In chemistry, a
suspension is a heterogeneous fluid containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometer.
The internal phase (solid) is dispersed throughout the external phase (fluid) through mechanical agitation, with the use of certain excipients or suspending agents. Unlike colloids, suspensions will eventually settle. An example of a suspension would be sand in water. The suspended particles are visible under a microscope and will settle over time if left undisturbed. This distinguishes a suspension from a colloid, in which the suspended particles are smaller and do not settle.
Colloids and suspensions are different from solutions, in which the dissolved substance (solute) does not exist as a solid, and solvent and solute are homogeneously mixed.
A suspension of liquid droplets or fine solid particles in a gas is called an aerosol or particulate. In the atmosphere these consist of fine dust and soot particles, sea salt, biogenic and volcanogenic sulfates, nitrates, and cloud droplets.
Suspensions are classified on the basis of the dispersed phase and the dispersion medium, where the former is essentially solid while the latter may either be a solid, a liquid, or a gas.
In modern chemical process industries, high shear mixing technology has been used to create many novel suspensions.
Suspensions are unstable from the thermodynamic poin of view; however, they can be kinetically stable over a large period of time, which determines their shelf life. This time span needs to be measured to ensure the best product quality to the final consumer. “Dispersion stability refers to the ability of a dispersion to resist change in its properties over time.” D.J. McClements.
Destabilisation phenomenon of dispersion:
These destabilisations can be classified into two major processes:
- 1-Migration phenomena : whereby the difference in density between the continuous and dispersed phase, leads to gravitational phase separation. In the case of suspensions sedimentation occurs as the dispersed phase is denser than the continuous phase.
- 2-Particle size increase phenomena: whereby the suspended particles join together and increase in size. Below are the two types of this phenomena.
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- reversibly (flocculation)
- irreversibly (aggregation)
Technique monitoring physical stability:
Multiple light scattering coupled with vertical scanning is the most widely used technique to monitor the dispersion state of a product, hence identifying and quantifying destabilisation phenomena. It works on concentrated dispersions without dilution. When light is sent through the sample, it is backscattered by the particles. The backscattering intensity is directly proportional to the size and volume fraction of the dispersed phase. Therefore, local changes in concentration (sedimentation) and global changes in size (flocculation, aggregation) are detected and monitored.
Accelerating methods for shelf life protection:
The kinetic process of destabilisation can be rather long (up to several months or even years for some products) and it is often required for the formulator to use further accelerating methods in order to reach reasonable development time for new product design. Thermal methods are the most commonly used and consists in increasing temperature to accelerate destabilisation (below critical temperatures of phase inversion or chemical degradation). Temperature affects not only the viscosity, but also interfacial tension in the case of non-ionic surfactants or more generally interactions forces inside the system. Storing a dispersion at high temperatures enables to simulate real life conditions for a product (e.g. tube of sunscreen cream in a car in the summer), but also to accelerate destabilisation processes up to 200 times.
Mechanical acceleration including vibration, centrifugation and agitation are sometimes used. They subject the product to different forces that pushes the particles / droplets against one another, hence helping in the film drainage. However, some emulsions would never coalesce in normal gravity, while they do under artificial gravity. Moreover, segregation of different populations of particles have been highlighted when using centrifugation and vibration.
Objective Type Questions for Suspensions in Pharmaceutics1. A pharmaceutical suspension is defined as a ………………….. ,in which insoluble drug particles are dispersed in liquid medium .
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2. In suspensions ,the partical size ranges b/w…………………..
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3. Following is not the important reason of the suspensions :
a) drug stability
b) taste improving
c) taste masking
d) compatibility
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4. Suspensions are preferred over tablets & capsules due to …………………in dosage forms -----------------------
5. The particle size is important while determining the rate of ……………………..
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6. During sedimentation , not to form a ………………… is an important feature of the good suspension -----------------------
7. A good suspension should be resistant to………………….
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8. The ……………….. of a good suspension should not be too much to pour.
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9. Such suspensions which are prepared just before dispensing to the patients are called …………………..
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10. Extemporaneous suspensions are prepared from:
a) tablets & capsules
b) just before dispensing to the patient
c) contents are crushed in mortar & pestle and a proper vehicle is added
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11.For preparation of the extemporaneous suspensions , a good quality suspending agent is added which is :
a) cheep in cost
b) less liable to microbial attack
c) less liable to be coagualated
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12. Carboxymethyl cellulose is an important …………………………. Used in preparation of the extemporaneous suspensions
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13. USP designs the extemporaneous suspensions as ………………………..
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14. Paediatric antibiotics suspensions are best examples of the ………………………
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15. Reconstituted suspensions are also called as ……………………………..
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16. Reconstituted suspensions are designated by usp as…………………………
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17. While preparing reconstituted suspensions ;active agents ,sweeteners, colorants ,flavouring agents ,stabilizers, suspending agents are mixed to prepare a ………………powder.
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18. Previously boiled & cooled water is added in …………………………… while preparing ------------------------
19. The commercial reconstituted suspensions available is ……………………..
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20. The condition in which the particles don’t aggregate and in which they remain uniformly distributed throughout the distributed throughout the dispersion and donot settle is celled …………………….
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21. Rate of the sedimentation is determined while preparing by ………………. Law .
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22. while preparing suspensions , all factors are adjusted so that the rate of the sedimentation is ……………..
a) maximum
b) minimum
c) medium
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23. While preparing the suspensions , the relation b/w density and sedimentation is …………………
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24. The particle size of the dispersed phase of the suspensions ranges b/w …………………….
a) 1 to 50 µm
b) 2 to 10 mm
c) 1 to 10 µm
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Answers to Objective Type Questions for Suspensions in Pharmaceutics
1. coarse dispersion
2. 10-15micrometer
3. b
4. flexibility
5. sedimentation
6. hard cake
7. microbial attack
8. viscosity
9. Extemporaneous suspensions
10. a
11. b
12. suspending agent
13. ORAL SUSPENSIONS
14. extemporaneous suspensions
15. powders for oral suspensions
16. for oral suspensions
17. homogenous
18. reconstituted suspension
19. Barium sulphate
20. physical stability
21. stokes'
22. b
23. inverse
24. a
(These Objective type questions are helpful for the preparation of Pharmacy Exams)
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Further Reading:
