medicinal emulsions

Define medicinal emulsions and explain the different types, routes of administration and methods used to identify emulsion type.

An emulsion is a liquid dosage form consisting of two immiscible liquids in which one liquid is dispersed as fine droplets (0.1–100 μm) within the other. The dispersed phase forms droplets, while the continuous phase surrounds them. Oral emulsions are almost always oil-in-water (O/W) emulsions, whereas external preparations are commonly called lotions, liniments or creams.

The three main emulsion types are:

• Oil-in-water (O/W): oil dispersed in water; used for oral medicines, intravenous emulsions and washable topical creams.

• Water-in-oil (W/O): water dispersed in oil; mainly used as greasy topical creams/ointments and depot intramuscular injections.

• Multiple emulsions (O/W/O or W/O/W): used for modified or delayed drug release.

A microemulsion differs from a crude emulsion because droplet size is only 10–200 nm (approximately 1 nm–1 μm). Microemulsions are transparent, homogeneous, thermodynamically stable and usually contain more than one surfactant.

Methods used to identify emulsion type include:

• Hydrophilic/hydrophobic dye test

• Conductivity test (O/W conducts electricity)

• Miscibility with water or oil

The three main emulsion types

• Oil-in-water (O/W): oil dispersed in water; used for oral medicines, intravenous emulsions and washable topical creams.

• Water-in-oil (W/O): water dispersed in oil; mainly used as greasy topical creams/ointments and depot intramuscular injections.

• Multiple emulsions (O/W/O or W/O/W): used for modified or delayed drug release.

how does a microemulsion differ from a crude emulsion

A microemulsion differs from a crude emulsion because droplet size is only 10–200 nm (approximately 1 nm–1 μm). Microemulsions are transparent, homogeneous, thermodynamically stable and usually contain more than one surfactant

Methods used to identify emulsion type include:

• Hydrophilic/hydrophobic dye test - hydrophilic o/w, hydrophobic w/o

• Conductivity test (O/W conducts electricity)

• Miscibility with water or oil

Explain interfacial tension, emulsion stabilisation and the characteristics of an ideal emulsion and emulsifying agent.

The interface is the boundary between oil and water. Because cohesive forces within each liquid are greater than adhesive forces between the liquids, a high interfacial tension exists, causing the phases to remain separate.

When oil and water are mixed, droplets are initially formed. These droplets rapidly coalesce, reducing surface area and interfacial energy until complete phase separation occurs unless an emulsifying agent is added.

An ideal emulsion should:

• Maintain droplet size and shape.

• Prevent coalescence.

• Prevent creaming and sedimentation.

• Resist flocculation and deflocculation.

• Prevent phase inversion.

• Resist microbial growth.

An ideal emulsifying agent should be:

• Colourless

• Odourless

• Tasteless

• Non-toxic

• Non-irritant

• Effective at low concentrations

• Produce stable emulsions over a wide temperature range

• Prevent creaming, cracking and phase inversion.

what is interfaciial tension

The interface is the boundary between oil and water. Because cohesive forces within each liquid are greater than adhesive forces between the liquids, a high interfacial tension exists, causing the phases to remain separate.

When oil and water are mixed, droplets are initially formed. These droplets rapidly coalesce, reducing surface area and interfacial energy until complete phase separation occurs unless an emulsifying agent is added.

An ideal emulsion should: 6

• Maintain droplet size and shape.

• Prevent coalescence.

• Prevent creaming and sedimentation.

• Resist flocculation and deflocculation.

• Prevent phase inversion.

• Resist microbial growth

An ideal emulsifying agent should be: 10

colourless, tasteless, odorless, non-toxic, non-irritant, effective at low concentrations, produces stable emulsions over a wide temperature range, prevent creaming, cracking and phase inversion

what do emulsfying agents do in interfacial tension

create a a protective barrier around droplets

why aare oral emulsions always o/w

fats or oils or vehicles for oil solubel drugs are pleasent to take. the sugar masks any unplesant taste.

what is total parenteal nutrition

iv use of emulsion strictly for patients that cant feed orally. the disperse phase will solubise many lipohilic vitamins which are present in the oil

examples of tpn and why is it limited

cottonseed olive soya bean, limited due to toxicity concerns except for lechin from egg yolk phospholipids

Explain the formulation and uses of medicinal emulsions for oral, intravenous and topical administration.

