems
 All kinds of dispersed phases might for
colloids in all possible kinds of media , except
for a gas–gas combination.
 Because all gases mix uniformly at molecular
level, gases only form solutions with each other.
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Classification
April 5, 2022
8
Dispersed
Phase
Dispersion
Medium
Colloid
Type
Examples
Solid Solid Solid sol. Pearls, opals
Liquid Solid Solid emulsion Cheese, butter
Gas Solid Solid foam Pumice, marshmallow,
sponge
Solid Liquid Sol, gel Jelly, paint, blood
Liquid Liquid Emulsion Milk, mayonnaise
Gas Liquid Foam Whipped cream, shaving
cream
Solid Gas Solid aerosol Smoke, dust
Liquid Gas Liquid aerosols Clouds, fog
Gas Gas …………………. None (A gas in a gas always
produces a solution)
A. Colloidal system can be classified based on the basis of the physical state
of two phases:

Classification,…
B. Colloidal system depending on the nature of attraction between the
dispersed phase and the dispersion medium are classified into:
a. Lyophilic Colloids (solvent loving)
b. Lyophobic Colloid (solvent hating)
c. Association Colloids (amphiphilic)
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a. Lyophilic Colloids
 Systems containing colloidal particles that readily interact
with the dispersion medium (solvent loving)
 Due to their affinity for the dispersion medium, such
 materials can easily form colloidal dispersions;
 simply by dissolving the material in the solvent being used.
 The dispersed phase does not precipitate easily
 If the dispersion medium is separated from the dispersed
phase,
 the solution can be reconstituted by simply remixing with the
dispersion medium.
 Hence, these solutions are called reversible solutions
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Lyophilic Colloids,…
o Thermodynamically stable systems.
o Form spontaneously on adding to the appropriate
dispersion medium
o The viscosity of dispersion medium increases greatly on
addition of dispersed phase.
o The dispersions are generally stable even in the presence
of electrolytes.
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Lyophilic Colloids,…
Types of lyophilic colloids;
─ According to the type of solvent
 Hydrophilic
 Solvent:- water
 Colloidal particles: organic molecules (Acacia, gelatin, insulin,
albumin…)in water
 Lipophylic
 Solvent: non aqueous organic solvent (benzene)
 Colloidal particles: organic molecules (Rubber & polystyrene)
 SO; material that form lyophilic colloid in a certain solvent may not
do so in another solvent, e.g.
 acacia + water => lyophilic colloid (hydrophilic type).
 acacia + benzene => NO lyophilic colloid formed
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b. Lyophobic Colloids
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o little interaction between dispersed phase and dispersion medium
(solvent-hating).
o due to the absence of a solvent sheath around the particle.
o Dispersion medium: water.
o Colloidal particles: inorganic particles (e.g.- gold, silver,
sulfur….)
o Thermodynamically unstable systems.
o The viscosity of dispersion medium does not increase greatly on
addition of dispersed phase.

Lyophobic Colloids…
o Easily ppted on addition of electrolyte.
o Once ppted, is not easily reconstituted by simple mixing with
the dispersion medium
 needs stabilizing agents to keep the dispersed phase from ppting out
o Need special method for preparation,
o because the process doesn't take place spontaneously
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 Methods to prepare lyophobic colloids:
oDispersion
oCondensation
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A) Dispersion methods:
─ coarse particles are reduced in size and dispersing by;
 Ultrasonic dispersion
 Dispersion achieved by high intensity ultrasonic dispersion at
frequency more than 20,000 cycles/second.
 Electric dispersion
 Involves production of an electric dispersion within the liquid
and
 dispersion achieved by intense heat generated by the
electrical dispersion so some metal of the electrodes
dispersed as vapor then condense to colloidal particles.
 Colloid mill
 Material sheared between 2 rapidly rotating close plates.
 Low efficiency & reduce the size of small proportion of
April 5, 2022
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B) Condensation methods:
─ materials of sub colloidal dimensions are
caused to aggregate into particles with
colloidal size range.
 Factors affecting the particle size are
 Solubility and concentration of the dispersed phase
 Viscosity of the medium and
 Temperature
 achieved by either changing
 temperature,
 solvent or
 chemical rxn. April 5, 2022
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c. Association Colloids
 Systems composed of amphiphiles or surface active agents
, that are characterized
 by having two distinct regions of opposing solution affinities with
in the same molecule or ion.
 When present in a liquid medium at low concentrations
(below the CMC),
 the amphiphiles exist separately and
 are sub-colloidal size.
 As the concentration is increased (above - the CMC),
 micelles are formed which may contain 50 or more monomers,
 the diameter of each micelleis of 5 nm (colloidal size).
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Association Colloids,…
 The formation of amphiphilic colloids is spontaneous,
 if the concentration of the amphiphile exceeds the CMC.
 Thermodynamically stable systems
 The viscosity of dispersion increases with increase in the
concentration of amphiphiles.
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April 5, 2022
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Lyophilic
(solvent-loving)
Lyophobic
(solvent-hating).
Association (amphophilic).
Dispersed
phase
Large organic mol.
lying within colloidal
size.
Inorganic particles
such as gold or silver
Aggregates (micelles) of small
organic mol. Or ions whose size
is below the colloidal size
Solvation
Mol. of dispersed
phase are solvated.
little
Hydrophilic or lipophilic portion
of the molecules is solvated
depending on the medium.
Preparati
on
Mol. disperse
spontaneously to form
colloidal solution.
Does not disperse
spontaneously.
Needs special
procedure
Spontaneous when conc. of
amphiphiles exceeds cmc.
Viscosity
Viscosity is increased
as the dispersed phase
conc. increase. At
certain conc. Gel sol
formation.
Viscosity is not greatly
increased by presence
of lyophobic colloidal
particles
Viscosity is Increased as conc. of
amphiphile increase as micelles
no. increase & become
asymmetric
Effect of
electrolyt
es
Stable in presence of
electrolytes
Desolvation and
salting out in high
conc.
Unstable in the
presence of even small
conc.of electrolytes;
due to neutralization
of charges on particles
Cmc is reduced by the addition
of electrolytes and salting
out occur at high salt conc.
Comparison of properties of colloidal Sol.

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Size and shape of colloidal particles:
Size may affect:
o drug release from DFs,
o drug bioavailability and
o the possession of large specific surface area results in
many of the unique properties
E.g. platinum is effective as a catalyst only when in the colloidal
form as platinum black
Properties of Colloids….

shape can affect
 the specific surface area exposed
 the more extended the particles the greater its specific
SA and greater opportunity for attraction forces to be
developed b/n particles of dispersed phase and
dispersion medium
 the flow, compressibility and sedimentation properties of
colloids and pharmacological action
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Properties of Colloids
 Optical Properties
 Kinetic Properties
 Electrical Properties
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A. Optical Properties
o When a beam of light is passed through a colloidal solution
 some of the light may be absorbed
 some is scattered and
 the remainder is transmitted undisturbed through the sample
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1) Light scattering (Tyndall effect)
2) Ultra microscope
3) Electron microscope
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Optical Properties…
1. Light scattering (Tyndall effect)
 True solutions do not scatter light and appear clear but
 colloidal dispersions contain opaque particles that do
scatter light and thus appear turbid.
The Faraday—Tyndall Effect
 When a strong beam of light is passed through a
colloidal sol, a visible cone, resulting from the
scattering of light by the colloidal particles, is
formed.
o This is the Faraday-Tyndall effect
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Fig. Faraday-Tyndall effect

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o The intensity of the scattered light depends on the:
 difference between the refractive indices of the dispersed phase and
the dispersion medium.
o In lyophobic colloids, the difference is appreciable and,
therefore,
the Faraday-Tyndall effect is well – defined.
o But in lyophilic sols, the difference is very small and
othe Faraday-Tyndall effect is very weak
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 Importance of light scattering
o measurements:
1) Estimate particle size.
2) Estimate particle shape.
3) Estimate particles interactions.
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2. Ultramicroscopy
─ The ultra microscope, allows to examine the light points (bright spots
corresponding to particles) responsible for the Tyndall cone.
─ Used in the technique of micro electrophoresis for measuring
particle charge.
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3. Electron microscopy
─ Give actual picture of the particles (up to 5A).
─ Used to observe the size, shape and structure of sols.
─ High energy electron beams are used (have greater resolving power)
─ One disadvantage is;
o only dried samples can be examined, not give information on
solvation.
April 5, 2022
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April 5, 2022 Colloidal Systems by Aliyi G.
Bule Hora University
32
B. Kinetic properties
o deals with motion of particles with respect to the dispersion medium
o the kinetic properties includes:
 Brownian motion,
 diffusion
 Osmotic pressure
 Sedimentation
 Viscosity
o The measurement of these properties is used to determine molecular
weight or particle size.

