Physical Pharmacy of Semisolids - Rheology Flashcards
Rheology
- Science of deformation and flow properties of matter
- Can be applied to all forms of matter, focus on liquids and semisolids in pharmaceuticals
Product Consistency Terms (2)
- Viscosity - measure of resistance to flow or thickness’
2. Elasticity - stickiness or structure
Rheology Types (3)
- Newtonian
- Non-Newtonian
- Thixotropy
Types of Non-Newtonian Rheology (3)
- Plastic
- Pseudoplastic
- Dilitant
Newtonian Rheology
dV/dx is proportional to F/A
F/A = n(dV/dx)
dV = velocity dx = distance (small) n = coefficient of viscosity
-Velocity of a material traveling a small distance is directly proportional to sheer stress (F/A)
Units
Shear Stress = dynes/cm^2
Rate of Shear = 1/sec
Viscosity (n) = (dynes/cm^2) * sec OR poise
Poise
Used for low viscosity liquids
1 cps = 1/100 poise
Fluidity
- Reciprocate of viscosity
- 1/n = O (O = fluidity)
Rheogram
- Graphical representation of rate of shear (y) v.s. shear stress (x)
- slope = fluidity
- Shear stress/Rate of shear = n
Newtonian Fluids
-Viscosity is independent of shear stress (absolute viscosity)
-Slope = constant, linear relationship
-Commonly have low MW and molecular homogeneity
-Viscosity SHARPLY drops with increases of temperature
EX: acetone, water, olive oil, glycerin
Viscosity + Temp (Newtonian)
n = Ae^(Ev/RT) ln(n) = ln(A) + Ev/RT
Ev = activation energy A = constant (same units as viscosity) R = universal gas constant T = absolute temperature (C + 273)
Non-Newtonian Rheology
- Observed in complex heterogeneous systems
- Relationship of shear stress v.s. rate of shear is NON LINEAR
Plastic Rheology
- Describes a situation where NO FLOW occurs in response to shear stress until transition point is reached
- Yield Value (Psi) is the transition
Yield Value (Psi)
- Minimum shear stress required before flow begins
- Once reached, shear stress v.s. rate of shear relationship is LINEAR (x-intercept of linear portion = psi)
- Too high is not easily rubbed in by patient
- Too low creates a runny product
Bingham Bodies
- Seen in gels, ointments, and creams
- Important to determine yield value
- Magnitude of stress is critical in terms of patient use
Pseudoplastic Rheology
-Observed in systems that deform and flow instantaneously with applied stress
-NOT LINEAR
-Shear-thinning system (viscosity closer to the origin is larger than a point further away from origin)
EX: Tears, ketchup, sodium alginate, methylcellulose
-Goes from globular form (higher viscosity) to uncoiled form (lower viscosity)
Diltant Rheology
-Observed in systems that deform and flow instantaneously with applied stress
-NOT LINEAR
-Shear-thickening systems (viscosity of point closer to origin is less than one further away)
EX: Whipped cream and suspensions containing high concentrations of particles (>50%)
Thixotropy
-Comparatively slow recovery of material structure on standing that was lost by shear thinning
-Time required to recover configuration once stress is removed
EX: Suspensions and emulsions
Degree of Hysteresis
May take shorter or longer time to reacquire structure and become viscous upon standing
Rheology Application + Semisolids (4)
- Spreading and adherence to the skin
- Removal from jars and extrusion from tubes
- Capacity of solids to mix with miscible liquids
- Release of drug from base
