Lecture 4 - Disperse Forms (Part 2 - Emulsions, Colloidal dispensions, Gels, & Use of Dispense systems) Flashcards
What are Emulsions?
Dispersion of small droplets of liquid in another immiscible liquid
- Emulsifying agent often a third phase
- If continuous phase aqueous o/w emulsion
- If continuous phase oleaginous then w/o emulsion
What can Emulsions be?
May be oral, topical, parenteral, ophthalmic
- Most are topical
- Not common for oral use
- Topical may be o/w or w/o and medication usually in internal phase
- w/o more occlusive; o/w better drug release (more protective/blockage)
What is the stability of Emulsions?
physically UNstable
Will revert to two immiscible phases
- Creaming when internal phase droplets merge and rise to top (o/w) or settle to bottom (w/o)
- After creaming has occurred can re-disperse with shaking
- Cracking or breaking is coalescence or internal phase and irreversible separation
May stabilize an emulsion through use of surfactants, hydrocolloids or solid particles
What are Surfactants?
Hydrophilic and lipophilic portions of molecule and concentrate at oil : water interface
What do Surfactants do?
Lowers interfacial tension between phases which reduces repellant forces between them and reduces attractive forces within each phase
- Facilitates breaking of globules into smaller droplets
- Surfactants may be anionic (sodium docusate, sodium oleate) or non-ionic (Spans and Tweens)
- Cationic (quaternary ammonium halides) seldom used due to irritation and toxicity in amount required
What are Hydrocolloids?
Provide protective multi-molecule layers around dispersed droplets (aim: have internal phase be protects & separated/suspended)
What can Hydrocolloids provide?
- May provide a charge which causes droplets to repel each other
- May swell to increase viscosity of system
- Hydrocolloids may be natural (gelatin), semi- synthetic (methylcellulose) or synthetic (Carbopol)
What are solid particles?
Tend to form layer around droplet and swell
Increases viscosity and reduces attraction between droplets
What do solid particle agents include?
Bentonite
Aluminum hydroxide
Magnesium trisilicate
Occasionally an auxiliary agent may be used
- Not emulsifying agents and tend to thicken
- Includes fatty acids (stearic acid), fatty alcohols (cetyl alcohol) and fatty esters (glyceryl monostearate)
What can occasionally be used in solid particles?
Occasionally an auxiliary agent may be used
- Not emulsifying agents and tend to thicken
- Includes fatty acids (stearic acid), fatty alcohols (cetyl alcohol) and fatty esters (glyceryl monostearate)
boost stability of emulsions
What is Hydrophilic-lipophilic balance (HLB)?
System designed to aid in determining type and amount of emulsifier needed to prepare stable emulsions
- HLB values experimentally determined and on a scale of 1 to 18 (tells how much hydrophilic & lipophilic is in it)
- Low HLB values indicate emulsifier oil-soluble so oil phase will predominate and w/o emulsion will form
- HLB values also assigned to oils and oil-like materials
lower - oily
higher - water-based product
How should you select an HLB?
Select a blend of emulsifying agents with HLB value close to that of the oil phase
What is HLB of a mixture of 20 g Span 20 (HLB 8.6) and 5 g of Tween 21 (HLB 13.3)?
HLB = (20 g)(8.6) + (5 g)(13.3) = 9.54 (20 g + 5 g)
HLB = (Qty surfactant 1)(HLB surfactant 1) + (Qty surfactant 2)(HLB surfactant2) (Qty surfactant 1 + Qty surfactant 2)
What is the method of preparation?
In industry done with large tanks and high-speed homogenizers or colloid mills
Energy must be put into the system
Homogenization
High-speed sheer
Trituration
Four general methods for extemporaneous preparation:
Continental
English
Bottle
Beaker
What is the Continental method?
Primary emulsion from oil (4 parts), water (2 parts), hydrocolloid or gum emulsifier (1 part)
- Acacia generally used as gum and porcelain mortar and pestle
- Acacia is levigated with the oil until they are thoroughly mixed and then the water is added all at once with vigorous trituration
- Additional solid materials are dissolved in some of the remaining water if they are soluble; oil soluble material would have been dissolved in the oil.
- Preparation transferred to a measuring cylinder and made to volume then mixed
What is the English method?
PROPORTIONS of materials for the primary emulsion are the SAME but the ORDER OF MIXING DIFFERENT
- The gum is triturated with the water to make a mucilage then the oil is added in portions with vigorous mixing
AKA wet gum method - add water 1st to wet gum instead of last
What is the bottle method? When does it work best?
- Works best if oil has low viscosity
- Same proportions of oil, water and gum (1:2:4)
- Acacia put into bottle, oil added and vigorously mixed then water and vigorous mixing
- Method can also refer to mixture of lime water and oil as it forms an in situ soap
What is the Beaker method?
With synthetic emulsifiers only about 5% is needed
- All the water soluble materials are dissolved in the water and all the oil soluble materials dissolved in the oil using beakers
Both solutions heated to about 70°C on a water bath
- Internal phase added to the external phase with
mixing and stirred until cooled to room temperature
Sometimes hand-held blender or homogenizer helpful