1)Brownian motion:
Definition:
 colloidal particles are subjected to random collision with molecules of
the dispersion medium (solvent), so each particle move in irregular and
complicated zigzag pathway.
 First observed by Robert Brown (1827) with pollen grains suspended in
water.
 The velocity of particles increases with decreasing particle size and
viscosity.
 Increasing the viscosity of dispersion medium (by glycerin) decrease
and then stop Brownian motion.
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o Consequences of Brownian movement
 Stable colloids systems in which the dispersed particles do not
settle,
 because the force of gravity is counteracted by Brownian movement.
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 Particles diffuse spontaneously from a region of higher
concentration to one of lower concentration until the
concentration of the system is uniform .
 Diffusion is a direct result of Brownian movement.
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2) Diffusion:

 The rate of diffusion is expressed by Fick's first law
where
 dm is the mass of substance diffusing in time
 dt across an area A under the influence of a concentration
gradient dC/dx
D is the diffusion coefficient and has the dimensions of area
per unit time
The minus sign denotes that diffusion takes
place in the direction of decreasing concentration
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 Diffusion coefficient can be given by
where η is the viscosity of the medium and a the
radius of the particle
K is the Boltzmann constant and T is temperature
NAAvogadro's number
R is the molar gas constant
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 The diffusion coefficient can be used to obtain the molecular
weight of an approximately spherical particle as follows
 where M is the molecular weight
 v the partial specific volume of the colloidal material
April 5, 2022
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3) Osmotic pressure
o The osmotic pressure, π, of a dilute colloidal solution is
described by the van't Hoff equation:
 Where c is the molar concentration solute
o It can be used to calculate the Mwt of colloidal material
 where C is the concentration of the solution
¶ is the osmotic pressure
M is the molecular weight of the particle and
B is a constant depending on the degree of interaction between the dispersion
medium and dispersed phase
April 5, 2022
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4) Sedimentation
 Velocity of sedimentation of spherical particles having
a density of ρ s in a medium of density ρo and a
viscosity  is given by stokes’ law,
V= 2r2 (ρ s- ρ o ) g or V= d2 (ρ s- ρ o ) g
9 18
 The equation is used to determine particle size
larger than 0.5μm.
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 Particle slower than 0.5μ min size don’t obey Stokes's
equation because Brownian movement becomes
significant and tends to offset gravity sedimentation.
 Stronger force (ultracentrifugation) must be applied to
generate the sedimentation of colloidal particles in a
measurable manner.
April 5, 2022
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 Stokes's equationis modified to:
 Where
 the acceleration of gravity is replaced by ω2x,
 ω is the angular velocity and
 x is the distance of the particle from the center of
rotation.
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5) Viscosity
o Viscosity of colloids depend upon the shape of the colloidal
material.
o An Einstein equation provide quantitative expression for the
flow of colloidal dispersions of spherical particles
 where
 ηo is the viscosity of the dispersion medium and
 η the viscosity of the dispersion when the volume fraction of
colloidal particles present is φ.
April 5, 2022
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April 5, 2022
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 Presence/absence and magnitude of charge is an important factor
in stability of colloids.
A. Electrical properties of interfaces
B. Electrical double layer
C. Electrical Properties

A) Electrical properties of interfaces:
─ Most colloidal surfaces acquire a surface electric charge when brought
into contact with an aqueous medium, the principal charging
mechanisms being as follows:
 Adsorption of a particular ionic species present in solution.
 Ionization of groups (such as COOH) that may be situated at the
surface of the particle.
 Ion dissolution.
 As a result ,dispersed solid particles usually are surrounded by a double
layer of electric charge made of ions.
April 5, 2022
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1) Ion dissolution:
 Surface charge of colloidal particle is controlled by the charge of
ion present in excess in the medium.
─ Examples; AgNO3 + NaI =>AgI +NaNO3
A. Silver iodide in a solution with excess iodide =>Particles acquire –ve
charge & vice versa, i.e., if excess Ag the charge will be +ve
 since the conc. of Ag and I determine the electric potential
B. Aluminum hydroxide in a solution with excess hydroxide =>particles
acquire –ve charge & vice versa.
─ Potential determining ions:
 ions whose conc. determine the electric potential at the particle surface (e.g. Ag+ ,
I -, H+, OH-)
April 5, 2022
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2) Ionization
─ Surface charge of colloidal particle is controlled by the
ionization of surface groupings such as COOH on the
particles.
o charge formation is a function of PH and PK.
─ Examples;
 Eg, amino acids acquire their charge mainly through the ionization of
carboxyl and amino groups to give -COO- and NH+
3 .
April 5, 2022
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3) Ion adsorption:
─ Surface charge of colloidal particle is controlled by the unequal
adsorption of oppositely charged ions.
─ Examples;
 Surfaces of sol in water are more often –ve charged than +ve charged
 Because cations are more hydrated than anions
 so cations reside in the bulk while less hydrated anions adsorbed on the
surface.
April 5, 2022
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B) The electrical double layer:
─ A double layer (DL, also called an electrical double layer, EDL) is
a structure that appears on the surface of an object when it is exposed to
a fluid.
─ The DL refers to two parallel layers of charge surrounding the object.
 Two layer : stern layer and diffusion layer
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 The first layer, the surface charge
(either positive or negative),
comprises ions adsorbed on to the
object
 due to chemical interactions.
 The electric double layer consists
of Layer of ions bounded firmly
to the surface called Stern layer,
surrounded by oppositely charged
ions that form a loose diffuse layer
in the adjacent liquid phase.
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 The second layer is composed of ions attracted to the
surface charge via the coulomb force, electrically
screening the first layer.
 This second layer is loosely associated with the
object.
 It is made of free ions that move in the fluid under
the influence of electric attraction and thermal
motion.
• It is thus called the "diffuse layer“
 The surface separating the two layers is called (shear or
slipping plane).
 The region out side the double layer with equal
distribution of anions and cations is called electro
neutral region. April 5, 2022
51

