Evaluation of
Suspension
Learning
Objectives
At the end of this
lecture, student will be able to
• Enlist the instabilities in emulsions
• Recommend suitable measures for prevention of
instabilities
• Explain the methods of evaluation of suspensions
Instabilities
in Suspensions
1. Sedimentation
2. Cake formation
Prevention
of Instabilities
1. Controlled flocculation
2. Use of right amounts of suspending / thickening agents
Evaluation
of Suspension
1. Determination of rate of sedimentation
2. Determination of viscosity (Rheological method)
3. Determination of zeta potential
4. Determination of particle size change
1. Determination
of rate of sedimentation
• Measurement of the sedimentation volume and ease of
redispersion
Sedimentation volume = Ultimate height of sediment (Hu) / Initial
height of sediment (Ho)
• Carried out in a measuring cylinder
• Larger this fraction, better is the suspendability.
• If the initial ht is 50 ml and after 1 day if the height
of the sediment is 40 ml
Then S.V = 40/50 = 0.8
If the initial ht is 50 ml and after 1 day if the height of
the sediment is 20 ml
Then S.V = 20/50 = 0.4
– Evaluation of redispersibility: is done using a mechanical shaking device
which simulates the human arm motion during the shaking process.
2.
Determination of Viscosity (Rheological method)
Determined using a Brookefield’s viscometer.
3.
Determination of Zeta potential
• Zeta potential controls flocculation of the dispersed
phase
• Determined by an instrument called “Zetameter”
• Measures the electrophoretic mobility of particles in
suspension.
Zeta Potential
• Zeta potential is a scientific term for electrokinetic
potential in colloidal dispersions. It is denoted using the Greek letter zeta
(ζ), hence ζ- potential.
• ZP (ζ) is related to surface charge
• A high ζ will produce a well-dispersed suspension; this,
in turn results in good sedimentation stability and solids loading capacity.
• In contrast, a low ζ results in particle association.
Effect of Zeta
Potential on Suspension Properties
Zeta Potential Values
|
Zeta potential [mV] |
Stability behavior of the colloid |
|
from 0 to |
Rapid coagulation or flocculation |
|
from ±10 |
Incipient instability |
|
from ±30 |
Moderate stability |
|
from ±40 |
Good stability |
|
more than |
Excellent |
4.
Determination of particle size change
• Studied by Coulter Counter and Optical Microscopy
• Determines the changes in particle size of the dispersed
phase
• Changes that occur due to temperature fluctuations or long
storage may result in change in particle size distribution and the crystal
growth
Labeling
for Suspensions
“SHAKE WELL BEFORE
USE”
Summary
1. Instabilities in
suspensions- Sedimentation and cake formation
2. Prevention of
instabilities- By controlled flocculation and use of right amount of
suspending and thickening agents
3. Evaluation of
suspensions:
– Determination of rate of sedimentation
– Determination of viscosity (Rheological method)
– Determination of zeta potential
– Determination of particle size change