Podcast
Questions and Answers
Which of the following factors does NOT influence the velocity of sedimentation in suspensions?
Stokes's law is applicable when particles are irregularly shaped.
False
What phenomenon is described as the settling of particles or floccules under gravitational force in liquid dosage forms?
Sedimentation
In deflocculated suspension, larger particles settle fast while smaller particles remain in the __________ liquid, making it appear cloudy.
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Match the following terms with their definitions:
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What occurs when particles in a suspension interfere with one another during sedimentation?
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The velocity of fall of a suspended particle is greater for __________ particles than for smaller particles.
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What does the variable F represent in the sedimentation volume equation?
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The minimum value of the degree of flocculation (β) is 0.
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What equipment is used to study the viscosity of suspensions in the rheological method?
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The electrokinetic method determines the __________ of the formulated suspensions.
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Match the following terms with their definitions:
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What factors influence Van der Waals forces?
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Zeta potential is related to the degree of attraction between charged particles.
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What is the term used to describe the difference in electric potential at the surface of a particle and the electroneutral region?
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The two layers of ions at the interface constituted by charged particles are known as the _____.
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Match the following terms with their definitions:
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When does deflocculation occur?
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Adding a small amount of electrolyte can increase the zeta potential.
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What are counter ions also known as?
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The ____ of the particle is determined by potential determining ions.
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What property do structured vehicles typically exhibit?
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Hydrophobic materials are easily wetted by water.
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What is the typical concentration of wetting agents used in formulations?
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Surfactants reduce the _______ tension between drug particles and liquid.
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Match the type of wetting agent with its characteristic:
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Which of the following is a common example of a wetting agent?
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Non-ionic surfactants are generally preferred for wetting applications.
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Why is polysorbate 80 widely used as a wetting agent?
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The HLB value of suitable wetting agents falls between _______ and _______.
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What happens when the interfacial tension between the solid and liquid is reduced?
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What is a commonly used surfactant for oral use?
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Hydrophilic colloids are typically cationic in nature.
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Name one disadvantage of using surfactants as wetting agents.
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The process of ensuring proper flocculation in a formulation is usually achieved by controlling particle size and adding _____ to enable crosslinking.
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Match the following wetting agents with their type:
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Which of the following is not a type of material used as a wetting agent?
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Flocculating agents can help produce a deflocculated system.
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What type of agents do hydrophilic colloids mainly behave as?
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_____, glycerin, and propylene glycol are examples of hygroscopic liquids used as wetting agents.
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Which of the following statement is true about polyelectrolytes?
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What happens to the sedimentation velocity of a particle if its diameter is decreased?
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Increasing the viscosity of the dispersion medium will enhance the settling rate of particles.
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What substances can be added to increase the density of the vehicle in a suspension?
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The velocity of sedimentation is inversely proportional to the __________ of the dispersion medium.
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Match the following parameters with their effects on sedimentation:
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Which of the following is a disadvantage of high viscosity in a suspension?
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Particles with a density less than the dispersion medium will sink rapidly.
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What parameter's increase can cause problems like pouring and syringibility in a suspension?
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What occurs when the zeta potential is reduced below the critical value?
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Brownian movement helps prevent sedimentation in dispersed systems.
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What type of forces arise from the electric double layers surrounding particles?
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For efficient Brownian movement, the particle size should be below the critical radius (r) of approximately _____ to _____ µm.
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Match the following types of forces with their descriptions:
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Which of the following is a method for stabilizing suspensions?
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Increasing the viscosity of the medium always leads to enhanced Brownian motion.
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How does the density of the dispersed phase affect Brownian movement?
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What is the primary purpose of wetting agents in a suspension formulation?
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A water-insoluble drug is typically the dispersed phase in an aqueous suspension.
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What should the particle size of the drug be for optimal performance in a suspension?
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What is the sedimentation volume (F) equal to when the ultimate volume of sediment is smaller than the original volume of suspension?
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The primary vehicle used in suspension formulations is typically __________ or __________ water.
