Suspensions Flashcards
Suspension
liquid preparations that consist of solid particles dispersed throughout a liquid phase
Suspensions vs. Solutions
-takes time to dissolve
-zero order (degradation is independent of conc.)
-used for bitter drugs (since it takes time to dissolve)
Suspensions vs Tablets
-has more dose flexibility
-easier to swallow
-good dissolution rate
Desirable properties of suspensions
-should NOT settle rapidly
-when particles settle they should NOT form a hard cake
-readily dispersed
-easy to administer (balance of viscosity)
-particle size remains constant during storage (maintains dissolution rate)
Settling
LARGE particles settle QUICKLY
if the difference of density is large then it SETTLES QUICKLY (solid is more dense)
Stokes Law
if V is POSITIVE = settles quickly
if V is NEGATIVE = settles slowly
What are the different types of suspensions?
-dispersed suspension
-flocculated suspension
-structured vehicle system
Thermodynamic stability of suspensions?
-the most stable when delta G = 0
-get there through: aggregation and crystal growth
Van der Waals attractive force
-happens in moderate distance of the molecules, stronger on the surface
-NOT AFFECTED by formulation
Hydration repulsive force
-force generated by having a water layer on the particles (helps with repulsion)
-NOT AFFECTED by formulation
Electrostatic repulsive force
-surface charge on the particles
-AFFECTED by formulation (pH or solution conditions)
Steric repulsive force
-generated due to polymers on the surface of the particle
-AFFECTED by formulation (pH or solution conditions)
Mechanism of Dispersed suspension
-make the repulsive forces dominant and they don’t aggregate
-although when they settle it forms a cake which is difficult to resuspend (due to high attractive forces)
-CLOUDY suspension, slow rate of suspension
Mechanism of Controlled flocculation
-repulsive and attractive forces are in BALANCE
-attracted to each other at the secondary minimum
-produce a sediment at high volume when they settle
-easy to redisperse
-rapid rate of sedimentation
-clear supernantant
Types of flocculating agents
-clay (bentonite magma)
-alternation of pH
-reduce the electrical barrier b/t particles
-nonionic or ionic surfactants
Structured vehicle system
-purpose is to thicken the dispersion medium and suspend particles
-shouldn’t affect the bioavailability
-shouldn’t make it too viscous
-appear as semi-solid when undisturbed but fluid when shaken
-no sedimentation
Examples of structured vehicle system
carboxymethylcellulose (CMC), methycellulose, microcrystalline cellulose, polyvinylpyrrolidone, xanthan gum, bentonite, magnesium aluminum silicate
Rheology
the study of flow characteristics
Shear rate
how FAR the liquid will go given a force
Shear stress
how much FORCE is applied
Types of Rheology flows
-newtonian flow
-plastic (non-newtonian flow)
-pseudoplastic (non-newtonian flow)
-dilatant (non-newtonian flow)
Newtonian flow
-ex: water
-graph is straight and linear
-viscosity graph is a straight horizontal line
Plastic non-newtonian flow
-graph is characterized by yield value
-viscosity decreases when shear rate increases (shear thinning)
Pseudoplastic non-newtonian flow
-graph is linearish with a slight curve, no yield value
-viscosity decreases when shear rate increases (shear thinning)
