Suspensions

Jim McElroy

Lincoln, 2011

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.

INTERPARTICLE FORCES THAT CAN BE CONTROLLED BY FORMULATION

Electrostatic repulsive force – charged particles exert a force on one another.

Steric Repulsive force – arises from the adsorption of large molecules. Can be controlled by formulation

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.

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.

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.

GRAPH

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

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.

PROPERTIES OF FLOCCULATED SUSPENSIONS

Rapid rate of sedimentation due to large size of floccules

Clear supernatant as all particles are incorporated into floccules

High sediment volume Sediment easily re-dispersed by shaking

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

HeteroflocculationAdd small oppositely charged particles to produce a particle network

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

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.

PSEUDOPLASTIC FLOW

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

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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.

STRUCTURED VEHICLEFORMULATION 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).

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

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

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