Suspensions Formulation Overview
- 2. Suspensions - DefinitionSuspensions 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. Interparticleforces that can be controlled by formulationElectrostatic 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
- 5. Interparticulate forces that cannot be controlled by formulationVan 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 ForcesColloid science has held that electrostatic and electrodynamic (van derwaals) 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 ForceHydration 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 GraphNernst Potential- charge at the true surfaceStern Layer- adsorbed counterions tightly bound and move with the solidDiffuse Layer- complete neutralization of the surface chargeDouble Layer- complete neutralization of the Nernst potential. Requires both the Stern Layer and Diffuse Layer i.e. the slipping planeDebye Length- thickness of the double layer is inversely related to ionic strength and ion valence
- 10. Flocculated SuspensionParticles 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 SuspensionsRapid rate of sedimentation due to large size of flocculesClear supernatant as all particles are incorporated into flocculesHigh sediment volumeSediment easily re-dispersed by shaking
- 13. Flocculation Formulation ApproachAdjust electrostatic repulsive force use an electrolyteModify the Nernst (equilibrium) potential reduce surface charge by adsorbing anions to itAdjust steric repulsive force adsorb a neutral polymerHeteroflocculationAdd 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 anionsthe electrostatic repulsive force by using an electrolyte like sodium chlorideThe steric repulsive force adsorbing a neutral polymer like polyvinyl alcohol
- 15. Structured VehicleProduce 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 FlowExhibited by polymer solutions. Increasing flow as the shear stress is increased. The viscosity decreases as the shear stress is increased.
- 17. Dilantant FlowThe 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 VehicleFormulation ApproachAddition of “inert” small particles such as clays like montmorillonite or silica dioxideMixture of polymers and “inert” small particles like sodium carboxymethycellulose with montmorillonite or silica dioxideUse of liquid-crystalline phases like surfactants at concentrations above the Critical Micelle Concentration (CMC).
- 19. Properties of Structured Vehicle SuspensionsMay appear as a semi-solid when undisturbedFluid when shakenThixatropic (becomes fluid when stirred or shaken and returning to the semisolid state upon standing )No sedimentation
- 20. ConclusionSuspensions 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