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In deflocculated systems, no clear boundary is formed between the sediment and the supernatant liquid.
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What is the term used to describe the ratio of the final volume of sediment to the original volume of the suspension?
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Match the following components of suspensions with their functions:
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In a flocculated system, the flocs tend to fall together, resulting in a distinct boundary between the _____ and the supernatant.
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Which of the following is NOT a common ingredient in suspension formulations?
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Once the powder in a suspension is wetted, all soluble components can be blended into the mixture.
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Match the following terms with their definitions:
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What happens when the ultimate volume of sediment in a flocculated suspension equals the original volume of suspension?
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What is the purpose of using flocculating agents in suspension formulations?
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Larger particles in a deflocculated suspension settle slower than smaller particles.
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Name one method for the formulation of suspensions.
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Which of the following is a function of wetting agents?
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The concentration of wetting agents used in formulations is typically greater than 0.5%.
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What is the HLB range appropriate for wetting agents?
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Surfactants reduce the interfacial tension between drug particles and ________.
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Match the following wetting agents with their characteristics:
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What is a primary characteristic of hydrophilic materials?
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Surfactants must always be ionic to effectively function as wetting agents.
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Name one disadvantage of using surfactants as wetting agents.
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Which of the following surfactants is commonly used for oral applications?
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Hydrophilic colloids typically behave as cationic substances.
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Name one disadvantage of using surfactants as wetting agents.
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Alcohol, glycerin, and propylene glycol are examples of _____ used as wetting agents.
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Match the following materials with their primary function:
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What is primarily achieved by using flocculating agents in formulations?
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Hydrophilic materials are incompatible with quaternary ammonium compounds.
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What effect do hydrophilic colloids have on the viscosity of water?
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What is zeta potential primarily related to in a suspension?
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Flocculated suspensions exhibit dilatant flow at low shearing stress.
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What effect does a reduction in zeta potential below a certain value have on particle interactions?
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A well-formulated thixotropic suspension is ________ during storage but becomes fluid upon shaking.
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Match the type of suspension to its description:
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What method can be used to study changes in particle size in suspensions?
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Higher concentrations in deflocculated suspensions lead to decreased viscosity with increased shearing stress.
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What does the rheological behavior of flocculated suspensions depend on?
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What is the primary goal of packaging suspensions?
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Nanosuspension particles usually have a size greater than one micron.
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What should be done to suspensions before each use?
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Suspensions for __________ use are formulated to control the rate of absorption of the drug.
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Match the following types of suspensions with their intended use:
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What is NOT a recent advance in suspensions?
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A higher viscosity in a suspension can enhance the settling rate of particles.
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The average particle size of solid particles in nanosuspensions ranges between __________ and __________ nm.
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What is the term used to describe the difference in electric potential between the surface of the particle and the electroneutral region?
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Deflocculation occurs when attractive forces exceed repulsive forces.
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What are potential determining ions?
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Zeta potential is defined as the difference between the surface of the tightly bound layer and the __________ region of the solution.
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Match the following terms with their definitions:
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What effect does adding a small amount of electrolyte have on zeta potential?
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Cations are known as potential determining ions.
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What governs the degree of repulsion between similarly charged particles?
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What is one of the primary functions of electrolytes in flocculation?
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Surfactants can only be used as wetting agents and not as flocculating agents.
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What happens to the zeta potential of bismuth subnitrate particles when negatively charged electrolytes are added continuously?
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Structured vehicles are aqueous solutions of natural and synthetic ________.
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What type of agents do long-chained polymers primarily act as in flocculation?
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Match the following flocculating agents with their characteristic:
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Higher viscosity in a suspension always enhances drug absorption.
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Too high viscosity can cause difficulties in ________ and ________.
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What is the primary role of wetting agents in a formulation?
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Hydrophobic materials can be easily wetted by water.
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What is the typical concentration of wetting agents used in formulations?
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Polysorbate 80 is a commonly used wetting agent due to its _______ nature.
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Match the following surfactants with their characteristics:
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Which of the following statements is true regarding thixotropy?