Oral emulsions are almost always O/W emulsions because they are more palatable. Oils such as cod liver oil, castor oil and liquid paraffin can be administered more easily when emulsified. Water-soluble flavourings improve taste, while fine emulsification enhances absorption of lipid-soluble drugs if absorbable oils are used.

Common oral emulsifying agents include:

• Polyoxyethylene sorbitan esters (Tweens)

• Acacia

• Tragacanth

• Gelatine

Intravenous emulsions are also O/W emulsions and are used in Total Parenteral Nutrition (TPN).

Typical oils include:

• Cottonseed oil

• Olive oil

• Soybean oil

• Safflower oil

Lecithin is commonly used as the emulsifier because of its low toxicity.

Topical emulsions include:

• O/W creams and lotions (washable)

• W/O creams and ointments (greasy, occlusive)

Paraffins are commonly used as ointment bases. Emulsions also act as carriers for topical drugs such as corticosteroids.

Explain Hydrophilic-Lipophilic Balance (HLB) and Required HLB (RHLB),

HLB (Hydrophilic-Lipophilic Balance) is a numerical scale describing the balance between the hydrophilic and lipophilic properties of a surfactant.

• High HLB = more hydrophilic

• Low HLB = more lipophilic

Examples include:

• Spans (sorbitan esters): HLB 1.8–8.6 (lipophilic)

• Tweens (polyoxyethylene sorbitan esters): HLB 9.6–16.7 (hydrophilic)

Discuss the stability of emulsions, methods of evaluation and factors affecting viscosity.

Emulsion stability is assessed by:

• Macroscopic examination

• Microscopic examination

• Particle size measurement using optical microscopy, Coulter Counter or laser diffraction

• Accelerated stability testing by centrifugation or storage at high and low temperatures

Creaming is calculated as:

Volume of cream/Total volume​

Most pharmaceutical emulsions exhibit non-Newtonian (pseudoplastic) flow rather than Newtonian flow.

Viscosity is affected by:

• Phase-volume ratio

• Particle size (smaller particles increase viscosity)

• Particle size distribution

• Viscosity of dispersed phase

• Emulsifying agent concentration

• Electrical double layer

• Distance between dispersed droplets

Increasing internal phase volume changes flow from:

Explain preservation of emulsions

Because emulsions contain water, they support microbial growth. Proteins, carbohydrates and many natural surfactants provide nutrients for bacteria and fungi.

Microbial contamination may cause:

• Phase separation

• Colour changes

• Gas production

• Changes in viscosity

• Product spoilage

• Infection of patients

Bacteria mainly grow in the aqueous phase, so preservatives must partition correctly into this phase.

An effective preservative should:

• Be un-ionised

• Remain unbound to formulation components

• Be bactericidal rather than bacteriostatic

• Have broad antimicrobial activity

• Have low toxicity

• Maintain activity despite formulation ingredients

Discuss emulsifying agents and explain the properties of glycerol monostearate.

Emulsifying agents reduce interfacial tension between oil and water, allowing stable droplets to form and preventing coalescence.

Surfactants are classified according to HLB:

• Low HLB agents stabilise W/O emulsions.

• High HLB agents stabilise O/W emulsions.

Common emulsifying agents include:

• Tweens

• Spans

• Lecithin

• Acacia

• Tragacanth

• Gelatine

Glycerol monostearate is a non-ionic surfactant.

It is a monosubstituted ester formed from glycerol and stearic acid.

Advantages of non-ionic surfactants include:

• Low toxicity

• Good stability

• Wide pH compatibility

• Reduced irritation

• Suitable for oral, topical and many pharmaceutical emulsions