Emulsions
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- Created by: LBCW0502
- Created on: 08-02-18 12:45
Give examples of pharmaceutical applications of emulsions (5)
Topical delivery (cream), total parenteral nutrition (IV feeding of fat emulsions), oral delivery/taste masking (e.g. cod liver oil), vehicle for drug (propofol) and as a aid for absorption of fat soluble materials from G.I.T
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What is an emulsion?
A 'pseudo-stable' dispersion of at least two immiscible liquids where one is dispersed throughout the other in the form of fine droplets (e.g. o/w) stabilised by an emulsifying agent
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Give examples of emulsifying agents (4)
Proteins/egg yolk (lecithin), gums/carbohydrates (hydrophilic colloids), solid powders (graphite w/o, bentonite o/w), electrolytes (thiocyanates and iodides) - surfactants/co-surfactant
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Describe how an oil-in-water emulsion is produced
Oil and water placed in a container. Add emulsifier and energy input. Oil-in-water emulsion is formed
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What is the size of an oil droplet?
0.1-100 micrometres in diameter (can be 100s nm in special cases)
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How do surfactants as emulsifiers affect interfacial tension?
Aids dispersion of oil and lowers interfacial tension between water and oil droplets (emulsion best stabilised by surfactant combined with non-ionic or co-surfactant)
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Describe the appearance of emulsions (6)
Cloudy/milky solution, polydisperse droplet sizes (range from nm to micrometres), low concentration of surfactant/co-surfactant needed, thermodynamically unstable (metastable), kinetically stable, input of energy (homogenisation/sonication/heat)
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What are the four types of emulsions?
Normal (o/w), reverse (w/o), multiple (o/w/o or w/o/w) and semi-solid (cream)
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Compare the orientation of the surfactant films in a o/w emulsion and in a w/o emulsion
o/w emulsion (head groups on outside, tails adsorbed in oil), w/o emulsion (head groups inwards, tails outwards)
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Describe the thermodynamics of emulsion formation
Large increase in SA between oil and water leads to decrease in interfacial tension with surfactant (but still high). Entropy increases due to dispersion of one phase into another (lower free energy)
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Is emulsion formation thermodynamically favoured?
No
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Is coalescence thermodynamically favoured?
Yes
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What are the three methods used to determine which surfactant to use to stabilise an emulsion?
HLB system, Schulman and Cockbain model and DVLO theory of emulsion stability
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What is the HLB system?
Hydrophilic-lipophilic balance (empirical/experimental). Match HLB of surfactant to HLB of oil. HLB = 7 + (no. of hydrophilic groups) - (no. of hydrophobic groups)
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What does the HLB system not take into account?
Temperature, pH, presence of additives or concentration of surfactant used (not a reliable method to predict emulsion stability)
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Antifoams have which range of HLB values?
1-3.5 (hydrophobic/oil soluble)
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Water-in-oil emulsifying agents have which range of HLB values?
3.5-8 (hydrophobic/oil soluble)
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Wetting and spreading agents have which range of HLB values?
7-9 (slightly water dispersible)
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Detergents have which range of HLB values?
13-16 (more water dispersible/hydrophilic)
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Solubilising agents have which range of HLB values?
15-40 (hydrophilic/water soluble)
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What are the HLB values for Span 80 (sorbitan monooleate), Span 60 (sorbitan monostearate) and Span 20 (sorbitan monolaurate)?
4.3, 4.7 and 8.6
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What are the HLB values for Tween 80, Tween 60 and Tween 20 (polyoxyethylene sorbitan monooleate/monostearate/monolaurate)
15, 14.9 and 16.7
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What is the HLB value for sodium dodecyl sulphate (SDS)?
40
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What type of surfactant is Span 60?
Non-ionic surfactant
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What type of surfactant is Tween 60?
Non-ionic surfactant (more hydrophilic than Span 60 due to structure)
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What type of surfactant is SDS?
Ionic surfactant
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How do you calculate HLB mix?
