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Pharmaceutical Formulation > Surface Phenomena 4 > Flashcards

Surface Phenomena 4 Flashcards

1
Q

Pharmaceutical applications of surfactants: wetting agents (HLB 7-9)

A
  • adsorb at L/V & S/L interfaces & reduce interfacial tension.
  • lowers contact angle
  • Aids in displacing an air phase with a liquid phase at the surface of a solid.
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2
Q

Pharmaceutical application of wetting

A
  • Displace air from surface of Sulfur, charcoal & otherpowders for dispersing drug powders in liquid vehicles
  • Cotton pad & dressings
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3
Q

Wetting agents

A
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4
Q

What is the Important action of wetting agent

A

Lower the contact angle between the surface and wetting liquid

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5
Q

What is the Contact angle θ?

A

Angle between a liquid droplet and surface over which it spreads. It can be used to describe the wettability of a powder.

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6
Q

How does Wetting agent increases the dissolution rate of a solid
drug?

A
  • aiding penetration of fluid.

- reducing the tendency of particle aggregation, effectively increases surface area of solids exposed to fluid.

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7
Q

What does not wetting agent affect?

A

Wetting agent does not alter solubility of a solid

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8
Q

Pharmaceutical applications of surfactants: solubilising agents

A

eg polysorbates in aqueous formulation of water- insoluble vitamins (Vit A, D E & K); iodophores: iodine- surfactant

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9
Q

Pharmaceutical applications of surfactants: Emulsifying agents

A

eg cetomacrogol emulsifying wax used in lotion or creams.

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10
Q

Pharmaceutical applications of surfactants: Suspension Stabilisers

A

eg polyoxyethylene-polyoxypropylene used for steric stabilisation and controlled flocculation

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11
Q

Pharmaceutical applications of surfactants: detergents

A

eg cetrimide shampoo

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12
Q

Pharmaceutical applications of surfactants: disinfectants

A

Ampholytic surfactant such as Dodicin, derivatives of long chain N-substituted amino acids

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13
Q

Pharmaceutical applications of surfactants: Stabilising Drugs

A

Solubilisation of drug by micelles can also improve the stability of drug. The deeper into the hydrocarbon core of a micelle the drug is located, the more likely it is to be better protected. eg benzocaine vs homatropine.

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14
Q

Base-catalysed Hydrolysis

A
  • Anionic micelles gives an enhanced protection due to the repulsion of the attacking OH- groups.
  • Cationic micelles were found also to be protective, suggesting the positively charged head groups hold the OH- groups thus blocking their penetration into the micelle.

Surface active drugs: eg micellar solution of penicillin G is 2.5 times as stable as monomeric solution

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15
Q

What are the Biological effects of surfactants & drug absorption/bioavailability

A
  • not inert
  • capable of increasing, decreasing or exerting no influence on bioavailability of drug

Affects biological activity of drug by

  • Influencing drug metabolising enzymes or
  • Drug binding to receptors
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16
Q

The ability of surfactant to have influence is dependent upon

A
  • Properties & conc. of surfactant
  • Nature of drug
  • Type of biological membrane is involved.
  • Surfactant may exert drug action itself.
  • eg quarternary ammonium compounds
17
Q

What are emulsions?

A

consist of droplets of one phase (oil or water) dispersed throughout another phase (water or oil)

18
Q

Why are emulsions thermodynamically unstable

A
  • Large surface area due to small size and large number of liquid
    droplets.
  • Interfacial tension between inner (disperse) and outer
    (continuous) phases. (Cohesive force between molecules of each
    separate phase are greater than the adhesive forces between the
    two liquids).
19
Q

What does emulsifying agent improve?

A

Stability can be improved by introducing an emulsifying agent - typically a mixture of surfactants, hydrophilic colloids or fine particles.

Emulsifying agents form films at the interface between the phases – protective layer

20
Q

What types of emulsifying agents?

A

Surfactants - adsorbed at o/w interfaces to form mono-molecular films.

Hydrophilic colloids - form a multi-molecular film around the dispersed droplets of oil in an o/w emulsion.

Finely divided solid particles - adsorbed at the interface between two immiscible liquid phases and form a film of particles around the dispersed globules

21
Q

The ways emulsifying agents may stabilise emulsions

A
  • reduction of interfacial tension –> reduces surface free energy
  • provision of electrostatic repulsion – if emulsifying agent is charged
  • steric stabilisation – prevents droplets from approaching each other
  • some surfactants can also increase the viscosity of emulsions – impedes movement of droplets
22
Q

What is the Surface tension theory?

A

Surface active agents lower interfacial tension of two immiscible liquids; as a result, they facilitate the break up of large droplets into small ones and maintain their dispersion

23
Q

What is the Interfacial film theory?

A

Emulsifying agents adsorb at the interface between oil & water, surrounding the internal phase as a thin layer of film. The film, acting as a physical barrier, prevents the contact and coalescing of the dispersed phase

24
Q

What is the Oriented-wedge theory?

A

Surfactant emulsifying agents form a monomolecular layer which curves around the droplet of the internal (disperse) phase of the emulsion. The emulsifying agents orientate themselves in a manner reflective of their solubility in the particular phase. If it is more water soluble, it would be embedded more deeply in water phase.

25
Q

Emulsifying agents: Surfactants

A
  • Reduce interfacial tension
  • Form a coherent monolayer (interfacial film) at the surface of droplets to reduce the possibility of coalescence – the main factor for emulsion stabilisation
  • Ideally the film should be tough, flexible, closely packed & reforming rapidly if broken or disturbed.
  • Whether or not a surface active agent will stabilise an emulsion depends on the type of film formed at o/w interface.
26
Q

What do Ionic Surfactants provide

A

electrostatic repulsion

27
Q

How does Combinations of emulsifying agents work?

A

A hydrophilic emulsifier in the aq. phase & a hydrophobic agent in the oil phase to form a complex film at interface.

eg combination of sodium cetyl sulfate & cholesterols: Complex film formed is of high viscosity, flexible,resists rupture and gives an interfacial tension lower than that produced by either component alone

28
Q

What are three types of combinations of emulgents

A
  • Anionic Emulsifying Wax : 10% SDS, 90% cetostearyl alcohol
  • Cationic emulsifying wax: 10% cetrimide,90% cetostearyl alcohol
  • Non-ionic emulsifying wax: 20% cetomacrogol 1000, 80% cetostearyl alcohol
29
Q

How are Hydrophilic colloids as emulsifying agents?

A
  • proteins (eg gelatin) or polysaccharides (eg Acacia, methylcellulose).
  • Exhibit little surface activity.
  • Adsorb at the o/w interface but do not cause an appreciable lowering of interfacial tension.
  • Form multilayers or multi-molecular film at the interface - mechanical barriers to coalescence.
  • If they contain ionic group (eg proteins and Acacia), they also provide electrostatic repulsion as an additional barrier.
  • Tend to promote the formation of o/w emulsion (hydrophilic
    nature)
  • Increase the viscosity of an emulsion to improve its stability.
  • Examples: Liquid paraffin emulsions (Acacia & methylcellulose mucilage as emulsifiers)
30
Q

How are Solid particles as emulsifying agents?

A
  • Finely divided solid particles that are wetted to some degree by both oil and water can act as emulsifying agents.
  • They are concentrated at the interface and form a particulate film around dispersed droplet to prevent coalescence.
  • Those wetted preferentially by water form o/w emulsions, and vice versa.
  • eg aluminium and magnesium hydroxides or bentonite are preferentially wetted by water thus form/stabilise o/w emulsions

Pharmaceutical Formulation (3 decks)

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