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Questions and Answers
What happens to the viscosity of dilatant liquids when shear rate increases?
What happens to the viscosity of dilatant liquids when shear rate increases?
The viscosity increases.
Define thixotropy and describe the changes it induces in materials when shear stress is applied and then removed.
Define thixotropy and describe the changes it induces in materials when shear stress is applied and then removed.
Thixotropy is the isothermal slow reversible conversion of gel to sol. When shear stress is applied, the material converts to sol, and it slowly returns to gel when the stress is removed.
Why must care be taken when selecting a suspending agent for oral preparations?
Why must care be taken when selecting a suspending agent for oral preparations?
Because the acid environment of the stomach may alter the physical characteristics of the suspension and affect the drug release rate.
How do suspending agents prevent agglomeration of particles in a suspension?
How do suspending agents prevent agglomeration of particles in a suspension?
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What are two main functions performed by most suspending agents?
What are two main functions performed by most suspending agents?
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What is the desirable property of a good suspension regarding its viscosity at rest and during agitation?
What is the desirable property of a good suspension regarding its viscosity at rest and during agitation?
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Name the three classifications of suspending agents.
Name the three classifications of suspending agents.
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Which classification of suspending agents includes gelatin and bentonite?
Which classification of suspending agents includes gelatin and bentonite?
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What is the purpose of adding chemical preservatives in suspension formulations?
What is the purpose of adding chemical preservatives in suspension formulations?
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How is the redispersion pattern of a suspension typically assessed during shelf-life determinations?
How is the redispersion pattern of a suspension typically assessed during shelf-life determinations?
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Study Notes
Dilatant Flow
- Resistance to flow (viscosity) increases with an increase in shear rate
- Volume increases when shear stress is applied, hence called Dilatant
- Also known as shear thickening
- Observed in suspensions containing more than 50% v/v of solids
Thixotropy
- Defined as the isothermal slow reversible conversion of gel to sol
- Thixotropic substances convert to sol (fluid) when shear stress is applied and slowly turn to gel (semisolid) when standing
Suspending Agents
- Substances used to keep finely divided insoluble materials suspended in a liquid media
- Prevent agglomeration and impart viscosity to the dispersion media
- Care must be taken when selecting a suspending agent for oral preparations due to the acid environment of the stomach
- Most suspending agents perform two functions: suspending and imparting viscosity
- Form a film around the particle and decrease interparticle attraction
- A good suspension should have well-developed thixotropy
Classification of Suspending Agents
- Natural: animal origin (Gelatin), plant origin (Accacia, Tragacanth, Starch, sea weed), mineral sources (Bentonite, Kaoline)
- Semi-synthetic: substituted cellulos (minerals), e.g., Hydroxyethyl cellulose, Sodium Carboxymethylcellulose, methylcellulose, Microcrystalline cellulose
- Synthetic: synthetic polymers, e.g., carboxypolymethylene (Carbopol), Polyvinyl Alcohol, Polyvinyl Pyrrolidone iodine complex (PVC)
Suspension Stability
- Chemical stability considerations apply to suspensions, ensuring drug remains chemically stable over the intended shelf-life
- Specifications are in place for maximum permitted levels of specified degradation products
- Chemical preservatives can be added to the suspension based on the determined chemical degradation pathway of the drug
- Effect of temperature on the chemical stability of the drug needs to be established
- Physical stability is equally important, with sedimentation kept to a minimum and easy redispersion of the sediment necessary
- Redispersion pattern should be established, testing at suitable time intervals and under various storage conditions
- Visual assessment of sediment redispersion on shaking is useful, but a more quantitative approach is taken by assessing particle size distribution and drug content of representative samples
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Description
Learn about the properties of non-Newtonian fluids, including dilatant flow and thixotropy, and their behavior under shear stress.