HLBA(X) + HLBB(1-X) where X is the fraction of A
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Outline the Schulman and Cockbain model
Made large numbers of liquid paraffin in water emulsions. Used single/combination of surfactants/co-surfactants. Found that combinations were more stable. Larger droplets led to smaller curvature (vice versa)
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Is the air-water interface flat/planar?
Yes
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Is the oil-water interface curved?
Yes
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Describe the Langmuir trough experiment
Dissolve (co-)surfactant in volatile solvent (e.g. chloroform). Inject solution onto surface of trough containing pure water. Let solvent evaporate to produce a uniform monolayer of (co)surfactant on pure water surface. Wilhelmy plate/bars (compress)
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What measurements are taken from the Langmuir trough experiment
Surface pressure = surface tension of pure water - surface tension of water in the presence of surfactant film
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What is the surface tension of pure water?
72.8 mN/m
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Describe features of surface pressure isotherm
Able to determine number of molecules added given the amount of (co)surfactant added. Area of monolayer is known. Able to work out average area per molecule of surfactant. Plot surface pressure against area per molecule (surface pressure isotherm)
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What are the four phases for the surface film (2D)?
Condensed, liquid expanded, vapour expanded, gaseous (see graph)
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Describe features of condensed film
Low surface pressure at large area per molecule (area per molecule decreases/no contact). Reaches a specific area per molecule and there is a sudden increase in surface pressure (contact)
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Give examples of co-surfactants which form a condensed film
Cholesterol and dodecanol - large hydrophobic tail (out of aqueous phase), small hydrophilic head group (in water)
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Describe features of gaseous film
Low surface pressure at large area per molecule (gradual increase). Molecules orientate by lying flat on the surface
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Give examples of surfactants which form a gaseous film
Steroids, dibasic esters, ionic surfactants
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What is the intermediate between gaseous and condensed films?
Expanded films - transition due to change in temperature or mixing surfactants
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Describe features of vapour expanded
Large area per molecule (like a gas). When area per molecule decreases, film condenses. Condensation occurs at large areas per molecule (close packing of molecules prohibited by bulky side chains/kink in hydrophobic tail)
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Give an example of a vapour expanded surfactant
Oleyl alcohol (cis configuration)
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Give features of liquid expanded
Molecules don't orientate vertically. Similar trend to vapour expanded but condensation occurs at smaller areas per molecule (large than with condensed film)
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What were the conclusions made from Schulman and Cockbain combining surfactants for emulsion formation?
Gaseous films and vapour expanded films did not form emulsions (molecules far apart to cover whole droplet). Condensed films formed unstable films (molecules too close/can't take up curvature on droplet). Liquid expanded films formed emulsions
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Why do liquid expanded films form stable emulsions?
Molecules cover the whole of the droplet surface and can take up the curvature required to cover the emulsion droplet. Surfactant/co-surfactant mixture forms an efficient packed film to cover whole droplet
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Give pharmaceutical examples of emulsifying agents
Cetrimide emulsifying wax (cetrimide as cationic surfactant, CSA as non-ionic surfactant). Emulsifying wax (SDS as anionic surfactant, CSA as non-ionic surfactant). Non-ionic emulsifying wax (cetomacrogol as non-ionic surfactant, CSA/non-ionic)
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Describe features of the DVLO theory
Considers balance of attractive (van der Waals) and repulsive (electrostatic). Applies to all colloidal systems. Explains coalescence/flocculation or why particles stay separate. Applies to ionic surfactants
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What are the four main types of forces
Van der Waals (attractive), electrostatic (repulsive), solvation forces (repulsive), steric forces (repulsive). Charged colloids (ionic surfactant/van der Waals/electrostatic). Uncharged colloids (non-ionic surfactant/van der Waals/solvation)
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Describe features of van der Waals forces
Describes energy of attraction between pairs of atoms or molecules of neighbouring particles. Energy of attraction varies with the distance (H) between pairs of atoms/molecules/particles
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Describe features of electrostatic forces
Repulsive forces arising from the interaction of the electrical double layers surrounding pairs of particles. Repulsive forces decay exponentially with distance/more rapidly than attractive forces (attractive forces predominate over longer distances)
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What does the combination of the two forces lead to?
