Emulsions: Evaluation methods, IP emulsion and MCQs for GPAT, NIPER, Pharmacist and Drug Inspector exam
EVALUATION TESTS :
1.Determination of particle size and particle count: It is performed by optical microscopy, sedimentation by using Andreasen apparatus and Coulter counter apparatus.
2.Determination of viscosity: For viscous emulsions, the use of penetrometer instrument is used.
3.Determination of phase separation: This is another parameter used for assessing the stability of the formulation. Phase separation may be observed visually or by measuring the volume of the separated phases.
4.Determination of electrophoretic properties: Determination of electrophoretic properties like zeta potential is useful for assessing flocculation since electrical charges on particles influence the rate of flocculation. O/W emulsion having a fine particle size will exhibit low resistance but if the particle size increase, then it indicates a sign of oil droplet aggregation and instability.
ASSESSMENT OF EMULSION SHELF LIFE: No quick and sensitive methods for determining potential instability in an emulsion are available to formulators. Instead, they are forced to wait for interminable periods at ambient conditions before signs of poor shelf life become clearly apparent in an emulsion. To speed up the stability program, formulators commonly place the emulsion under some sort of stress. Alternately, they may seek a test or parameter that is more sensitive for the detection of instability than mere macroscopic observations. Both approaches may be faulty. The first one may eliminate many good emulsions because excessive artificial stress has been applied and it will speed up the abnormal processes involved in instability. The second one may eliminate only those emulsions that are extremely poor unless the parameter correlates well with shelf life. It is therefore essential to use sound judgment and great care in setting up a meaningful stability program for a given emulsion.
Stress Conditions – Stress conditions normally used for evaluating the stability of emulsions include (1) ageing and temperature, (2) centrifugation and (3) agitation.
1.Ageing and temperature – The Arrhenius equation, which predicts that a 10°C increase in the temperature doubles the rate of chemical reaction, is not applicable to emulsions. In case of emulsions, exposures to unrealistically high temperatures bring into play new reactions that may produce meaningless results. It is clearly established that many emulsions may be perfectly stable at 40 or 45°C but cannot tolerate temperatures in excess of 55 or 60°C even for a few hours. A particularly useful means of evaluating shelf life is cycling between two temperatures. Again, extremes should be avoided, and cycling should be conducted between 4 and 45°C. This type of cycling approaches realistic shelf conditions but places the emulsion under enough stress to alter various emulsion parameters. From practical aspects, an emulsion should be stable for at least 60–90 days at 45 or 50°C, 5–6 months at 37°C and 12–18 months at room temperature. Similarly, an emulsion should survive at least six or eight heating/cooling cycles between refrigerator temperature and 45°C, with storage at each temperature of no less than 48 h.
2.Centrifugation – It is commonly accepted that shelf life under normal storage conditions can be predicted rapidly by observing the separation of the dispersed phase due to either creaming or coalescence when the emulsion is exposed to centrifugation. Stokes’ law shows that creaming is a function of gravity, and an increase in gravity therefore accelerates separation. Centrifugation, if used judiciously, is an extremely useful tool for evaluating and predicting shelf life of emulsions. Centrifugation at 3750 rpm in a 10-cm radius for a period of 5 h is equivalent to the effect of gravity for about 1 year. On the other hand, ultracentrifugation at extremely high speeds (approximately 25,000 rpm or more) can be expected to cause effects that are not observed during normal ageing of an emulsion. From practical aspects, a stable emulsion should show no serious deterioration by centrifuging at 2000–3000 rpm at room temperature.
3.Agitation – It is a paradigm of emulsion science that the droplets in an emulsion exhibit Brownian movement. In fact, it is believed that no coalescence of droplets takes place unless droplets impinge upon each other owing to their Brownian movement. Simple mechanical agitation can contribute to the energy with which two droplets impinge upon each other. The emulsion should not be adversely affected by agitation for 24–48 h on a reciprocating shaker (~60 cycles per minute at room temperature and at 45°C). However, such an evaluation of emulsion by agitation is rarely appreciated.
During the testing period as described previously, the samples stored at various conditions should be observed critically for separation and, in addition, monitored at reasonable time intervals for the following characteristics:
- Change in electrical conductivity
- Change in light reflection
- Change in viscosity
- Change in particle size
In addition to these physical measurements, a shelf-life program for emulsions should include testing of the emulsion for microbiologic contamination at appropriate intervals.
Quality control testing for emulsions:
Most dosage forms containing active pharmaceutical ingredients require chemical and physical tests to ensure potency, purity and stability of the active pharmaceutical ingredient.
Suspension and emulsion dosage forms typically require various physical tests to ensure consistent particle size and distribution as well as in-use tests to ensure that patients will be able to use the drug product effectively over the shelf-life of the product. Suspensions and fluid emulsions are generally tested for average particle size as well as some measure of particle size distribution. The latter could be a statistical parameter, such as standard deviation, but may also be a limit on the upper or lower particle size, such as “99% of particles must be less than X micrometers.” Rheological properties such as viscosity are also useful because they indicate ease of use, such as spreadability for a topical emulsion or suspension.
