Test 3- Rheology/Dispersed Systems Flashcards
Rheology
the study of deformation and the flow of matter
Why study rheology?
- understanding uniformity of liquid and semi-solid dosage forms
- affects patient’s acceptability of the product
- pharmaceutical manufacturing
- needed to determine the type of packaging required
viscosity
the resistance of a fluid to flow; the higher the viscosity, the greater the resistance
How does viscosity relate to temperature?
they are inversely related
Newtonian flow
the rate of flow of a liquid (G) was directly proportional to the applied force (F), where n is the viscosity
Shearing
When the top layer of a cube is subjected to force, each lower layer moves with velocity directly proportional to its distance from the bottom layer
Shear rate
the difference in velocity (dv) between the two planes of liquid separated by distance (dr); this is equivalent to flow rate; difference between moving plane and stationary phase
Shear stress
force per unit area required to bring about the flow
Liquids that demonstrate Newtonian flow
water, ethanol, glycerin (Pure liquids); syrup, normal saline, 5% dextrose (Molecular solutions)
Non-Newtonian flow
the viscosity of non-Newtonian fluids changes under stress; can cause the fluid to get runnier or thicker
3 types of non-Newtonian flow
- Plastic flow
- Pseudoplastic flow
- Dilantin flow
Plastic flow
-does not flow at low values of force
-when force reaches the yield value, the liquid will flow proportionate to the applied force
-the rheogram does not pass through the origin
-at stresses below the yield value, behaves like a solid
-e.g. semi-solid creams and ointments
Pseudoplastic flow
-do not need to reach a yield value
-termed “shear thinning”
-as greater force is applied, the viscosity of the liquid decreases
-e.g. suspensions, liquid emulsions, hydrophilic polymers in aqueous solutions
Shear thinning
the viscosity of a liquid decreases with increasing shear stress
Dilantin flow
-viscosity increases as force applied increases
-termed shear thickening
-e.g. suspensions with high concentrations of undissolved solids (>40%) demonstrate dilantin flow
Shear thickening
the viscosity of a liquid increases with increasing shear stress
Solutions
mixtures of ions or small molecules that form a single phase that is homogenous down to the molecular level
Disperse system
-consists of a disperse phase dispersed as particles or droplets throughout the continuous phase
-the solubility of the dispersed phase within the continuous phase is low
Molecular solutions
-homogenous down to molecular level
- <1 nm
- e.g. syrups, elixirs, solutions for injection, nasal, otic, ophthalmic solutions
-solutes exhibit Brownian motion
-diffuse easily from an area of high concentration to an area of low concentration
-do not scatter light
-demonstrate Newtonian flow
Colloidal
-Large MW compounds or aggregates of smaller MW compounds
-1nm to 0.5 micrometer
-e.g. gels and vehicles for suspensions
-exhibit Brownian motion
-diffuse from an area of high concentration to an area of low concentration
-scatter light
-exhibit non-Newtonian flow
-do not settle due to gravity
Coarse dispersions
-contains particles large enough to see with the naked eye
- >5 micrometer
-e.g. suspensions, emultions, foams, aerosols
-too large to exhibit Brownian motion
-too large to diffuse
-scatter light
-exhibit non-Newtonian flow
-dispersed particles tend to aggregate (can cause stability issues)
Brownian motion
random kinetic motion of particles
Hydrophilic polymers
-large chains of macromolecules in solutions
-swell when they become hydrated (will increase the viscosity of solution)
-reform upon rest
-associations untangle upon shaking