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Surfactants with an HLB value between 7 and 9 are considered unsuitable as wetting agents.
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What effect do surfactants have on interfacial tension?
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What does the degree of flocculation (β) indicate?
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The minimum value of β is 0 when the sedimentation volumes are equal.
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What instrument is used in the rheological method to measure the viscosity of suspensions?
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In the electrokinetic method, the __________ potential of formulated suspensions is measured.
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Match the following methods with their descriptions:
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What is the role of a T-bar spindle in the Brookfield viscometer?
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Cup and Bob viscometers are suitable for analyzing flocculated suspensions.
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What parameter does the rheological evaluation serve as a quality control measure for?
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Study Notes
Suspensions
- Dispersed systems consist of particulate matter (dispersed phase) distributed throughout a continuous medium.
- Dispersed material ranges in size from atomic/molecular dimensions to millimeters.
- "Disperse System" describes a system where one substance (dispersed phase) is distributed in discrete units throughout a second substance (dispersed medium).
- Pharmaceutical suspensions are coarse dispersions (heterogeneous systems) where the internal phase (active ingredient) is evenly dispersed throughout the external phase.
- Suspensions contain insoluble solid particles (0.5-5 microns) dispersed with a suspending agent.
- The external phase (suspending medium) is usually aqueous, but can be organic/oily for non-oral use.
- Finer particles may self-suspend if Brownian motion energy exceeds gravitational force, leading to low viscosity.
- Suspensions are used as drug products and in-process materials.
Types of Suspensions
- Colloidal Dispersion: Particle size <1 nm, visible with electron microscope; diffuse slowly (e.g., colloidal silver, cheese, milk).
- Coarse Dispersion: Particle size 0.1-0.2 µm or >0.2 µm, visible with microscope; do not diffuse (e.g., sand, pharmaceutical emulsions, red blood cells).
- Nano suspensions: Biphasic colloidal dispersions of nanosized drug particles stabilized by surfactants; particle diameter typically 10-50 nanometers.
Classification of Suspensions
-
Administration:
- Oral (e.g., paracetamol, antacid).
- Topical (e.g., calamine lotion).
- Ophthalmic (e.g., prednisolone).
- Otic (e.g., hydrocortisone-neomycin-polymyxin).
- Rectal (e.g., barium sulfate).
- Aerosols (e.g., tolnaftate).
- Parenteral (e.g., vaccines, insulin zinc).
- Physical State: Suspensions, Aerosols, Gels, Foams
- Proportion of solid particles: Dilute (2-10% w/v solid), Concentrated (50% w/v solid).
- Electro-kinetic nature of solid particles: Flocculated, Deflocculated
Advantages and Disadvantages of Suspensions
- Advantages*
- Improved chemical stability of certain drugs (e.g., procaine penicillin G).
- Higher bioavailability compared to other dosage forms (solution > suspension > capsule > compressed tablet > coated tablet).
- Controlled duration and onset of action (e.g., protamine zinc-insulin suspension).
- Masking of unpleasant/bitter taste (e.g., chloramphenicol).
- Disadvantages*
- Physical stability problems (sedimentation, compaction).
- Bulky nature requiring careful handling and transport.
- Formulation difficulty.
- Inaccurate dosage unless packed in unit form.
Qualities of a Good Suspension
- Uniform content.
- Low settling volume.
- Absence of API crystal growth.
- Palatability.
- Resuspendability.
- Absence of caking.
- Deliverability.
- Flow.
- Lack of microbial growth.
- Physical integrity.
- Physical stability.
- Particle adhesion to the package.
- Polymorphic integrity.
- Chemical stability.
- Drug release.
Desired Features in a Pharmaceutical Suspension
- Particles settle slowly and are readily redispersed.
- Viscosity allows easy pouring.
- Chemically and physically stable.
- Palatable (if oral).
- Free from gritting particles.
- Consistent particle size.
Reasons for Formulating a Pharmaceutical Suspension
- When the drug is insoluble in the delivery vehicle, e.g., prednisolone suspension.
