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Suspensions Formulation Overview


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Suspensions Formulation Overview

  1. 1. Suspensions<br />Jim McElroy<br />Lincoln, 2011<br />
  2. 2. Suspensions - Definition<br />Suspensions are classified on the basis of the dispersed phase and the dispersion medium.<br />The former is essentially solid while the latter may either be a solid, a liquid or a gas. <br />
  3. 3. Interparticleforces that can be controlled by formulation<br />Electrostatic repulsive force – charged particles exert a force on one another. <br />Steric Repulsive force – arises from the adsorption of large molecules. Can be controlled by formulation<br />
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  5. 5. Interparticulate forces that cannot be controlled by formulation<br />Van de Waals force - attractions between atoms, molecules, and surfaces. <br />Repulsive Hydration force – arises from the structuring of water in the interfacial region. Operates over short distances.<br />
  6. 6. Van der Waals Forces<br />Colloid science has held that electrostatic and electrodynamic (van derwaals) forces are principle determinants of colloid systems.<br />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.<br />
  7. 7. Repulsive Hydration Force<br />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.<br />
  8. 8. Graph<br />
  9. 9. How to Interpret the Graph<br />Nernst Potential- charge at the true surface<br />Stern Layer- adsorbed counterions tightly bound and move with the solid<br />Diffuse Layer- complete neutralization of the surface charge<br />Double Layer- complete neutralization of the Nernst potential. Requires both the Stern Layer and Diffuse Layer i.e. the slipping plane<br />Debye Length- thickness of the double layer is inversely related to ionic strength and ion valence<br />
  10. 10. Flocculated Suspension<br />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. <br />The distance between particles is approximately 100 to 200 A.<br />The network is easily disrupted by shaking but it reforms when the turbulence stops.<br />
  11. 11. Properties of Flocculated Suspensions<br />Rapid rate of sedimentation due to large size of floccules<br />Clear supernatant as all particles are incorporated into floccules<br />High sediment volume<br />Sediment easily re-dispersed by shaking<br />
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  13. 13. Flocculation Formulation Approach<br />Adjust electrostatic repulsive force <br />use an electrolyte<br />Modify the Nernst (equilibrium) potential <br />reduce surface charge by adsorbing anions to it<br />Adjust steric repulsive force <br />adsorb a neutral polymer<br />Heteroflocculation<br />Add small oppositely charged particles to produce a particle network<br />
  14. 14. Flocculation Example: <br />Adjust or modify:<br />the Nernst Potential using an ionic species such as phosphate anions<br />the electrostatic repulsive force by using an electrolyte like sodium chloride<br />The steric repulsive force adsorbing a neutral polymer like polyvinyl alcohol<br />
  15. 15. Structured Vehicle<br />Produce a liquid phase which exhibits shear thinning rheology, i.e. very viscous on the shelf to prevent settling and fluid when shaken.<br />Usually contains a polymer and a clay (or several polymers) in order to produce a shear-thinning system.<br />
  16. 16. Pseudoplastic Flow<br />Exhibited by polymer solutions. Increasing flow as the shear stress is increased. The viscosity decreases as the shear stress is increased.<br />
  17. 17. Dilantant Flow<br />The system becomes more viscous as the shear stress is increased. <br />* can be a problem on scale-up. Production equipment often introduce more shear than laboratory equipment.<br />
  18. 18. Structured VehicleFormulation Approach<br />Addition of “inert” small particles such as clays like montmorillonite or silica dioxide<br />Mixture of polymers and “inert” small particles like sodium carboxymethycellulose with montmorillonite or silica dioxide<br />Use of liquid-crystalline phases like surfactants at concentrations above the Critical Micelle Concentration (CMC).<br />
  19. 19. Properties of Structured Vehicle Suspensions<br />May appear as a semi-solid when undisturbed<br />Fluid when shaken<br />Thixatropic (becomes fluid when stirred or shaken and returning to the semisolid state upon standing )<br />No sedimentation<br />
  20. 20. Conclusion<br />Suspensions are complex systems that require an understanding of their basic chemistry for proper development and understanding.<br />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<br />
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