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PMP102 - Drugs and Medicines > stability of micro/nano > Flashcards

stability of micro/nano Flashcards

1
Q

What are droplets like in a stable emulsion?

A

In a stable emulsion, droplets retain their initial character and
remain uniformly distributed throughout the continuous phase

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2
Q

What can cause emulsion instability?

A
  • Phase inversion
  • Creaming
  • Flocculation
  • Coalescence
  • Ostwald ripening
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3
Q

What happens in phase inversion/ oil in water?

A
  • Oil-in-water emulsion stabilised by ionic surfactant/co-surfactant
  • If charge on emulsion droplet is reduced
    (with the addition of ions through buffer or drug), emulsion droplets will come
    together
  • Once droplets are in contact, interfacial surfactant film re-aligns forming water-in-oil droplets and phase inversion occurs
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4
Q

What is phase inversion?

A

Conversion from oil-in-water emulsion to water-in-oil emulsion

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5
Q

What is creaming?

A

Fat globules (coloured with dye) tend to accumulate as a cream
layer on top of the milk
* Due to the differences in densities between fat globules and the plasma
phase of milk

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6
Q

Why does creaming happen?

A
  • Due to density difference between oil and water, the oil droplets
    tend to concentrate at the top of the emulsion
  • To avoid this, increase the oil density or viscosity of the
    emulsion
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7
Q

What is flocculation?

A
  • Flocculation is when two or more emulsion droplets aggregate
    without losing their individual identity
  • Larger droplets (> 2 μm) flocculate fastest and flocculation is
    promoted by creaming
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8
Q

What is an example of flocculation?

A

adding salt (Na3PO4) causes flocculation

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9
Q

When does coalescence occur?

A
  • Coalescence occurs when two or more droplets collide and
    form one larger droplet and is irreversible
  • It is caused by various factors, including surfactant type and
    concentration, pH, temperature etc
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10
Q

What happens in Ostwald ripening?

A
  • With polydispersed droplets, collision between two droplets may
    cause one bigger droplet and one smaller droplet
  • Upon repeated collisions, the small droplets become very small
    and become solubilised in the continuous medium. They
    eventually diffuse and re-deposit on larger droplets making
    them even larger in size
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11
Q

What is van Der walls + how it works? (DVLO)

A

Energy of attraction varies with the distance (H) between pairs of atoms or molecules or neighbouring particles with the inverse of the 6th power. A= 1/H⁶

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12
Q

Vr (DVLO) how it arrises, what happens?

A
  • Arises from the interaction of
    the electrical double layers
    surrounding pairs of particles
  • Repulsive forces decay
    exponentially with distance
  • Repulsive forces decay more
    rapidly than attractive forces
    therefore the attractive forces
    predominate over longer
    distances
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13
Q

What will increasing the charge on the double layer do? (VR)

A
  • Optimise the concentration of
    surfactant (don’t forget about
    the associated counterions)
  • Optimise the pH
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14
Q

What does 0 to +5 zeta potential mean in terms of stability?

A

Rapid coagulation or flocculation

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15
Q

What does +10 to +30 zeta potential mean in terms of stability?

A

Incipient instability

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16
Q

What does +30 to +40 zeta potential mean in terms of stability?

A

Moderate stability

17
Q

What does +40 to +60 zeta potential mean in terms of stability?

A

Good stability

18
Q

What does +61+ zeta potential mean in terms of stability?

A

Excellent stability

19
Q

What does Va in DVLO prove in contrast to Vr?

A
  • Explains why some colloidal
    particles aggregate
  • Emulsions → coalesce
  • Suspension → floc

Whereas the Vr =* Explains why some colloidal particles stay separate

20
Q

What is the potential energy diagram of VT= Va + Vr?

A

picture

21
Q

What does the secondary minimum mean?

A
  • At large distances of
    separation, particles
    experience a minimal
    attraction
  • Forces of attraction are weak,
    flocculation not coalescence,
    i.e. can redisperse upon
    shaking
22
Q

What happens at the primary maximum?

A
  • As particles come closer
    together, they start to
    experience some repulsion
    which will peak at the primary
    maximum
  • The height of this repulsive
    force (Vmax) determines the
    stability of the system
  • The height varies with different
    surfactants and electrolyte
    concentration
  • A high value will ensure
    coagulation is so slow that the
    system displays long term stability
  • The energy barrier that leads to
    irreversible particle aggregation
23
Q

What if you add salt during primary maximum?

A
  • Addition of electrolyte (e.g.
    NaCl) induces:
  • Neutralisation or reduction of
    charge on droplets
  • Decrease in Vmax
  • Destabilisation of the emulsion
24
Q

What happens at the primary minimum?

A
  • At close approach, van der
    Waals forces always
    dominate over repulsive
    electrostatic forces
  • A deep primary minimum is
    present
  • At this short inter-particle
    distance, particles/droplets
    coagulate irreversibly
25
Q

What does DVLO only apply to?

A

ionic surfactants

26
Q

What if a non-ionic surfactant were to stabilise an emulsion?

A
  • When a non-ionic surfactant stabilises an emulsion, no
    electrostatic charge is present to stabilise the droplet (the
    charge on the droplet is neutral)
27
Q

How do hydrophilic polymer chains stabilise emulsions?

A

one of the two ways:
* Entropic (steric) effects
* Osmotic (solvation) forces

28
Q

What is the entropic steric effect and what happens in it?

A
  • When two particles come into close contact, the polymer chains start
    to overlap
  • This leads to a loss in the freedom of motion of the polymer chains,
    i.e. a loss of entropy
  • This situation is thermodynamically unfavourable and forces the
    droplets apart again
29
Q

What is the osmotic salvation force, what happens in it?

A
  • When two particles come into close contact the polymer chains
    start to overlap, effectively leading to a concentrated polymer
    solution
  • This induces an osmotic gradient in the solution: a concentrated
    polymer solution in the overlap region and a dilute solution in
    the bulk solution
  • Water enters the concentrated region in an attempt to dilute it
    and in doing so forces the polymer chains (and droplets) apart
30
Q

How does steric stabilisation apply? + new equation?

A
  • Modified equation
    VT = VA + VS
  • A = attractive
  • S = steric & solvation forces
  • Generally when mixtures of
    surfactants are used,
    electrostatic forces are more
    efficient at stabilising
    emulsion droplets than
    steric/solvation forces alone
31
Q

What are the 4 main types of forces?

A
  • Van der Waals (attractive)
  • Electrostatic (repulsive)
  • Steric forces (repulsive)
  • Solvation forces (repulsive)
32
Q

What forces apply for charged colloids?

A
  • For charged colloids (emulsions stabilised by ionic surfactants),
    van der Waals and electrostatic forces are the most important
33
Q

What forces apply for uncharged colloids?

A
  • For uncharged colloids (emulsions stabilised by non-ionic
    surfactants) van der Waals and steric and solvation forces are
    the most important
34
Q

What equation is used when both ionic and nonionic stabilise an emulsion?

A
  • DLVO equation becomes
    VT = VA + VS + VR
    BEST STABILITY

PMP102 - Drugs and Medicines (20 decks)

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