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MPharm 2 - PH2107 > PH2107 - suspension, colloids & nanoparticles 3 > Flashcards

PH2107 - suspension, colloids & nanoparticles 3 Flashcards

1
Q

What are the two possible outcomes for particles when they collide?

A
  • rebound

- associate

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2
Q

What are the two states of particles when they associate after a collision?

A
  • temporary

- permanent

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3
Q

What happens when particles become associated with each other after a collision?

A

Clumps increase in overall size and may sediment

- at which point the system may become impossible to re-disperse

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4
Q

What determines whether particles rebound and stay separated after collisions, or get permanently attached when they collide?

A

The balance of attractive and repulsive forces

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5
Q

What is of great importance in the stability of disperse systems?

A

Charge

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6
Q

How do particle surfaces acquire an electrical charge?

A
  1. Particle charge by differential ion dissolution
  2. Particle charge by ionisation
  3. Particle charge by ion adsorption
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7
Q

What property do different ions have?

A

Different solubilities

- silver ions (Ag+) are more soluble in water than iodide ions (I-)

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8
Q

How does differential ion dissolution work?

A

Adding silver iodide to water, the more soluble silver (Ag+) would dissolve into the bulk of the water faster, leaving behind iodide ions (I-)
Remaining particle surface therefore acquires an overall negative charge

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9
Q

How can the molecules at the surface of a structure become charged?

A

If a particle is comprised of molecule/moieties that can be ionised

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10
Q

What is particle charge by ionisation influenced by?

A

By the pKa of the ionisable groups

- can be manipulated by adjusting the pH of the dispersant / continuous phase

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11
Q

How can a surface charge of a structure be aquired?

A

By the adsorption of ions onto the surface of an electrically neutral particle
- i.e. if more anions adsorb to a surface, the surface of the particle will develop a net negative charge

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12
Q

Which charge is more often gained in water?

A

In water, surfaces tend more often to become negatively charged than positively charged

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13
Q

Why does the surface of water more often tend to become negatively charged?

A
  • cations are generally more hydrated than anions so more favourable for them to stay in the water phase
  • this leaves anions which are more likely to adsorb onto the particle surface
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14
Q

What charge must a disperse system have?

A

It must be electrically neutral overall

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15
Q

What influences the distribution of ions in the rest of the volume of liquid in a disperse system?

A

The surface charge of the particles

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16
Q

What forms around each charged particle in a disperse system?

A

Distinct layers

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17
Q

What is an EDL?

A

Electrical Double Layer

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18
Q

What are the three distinct regions of an electrical double layer?

A
  • inner region - includes charged surface and adsorbed ions
  • diffuse region - beyond adsorbed ions and up to the edge of the electrically neutral region
  • electrically neutral region - bulk solvent, outside the EDL
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19
Q

How many distinct regions make up an electrical double layer?

A

Three

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20
Q

What is a stern layer?

A

A monolayer of ions adsorbed onto particle surface

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21
Q

What is a stern plane?

A

The boundary between the tightly bound stern layer and diffuse layer

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22
Q

What is the diffuse layer?

A

Contains molecules loosely bound to the stern layer, extending to a distance
- debye length

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23
Q

What is the shear plane?

A

An imaginary line through the middle of the diffuse layer

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24
Q

What is the electrical double layer (EDL)?

A

Comprised of stern layer and diffuse layer

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25
Q

What is the electroneutral region (ENR)?

A

Non-attracted bulk solvent molecules throughout the continuous phase up to the EDL of the next particle

26
Q

What is the DeBye length?

A

The distance between Stern layer and EDL limit

- the thickness of the exponential region of the plot

27
Q

What is k dependent on in a charged system?

A

k is a constant that varies between systems which is dependent on the electrolyte concentration of the liquid phase
- adding electrolyte (increasing ionic strength) increases k and therefore decreases 1/k and that means that the thickness of the EDL is decreased

28
Q

What does the EDL theory show?

A

There is an uneven distribution of ions in a disperse system

29
Q

What is an electrical potential?

A

A difference in charge between two areas

30
Q

What is an electrical potential measured against?

A

The bulk ENR

31
Q

What is the relationship between distance from the surface and electrical potential?

A

Potential decreases with the distance from the surface

  • drops rapidly and linearly between particle surface and the Stern plane
  • then has a more gradual, exponential decrease towards zero
32
Q

Why does the electrical potential decrease with distance from the surface?

A

The counterions close to the surface act as a screen to other ions
- reduces the attraction between the charged surface and the counterions further into the bulk solution

33
Q

What is the Nerst potential (E)?

A

The potential between the true solution and the electroneutral region of the solution

34
Q

What is the Stern potential?

A

The potential between the Stern plane and the ENR

35
Q

What is the zeta potential?

A

The charge at the Shear plane, measured in mV

  • quantities the degree of repulsion between similarly charged particles and the likely (in)stability
  • hence the fundamental determinant in stability determinations
36
Q

What varies between formulations and can be manipulated to achieve stability?

A

Zeta potential

37
Q

What are the zeta determinations?