- To mask the bitter taste of the drug, e.g., chloramphenicol palmitate suspension.
- To increase drug stability, e.g., oxytetracycline suspension.
- To achieve controlled/sustained drug release, e.g., penicillin procaine.
Dry Powders for Oral Suspension
- Commercial preparations consist of dry powder mixtures or granules to be suspended in water, or another oral vehicle.
- Most prepared products used for oral suspensions are antibiotics.
- These products contain the antibiotic drug, colorants, flavorings, sweeteners, stabilizing agents, suspending agents, and preservatives.
Flocculated and Deflocculated Systems
-
Flocculated Suspension:
- Weak particle bonds form flocs.
- Rapid settling and easy re-dispersion.
- Less prone to compaction.
- Flocculated particles form agglomerations that resist complete settling.
-
Deflocculated Suspension:
- Strong particle bonds form a hard cake.
- Slow settling and difficult re-dispersion.
- Prone to compaction.
Theoretical Considerations for Suspensions
- Settling in suspensions is important to physical stability.
- Factors influencing sedimentation velocity include:
- Theory of Sedimentation.
- Particle size.
- Brownian movement.
- Interfacial properties.
- Electrokinetic properties.
- Sedimentation of flocculated particles.
- Sedimentation parameters.
Theory of Sedimentation
- Sedimentation is particle or floc settling under gravity in liquid.
- Brownian motion counteracts this settling.
- Related to particle size, density, and dispersion medium viscosity.
- Stokes Equation describes sedimentation velocity.
Stoke Law
- Not applicable to irregular particles or those with hindered settling, or high concentration.
- Sedimentation velocity is proportional to d2 (density difference).
Particle Size
- Critical for suspension stability.
- Reduced particle size enhances re-dispersibility and prevents caking.
- Symmetrical particles tend to be stable.
- Asymmetrical particles form hard cakes.
Density of the Vehicle
- Increasing vehicle density can reduce sedimentation velocity.
- Adding substances (polyethylene glycol, polyvinyl pyrrolidone, glycerin, sorbitol, sugar) to the vehicle increase its density.
- If the density of the dispersed phase and dispersion medium are equal, the rate of settling is zero.
Viscosity of Dispersion Medium
- High viscosity reduces settling due to increased particles' suspending time.
- Lower viscosity may pose problems with pouring, syringeability and re-dispersibility.
Viscosity of Suspensions
- Viscosity plays a key role in suspension stability and pourability.
- Increasing viscosity decreases settling rate, hence increasing stability.
Interfacial Properties of Suspended Particles
- The large surface area of solid particles results in high surface free energy thus instability.
- Flocculation (formation of flocs, light and fluffy, aggregates) occurs to reduce surface energy.
- Wetting of particles by a vehicle is crucial to disperse the solid particles.
Wetting of Particles
- Wetting agents decrease interfacial tension, allowing vehicle penetration into pores.
- Alcohol, glycerin, propylene glycol are common agents for aqueous vehicles. Mineral oil is used for non-aqueous vehicles.
- Hydrophobic particles require more effort to wet.
DLVO Theory
- Describes the stability of colloidal systems based on attractive and repulsive forces between particles.
Electrical Properties of Interfaces
- Particles often have surface charges, so surrounding ions (cations) are attracted to create the electric double layer.
- The adsorbed ions (anions) determine the surface potential.
- Adhesion / clumping of these particles is minimized with an electric double layer.
Electro-thermodynamic (Nernst) Potential
- The difference in electric potential between a particle's surface and the electroneutral region.
- Controlled by the electrical potential determined by the ions on the particle's surface.
Electrokinetic or Zeta Potential
- Potential difference between the tightly bound layer (shear plane) and the electroneutral region of the solution.
- Governs the degree of repulsion between similarly charged particles.
- A high zeta potential reduces attraction forces, preventing aggregation and enhancing stability.
Deflocculation and Flocculation
- Deflocculation: Higher zeta potential; strong repulsive forces; particles dispersed.
