Mixing Flashcards
What is the aim of mixing?
To rearrange particles to distribute them homogeneously.
Positive Mixture is
mixes spontaneously, energy input required to separate to components
Negative Mixture is
segregates spontaneously, energy required to mix and keep mixed
Neutral Mixture is
components neither mix nor segregate, energy required to mix or segregate.
Mixing in Liquids types (3)
bulk transport
- large degree of mixing in short period of time
turbulent mixing
- group of molecules move with constant velocity changes
molecular diffusion
- movement down a conc gradient
Mixing in Solids types (3)
connective mixing
- like bulk transport in liquids
shear mixing
- like turbulent in liquids
diffusive mixing
- like MD in liquids
What is a free flowing powder, is it easy to mix?
easy to mix especially if monosize
susceptible to segregation if different size or density
What is a cohesive powder, is it easy to mix?
Fine particles which are cohesive
need high energy de-agglomeration for adequate mixing but need to consider the friability of particles.
Is a perfect mixture attainable in powders?
No.
Random mixture is most likely.
What is the random mixture?
Probability of selecting a particle type is the same in all positions of the mixture and equal to the proportion of the particles in the totalmixture.
Equation for theoretical standard deviation of a random mixture =
O R = Square root of p(1-p) /n
p = proportion go drug/tracer in mixture n = total number of particles in mixture
Mixing index =
o (standard deviation) / theoretical standard deviation
If mixing index is 1…
mixing complete - homogenous sample attained.
What is the scale of scrutiny?
the amount of powder per sample for assessing the extent of mixing = amount in 1 dosage unit e.g. 1 tablet.
If you sample too little..
you will mistakenly believe that the powder is not mixed enough
if you sample too much…
you will mistakenly believe the powder is mixed enough
What is an ordered mixture?
where the micronised particles are adsorbed on the surface of larger carrier particles.
Segregation is likely if… (3)
particle size non uniform
particle density non-uniform
spherical shape - free flowing.
Mechanisms of segregation:
Percolation segregation
- small particles fall through voids.
Trajectory segregation
- larger particles go further as they have more mass based inertia
Elutriation segregation
- fine particles settling on top after motion has ceased
Minimising segregation: (4)
ordered mixture
reduce vibration
inform particle size
excipients of similar density
mechanism of mixing in a powder is:
convective mixing
