Upload
jim-mcelroy
View
633
Download
0
Embed Size (px)
DESCRIPTION
A primer for suspension Formulat
Citation preview
Suspension Formulation
Jim McElroy
2014
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 Heteroflocculation
Add 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.
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.
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