PROPERTIES OF COLLIODS used in pharmaceutical

PROPERTIES OF COLLIODS
OPTICAL PROPERTIES
KINETIC PROPERTIES
ELECTRICAL PROPERTIES

 PROPERTIES OF COLLIODS
 Optical Properties, Kinetic Properties, Electrical
Properties helps in obtaining information regarding
Size, shape, Structure and molecular wt. of colloids.
OPTICAL PROPERTIES
1. Faraday –Tyndall Effect
2. Electron Microscope
3. Turbidity
4. Scattering of Light -Molecular weight determination

Faraday –Tyndall Effect 2 marks
 When a Intense narrow beam of light passed
through Colloidal solution – the beam passes
 A Visible cone is formed when the particles of the
colloidal solution scatters the light –Tyndall cone or
beam.
 Phenomenon known as Faraday –Tyndall Effect.
 The beam is visible when viewed against dark
background, Perpendicular to the incident beam.
 Observed with the help of ultra-microscope (dark
field Microscope) – dispersed particles appear as
bright dark spots.

Faraday –Tyndall Effect (contd…)

Electron microscope
 Used to study the Particle size, shape and structure of colloidal
particles
 High resolving power about 5 A with using wavelength of light about
0.1 A. (Normal microscope 2000 A (0.2 µ).
 Shorter the wavelength of light used better is the resolution power.
 It can give photographs of actual particles.
Turbidity
 Turbidity (T) – fractional decrease in intensity of transmitted light in
relation to incident light passing through 1 cm solution.
 Used to estimate the concentration of dispersed particles and
molecular weight.
 Measurement is done by using spectrophotometer and
Nephelometer.

Light scattering 5 marks
 Useful to obtain information about shape and size of
the particles –Proteins, polymers, association and
lyophobic colloids.
 Determination of Molecular weight - polymers,
Micelles.
 Spherical particles –scattering of light uniform all
directions.
 Rod shaped - scattering of light right angles to the
direction of flow.
 At a given concentration of dispersed phase, the
turbidity is proportional to molecular weight of
lyophilic colloid (low turbidity).

Light scattering
 Scattering of light depends on Wavelength of incident beam,
intensity of incident beam, and difference in Refractive index
of particles and medium
 In lyophilic colloids it is easy to measure the scattered light
than the transmitted light. provided the dimensions of particles
are small compared to wavelength of light used.
 Highly colored nature of many colloids is due to
wavelength of scattered of light. 2 marks
 Reduced gold chloride is red in color.
 Colloidal gold is deep red in color.
 Intermediate size is violet in color.
 Coarse dispersion appears blue in color.
 Silver iodide is yellow in colour

Light scattering (contd…)
 Light source having λ higher than the dimension of
particles used.
 Turbidity is measured from the scattered light by
viewing through a particular angle.
 τ = (16π/3) R90
 R90= Ir2/ I0 (Rayleigh ratio)
τ - Turbidity
I- intensity of incidence light.
I0- intensity of the Scattered light.
r - distance from scattered particle to the point of
observation.

 When light scattering treated as a consequence of random.
 The relationship between turbidity and Mol.
Wt. derived by
 Molecular wt. colloid
Hc = 1 + 2 Bc
τ M
(τ – Turbidity, c –Con. of solute in gm/cc of solution.
M – Molecular weight, H – constant for particular
system, B- interaction constant for solute solvent
system)

H = 32 π3 n2 (dn/dc)2/3λ4N
Where N = Refractive Index of solution
C – concentration, λ - wavelength, dn/dc= change in
RI with concentration at c
N – Avogadro’s Number.

 A plot of HC/τ against C result in a straight line
with a slope of 2B and intercept is 1/M.
 The reciprocal of this yield Molecular weight.

A plot of HC/τ against C result in a straight line with a
slope of 2B and intercept is 1/M.
Slope= 2B
1/M
HC/τ
C

 Applications
 Light scattering has been used for the study of
properties of
 proteins, synthetic polymers, Association colloids and
Lyophobic colloids.
 Used to determine molecular weight.

Electrical/ Electro kinetic Properties of
colloids
 Colloidal particles posses charge on their surfaces.
 Acacia, Sulfur, Kaolin, Aresenious colloids (-ve charged);
 Ferric hydroxide, Bismuth, Aluminium (metal oxides) (+ve
charged)
 Proteins both charges or neutral depending on pH of
medium.
 The basic principle is Movement of charged surface
(dispersed phase) with respect to an adjacent liquid
phase (dispersion Media)
1. Surface charge-zeta potential
 Distribution of ions in the environment of a charged
particles explained by – Electrical double layer concept.