For a parenteral suspension or emulsion, ease of injectability may also be a valuable test because the product may need to consistently be injected through a 21–27 Ga needle. Changes in viscosity, particle size or floccule size may alter the ability to effectively use the product. Other physical tests may include sedimentation rate/volume, zeta potential, redispersibility, pH, color, and possibly microscopic appearance. Microbiological testing may also be conducted, not only for sterile suspensions and emulsions, but whenever the product is capable of supporting microbial growth.
When an antimicrobial preservative is required for the product, preservative effectiveness testing should be conducted and the product must be tested to be sure adequate concentrations of the preservative are present throughout the product shelf-life. It is also important to demonstrate dose uniformity of the suspension or emulsion by testing the drug content of aliquots of product that simulate dose volumes used by the patient.
For suspension dosage forms, a dissolution test will most likely need to be performed because dissolution rate is an important step controlling rate and extent of absorption for poorly water soluble drugs. As with other pharmaceutical dosage forms, chemical, physical and microbiological tests are performed at initial release of the product and during product stability studies. Key physical tests, such as particle size and viscosity, can also be conducted during manufacture of the product to ensure that the manufacturing process is producing product with acceptable physical characteristics.
IP emulsions:
Simethicone IP – Simethicone IP/BP/USP is also known as Activated Dimethicone. The chemical is used in the preparation of antacid where it acts as the anti-flatulent agent. The clinical use of this chemical is based upon its anti- foam property that is beneficial in the fermentation process to control the foam.
Uses –
- Used as anti-flatulent agent to relieve symptoms referred to as gas
- Used as anti-foaming agent in percolation, maceration and fermentation
- Used as defoaming/ deareating compound in various application or as an anti-flatulating agent in antacid preparation.
Multiple choice questions:
1.Which of the following evaluation tests are done for emulsions?
a)Determination of particle size and particle count
b)Determination of viscosity
c)Determination of phase separation
d)All of these
2.Determination of particle size and particle count of emulsions is performed by
a)Optical microscopy
b)Andreasen apparatus
c)Coulter counter apparatus
d)All of these
3.Determination of viscosity of emulsions is performed by
a)Optical microscopy
b)Andreasen apparatus
c)Coulter counter apparatus
d)Penetrometer
4.Phase separation may be observed
a)visually
b)by measuring the volume of the separated phases
c)both of these
d)none of these
5.Determination of electrophoretic properties like zeta potential is useful for assessing flocculation since electrical charges on particles influence the rate of flocculation.
a)true
b)false
6.O/W emulsion having a fine particle size will exhibit low resistance but if the particle size increase, then it indicates a sign of oil droplet aggregation and instability.
a)true
b)false
7.Stress conditions normally used for
a)evaluating the stability of emulsions
b)evaluating the viscosity of emulsions
c)evaluating the flow properties of emulsions
d)all of these
8.Which of the following stress conditions are used for emulsions?
a)ageing and temperature
b)centrifugation
c)agitation
d)all of these
9.The Arrhenius equation, which predicts that a 10°C increase in the temperature doubles the rate of chemical reaction, is not applicable to emulsions.
a)true
b)false
10.An emulsion should survive at least ____ heating/cooling cycles between refrigerator temperature and 45°C, with storage at each temperature of no less than 48 h.
a)2-4
b)6 or 8
c)10
d)12
11.From practical aspects, a stable emulsion should show no serious deterioration by centrifuging at ____ rpm at room temperature.
a)500-1000
b)1000-2000
c)2000-3000
d)3000-5000
12.It is a paradigm of emulsion science that the droplets in an emulsion exhibit ____ movement.
a)brownian
b)streamline
c)layered
d)none of these
13.During the testing period as described previously, the samples stored at various conditions should be observed critically for separation and, in addition, monitored at reasonable time intervals for which of the following characteristics?
a)Change in electrical conductivity
b)Change in light reflection
c)Change in viscosity
d)All of these
14.Rheological properties such as viscosity are also useful because they indicate ease of use, such as spreadability for a topical emulsion.
a)true
b)false
15.Physical tests of emulsions do not include
a)zeta potential
b)redispersibility
c)pH
d)microbiologic contamination
Solutions:
- d)All of these
- d)All of these
- d)Penetrometer
- c)both of these
- a)true
- a)true
- a)evaluating the stability of emulsions
- d)all of these
- a)true
- b)6 or 8
- c)2000-3000
- a)Brownian
- d)All of these
- a)true
- d)microbiologic contamination
References:
- Remington Essential of Pharmaceutics, 1st edition 2013, page no. 390.
- Gaurav K. Jain, Theory and Practice of Physical Pharmacy, 1st edition 2012 Elsevier, page no. 244-246.
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