A
  • the closer the zeta potential to zero, the greater the tendency to floc
  • the higher the zeta potential, the more stable the system
  • for a dispersion to remain stable, the zeta potential should be either > +30 mV or < -30 mV
38
Q

What are the two type of particle-particle interactions in disperse systems?

A
  • attractive interactions (VA)

- repulsive interactions (VR)

39
Q

What causes attractive interactions (VA)?

A

Van der Waals (vdW) forces between molecules in the surface layers of the interacting particles

40
Q

How can the strength of Van der Waals forces be measured?

A

de Boer and Hamaker calculated the attractive forces between spheres of the same radius

VA = -Aa/12H

Attractive Interactions (VA)
A Hamaker constant
a particle radius
H distance between particles

41
Q

What is the relationship between the attractive force and distance?

A

The attraction decreases as the distance increases

42
Q

What causes repulsive interactions (VR)?

A

Electrical charges on the surfaces of particles, due to

  • adsorption of charged polymers or surfactants at the interface
  • polarity differences between the solid and the liquid
  • ionisation of chemical groups at the surface of the particles
  • adsorption of small inorganic ions onto particle surfaces
43
Q

Where do repulsive interactions act?

A

Over approximately the thickness of the double layer

44
Q

What is the relationship between distance between particles and repulsive force?

A

VR decreases with distance between particles, more sharply than van der Waals, VA

45
Q

What does DVLO stand for?

A

Derjaguin and Landau, and Verwey and Overbeek

46
Q

What is DVLO?

A

The interaction between two particles (VT) in terms of van der Waals attractive forces (VA) and electrical repulsive forces (VR)

VT = VA + VR

47
Q

What happens as particles approach each other?rs

A

They will come under the attractive influence of van der Waals forces, which will be opposed by the repulsive forces of overlapping diffuse layers

48
Q

What regions are there between two particles?

A

Four distinct regions between two particles

  • primary minimum
  • primary maximum
  • secondary minimum
  • secondary maximum
49
Q

What happens at primary minimum?

A
  • VA predominates at very low H i.e. when particles are close togther
  • irreversible aggregation occurs - particles clump together to form compact flocs and become sufficiently heavy to settle
    Undesirable in a formulation!
50
Q

What happens at primary maximum?

A
  • occurs within the EDL - the repulsive forces act as an energy barrier to aggregation and particles remain separate from each other
  • after settling, they can be hard to re-disperse
    Undesirable in a formulation!
51
Q

When does primary maximum occur?

A

At intermediate distances (within the thickness of the electrical double layer) due to a high VR. The height (i.e. degree of repulsion) depends on the VR and therefore surface and zeta potential

52
Q

What happens at secondary minimum?

A
  • an open loose structure develops and the particles remain suspended
  • any flocs and partial flocs are large and can usually be broken up by shaking
    Desirable for a formulation!
53
Q

When does secondary minimum occur?

A

At large H because, even though the VA decreases with distance, VR decreases even more sharply. The depth of the trough on the graph depends on particle size

54
Q

What happens at secondary maximum?

A
  • an open loose structure develops and the particles remain suspended
  • flocs cannot form
    Desirable state for a pharmaceutical
55
Q

What are the three states of aggregation (groupings of particles)?

A 
Full flocculation (irreversible)
Deflocculation
Partial flocculation (reversible)
56
Q

What is full flocculation?

A
  • primary minimum phenomenon
  • particles settle very quickly
  • irreversible
  • particles are closely aggregated
  • some continuous phase vehicle is trapped, reducing the amount of free vehicle and making the formulation very viscous and difficult to pour
  • a single large aggregate is formed on standing
  • shaking may be enough to deform the aggregate but it is very difficult or impossible to redisperse (i.e. it is clayed)
57
Q

What is deflocculation?

A
  • primary maximum phenomenon
  • particles remain separate
  • particles are small so settle slowly (Stoke’s Law)
  • VR allow particles to ‘slide’ or ‘slip’ past each other as they sediment
  • these characteristics prevent liquid being trapped in sediment
  • compact sediments of low sedimentation volume are produced (caked)
  • sediment does not easily redisperse on shaking
58
Q

What is flocculation in terms of particles?

A
  • secondary minimum phenomenon
  • formation of loose aggregates (groups) of particles which constantly break up and reform
  • aggregates are relatively large so sedimentation is rapid
  • liquid is trapped within and between aggregates
  • sediment has a high sedimentation volume
  • shaking can re-disperse the sediment and a homogenous system may be obtained
59
Q

What is sedimentation volume determination, F?

A

The ratio of the ultimate volume to the original volume

60
Q

When does sedimentation volume determination arise > 1?

A

When networks of flocs so loose and fluffy that they expand, and so ultimate volume > the original volume

F=Vu/Vo

Vu=equilibrium volume of sediment
Vo=Total volume of suspension

61
Q

Which patients may need to be prescribed an oral liquid medicine?

A

Patients who are unable to swallow oral solids easily
Patients who require a dose which is difficult to deliver accurately through available solid oral dosage forms
Also in certain patients / conditions where a faster onset of action is required vs. solid oral form

MPharm 2 - PH2107 (26 decks)

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