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Form & Tech II > Suspensions > Flashcards

Suspensions Flashcards

1
Q

Define suspension

A

Dispersion of finely divided solid particles in a liquid medium. The liquid medium may be aqueous or non-aqueous

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2
Q

Distinguish between suspension and emulsion

A
  • Suspensions: solid particles in liquid medium

- Emulsions: 2 immiscible liquids stabilised by surfactants

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3
Q

The two broad categories of suspensions and their solid particle size?

A
  1. Coarse suspension: particle size >1 micron

2. Colloidal: particle size ≤ 1 micron

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4
Q

List some applications of suspensions as a pharmaceutical preparation

A
  1. PO
    - Administer insoluble drugs in liquid form
    - Mask distasteful drugs
    - Effective in administering insoluble compounds for absorptive or antacid properties to treat gastric disorders
  2. Injection: depot therapy (particle sdissolve slowly)
  3. External uses such as topical applications
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5
Q

List the ideal/desirable properties of suspensions

A
  1. Chemically stable
  2. Low sedimentation rate of particles
  3. Particles readily re-dispersed upon gentle shaking of container
  4. Constant particle size throughout storage
  5. Readily poured from container
  6. For injection: flow readily through syringe needle
  7. External use: spread easily, but not too mobile so that it will not run off surfaces when applied
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6
Q

What are the factors affecting the rate of sedimentation (Hint: Stoke’s Law)

A

Using Stoke’s law, velocity of the fall/rise of particle is:

  • Proportional to: radius of particles and difference in densities between the particle and the dispersion medium
  • Inversely proportional to the viscosity of the dispersion medium
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7
Q

What are floccules?

A

Solid particles in a suspension stick together

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8
Q

Distinguish between flocculated suspension and deflocculated suspension in terms of:

  1. Nature of dispersed phase
  2. Sedimentation rate
  3. Nature of final sediment
  4. nature of supernatant liquid
  5. Rheological property of concentrated suspension
A

Flocculated VS deflocculated suspension:

  1. Aggregates VS individual particles
  2. Rapid VS Slow
  3. Porous sediment on prolonged standing with relatively large volume and easy to re-disperse VS compact sediment, small volume and difficult to re-disperse
  4. Clear VS Cloudy
  5. Plastic flow with thixotropy VS dilatant flow
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9
Q

Name the two main components required in the formulation of a suspension, and list some other additives that could be added to the preparation

A

Main components:
- Dispersed phase, liquid vehicle

Other additives:
- Wetting agent, flocculating agent, thickening agent, pH buffer, preservative, colorant, flavour and fragrance

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10
Q

The dispersed phase of a suspension is also known as

A

Suspensoid

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11
Q

What are the two types of insoluble solid particles in the dispersed phase of a suspension, and what are their properties?

A
  1. Lyophilic solids:
    - No need wetting agent to disperse in liquid vehicle
    - May increase viscosity of suspension
  2. Lyophobic solids:
    - Require wetting agent for dispersion in liquid vehicle
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12
Q

What is the function of a wetting agent, and how does it carry out its function?

A

Function: Make solid particles more penetrable by dispersion medium

Modes of action:
1. Reduce solid-liquid interfacial tension
2. Imparts hydrophilicity to particle surface
which aids water in displacing the air pockets surrounding the crevices of drug particles

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13
Q

Wetting agents are usually pre-mixed with solid particles in preparation of suspensions.

What are some categories of wetting agents and some examples for each of the categories?

A
  1. Surfactants: HLB 7-9
    - E.g. Spans, used at concentration below CMC
  2. Hydrophilic polymers
    - E.g. acacia, tragacanth, xanthan gum, povidone
  3. Polyhydric alcohols
    - E.g. glycerin, PEG, propylene glycol
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14
Q

List the three flocculating agents

A
  1. Electrolytes
  2. Surface active agents
  3. Hydrophilic polymers
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15
Q

In terms of zeta potential and other forces of attraction, explain microscopically what happens to solid particles in a suspension, if it was initially deflocculated

A
  • Initially, zeta potential that causes interparticulate repulsion is more than the van der waals attractive forces, hence particles are repelled from each other and are deflocculated
  • When low concentration of electrolytes are added, zeta potential is reduced below critical value, such that attractive VDW forces > repulsion by zeta potential, hence causing aggregation of particles and flocculation
  • When high concentration of electrolytes are added, the polarity of zeta potential is reversed, causing a magnitude of interparticulate repulsion > VDW attractive forces, forming deflocculated suspension
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16
Q

What does Schulze-Hardy rule state about the relationship between effectiveness of electrolytes in aggregating particles and its valency?

A

Effectiveness of electrolyte in aggregating particles increases with valence of the ions (E.g. +2 ions are 10 times more effective than +1 ions)

Hence monovalent ions are sufficient to induce flocculation of weakly charged particles

17
Q

What are the two types of surface active agents used as flocculating agents in a suspension?

A
  1. Ionic agents (which behaves like electrolytes)

2. Non-ionic agents

18
Q

How does non-ionic surface active agents act as a flocculating agent?

A
  • Used at a concentration above CMC
  • Micelles formed and adsorbed at the surface of particles where they serve as bridge between particles, hence causing flocculation by bridging mechanism
19
Q

How do hydrophilic polymers function as a flocculating agent?

A

Bridging mechanism

20
Q

Explain microscopically what happens to the solid particles in a suspension as hydrophilic polymers are added

A
  • At low conc. (<0.05%), hydrophilic polymers adsorbed at the surface of particles, serving as bridges between particles hence causing flocculation
  • At higher concentration (0.05 - 0.1%), the polymers forms a protective coat around each drug particle, separating them from one another. This is known as protective colloid effect
  • At even higher concentrations (>0.1%), polymers increases the viscosity of the medium by forming a network in the liquid medium of the suspension, improving stability but reducing flowability of the suspension (viscosity-enhancing effect)
21
Q

What is the purpose of thickening agent in suspensions?

A

Assist the suspension of solid particles in the suspension

22
Q

Which variable in the Stokes’ law is modified when a thickening agent is added into a suspension?

A

Viscosity of the medium

23
Q

What are the two techniques employed in preparing a suspension?

A
  1. Dispersion technique

2. Precipitation technique

24
Q

Why are colloid mills not very appropriate in the large-scale manufacture of suspensions?

A

Impart high shear, which can cause the size of the drug particle to be modified hence affecting the drug’s release profile

25
Q

pH-induced precipitation technique is only applicable to drugs whose solubility is affected by _________

A

pH

since technique employs pH-induced precipitation

26
Q

What are some ways in which a suspension can be evaluated?

A
  1. Stability

2. Re-dispersibility

27
Q

Outline the method used to determine the stability of a suspension

A
  • Freshly prepped suspension is transferred to a measuring cylinder, and volume of sediment determined at different time intervals

F = Vt/Vo, where F is index of stability, Vt is volume of sediment at time t and Vo is initial volume of total suspension

F value starts with 1 and decreases as particles start to sediment

28
Q

On a graph of F value against time, the line with the steepest gradient corresponds to the more or less stable suspension?

A

Less stable

29
Q

Outline the method used to determine the redispersibility of a suspension

A
  • Suspension first left to settle down for awhile
  • Mechanical device shake suspension
  • Number of shakes required to disperse sediment determined
  • Ease of redispersibility is inversely proportional to the number of shakes required to disperse the sediment

Form & Tech II (6 decks)

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