V total = sum of attractive force (Va) and repulsive forces (Vr) acting on a pair of particles (or emulsion droplets)
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Describe features of a potential energy diagram
Attractive/repulsive curves. Net energy curve (from combination of forces). Secondary minimum. Primary maximum. Primary minimum
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Describe features of the secondary minimum
At large distances, there is minimal attraction. Attractive forces are weak, don't lead to coalescence of droplets e.g. can be re-dispersed by shaking
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Describe features of the primary maximum
Particles are closer together and experience some repulsion. Height of repulsive force determines stability of system (15-25, slow coagulation/metastability). Height varies with different surfactants/electrolyte concentration etc.
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Describe features of the primary minimum
Close distance, van der Waals forces dominate over repulsive electrostatic forces. Deep primary minimum present. At short inter-particle distance, particles/droplets coagulate irreversibly
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Describe features of electrostatic forces
- charged surfactant bound to + counterions. Surrounded by diffuse layer (double layer). Counterions move with droplet. Plane of shear (outer layer measured). Measure of charge (zeta potential)
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What does the addition of a drug or electrolyte induce?
Neutralisation/reduction of charge in droplets. Decrease in Vmax. Destabilisation of emulsion
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Describe how non-ionic surfactants work?
Stabilise emulsion using either osmotic (solvation) forces or entropic (steric) effects
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Describe osmotic (solvation) forces
Two droplets are in close contact. Concentrated polymer solution in overlap region. Dilute in bulk solution. Water enters concentrated region to dilute it so polymer chains/droplets are forced apart
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Describe entropic (steric) effects
Two droplets in close contact, polymer chains overlap. Loss in freedom of motion of polymer chains (lose entropy). Thermodynamically unfavoured and forces droplets apart again
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Describe features of steric stabilisation
Vt = Va + Vs. Graph. Mixtures of surfactants are used (electrostatic forces and steric forces used) - add to DVLO equation
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Summarise the methods used to determine which surfactants to use
HLB (unreliable). Schulman and Cockbain model (combination used to lower surface tension/free energy). DVLO (ionic surfactants/some non-ionic surfactants used as combination).
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State the five forms of emulsion instability
Phase inversion, creaming, flocculation, coalescence, ostwald ripening
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Describe features of phase inversion
o/w stabilised by surfactants. Add ions, charge on droplet reduced, droplets come into contact. Surfactant film re-aligns to form w/o droplets (increased temperature can cause o/w stabilised by non-ionic surfactants to phase invert)
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Describe features of creaming
Oil droplets concentrate at the top of the emulsion (avoid this by increasing oil density or viscosity of emulsion)
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Describe features of flocculation
When two or more droplets aggregate without losing their individual identity. Larger droplets flocculate faster and flocculation is promoted by creaming
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Describe features of coalescene
When two or more droplets colloid and form one larger droplet (irreversible). Caused by various factors e.g. pH, salt, emulsifier concentration, phase-volume ratio, temperature etc.
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Describe features of ostwald ripening
Emulsions with polydisperse droplets. Collisions between two droplets cause one bigger droplet and one smaller droplet. Repeated collisions. Small droplets become very small/solubilised in continuous medium. Diffuse/re-deposit on large droplets
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Other cards in this set
Card 2
Front
What is an emulsion?
Back
A 'pseudo-stable' dispersion of at least two immiscible liquids where one is dispersed throughout the other in the form of fine droplets (e.g. o/w) stabilised by an emulsifying agent
Card 3
Front
Give examples of emulsifying agents (4)
Back
Card 4
Front
Describe how an oil-in-water emulsion is produced
Back
Card 5
Front
What is the size of an oil droplet?
Back
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