This document discusses suspension formulations. Suspensions have a dispersed solid phase and a dispersion medium that can be solid, liquid, or gas. Particle interactions can be controlled through electrostatic or steric repulsive forces using formulation techniques. Forces like van der Waals attractions cannot be controlled. Structured vehicles use shear thinning polymers and clays to create suspensions that are semi-solid at rest but fluid with shaking, preventing settling. Flocculation uses electrolytes or polymers to reduce repulsive forces and promote particle aggregation and settling.

1. Suspension Formulation
Jim McElroy
2014

2. Suspensions - Definition
 Suspensions are classified on the basis of the dispersed phase and
the dispersion medium.
 The former is essentially solid while the latter may either be a solid, a
liquid or a gas.

3. Interparticle forces that can
be controlled particles exert a force on one
by formulation
 Electrostatic repulsive force – charged
another.
 Steric Repulsive force – arises from the adsorption of large molecules.
Can be controlled by formulation

5. Interparticulate forces that
cannot be controlled by
formulation
 Van de Waals force - attractions between atoms,
molecules, and surfaces.
 Repulsive Hydration force – arises from the structuring of
water in the interfacial region. Operates over short
distances.

6. Van der Waals Forces
 Colloid science has held that electrostatic and
electrodynamic (van der waals) forces are principle
determinants of colloid systems.
 Interaction between two dipoles that are either
permanent or induced. The temporary dipole and the
induced dipoles are attracted to each other. It is
always present, it is short-range, and it is attractive.

7. Repulsive Hydration Force
 Hydration repulsion is due to the work needed to
remove water molecules from hydrophilic surfaces at
small film thicknesses and is described by an
exponentially decaying interaction potential.

8. Graph

9. How to Interpret the Graph
 Nernst Potential- charge at the true surface
 Stern Layer- adsorbed counterions tightly bound
and move with the solid
 Diffuse Layer- complete neutralization of the
surface charge
 Double Layer- complete neutralization of the
Nernst potential. Requires both the Stern Layer
and Diffuse Layer i.e. the slipping plane
 Debye Length- thickness of the double layer is
inversely related to ionic strength and ion
valence

10. Flocculated Suspension
Particles finer than 0.1 µm in water remain
continuously in motion due to
electrostatic charge (often negative)
which causes them to repel each other.
The distance between particles is
approximately 100 to 200 A.
The network is easily disrupted by shaking
but it reforms when the turbulence stops.

11. 
Properties of Flocculated
Suspensionsto large size of floccules
Rapid rate of sedimentation due
 Clear supernatant as all particles are incorporated into floccules
 High sediment volume
 Sediment easily re-dispersed by shaking

13. Flocculation
Formulation Approach
Adjust electrostatic repulsive force
use an electrolyte
Modify the Nernst (equilibrium) potential
reduce surface charge by adsorbing
anions to it
Adjust steric repulsive force
adsorb a neutral polymer
Heteroflocculation
Add small oppositely charged particles
to produce a particle network

14. Flocculation Example:
 Adjust or modify:
 the Nernst Potential using an ionic species such as
phosphate anions
 the electrostatic repulsive force by using an
electrolyte like sodium chloride
 The steric repulsive force adsorbing a neutral polymer
like polyvinyl alcohol

15. Structured Vehicle
 Produce a liquid phase which exhibits shear thinning rheology, i.e. very
viscous on the shelf to prevent settling and fluid when shaken.
 Usually contains a polymer and a clay (or several polymers) in order to
produce a shear-thinning system.

16. Pseudoplastic Flow
 Exhibited by polymer solutions. Increasing flow as the
shear stress is increased. The viscosity decreases as
the shear stress is increased.

17. Dilantant Flow
The system
becomes more
viscous as the
shear stress is
increased.
* can be a problem
on scale-up.
Production equipment
often introduce more
shear than laboratory
equipment.

18. Structured Vehicle
Formulation Approach
Addition of “inert” small particles such as
clays like montmorillonite or silica dioxide
Mixture of polymers and “inert” small
particles like sodium
carboxymethycellulose with
montmorillonite or silica dioxide
Use of liquid-crystalline phases like
surfactants at concentrations above the
Critical Micelle Concentration (CMC).

19. Properties of Structured
Vehicle Suspensions
 May appear as a semi-solid when undisturbed
 Fluid when shaken
 Thixatropic (becomes fluid when stirred or shaken and returning to the
semisolid state upon standing )
 No sedimentation

20. Conclusion
 Suspensions are complex systems that require an
understanding of their basic chemistry for proper
development and understanding.
 It is important to take into consideration all aspects of
the formula before considering a preservative system.
This includes type of formula, bulk handling and
packaging