Suspensions Flashcards
Suspensions
-solid particles dispersed in a liquid phase in which they are not soluble
Peptobismol example
-active ingredient: bismuth subsalicylate
-suspension of insoluble salt of salicylic acid linked to bismuth cation
suspensions vs solutions
-solubility
-chemical stability (0 vs1st order)
-palatability (taste)
suspesions vs tablets
-flexibility of dose
-ease of swallowing
-dissolution rate
Components of Suspension
-active ingredient (solid particles)
-vehicle
-buffer
-preservative
-flocculating agent
-structured vehicle system
-wetting agent
-antifoaming
-flavor and sweetener
Desirable properties of Suspensions
-suspended material should not settle rapidly
-particles that settle must not form a hard cake and readily be redispersed when shaken
-easy to admin
-particle size remains constant in storage
Viscosity of suspension
-should not flow thru syringe needle
-fluid enough to spread over affected area
Settling equations
-Stokes Law
Particle size
-micropulverization (10-50mcm)
-fluid energy grinding (10mcm)
-spray drying (5 mcm)
Micropulverization
-10-50mcm
-high speed attrition or impact mills
fluid energy grinding
-less than 10 mcm
-jet milling, micronizing
-shearing action of high-velocity compressed airstreams on the particles in a confined space
Spray drying
-5mcm
-spray dryer: cone-shaped apparatus into which a solution of drug is sprayed and rapidly dried by current of warm, dry air circulating in the cone
Suspension Formulation Designs
-dispersed phase (solid particles)
-dispersion medium
Types of suspension
-dispersed suspension
-flocculated suspension
-structured vehicle system
Thermodynamic stability
slide 13
Interparticle forces
-van der Waals ATTRACTIVE
-hydration REPULSIVE
-electrostatic REPULSIVE
-steric REPULSIVE
van der waals attractive force
-operates at moderate distance from the surface but becomes very strong close to the surface
-NOT affected by formulation factors
Hydration REPULSIVE force
-due to absorbed water molecules at surface of particle
-NOT affected by formulation factors
Electrostatic repulsive force
-due to surface charge on the particles
-may be affected by formulation
Steric REPULSIVE force
-due to an absorbed layer of neutral polymer at surface of a particle
-may be affected by formulation
forces NOT affected by formulation factors
-van der waals ATTRACTICE
-hydration REPULSIVE
forces that might be affected by formulation
-electrostatic REPULSIVE force
-steric REPULSIVE force
Net effect of interparticle forces
slifde 15?
Dispersed suspension
-make REPULSIVE forces dominant
-particles repel each other
-no aggregation
Problems with dispersed suspensions
-particles settle as individual particles
-leads to dense compact sediment (cake)
Controlled Flocculation
-repulsive and attractive forces are in BALANCE
-particles attracted to each other at the secondary minimum to form aggregates (aka floccules)
-settles as a sediment with a high volume
-type of sediment is easy to redisperse
slide 18
slide 18
choice of Flocculating agents
-depend on type of drug and type of product desired
Flocculating agents
-clay (diluted bentonite magma)
-change pH
-Electrolytes
-non-ionic or ionic surface-active agents
Clay (diluted bentonite magma)
-flocculating agent
-oral suspensions
changing pH of suspension
-to region of minimum drug solubility
-flocculating agent
-parenteral solution
Electrolytes
-flocculating agents
-reduce the electrical barrier between the particles
Why dont we want suspension to settle too rapidly
-hinders accurate measurement of dosage
-esthetically not good
structured vehicle
-thicjen the dispersion medium to help suspend particles
-polymer or clay
-should not interfere w availability of drug
-should not make the suspension too viscous to agitate or to pour
Rheology
-study of flow characteristics
-shear rate (dv/dr)
-shear stress (F)
Shear rate (dv/dr)
-difference of velocity (dv) between two planes of liquid separated by a distance (dr)
Shear Stress (F)
-force per unit (Fâ/A) required to bring about flow
Newtonian flow
slide 16
Non-newtonian flow
-plastic
-pseudoplastic
-dilatant
Plastic non-newtonian flow
-typical of flocculated suspensions
-f (yield value)
f (yield value)
-threshhold of shear stress necessary to initiate flow
-shear-thinning
Psuedoplastic non-newtonian flow
-typical of polymer solutions such as methyl cellulose, polyvinyl alcohol, sodium carboxymethylcellulose, xanthan
-shear-thinning
Dilatant non-newtonian flow
-exhibited by suspension having a high solids content
-shear-thickening
Thixotropy
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