Surface Phenomena 4 Flashcards
Pharmaceutical applications of surfactants: wetting agents (HLB 7-9)
- 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.
Pharmaceutical application of wetting
- Displace air from surface of Sulfur, charcoal & otherpowders for dispersing drug powders in liquid vehicles
- Cotton pad & dressings
Wetting agents
What is the Important action of wetting agent
Lower the contact angle between the surface and wetting liquid
What is the Contact angle θ?
Angle between a liquid droplet and surface over which it spreads. It can be used to describe the wettability of a powder.
How does Wetting agent increases the dissolution rate of a solid
drug?
- aiding penetration of fluid.
- reducing the tendency of particle aggregation, effectively increases surface area of solids exposed to fluid.
What does not wetting agent affect?
Wetting agent does not alter solubility of a solid
Pharmaceutical applications of surfactants: solubilising agents
eg polysorbates in aqueous formulation of water- insoluble vitamins (Vit A, D E & K); iodophores: iodine- surfactant
Pharmaceutical applications of surfactants: Emulsifying agents
eg cetomacrogol emulsifying wax used in lotion or creams.
Pharmaceutical applications of surfactants: Suspension Stabilisers
eg polyoxyethylene-polyoxypropylene used for steric stabilisation and controlled flocculation
Pharmaceutical applications of surfactants: detergents
eg cetrimide shampoo
Pharmaceutical applications of surfactants: disinfectants
Ampholytic surfactant such as Dodicin, derivatives of long chain N-substituted amino acids
Pharmaceutical applications of surfactants: Stabilising Drugs
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.
Base-catalysed Hydrolysis
- 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
What are the Biological effects of surfactants & drug absorption/bioavailability
- 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
The ability of surfactant to have influence is dependent upon
- Properties & conc. of surfactant
- Nature of drug
- Type of biological membrane is involved.
- Surfactant may exert drug action itself.
- eg quarternary ammonium compounds
What are emulsions?
consist of droplets of one phase (oil or water) dispersed throughout another phase (water or oil)
Why are emulsions thermodynamically unstable
- 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).
What does emulsifying agent improve?
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
What types of emulsifying agents?
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
The ways emulsifying agents may stabilise emulsions
- 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
What is the Surface tension theory?
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
What is the Interfacial film theory?
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
What is the Oriented-wedge theory?
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.
Emulsifying agents: Surfactants
- 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.
What do Ionic Surfactants provide
electrostatic repulsion
How does Combinations of emulsifying agents work?
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
What are three types of combinations of emulgents
- 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
How are Hydrophilic colloids as emulsifying agents?
- 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)
How are Solid particles as emulsifying agents?
- 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