Electrical Double Layer at solid-liquid interface
Distribution of ions
a b c d
a’ b` Shear Plan c’ d’
Solid
Liquid
interface
Tightly
Bound
layer
Diffuse second
layer
Bulk liquid Phase
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
-
+
+ +
-
-
- -
-
-
-
-
- -
-
-
aa`- Positive charge
bb`- Positive charge
cc`- eclectically neutral
The entire system is
electrically neutral.
-
-
-
-
-
+
+
Zeta pot
+
+
+
+
+

Nernst Potential (E)
 Potential of solid surface itself aa`, Due to presence
of potential determining ions.
 Defined as the potential difference between the
actual surface and the electro-neutral region of
the solution.
Zeta Potential (ζ) 2 marks
 Potential observed at shear plane (bb` line)
 Defined as the potential difference between the
surface of tightly bound layer (shear plane) and the
electro-neutral region of the solution.

Applications Zeta Potential
 Stability of systems having dispersed systems
(solutes). Optimum Zeta Potential required maximum
stability.
 Plays a important role in flocculation : when it is
below certain value, the attractive forces exceeds the
repulsive forces, results in agglomeration of particles.

Electrophoresis 2/5 marks
 Used to determine the sign and magnitude of zeta
potential.
 It involves movement of charged particles through a
liquid under the influence of an applied potential
difference.

 Electrophoresis cell fitted with two electrodes.
 Particles migrate towards oppositely charged
electrodes.
 If the particles moves towards anode, charge on it is
negative; If it is moves towards cathode, charge on
it is positive.
 Electrophoretic mobility may be obsereved using
electrophoresis cell
 thus the sign of zeta potential determined.

Electrophoresis (contd…)
 Ultramicroscope is used to observe the rate migration of
particles and is function of charge on the particle.
 Rate determining potential is zeta potential.
 From direction and rate of migration. The sign and the
magnitude of zeta potential can be determined
 Velocity of Migration ∝ Pot. Gradient across the
particles
v ∝ E
v = ζ x E
ζ = v/E
ζ - zeta potential

Electro-osmosis
 If Charged colloidal particles rendered immobile,
the counter ions in the free water move towards the
electrodes of opposite charge dragging the water
molecule along with it.
 The flow of liquid medium under influence of
electric field is called electro-osmosis.
 Pressure produced is called electro osmotic
pressure.
 Opposite to the principle of electrophoresis.

 When electrophoresis of dispersed particle in a
colloidal system is prevented by some suitable
means, it is observed that dispersion medium itself
begins to move in an electric field. This phenomenon
is known as electro osmosis.

Streaming potential
 It is the converse of electro osmosis.
 In this a potential difference is created, if the liquid
is forced to flow past a plug or bed of charged
particles by applying hydrostatic pressure.
 The potential difference is created because of
displacement of counter ions in the free water.
 The method can be used to determining the zeta
potential.

Sedimentation Potential
 It is the reverse of electrophoresis.
 When colloidal particles are caused to under go
sedimentation, a potential difference is developed.

Donnan Membrane Equilibrium 5 marks
 Principle used to enhance the absorption of drugs across
the membrane. (drugs like sodium salicylate and
Potassium benzyl penicillin) by the use colloids.
 Sodium carboxy methyl cellulose (anionic
polyelectrolyte) is not diffusible through semi permeable
membrane but enhance the drug absorption, when it
combines another anionic drug provided the drug is
diffusible.
 Similarly ion exchange resins, Phosphate and sulfate ions.

Donnan Membrane Equilibrium (contd….)
 Principle
 Solution of sodium chloride placed on one side of
semi permeable membrane and on other side
solution of negatively charged colloid together with
its counter ions is placed.
 Volume of solution is equal on both the sides.
 Initial state
Out side (o) Inside (i)
Na+ Na+
Cl- R –

 Sodium and chloride ions moves freely across the
membrane. But not colloidal particles R –
 At equilibrium
Out side (o) Inside (i)
R –
Na+ Na+
Cl- Cl-
 The condition of electroneutrality, + ve and –ve charges
are balanced on either side
 Out side : [Na+]o = [Cl-]o -------------------------(1)
 Inside side : [Na+]i = [R-]i+ [Cl-]i -------------------------(2)
 At equilibrium as per the principle of escaping tendency
 [Na+]o [Cl-]o = [Na+]i [Cl-]i -------------------------(3)