Example:
 Silver iodide sols can be prepared by the reaction,
AgN03 + Nal => Agl + NaN03
 If the reaction is carried out with an excess silver
nitrate, there will be more Ag+ than l ions in the
surface of the particles
 The particles will thus be positively charged and
the counterions surrounding them will be N03-.
 If the reaction is carried out with an excess NaI,
there will be more l- than Ag+ ions in the surface
of the particles
 The particles will thus be negatively charged and the
counter ions surrounding them will be Na+.
April 5, 2022
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Different potentials
 Surface potential
 Stern potential
 Zeta potential
April 5, 2022
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Zeta potential
 The zeta potential is defined as the difference in potential
between the surface of the tightly bound layer (shear
plane) and electro-neutral region of the solution.
 The zeta potential is measured
 to monitor and predict the stability of dispersion systems.
April 5, 2022
54

 The significance of zeta potential
─ its value can be related to the stability of colloidal dispersions (e.g., a
multivitamin syrup).
─ It indicates the degree of repulsion between adjacent, similarly charged
particles in a dispersion.
─ For molecules and particles that are small enough, a high zeta potential
will confer stability, i.e., the solution or dispersion will resist aggregation.
─ When the potential is low, attraction exceeds repulsion and the dispersion
will break and flocculate.
 So,
 colloids with high zeta potential are electrically stabilized while
 colloids with low zeta potentials tend to coagulate or flocculate.
April 5, 2022
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Electrokinetic phenomena
o General description applied to the phenomena that arise when
attempts are made to shear off the mobile part of the electrical
double layer from a charged surface.
o Four type electrokinetic phenomena:
o Electrophoresis,
o sedimentation potential,
o streaming potential and
o electro-osmosis
o All of which can be used to measure the zeta potential.
April 5, 2022
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Electrophoresis (microelectrophoresis)
o The movement of a charged particle (plus attached ions) relative to a
stationary liquid under the influence of an applied electric field
o the movement of light spots scattered by particles
o is too small and
o is observed using an ultramicroscope
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o the speed of movement of the particle under the influence of a
known electric field is measured. (electrophoretic velocity, v)
o the electrophoretic mobility, u, is given by:
o where v is measured in cm/sec, and E, the applied field strength,
in volts , so that u has the dimensions of cm/sec.volts
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o the zeta potential is given as
 where ζ is zeta potential
 ε is the dielectric constant and
 η the viscosity of the medium
April 5, 2022
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o The technique of micro electrophoresis finds application in
 the measurement of zeta potentials of Colloidal and coarse
dispersions (e.g. suspensions and emulsions)
 to assess their stability and identification of charged groups.
April 5, 2022
60

