Lab 4: Formulation And Evaluation Of Suspensions

Question 0

What is a flocculated suspension like?

Answer 0

Uncharged particles are free to collide and aggregate resulting in a faster sedimentation rate. This results in some caking and is usually easy to redisperse. The supernatant is clear.

Question 1

What is a deflocculated suspension like?

Answer 1

Particles are charged and will repel each other, resulting in a slower sedimentation rate. This results in hard caking, and is usually hard to redisperse.
The supernatant is usually milky

Question 2

What is the sedimentation volume ratio?

Answer 2

F.
This is the volume of sedimentation divided by the total volume.
This is usually <1.

Question 3

What is the effect of electrolytes on the degree of flocculation?

Answer 3

Addition of electrolytes will neutralise the surface charges of the particles, preventing them from repelling each other.
At an optimal electrolyte concentration, controlled flocculation will be achieved. This is the most stable suspension system.
However upon further addition of the electrolyte, charge reversal will occur, this is when the particles repel each other again due to excess electrolyte present in the medium. The suspension will return to its deflocculated state causing caking

Question 4

What is a suspension?

Answer 4

A coarse dispersion in which insoluble particles ( usually of a diameter of 0.1μm) are dispersed in a liquid medium.

Question 5

What are the advantages of suspensions?

Answer 5

Taste masking option
Improved drug stability compared to solution
Potential for sustained drug release
Rapid drug absoprtion compared with tablets

Question 6

What are the ideal properties of suspensions?

Answer 6

Resuspendable
Homogenous
Uniform particle size
Easily measure able and transferred

Question 7

What is the ideal suspension system?

Answer 7

One with controlled flocculation

Question 8

How can we achieve controlled flocculation?

Answer 8

Using a wetting agent, flocculating agent or viscosity modifier

Question 9

Why does a wetting agent help achieve controlled flocculation?

Answer 9

Wetting agent displaces the air at the solid surface and replaces it with liquid.
This increases the surface area, increasing diffusion and reduces the surface tension at the solid liquid interface

Question 10

How does a flocculating agent help to achieve controlled flocculation?

Answer 10

They aid in the formation of loose aggregates by preventing electrostatic repulsion and cross linking between adjacent particles

Question 11

How does a viscosity modifier achieve controlled flocculation?

Answer 11

It reduces the sedimentation velocity in accordance to stokes law

Question 12

What is the role of tween 80 in a suspension formulation?

Answer 12

Wetting agent/surfactant

Question 13

What is the role of aluminium chloride in a suspension formulation?

Answer 13

Acts as an electrolyte/flocculation agent

Question 14

What is the role of methyl cellulose solution in a suspension formulation?

Answer 14

Acts as a viscosity enhancer/modifier
Weak surfactant (via a multi molecular adsorption film)
Polymeric flocculation agent (via steric stabilisation)

Question 15

What does the experiment of using one additive for each formulation tell us?

Answer 15

That only 1 additive alone is not sufficient and a combination of all additives at the right concentrations is required to produce an acceptable and stable suspending system

Question 16

What is the relationship between the SVR and electrolyte concentration?

Answer 16

Upon addition of electrolyte the SVR is constant, then gradually increases.
It will reach a range of concentrations of electrolyte of an optimal SVR.
Further additions of electrolyte will cause the SVR to decrease again

Question 17

Why do we have the double layer model?

Answer 17

It is used to visualise the ionic environment in the vicinity of a charged colloid and explains how the electrical repulsive forces occur

Question 18

What is the double layer?

Answer 18

This refers to the charged atmosphere in the diffuse layer AND the attached counterions in the stern layer

Question 19

Describe the double layer of a negatively charged particle?

Answer 19

The double layer: The stern layer is essentially a layer of firmly attached counter ions to the particle surface. This results in a charged atmosphere surrounding the particle known as the diffuse layer.

The bulk of the solution aka the medium, consists of co-ions and the counter ions in the bulk solution.

Question 20

What is the zeta potential?

Answer 20

The potential difference between the stern layer (layer of firmly attached counter ions) and the bulk electrolyte (equilibrium of co-ions and counter ions)

Question 21

What does the DLVO theory explain?

Answer 21

The tendency of colloids to agglomerate or remain discrete.
This depends on the distance between particles, and their repulsive and attractive forces.

Particles with their energy levels in the secondary minimum will achieve controlled flocculation

Question 22

What is the desired rheology of a suspension

Answer 22

Upon standing, we want suspended particles to settle slowly or remain permanently suspended. This requires a high apparent viscosity at low shear rates.
At a moderate shear rate (shaking), we want the apparent viscosity to fall sufficiently so the suspension on can be easily poured from the container.
Upon standing again we want the original apparent viscosity to be regained after a short time to maintain adequate physical stability

The resulting rheology of a suspension should be pseudoplastic Thixotropy.