emulsion dosage forms Flashcards
What are some pharaceutical applications of emulsions?
- Topical delivery (creams)
- Total parenteral nutrition (intravenous feeding of fat emulsions)
- Oral delivery and taste masking (cod liver oil)
- As vehicle for drug (emulsions containing propofol)
What is an emulsion?
A ‘pseudo-stable’ dispersion of at least two immiscible liquids, one of
which is dispersed throughout the other in the form of fine droplets
(generally oil and water) stabilised by the presence of an emulsifying
agent (known as emulsifier)
What are some characteristics of emulsions?
- Cloudy or milky
- Droplet sizes are polydispersed (nm to μm)
- Low concentration of surfactant/cosurfactant
- Thermodynamically unstable but kinetically stable
(feasible to separate, but happens very slowly) - Prepared by input of large amount of energy, e.g. high speed
homogenisation, sonication, heat
What are the emulsion types?
- Normal emulsion (o/w)
- Reverse emulsion (w/o)
- Multiple emulsion
(o/w/o or w/o/w)
What are some examples of w/o emulsions?
- w/o emulsions
- Sunscreen
- w/o/w emulsions
- Vancomycin hydrochloride – enhanced enteral bioavailability
What is an example of a o/w emulsion?
- Vehicles for lipophilic drugs
What are creams made up of in terms of emulsions?
Semi-solid emulsions of two immiscible phases stabilised by emulsifying
agent (either o/w or w/o)
What are some characteristics and properties of ‘watery’ (o/w) creams?
- Water is the continuous medium, while oil is the dispersed
phase, therefore o/w creams do not feel greasy - Can deposit lipids so restore skin hydration, however they
are non-occlusive - Rub into the skin, leaving behind a thin film of rapidly
releasing water-soluble drug - Readily diluted and miscible with water; easily rinsed off
- Conduct electricity
- Prepared using surfactants with higher HLB (8-16)
What are some characteristics and properties of ‘oily’ w/o creams?
- Are more greasy, since oil is the continuous phase
- More moisturising as they provide an oily barrier which
reduces water loss from the outer layer of the skin - Hydrophobic drugs are better formulated and more
readily released - Miscible with oil therefore not easily washable, however
more easily spreadable - Do not conduct electricity
- Prepared using low HLB surfactants (3.5-8)
What do semi-solid creams require?
Semi-solid creams require excess surfactant/co-surfactant compared to
that required for stabilising runny emulsions
* The excess surfactants form structures in the bulk phase producing complex
semi-solid multiphase systems
What are the four phases observed in an o/w cream prepared by cetostearatyl alcohol?
- Dispersed oil phase
- Bulk water phase
- Crystalline gel phase
containing interlamellar fixed
water - Phase composed of crystalline
hydrates of cetostearyl alcohol
What are the benefits to a greasier emollient? +downside
Generally, the greasier an emollient, the more effective it is, as it is able to
trap more moisture in the skin
* However greasy emollients are often less acceptable or tolerated
Why are creams better than ointments?
Creams are less greasy but generally more acceptable than ointments
Why are lotions better than creams and ointments?
Lotions are good for very mild dry skin and for the face. They can also be
used in hairy places where the application of ointments or thicker creams can
be quite messy
Why are ointments okay?
Ointments should not be used where an infection is present (unless it is an
antibiotic ointment)
* Over-use of greasy ointments can lead to folliculitis (blockage and inflammation of
hair follicles)
what are the different methods to differ between the two emulsion types?
*dye test
*dilution with water vs oil test
*conductivity
*Fluorescence
How can you differ between o/w and w/o emulsions? (dye test)
Dye solubility test
* Mix emulsion with a water-soluble dye (amaranth) and observed under the
microscope.
* Continuous phase appears red → o/w type
* Scattered globules appear red → w/o type
How can you differ between o/w or w/o? (dilution method)
Dilution of emulsions with water or oil
* If the emulsion is o/w type
* Diluted with water → remain stable as water is the dispersion medium
* Diluted with oil → the emulsion will break
How can you differ between the two different emulsion types? (conductivity)
Conductivity
* Based on the electrical conductivity of aqueous solutions, the electric current is
supplied through electrodes placed in the emulsion
* If the current is passed → o/w type
* Not passed → w/o type
How can you differ between the two emulsion types? (fluorescence)
Fluorescence
* Based on the fluorescence of oils under ultraviolet light, examined under the
light in the microscope
* Whole fluid is fluorescent → w/o type
* Spotty fluorescence → o/w type
What is a micro emulsion?
A system of water, oil, and an amphiphile (surfactant and co-
surfactant) which is a single optically isotropic (same optical properties
in all directions) and thermodynamically stable liquid solution
What do micro emulsions do?
Improves the solubility and bioavailability of drugs such as paclitaxel
(chemotherapy), cyclosporine (immunosuppressant) and acyclovir
(antiviral)
What is the difference between emulsions and micro emulsions?
EMULSION:
* Droplet diameter > 500 nm
Thermodynamically unstable but
kinetically stable
* Inefficient molecular packing
* Direct oil/water contact at the
interface
* High interfacial tension
* High viscosity
MICROEMULSION: Droplet diameter = 10 – 100 nm
*Thermodynamically stable and
long shelf-life
* Efficient molecular packing
* No direct oil/water contact at the
interface
* Ultra low interfacial tension
* Low viscosity with Newtonian
behaviour
What is the HLB system?
Hydrophile-Lipophile Balance (HLB) system
* An empirical approach to aid the choice of surfactant(s) for a particular
purpose, devised by Griffin in 1949
Why do we do the HLB system?
- For emulsification, the strategy is to match the HLB of the surfactants to the
HLB of the oil phase being emulsified - A number of surfactant(s) may possess the same HLB value, therefore
experiments are performed to find the best emulsifying system
How is HLB measured?
The HLB of a surfactant is expressed using an arbitrary scale
* For non-ionic surfactants HLB ranges from 0 to 20
* HLB = 7 + Σ(hydrophilic group no) – Σ(hydrophobic group no)
What does a high HLB of (>10) mean?
- Hydrophilic surfactants
- Act as solubilising agents, detergents and o/w emulsifiers
What does a low HLB of (1-10) mean?
- Lipophilic surfactants
- Act as w/o emulsifiers
What is the best HLB mix?
- Mixtures of surfactants with a high and low HLB usually provide more
stability
What is the HLB balance of antifoaming agents?
2-3
What is the HLB balance of w/o emulsifying agents?
3-6
What is the HLB balance of wetting and spreading agents?
7-9
What is the HLB balance of o/w emulsifying agents?
8-16
What is the HLB balance of detergents?
13-15
What is the HLB balance of solubilising agents?
15-18