Sedimentation potential
o The potential difference set up b/n top and bottom of a
suspension of solid particles in a liquid when the particles
settle under the influence of gravity
o the reverse of electrophoresis
o is the electric field created when particles sediment
April 5, 2022
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Streaming potential
o the electric field created when liquid is made to flow along a
stationary charged surface.
o Is due to the displacement of the charges equilibrated in the
double layer around the solid.
o Use to measure zeta potential of relatively coarse particles.
April 5, 2022
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Electro-osmosis
o the opposite of streaming potential
o The atmospheres of counter ion around particles confers a charge
to the dispersion medium
o The sign is opposite to that of the particles
o The particles move to one pole and liquid move toward the other
pole
o The pressure produced by the process of electro-osmosis is KA
electro-osmotic pressure
April 5, 2022
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Purification of colloids
1) Why?
 Many lyophobic sols contain more or less material in true
solution which may be undesirable for any number of reasons;
e.g., electrolyte impurities : cause the flocculation of the sol.
2) How?
a) Dialysis.
b)Electro dialysis.
c)Ultra filtration.
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Purification of colloids,…
a)-Dialysis:
 Depend on difference in size between
 colloidal particles & molecular particles (impurities).
• Technique;
 use semi permeable membrane (e.g.collodion (nitrocellulose),
cellophane).
 pore size of used semi permeable membrane prevent passage of
colloidal particles & permit passage of small molecules & ions
(impurities) such as urea, glucose, and sodium chloride, to pass
through.
 At equilibrium, the colloidal material is retained in compartment A,
while the sub-colloidal material is distributed equally on both sides
of the membrane.
 By continually removing the liquid in compartment B, it is possible
April 5, 2022
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b)-Electro dialysis:
• Technique;
 An electric potential may be used to increase the rate of
movement of ionic impurities through a dialyzing membrane
 Rapid purification
 Electro-dialysis is carried out in a three compartment vessel
with electrodes in the outer compartments containing water
and the
sol in the center compartment.
 A typical apparatus is shown in the figure.
 Application of electrical potential causes cations to migrate to
the negative electrode compartment and anions to move to
the positive electrode compartment, in both of which running
water ultimately removes the electrolyte. April 5, 2022
66

c)Ultra filtration:
Technique;
 Apply pressure (or suction)
 Solvent & small particles forced across a membrane while
colloidal particles are retained.
 N.B.
 The membrane must be supported on a sintered glass plate
to prevent rupture due to high pressure.
 Pore size of the membrane can be increased by soaking in a
solvent that cause swelling
o e.g. cellophane swell in zinc chloride solution.
o e.g. collodion (nitrocellulose) swell in alcohol.
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Physical stability of colloidal
systems
 Stability of colloidal system depends on the
forces of interaction between the particles.
 These forces can be divided into three groups:
─ electrical forces of repulsion,
─ forces of attraction, and
─ forces arising from solvation.
 An understanding of the first two explains the
stability of lyophobic systems, and all three
must be considered in a discussion of the
stability of lyophilic dispersions.
April 5, 2022
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systems,…
 Important terms to be considered in
colloid science
o aggregation, coagulation and
flocculation
 Aggregation
o is a general term signifying the
collection of particles into groups.
 Coagulation
o signifies that the particles are closely
aggregated and difficult to redisperse.
 Flocculation
o the aggregates have an open structure
in which the particles remain a small
April 5, 2022
69

systems,…
 Stabilization serves to prevent colloids from
aggregation.
 The presence or absence and magnitude 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
o surrounding each particle with a protective solvent
sheath which prevent adherence due to Brownian
movement
─ 2-electrostatic stabilization
o providing the particles with electric charge
April 5, 2022
70

Applications of colloidal
solutions:
Therapeutically purpose
─ Colloidal system are used as therapeutic
agents in different areas.
o e.g-
 Silver colloid => germicidal
 Copper colloid => anticancer
 Mercury colloid => Antisyphilis
April 5, 2022
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Applications of colloidal sol,…
Stability, solubility
─ Colloidal coatings to solid dosage forms are used
to protect drugs that are susceptible to atmospheric
moisture or degradation under the acid condition of
the stomach.
─ Association colloids (SAA) are used to increase
solubility & stability of certain compounds in
aqueous & oily pharmaceutical preparation
April 5, 2022
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Absorption
─ As colloidal dimensions are small enough, they have
a huge surface area.
 Hence, the drug constituted in colloidal form is released
in large amount.
e.g- sulphur colloid gives a large quantity of sulphur and
this often leads to sulphur toxicity
April 5, 2022
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Targeted Drug Delivery
─ Seven main types of colloidal drug delivery
systems in use are:
o Hydrogels, Microparticles, microemulsions,
liposomes, micelles, nanoparticles and nanocrystals.
─Colloid drug-delivery systems are used:
 to increase the bioavailability of drug substances,
 to improve drug stability
 to sustain and control drug-release rates
 to target drugs to specific sites in the body
April 5, 2022
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Thank You!!
April 5, 2022
75

Ch8. colloids system

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Ch8. colloids system