 Out side : [Na+]o = [Cl-]o -------------------------(1)
 Inside side : [Na+]i = [R-]i+ [Cl-]i -------------------------(2)
 At equilibrium as per the principle of escaping tendency
 [Na+]o [Cl-]o = [Na+]i [Cl-]i -------------------------(3)
 Substituting the terms in equation (1) and (2) in (3)
 [Cl-]o [Cl-]o = ([R-]i+ [Cl-]i) [Cl-]i
= [Cl-]i
2 + ([R-]i [Cl-]i)
= [Cl-]i
2 (1+ ([R-]i / [Cl-]i)
 [Cl-]o
2 /[Cl-]i
2= 1+ ([R-]i / [Cl-]i)
 [Cl-]o /[Cl-]i= √1+ ([R-]i / [Cl-]i) -------------------(4)
 Equation represents the Donnan membrane equilibrium.

 Applications
 Used to calculate the ratio of concentration of diffusible
anion outside and inside the membrane at equilibrium.
 Demonstrate use of sodium CMC in order to enhance
the absorption of drugs such as sodium salicylate.
 [R-]i– Con. of CMC (-ve charged and non-diffusible)
 [Cl-] = [D-] – Drug salicylate ion.
 At equilibrium, the ratio of diffusible drug on either side
of membrane
 [D-]o /[D-]i= √1+ ([R-]i/[D-]i)

 When ([R-]i/[D-]i) = 8, then [D-]o /[D-]i= 3
 Both CMC and Drug present on same side (GIT),
 In blood is comparatively 3 times that of the drug
present in the GIT.
 When ([R-]i/[D-]i) = 99, then [D-]o /[D-]i= 10
 The addition of anionic polyelectrolyte to a
diffusible drug enhance the diffusion of drug.
 A dosage form containing high concentration of
Sodium CMC is will drive away the drug from GIT
into blood.

Kinetic Properties of colloids
 Relates to motion of particles with respect to dispersion medium.
 Helpful in
 Predicting stability of the system.
 Determining the molecular weight.
 Studying Transport Kinetic properties of colloidal particles.
 Kinetic Properties Motion of particles may be
Thermally induced Gravitationally
induced
Externally
Induced
Electrically Induced
•Brownian movement,
•Diffusion,
•Osmosis
Sedimentation Viscosity Considered in
electrical properties
of colloids

1. Brownian Motion
2 marks
 Random and Erratic movement may be
observed with particles as large as
about 5µ.
 Velocity of molecules increases by
decreasing particle size.
 Increasing the viscosity of medium by
addition of Glycerin decreases
Brownian Motion.
 Brownian movement is not observed in
Larger particles.

Kinetic Properties of colloids (contd…)
2. Diffusion
 Particles spontaneously diffuse from a region of
higher concentration to a region of lower
concentration until the concentration of the system is
uniform throughout.
 Diffusion is direct result of Brownian Motion.
 Fick’s law of Diffusion
 The amt. (dq) of sub. Diffusing in time (dt) across a
plane area (S) is directly proportional to the
change of concentration (dc) with the distance
travelled (dx).

 Ficks law
 dq = - DS (dc/dx) dt.
 D = diffusion coefficient
 dc/dx = concentration gradient.
 Negative sign because diffusion occurs in the
decreasing conc.
 Coefficient may be obtained by diffusion experiments
in which the material is allowed to pass through a
porous disk and samples are analyzed periodically.

Determination of molecular weight
 Colloidal particles may be assumed to be
approximately spherical
 Radius of the particles or molecular weight can be
obtained by
 D= RT/6πηrN
 D – Diffusion coefficient obtained from Fick’s law.
 R- Molar gas constant.
 T- absolute temperature.
 η - Viscosity of solvent.
 r – radius.
 N- Avogaadro’s Number.

 By this method Molecular wt. of spherical particles
(albumin and Hemoglobin)
 D =(RT/6πrηN) 3 4πN/3M v
 M- Molecular weight
 v – partial specific volume = Volume in cm3 of one
gram of solute.

3. OSMOTIC PRESSURE
 Determination of Molecular weight
 Osmotic pressure of the colloidal solution is very small
and is a colligative property.
 Vant hoff equation may be used to calculate the
molecular weight of a colloid in a dilute solution.
π = cRT
c- is the grams of solute per lit of solution
 Replacing ‘c’ with (C/M)
π = (C/M)RT
π /c= RT/M
M- Molecular weight

OSMOTIC PRESSURE (contd..)
 π /C = RT/M
 π - Osmotic pressure exhibited by the colloid.
 C- Concentration
 R-Molar gas constant
 T-Absolute temperature.
 Equation holds good if the colloid is a very dilute
solution in which the interaction between solute and
solvent is better and particles are spherical.
 A plot of π/C against C generally results in one of
the three curves depending on whether the system is
ideal (I) or real (II and III).

