Emulsions and Creams Flashcards
What is a emulsion
A homogeneous liquid or semisolid preparation consisting of TWO immiscible liquids (oil and water) mixed together by the addition of an emulsifying agent
- one of the liquids (dispersed phase) is uniformly distributed as droplets throughout the other (continuous phase)
- o/w: oil droplets dispersed in aqueous continuous phase
- w/o: water droplets dispersed in oil continuous phase
- multiple emulsions, microemulsions, nanoemulsions
What are microemulsions and nanoemulsions?
- o/w nanoemulsions: used in cosmetics, pharmaceutical, food, agricultural industries as delivery systems for active lipophilic compounds and drugs
- Translucent nanoemulsions: extremely small droplet size provides long-term stability, high concentrations of active, solubilize lipophilic actives, improves skin absorption and enables the delivery of exceptionally high concentrations of active substances.
- High-power ultrasound and other techniques used in the preparation.
How to identify emulsions?
- Miscibility tests: add small quantity of emulsion to water or oil – will mix only if continuous phase is same as solvent
- Microscopic examination after staining with oil soluble dye (eg Scarlet Red: w/o -red; o/w – pink) or water soluble dye such as methylene blue
- Conductivity measurement: o/w will conduct electricity
What are types of emulsion instability?
- Creaming: dispersed phase rises or sediments - reversible
- Cracking/coalescence: irreversible (may be caused by any chemical, physical or biological effect that changes the nature of the interfacial film)
- Phase inversion: o/w becomes w/o or vice versa
What are some examples of oral emulsions? What are their advantages and disadvantages?
- Milk
- Rationale for pharmaceutical emulsions
- Oil in water (o/w)
General: oil phase divided into minute droplets, coated with emulsifying agent, then suspended in aqueous phase.
> Used for flavouring, colouring presrevative
Advantages
- Formulation of unpalatable drugs
- Aqueous phase easily flavoured
- Increased rate of absorption
- Can incorporate 2 incompatible ingredients
- Useful for enteral nutrition: high calorie
Disadvantages
- Require effective shaking before use
- Storage conditions may affect stability
- Liable to microbial contamination which can cause instability
What are some components of oral emulsions?
Emulgents
- Emulsifying gum eg acacia –
- Methylcellulose, magnesium hydroxide, aluminum hydroxide
Oil
- Fixed oils eg Arachis, castor, cod liver oil
- Mineral oils eg liquid paraffin
- Volatile oils eg cinnamon, peppermint oil
Water
- Freshly boiled and cooled purified water
- Preservative, flavoring, coloring agents
How to prepare oral emulsions?
- A thick, primary emulsion must by firstly prepared (ratios for this is in attached image)
- Wet gum method: triturate the acacia with water to produce a mucilage. Then add oil.
- Dry gum method: triturate acacia with oil first, just for a short time, then add all the water at once and triturate thoroughly
What are some examples of topical formulations and external emulsions
See attached image
- o/w and w/o emulsions
- Liniments, lotions, creams, foams
What are some formulation components of external emulsions?
Aqueous phase: Water, alcohol, preservative (e.g. benzoic acid, chlorocresol, phenoxyethanol, cetrimide, parabens)
Oil phase: – Mineral oils eg liquid paraffin – Vegetable oils eg arachis, coconut, olive oil – Synthetic oils eg dimethicone
Emulgent: determined using the HLB method by the type and quantity of oil present in the formula
Why mix both phases after warming
- Used for waxes that require heat to melt
- Emulgent added to phase in which miscible or soluble
- Water temperature slightly above oil temperature (≈65C)
- Phases mixed – water generally added to the oil and stirred to cool
- Slow cooling to prevent granular product
- Adjust to weight
How does emulsification by precipitation happen? Provide an example
- Oil dissolved in solvent such as ethanol containing emulgent
- Solution added to large volume of water
- Emulsion formed
Example: Coal tar solution to water with polysorbate 80 (Tween 80)
> Coal tar solution is 20% coal tar in alcohol with polysorbate 80
What is the method of adding the emulgent?
- Add in water phase or oil phase – dependent on the solubility of the emulgent
- Nascent soap (formed in situ)
> Formed from combination of fatty acid (in oil) and alkali (in aqueous phase)
> Type of soap formed determines o/w or w/o emulsion as product
> Eg potassium oleate – o/w
> Eg calcium oleate – w/o
What are some properties of emulsifying agents?
Surface-active agents or surfactants
- Polar group = hydrophilic
- Non-polar group = lipophilic
HLB: hydrophilic-lipophilic balance of surfactant effects type of emulsion:
- high HLB – high hydrophilicity
- low HLB – high lipophilicity
Ideal properties: colourless, odourless, tasteless, non-toxic, non-irritant, form stable emulsions at low conc
Uses: topical formulations, parenteral formulations, solubilisers, cleaning/disinfecting
What are the types of emulsifiyng agents?
