Physical Pharmacy of Semisolids - Rheology

Question 1

Rheology

Answer 1

• Science of deformation and flow properties of matter

- Can be applied to all forms of matter, focus on liquids and semisolids in pharmaceuticals

Question 2

Product Consistency Terms (2)

Answer 2

1. Viscosity - measure of resistance to flow or thickness’

2. Elasticity - stickiness or structure

Question 3

Rheology Types (3)

Answer 3

1. Newtonian
2. Non-Newtonian
3. Thixotropy

Question 4

Types of Non-Newtonian Rheology (3)

Answer 4

1. Plastic
2. Pseudoplastic
3. Dilitant

Question 5

Newtonian Rheology

Answer 5

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)

Question 6

Units

Answer 6

Shear Stress = dynes/cm^2
Rate of Shear = 1/sec
Viscosity (n) = (dynes/cm^2) * sec OR poise

Question 7

Poise

Answer 7

Used for low viscosity liquids

1 cps = 1/100 poise

Question 8

Fluidity

Answer 8

• Reciprocate of viscosity

- 1/n = O (O = fluidity)

Question 9

Rheogram

Answer 9

• Graphical representation of rate of shear (y) v.s. shear stress (x)
• slope = fluidity
• Shear stress/Rate of shear = n

Question 10

Newtonian Fluids

Answer 10

-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

Question 11

Viscosity + Temp (Newtonian)

Answer 11

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)

Question 12

Non-Newtonian Rheology

Answer 12

• Observed in complex heterogeneous systems

- Relationship of shear stress v.s. rate of shear is NON LINEAR

Question 13

Plastic Rheology

Answer 13

• Describes a situation where NO FLOW occurs in response to shear stress until transition point is reached
• Yield Value (Psi) is the transition

Question 14

Yield Value (Psi)

Answer 14

• 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

Question 15

Bingham Bodies

Answer 15

• Seen in gels, ointments, and creams
• Important to determine yield value
• Magnitude of stress is critical in terms of patient use

Question 16

Pseudoplastic Rheology

Answer 16

-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)

Question 17

Diltant Rheology

Answer 17

-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%)

Question 18

Thixotropy

Answer 18

-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

Question 19

Degree of Hysteresis

Answer 19

May take shorter or longer time to reacquire structure and become viscous upon standing

Question 20

Rheology Application + Semisolids (4)

Answer 20

1. Spreading and adherence to the skin
2. Removal from jars and extrusion from tubes
3. Capacity of solids to mix with miscible liquids
4. Release of drug from base