A pharmaceutical suspension is a coarse dispersion of insoluble solid particles in a liquid medium. ... However, it is difficult and also impractical to impose a sharp boundary between the suspensions and the dispersions having finer particles. Suspensions are an important class of pharmaceutical dosage forms

1. Suspension
Md. Saiful Islam
BPharm, MPharm (PCP)
North South University
Fb Group: Pharmacy Universe

5. Flocculated and deflocculated
suspension
Depending on the sedimentation behavior,
suspensions are of two types:
1) Flocculated suspension and
2) Deflocculated suspension

6. Flocculated suspension
• Flocculation results from the collision and
combination of primary particles in a suspension
• Not all collision result in binding due to presence
of protective barrier of electric charge or
adsorbed molecules around the particles
• The greater the protective barrier, the slower will
be the rate of flocculation

7. Properties of Flocculated
suspension
• Particles form loose aggregates
• Rate of sedimentation is high and particles
settle as floc
• A sediment is formed rapidly

8. Properties of Flocculated suspension
• The sediment is loosely packed. So does not
form a hard, dense cake
• Sediment is easy to re-disperse
• A clear supernatant region is observed due to
rapid sedimentation which is somewhat
unsightly

9. Properties of deflocculated
suspension
• Particles exist in suspension as separate entities
• Rate of sedimentation is slow, since each
particle settles separately and particle size is
minimal
• A sediment is formed slowly

10. Properties of deflocculated suspension
• A hard cake is formed because-
- Sediment eventually becomes very closely
packed due to weight of upper layers and
- Repulsive forces between particles are
overcome
• Difficult, if not impossible, to re-disperse
• Has a pleasing cloudy appearance, even when
settling is apparent

11. Physical properties of dispersed
particles
Uniform dispersion of particles at the time when patient
uses the suspension is very crucial and this is related
with the physical stability of the product that is affected
by-
1) Particle-vehicle interactions
2) Particle-particle interactions and
3) Rheological properties
So, factors affecting all the above are essential to
consider for the successful formulation of suspensions

12. Particle-vehicle interactions:
(Associated with wetting and dispersion of particles)
• Surface area is greatly increased with the size
reduction of particles
• Free energy associated with surface becomes larger
• Particles become highly energetic and tend to
agglomerate into larger masses to obtain a minimum
surface free energy
• This agglomeration is referred to as flocculation

13. Particle-vehicle interactions:
• This flocculation is responsible for clumping of solid
particles in suspension
• Because of this tendency, systems of finely dispersed
particles are unstable from the thermodynamic point of view
• The relevant expression is-
ΔF = ‫ע‬sl. ΔA
where,
‫ע‬sl= interfacial tension between the particles and the
liquid
ΔF = increase in surface free energy
ΔA = increase in the surface area

14. Particle-vehicle interactions:
• The suspension becomes thermodynamically stable
when ΔF approaches zero.
HOW TO ATTAIN THIS?
• This state of stability may be achieved by-
- either lowering the interfacial tension
- or reducing the interfacial area
• Surface active agent also known as wetting agent can
be added

15. Particle-vehicle interactions:
Surface active agent promote the dispersion of particle
within the vehicle by-
• Reducing the interfacial tension by reducing surface free
energy
• Thus reducing the tendency of the material to flocculate

16. Particle-vehicle interactions:
Particles may be of two types:
a) Lyophilic (solvent attracting) –easily wetted by the vehicle,
creates no difficulty in initial dispersion.
Ex- talc, magnesium carbonate in water
b) Lyophobic- difficult to wet by the vehicle, tends to clump
and float on the surface of the vehicle
Ex- sulfur, magnesium stearate in water

17. Particle-vehicle interactions:
Wettability of lyophobic powers can be
increased by-
• Mixing the powders with wetting agent( alcohol, glycerin
or other hygroscopic liquid) and
• Passing through a colloid mill which will cause-
- the displacement of air
- penetration of the vehicle into the powder mass
- the dispersion of particles

18. Particle-particle interactions
• Play and important role in flocculation and deflocculation
mechanism that is dependent on the presence of surface
electrical charges and the distribution of ions around the
particle
• Source of the charge on particle may arise from
- ionizable groups on the surfaces or
- adsorption of ions from the surrounding solution

19. Particle-particle interactions
• Regardless of the specific mechanism, the particles will
bear either positive or negative charges. Thus
electrostatic repulsion set up between adjacent particles
preventing them from adhering to one another
• Solvation of particle surfaces also helps to prevent
particles coming together
Accordingly deflocculation occurs

20. Rheological properties of dispersed particles
• Rheological property associated with different flow
of the suspension
• This provides-
1) prevention of sedimentation
2) ease of particle redispersion
3) development of a product that pours readily from
the container or spreads evenly over the skin

21. Rheological properties of dispersed
particles
In a deflocculated suspension( where the
concentration of particle is less than10%)
viscosity is essentially Newtonian which-
- Is only dependent on the composition and temperature
and
- Not dependent on shear stress or rate of shear

22. Rheological properties of dispersed
particles
In more concentrated dispersion, non-
Newtonian flow occurs which can be categorized
into three types. These are-
a) Plastic flow
b) Pseudo plastic flow and
c) Dilatant flow

23. - Associated with thixotropy, which is the
time-dependent sol-gel reversible transition
- Here dough like consistency is produced in
the suspension
There are three types of flow associated with
flocculation
Plastic flow

24. - At rest, molecules are in disorder
- With the increase of shearing stress, molecules
become aligned in the direction of flow and tend
toward an ordered arrangement
- Initial resistance to flow decreases and thus
viscosity reduced
Pseudo plastic flow

25. -Removal of shearing stress, molecules return
to their former arrangement and viscosity
increases
- Addition of a viscous suspending agent
changes the plastic viscosity into pseudoplastic
character
Pseudoplastic flow………

26. Involves changes in consistency of a product
with agitation,
- Found in deflocculated suspension
containing 50% or more of solid particles
- With agitation, particles crowd against one
another, displacing the medium and resistance
to flow increases markedly
Dilatant flow

27. -When shearing force removed, liquid medium
return to coat the particles and resistance to flow
reduced
- Dilatant materials are hazardous if
unrecognized. They can jam and overload high
speed roller and colloid mills
Dilatant flow………