III
B
I
II
π/C
RT/M
C

 The intercept is RT/M, the molecular weight of
solute can be calculated.
 In curve III the slope of line is B.
 B= 0 in curve I and in dilute spheroid solutions.
 Curve III is typical of linear colloid in a solvent
having high affinity for the dispersed particles. Such
a solvent referred to as good solvent for particular
colloid.

 Curve II depicts the situation when the same colloid
is present in a relatively poor solvent having
reduced affinity for the dispersed material.
 How ever that the extrapolated intercept on the π/c
axis is identical for both curves II and III showing
that the calculated molecular weight is independent
of the solvent used.

4. Sedimentation
 Velocity (V) of sedimentation of spherical particles
having a density (ρ) in a medium of density (ρo )and
a Viscosity of (ηo ) is given by stokes law.
 V = 2 r2 (ρ- ρo)g/ 9 ηo
 In colloids stronger force must be applied to bring
about sedimentation of colloidal particles in a
quantitative and measurable manner.
 Use of ultracentrifuge so in above equation
 V =dx/dt = 2 r2 (ρ- ρo) w2 x/ 9 ηo

 Instantaneous velocity
 V= dx/dt of a particle in a unit centrifugal field is
expressed in terms of sedimentation coefficient.
 S= (dx/dt)/w2 x
 S w2 x = (dx/dt)
 S w2 dt = (dx/x)
 Where S – Svedberg sedimentation coefficient.
 Integrating the above equation
 S= log (x2/x1)/w2 (t2-t1)
 X1 and X2 the distance of particle at time t1 and t2
depending on the molecular wt. Values can be obtained by
photographic plate.
 W2 – Angular Velocity.

 Molecular wt. obtained by
 M = RTS/D (1-v ρo)
 R-Molar gas constant
 T- Absolute temp., S- Svedberg sedimentation coefficient
 D – Diffusion coefficient
 V - Partial specific Volume
 ρo – Density of the Solvent
 Ultracentrifuge is used for the determination of mol. Wt. of
polymers. (proteins)
 To evaluate the extent of homogeneity

Viscosity 10 marks
 Viscosity of colloidal dispersion depends on
 Shape of the dispersed particles
 Affinity of particles to the medium
 Types of colloids
 Molecular wt. of the particles
 Resistance to flow of a system under an applied
stress.
 Applied to determine flow properties of colloids.
 Uses – Molecular wt. determination and to
determine shape of particles.

Viscosity (contd…)
 Einstein equation to determine flow dilute colloids
containing spherical particles.
 η = ηo (1+2.5φ)
 η – Viscosity of dispersion
 ηo – Viscosity of dispersion medium
 φ -Volume fraction of colloidal particles
 Rearranging the equation Various, other viscosity
coefficients can be defined
 η /ηo =(1+2.5φ) = ηrel
 Where, ηrel – Relative viscosity

Viscosity (contd…)
 (η /ηo ) -1=2.5φ = ηsp
 Where, ηsp – Specific viscosity
 = (η - ηo) /ηo = 2.5 φ
 = (ηsp/ φ) = 2.5
 φ can be expressed in terms of C
 (ηsp/ c) = 2.5 or K
 K – Intrinsic viscosity [η]
 C= con. Of colloidal particles (in grams) per 100 ml of
total dispersion.

A plot of (ηsp/c) against C result in a straight line with a
intercept is K - intrinsic viscosity).
K- Intrinsic
Viscosity
(ηsp/c)
C

 When viscosity of dispersion at various concentration (η) and
viscosity of dispersion medium (η0) is known the specific
viscosity (ηsp) can be easily determined.
 By knowing specific viscosity (ηsp) approximate mol. Wt of
dispersed phase can be calculated.
 According to Mark-hownink equation
 [η]=KMa
 [η]= intrinsic viscosity
 Where K and a are character of particular polymer – solvent
system.
 K and a are obtained by calibrating with a colloidal system of
known characteristics and molecular weight.
 (ηsp/c) = k1+ k2c+k3c2 (for highly polymeric substances)
 A capillary Viscometer used (0stwald)

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