SETTLING IN SUSPENSIONS :
One aspect of physical stability in pharmaceutical suspensions is concerned with keeping the particles uniformly distributed throughout the dispersion. It possible to prevent settling completely over a prolonged period of time, it is necessary to consider the factors that influence the velocity of sedimentation.
THEORY OF SEDIMENTATION:
The velocity of sedimentation is expressed by Stokes’s law:
V = d2(Ƿs – Ƿo)g / 18ƞo
v = terminal velocity in cm/sec,
d = diameter of the particle in cm,
ρs and ρo = the densities of the dispersed phase and dispersion medium, respectively,
g = acceleration due to gravity,
ηo = viscosity of the dispersion medium in poise.
STOKES LAW IS APPLICABLE IN FOLLOWING CONDITION –
- The particle should be spherical, but in suspensions particle are largely irregular.
- The particles do not interfere with one another during sedimentation, and free settling occurs. In most pharmaceutical suspensions that contain dispersed particles in concentrations of 5%, 10%, or higher percentages, the particles exhibit hindered settling. The particles interfere with one another as they fall, and Stokes’s law no longer applies.
The physical stability can be obtained by diluting the suspension so that it contains about 0.5% to 2.0% w/v of dispersed phase. This is not always recommended, as the addition of a diluent may affect the degree of flocculation (or deflocculation) of the system, thereby effectively changing the particle-size distribution.
EFFECT OF BROWNIAN MOVEMENT:
For particles having a diameter of about 2 to 5 μm (depending on the density of the particles and the density and viscosity of the suspending medium), Brownian movement counteracts sedimentation to a measurable extent at room temperature by keeping the dispersed material in random motion. The critical radius, r, below which particles will be kept in suspension by kinetic bombardment of the particles by the molecules of the suspending medium (Brownian movement) was worked out by Burton. It can be seen in the microscope that Brownian movement of the smallest particles in a field of particles of a pharmaceutical suspension is usually eliminated when the sample is dispersed in a 50% glycerin solution, having a viscosity of about 5 centipoise. Hence, it is unlikely that the particles in an ordinary pharmaceutical suspension containing suspending agents are in a state of vigorous Brownian motion.
Multiple choice questions(MCQs)
1.Each mixture below has particles dispersed in water. Which list has the particles increasing in size
a)Solution, colloid, suspension
b)Suspension, colloid, solution
c)Colloid, suspension, solution
d)Solution, suspension, colloid
2.The velocity of sedimentation is expressed by
3.V = d2(Ƿs – Ƿo)g / 18ƞ. This is the equation for
4.V = d2(Ƿs – Ƿo)g / 18ƞ. In this equation g stands for
a)terminal velocity in cm/sec
b)diameter of the particle in cm
c)acceleration due to gravity
d)viscosity of the dispersion medium in poise
5.Which of the following test is used to compare different suspensions?
d)None of the above
6.Stokes law is applicable in following condition
a)The particle should be spherical, but in suspensions particle are largely irregular
b)The particles do not interfere with one another during sedimentation, and free settling occurs. In most pharmaceutical suspensions that contain dispersed particles in concentrations of 5%, 10%, or higher percentages, the particles exhibit hindered settling. The particles interfere with one another as they fall, and Stokes’s law no longer applies
c)Both of these
d)None of these
7.The physical stability can be obtained by diluting the suspension so that it contains about
a)0.5% to 2.0% w/v
b)1% to 2% w/v
c)0.5% to 2.0% w/w
d)0.5% to 2.0% v/v
8.Properties applicable to suspension
9.Viscosity of suspension is determined by
d)Cone and plate viscometer
10.Sedimentation of solid is minimized by
b)Viscosity of suspension
d)Both b and c
11.Deflocculated suspension has ____ supernatant
12.Rate of sedimentation for flocculated suspension is
13.Anderson pipette is based on the principle of
c)Both a and b
d)None of the above
14.Flocculated suspension exhibit ____ flow type
15.In stoke’s law, rate of sedimentation is inversely proportional to
a)Density of particles
b)Viscosity of medium
c)Radius of particles
- a) Solution, colloid, suspension
- a)Stokes law
- a)Stokes law
- c)acceleration due to gravity
- c) Rheological evaluation
- c)Both of these
- a)0.5% to 2.0% w/v
- c)Stoke’s law
- c)Brookfield viscometer
- d)Both b and c
- b)Viscosity of medium
1. GAURAV KUMAR JAIN – THEORY & PRACTICE OF PHYSICAL PHARMACY, 1st edition 2012 Elsevier, page no. 204-207.
2. Martins Physical Pharmacy, 6th edition 2012, page no. 750-751.