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PMP102 - Drugs and Medicines > disperse systems - macro > Flashcards

disperse systems - macro Flashcards

1
Q

What are the key physical stability properties of suspensions?

A
  • Lyophobic systems have a poor interaction with the solvent
  • Suspensions are ‘coarse’ i.e. contain larger particles
  • Large particles sediment
  • Stokes’ law governs the sedimentation
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2
Q

in a suspension, two substances may not mix.. what is the first approach?

A

preventing sedimentation

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3
Q

in a suspension, two substances may not mix.. what is the second approach?

A

enabling redispersion
e.g shake the medicine for 10 seconds

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4
Q

What are the different types of instability phenomenon?

A
  • Aggregation
  • Coagulation
  • Flocculation
  • Sedimentation
  • caking
  • Ostwald ripening
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5
Q

What is aggregation?

A
  • Particles in groups
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6
Q

What is coagulation?

A
  • Closely aggregated and difficult to redisperse
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7
Q

What is flocculation?

A
  • Aggregates have an open structure with particles a
    small distance apart, attracted by weak forces to
    form flocs or flakes
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8
Q

What is sedimentation?

A
  • Process of settling or being deposited as a sediment
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9
Q

What is caking?

A
  • Deflocculated particles (fine separate particles)
    form cakes which are difficult to re-suspend
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10
Q

What is Ostwald ripening?

A
  • Dissolution of small crystals or sol particles and the
    re-deposition of dissolved species on the surfaces of
    larger crystals or sol particles
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11
Q

What does instability lead to?

A
  • Physical instability results in poor dosing reproducibility
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12
Q

What factors affect stability?

A
  • Kinetic properties (motion of the particles with respect to dispersion medium)
  • Brownian motion and diffusion
  • Sedimentation
  • Viscosity
  • Size/shape
  • Electrical properties
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13
Q

What is Brownian motion and diffusion?

A
  • Particles diffuse from a high concentration to a low concentration
  • Diffusion rate is based on Fick’s first law:
    𝑑𝑚/ 𝑑𝑡 = −𝐷𝐴 x 𝑑𝐶/𝑑𝑥
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14
Q

What is the diffusion co efficient/ stokes-einstein equation?

A

𝐷 = 𝑘𝐵𝑇/ 6𝜋𝜂𝑟
* kB = Boltzmann constant
* T = Absolute temperature
* η = Viscosity of medium
* r = Radius of the solute molecule

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15
Q

What is the rate of sedimentation equation?

A

𝑉 = 2𝑟2 × (𝜌 − 𝜌o) × 𝑔/9𝜂o
* v = sedimentation rate
* r = particle radius
*
P= density of the disperse phase
*
P0 = density of the continuous phase
* g = gravity
* η0 = viscosity of the continuous phase

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16
Q

What is sedimentation dependant on?

A
  • The rate of sedimentation is dependant on the combined forces of gravity and drag
  • Particle falling under the forces of gravity according to Stokes’ law
17
Q

what are some extra rules about the sedimentation rate?

A
  • only applies to > 0.5 μm
    *If 𝜌 − 𝜌o < 0 then creaming rather than caking
18
Q

What is viscosity?

A
  • Related to molecular weight of suspended particles/suspending agents
  • Resistance to flow under an applied stress
19
Q

What factors affect rheology suspensions?

A

a) High volume fractions, f
b) Particle size
c) Particle size distribution
d) Particle shape
e) Electrostatic interactions
f) Steric hindrance

20
Q

What are the shape of most molecules?

A

Many suspended particles are spherical
* Several measurement techniques assume a sphere

21
Q

small deviation shape?

A
  • Ellipsoidal model
22
Q

large deviation shape possibilities?

A
  • Hydroxyapatite (rod-shaped)
  • Clay suspension (plate)
  • Polymers in solution (coil)
23
Q

How to prevent sedimentation? (relating to the equation)

A
  • Form smaller particles
  • Decrease the density difference between the two phases
  • Increase viscosity of continuous phase
24
Q

What are the two layers of the electrical double layer of ions?

A
  • stern layer
  • diffuse layer
25
Q

explain zeta potential + trends depending on the value

A
  • Zeta potential = magnitude and
    type (+ or –) of the electrical
    potential at the slipping plane
  • Low zeta potential (0 to 5 mV) are
    prone to aggregate
  • Zeta potential > 30 mV tend to
    remain dispersed
26
Q

What factors affect zeta potential?

A
  • Ion concentration
  • Charge of ions determines magnitude
  • pH of continuous phase
  • Alters the ionisation of ionic species in the continuous phase and the surface
    charge of ionisable groups
27
Q

What is DVLO?

A
  • Established by Derjaguin, Landau, Verwey, and Overbeek in the 1940s
  • Quantitative approach to the stability of lyophobic systems
  • Assumes the only interactions involved are
  • Van der Waals forces of attraction (VA)
  • Electrostatic repulsive forces (VR)
28
Q

What is the DVLO equation?

A

VT = VA + VR

29
Q

What is VR meaning/ entail?

A
  • Repulsive forces from electrical charges on particles
  • Ionisation of surface groups
  • Adsorption of ions
  • A particle surface with a positive charge has a layer of negative ions
    attracted to its surface in the Stern layer

PMP102 - Drugs and Medicines (19 decks)

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