rheology Flashcards
What is rheology? What is is affected by?
rheology is the science of the flow properties of materials
- viscosity is the resistance to flow; the higher the viscosity, the greater is the resistance
Why is rheology important?
ease of packaging
- flow into a bottle or tube
ease of pouring
- flow from bottle or tube
flow through a syringe needle
smoothness/feel of an ointment, gel or cream when spread on skin
What is the difference between newtonian and non-newtonian systems?
newtonian
- follow newton’s law
- linear relationship
- as viscosity increases, shear stress increases = directly proportional
- viscosity is constant = no change
non-newtonian systems
- do not follow newton’s law
- non-linear relationship
- viscosity and shear stress are not directly proportional
What are the non-newtonian systems?
plastic - Bingham bodies
pseudoplastic
dilatant
How do plastic non-newtonian systems work?
does not pass through the origin
does not begin to flow until the shear stress exceeds a critical point (𝜎𝛾)
- value is known as the yield value
linear after yield value met
- below the yield value = elastic solid
- above the yield value = linear relationship
viscosity is not constant
What are examples of plastic non-newtonian systems?
creams, ointments, toothpaste
- yield value is the force required to spread them the skin
flocculated system
- yield value is the force needed to re-disperse sediment
How do pseudoplastic non-newtonian systems work?
shear thinning
- viscosity decreases as shear rate decreases
- viscosity is not constant
no part of the rheogram is linear
at rest
- intramolecular bonds form a dense network
- water entrapped in polymers
- viscous system
shear stress applied
- breaks intramolecular bonds
- polymer aligns in direction of flow
- decreased internal resistance and viscosity
How do dilatant non-newtonian systems work?
shear thickening
- viscosity increases as shear rate decreases
- viscosity is not constant
reversible process
- when shear stress is removed, it goes back to original viscosity
in deflocculated systems
What is thixotropy?
any material that exhibits a reversible time-dependent decrease in apparent viscosity is described as thixotropic
- regains viscosity
reducing the shear stress produces identical down curves
What is the effect of shear stress on molecules at rest and under shear stress?
at rest
- intra-macromolecular secondary bonds
- loose three-dimensional structure
- material is gel-like
shear stress applied
- shearing energy break down bonds
- macromolecules become aligned
- viscosity falls-sol-like structure
- a gel-sol transformation
shear stress relieved
- stress is removed → structure tend to reform
- process is not immediate but will increase with time molecules return to the original state
- time taken for recovery can vary from minutes to days = it is directly related to the length of shearing time as this will affect the degree of breakdown.
What is the partition coefficient?
molecule partitioning between two phases depends on its solubility in each phase and how easily it can diffuse between them
p = concentration in oil / concentration in water
log P = 0
- equal distribution of drug in both phases
log P > 0
- drug has higher affinity for lipid phase
- more hydrophobic
log P < 0
- drug has lower affinity for aqueous phase
- more hydrophilic
What is distribution coefficient?
log D
- Ionisable drugs (acids, bases) can also partition between octanol and water phases
- their partition will depend on water phase pH
an un-ionisable drug shows the same log P value regardless of the pH of the water phase
Why are weak acids absorbed in the stomach but not weak acids?
weak bases
- ionised at low pH
- ionised form will not be absorbed
weak acids
- un-ionised at low pH
- un-ionised form is absorbed