Synthetic or semi-synthetic emulgents
- Anionic, cationic, non-ionic, ampholytic surfactants
Natural products
- Polysaccharides, sterols
Finely divided solids
- Natural clays, bentonite
- Preferential wetting determines emulsion type
What are anionic surfactants? Provide some examples?
Dissociate in aqueous solution to form large surface active anion (negative)
- Alkali metal soaps
- Ammonium soaps
- Di and tri-valent metallic soaps
- Amine soaps
- Alkyl sulphate soaps
Discuss the properties and disadvantages of alkali metal soaps –> monovalent soaps (anionic surfactants)
Formed from monovalent metal (eg Na, K) or ammonium and long-chain fatty acid (C12-18)
> E.g linoleic, stearic acid
General formula: RCOO- M+
- Soluble or dispersible in water
- Form o/w emulsions
- Form alkaline emulsions (pH>8)
Disadvantages
- Require preservative and humectant (PG, glycerol)
- Sensitive to high temperature, high concentration of electrolytes, acids
- Toxic and unpleasant taste: external use only
- Incompatible with cationic actives (eg many antiseptics)
Discuss the properties and disadvantages of di and trivalent metallic soaps (anionic surfactants)
- Ca++, Mg++, Al+++ and Zn++ salts of fatty acids are w/o emulsifying agents
- Only Ca salts tend to be commonly used: –> Ca(OH)2 – lime water
General formula: (RCOO-)2 M2+
- Hydrophobic group dominant: w/o emulsions
- Do not need preservative
- Require stabiliser to form good quality emulsion eg wool fat
Disadvantages and incompatabilities
- Require humectant as water is internal phase
- Less alkaline and less sensitive to acid than monovalent soaps but high conc acids problem
- Incompatible with cationic actives (eg many antiseptics)
- Incompatible with o/w emulgents
- External use only
What are some examples of anionic emulgent based emulsion formulations
- Oily Calamine Cream BP
- Oily Calamine Lotion APF
- Zinc Cream BP
- Zinc Cream Oily APF
- Glycerol Cream Oily APF
- Aluminium Acetate Cream Oily APF
- Cold creams
Discuss the properties and disadvantages of amine soaps (anionic surfactants)
- Organic amines (eg. Triethanolamine) form salts with fatty acids
- Form fine-grained o/w emulsions pH 7.5-8
- More stable to pH, acids than monovalent soaps
- Superior emulgent than monovalent soaps
- Emulsions softer than NaOH, KOH based soaps – tend to be more widely used in cosmetics
Disadvantages and incompatabilities
- Similar to monovalent soaps
- More resistant to acids and electrolytes
- Incompatible with high conc NaCl – salts out soap and breaks emulsion
- Incompatible with cationic actives (eg many antiseptics)
- External use only
Discuss the properties of alkyl sulphate soaps (anionic surfactants)
- Esters of fatty alcohols with sulphuric or phosphoric acids, then neutralised with NaOH
General formula: R.O.SO2.Na
- Eg sodium lauryl sulphate C12H25OSO2Na (lauryl alcohol from coconut oil), sodium cetostearyl sulphate
- o/w emulsions of low stability: need stabiliser (fatty alcohol) – form high quality, stable emulsion
Aqueous cream is anionic, what is found in aqueous cream?
What are some properties of emulsions of emulsifying wax
- Compatible with Ca, Mg ions (hard water)
- Neutral – tolerate pH change
- Do not withstand heat (eg for sterilization): hydrolysis of ester linkage
- Incompatible with cationic agents
Discuss the properties and incompatibilities of cationic surfactants
See attached image for ingredients for cetrimide cream aqueous APF (cationic)
- Quaternary ammonium compounds
- o/w emulsifying agents
- General formula: (R1R2R3R4N)+. X-
- Eg. Cetrimide: C16H33CH3CH3CH3N+.Br -
- Emulsifying power poor if used alone
- Used in combination
Properties
- Water soluble (use hot water to reduce froth)
- Bactericidal (also benzalkonium chloride – antibacterial and preservative, not emulgent)
- Relatively toxic: external use only
- Stable pH 3-7 therefore good skin compatability
- Unaffected by di and trivalent cations
Incompatabilities
- Anionic agents, iodine, alkali hydroxides
- Strong w/o non-ionic emulsifiers
- Bacteriocidal activity reduced by Cetomacrogol emulsfiying wax
What are non-ionic surfactants?
- Balanced hydrophilic and lipophilic groups
- Minimal ionisation in solution
- Compatible with anionic and cationic
- Resistant to electrolytes, pH change, polyvalent cations
- Tend to inactivate preservatives (incompatible) with phenolic or carboxylic acid groups
- Esters, ethers and amides