- Flocculation: Lower zeta potential; attractive forces surpass repulsive forces; particles form aggregates.
Potential Energy Curves
- Shows attractive and repulsive forces, and net energy, as a function of the distance of separation between particles for better understanding of flocculation or deflocculation phenomena.
Kinetic Stability of Dispersed Systems
- Brownian movement counteracts sedimentation by keeping particles in random motion.
- Brownian motion depends on particle size, density of dispersed phase and dispersion medium viscosity.
- Particle size below a critical radius is key.
Effect of Brownian Movement
- Brownian motion counteracts particle settling.
- Significant for small particles, but the effect is reduced in concentrated suspensions (large volume).
Methods for Stabilizing Suspension
- Controlling particle charges (like charges repel, preventing agglomeration), and using a solvent sheath around the particle to prevent coming close.
Sedimentation in Different Systems
- Flocculated systems: Particles tend to fall together, with a clear boundary between sediment and supernatant liquid.
- Deflocculated systems: Larger particles settle faster, without a clear boundary, depending on Stokes' law.
Sedimentation Parameters
- Sedimentation volume (F): ratio of final sediment volume (Vu) to the initial suspension (Vo) volume.
- F < 1 implies less sediment volume.
- F > 1 indicates fluffy aggregates.
The Sedimentation Behavior of Flocculated and Deflocculated Suspensions
- Diagrams show the different settling behavior of flocculated and deflocculated suspensions.
Methods for Formulation of Suspensions
- Controlled flocculation using electrolytes, surfactants, polymers
- Structured vehicles to increase viscosity
- Combining both methods
Addition of Electrolytes
- Electrolytes reduce repulsive surface charges, leading to flocculation.
- The flocculating power is strongly dependant on the valency of the ions.
Addition of Surfactants
- Surfactants reduce the interfacial tension between solid and liquid phases leading to enhanced wetting; can also control flocculation by changing the surface charge.
Addition of Polymers
- Polymers adsorb onto particles, forming a gel-like network and increasing viscosity and suspension stability through bridge formation .
Use of Structured Vehicles
- Aqueous solutions of natural or synthetic gums to increase viscosity and inhibit settling.
- This is especially useful for topical suspensions.
General Procedure for Suspension Formulation
- Reduce particle size of insoluble drug - Levigate/grind to a smooth paste in a vehicle containing a wetting agent. - Dissolve all soluble ingredients in a portion of the vehicle and add. - Gradually add more portion of vehicle to obtain a finished suspension.
Formulation of Suspension
- Some dispersed phases have high affinity with the vehicle, thus readily wetted. - Other drugs or particles require initial wetting before suspension, to prevent clumping or floating. - Liquids may be mixed in phases before or after wettability.
Flow Chart for Formulation of Suspension
- A step-by-step process demonstrating the steps in formulating and creating a suspension
Ingredients of Suspensions
- Insoluble drug.
- Vehicle (suspending medium).
- Wetting agents.
- Compounds that control stability and sedimentation (flocculating/suspending agents).
- Additives for flow (e.g. viscosity modifiers). Other additives (e.g. flavour, color, sweeteners, preservatives).
Formulation Components
- Drug: Should be uniform particle size (1-50µm), water insoluble, surface charge characteristics must be considered.
- Vehicle: Mostly distilled water, deionized water, alcohol, glycerol solution, non-aqueous vehicles (for topical use), structured vehicles (pseudoplastic or plastic).
- Wetting Agents: Surfaces must be easily wetted by the vehicle, can include surfactants, hydrophilic colloids, certain solvents.
Wetting Agents
- Hydrophilic materials are wetted by water; hydrophobic materials require non-polar liquids.
- Wetting agents (surfactants) are used to reduce interfacial tension between particles and liquid to facilitate wetting.
- Examples: sodium lauryl sulfate, polysorbate 80.
Hydrophilic Colloids
- Examples: acacia, bentonite, tragacanth, alginates, xanthan gum.
- Coat hydrophobic particles which imparts a hydrophilic character, improves wetting, and increases viscosity of water.
- Act as protecting colloids, increasing stability.
Solvents
- Materials like alcohol, glycerol, glycols, are miscible with water to reduce interfacial tension, permitting penetration into the pores of the particles and enabling wetting.
- Common used in conjunction with aqueous vehicles and for hydrophobic particles.
Flocculating Agents
- Electrolytes, surfactants, polymers.
- Electrolytes reduce particle repulsion and induce aggregation.
- Surfactants affect surface charges and induce aggregation.
- Polymers bind particles to form aggregated flocs, leading to rapid settling.
Flocculating Agents
- Suspended particles with high charge density tend to deflocculate then cake.
- Neutralizing the charged particles results in flocculation (aggregation in loose clusters).
- Flocculated aggregates settle rapidly but are easily redispersed.
Zeta Potential
- Zeta potential is a function of a particle’s surface charge.
- High zeta potential = increased repulsion between particles, better stability.
- Low zeta potential = increased attraction between particles, leading to instability.
Flocculating Agents
- Electrolytes
- Surfactants
- Polymers
- pH adjustment
Viscosity Modifiers (Suspending Agents)
- Typically hydrophilic polymers used to increase viscosity and retard sedimentation.
- Hydrophilic regions interact with suspension particle surface and gel-like network is created.
Suspending Agents
- Polysaccharides (e.g., acacia, tragacanth, alginates, and xanthan gum).
- Water-soluble celluloses (e.g., methylcellulose and hydroxypropylcellulose).
- Hydrated silicates (e.g., bentonite).
- Carbomers (e.g., carboxymethylcellulose).
- Colloidal silicon dioxide (Aerosil).
Other Additives
- Buffers: Materials that resist pH change (e.g., carbonates, citrates, gluconates, phosphates). Ideal range 7.4-8.4 for suspensions
- Preservatives: Prevent microbial growth (e.g., propylene glycol, disodium EDTA, benzalkonium chloride).
- Sweeteners/Flavors/Colorants: Enhance palatability and appearance.
Preparation of Suspensions
- Precipitation Method: Insoluble drugs are dissolved in a water-miscible organic solvent, which then gets precipitated when added to water.
- Dispersion Method: Solid particle dispersion (often using particle reduction methods) into the vehicle (which may incorporate a wetting agent).
Evaluation of Suspensions
- Sedimentation Method: Measures sedimentation volume (F) and degree of flocculation (β) to determine stability.
- Rheological Method: Evaluates viscosity changes in response to shearing forces for thixotropic properties.
- Electrokinetic Method: Measures zeta potential to assess particle stability.
- Micromeritic Method: Measures particle size changes to determine stability.
Rheology of Suspensions
- Flocculated: Plastic or pseudoplastic flow, dependent on concentration.
- Deflocculated: Dilatant flow; apparent viscosity increases with shearing stress.
Thixotropic Properties of Suspensions
- Thixotropic suspensions are viscous when stored but become fluid when shaken. They are beneficial to topical and certain oral medications, as well as certain parenteral injections.
Packaging and Storage of Suspensions
- Wide-mouth containers with adequate headspace to allow thorough mixing; tightly sealed to protect from light heat and freezing.
Other Suspensions
- Topical: Often semisolid, high concentrations of dispersed powder in a paraffin base; examples include lotions and pastes.
- Parenteral: Formulated to control drug absorption rate based on size of dispensed particles; examples include, but are not limited to, eye drops, ear drops, and even some injections.
Innovations in Suspensions
- Nano suspensions: very fine, solid drug particles suspended for oral, topical, or parenteral administrations.
- Taste-masked suspensions: Uses various ingredients that cover up the bitter taste, increasing patient palatability.
- Sustained release suspensions: Coating with polymers that slow the absorption rate of the drug.
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Description
This quiz covers the essentials of suspensions, specifically focusing on dispersed systems in pharmaceutical applications. You'll explore the characteristics, types, and significance of suspensions in drug formulation. Test your understanding of how these systems are utilized in medicinal products and their operational contexts.