Pharmaceutical Significance of Viscosity

Vijay Ratna

Viscosity is an expression of the resistance to flow of a system under an applied
stress. The more viscous a system, the greater the applied force required to
make it flow at a particular rate.

The unit of viscosity is the poise, defined as the shearing force required to
produce a velocity of one centimeter per sec. between two parallel planes of
liquid each 1 centimeter square in area and separated by a distance of one
centimeter. The Cgs units for the poise are dyne sec cm^-2. The centipoises
are a more convenient unit of viscosity.

Viscosity is a property which characterizes the flow nature of a liquid. Study of
viscosity helps us in a number of situations and helps us in taking decisions.

1. For solutions of polymeric materials viscosity is related to molecular weight
   by the equation, known as Mark-Houwink equation,



M = Molecular weight of polymer
K and a are constants characteristic of the particular polymer – solvent system.
With the help of this equation we can determine the Molecular weight of polymeric
materials.

2. When we study the change in viscosity of a solution with a change in applied
   shearing stress, the rhelogical behavior of the solution is understood. We
   can decide whether the solution is Newtonian or Non-Newtonian and this helps
   us in formulation work.
3. When the change in viscosity with the change in concentration of a material
   is studied, its behavior (whether it can be used as a suspending agent, mucilage,
   gel, etc.) is understood. This is also useful in formulation.
4. The viscosity of a solution influences heat transfer through it. For example,
   the cooling of hot syrup, the evaporation of a viscous plant extract, all
   depend on their viscosities. So in unit processes such as evaporation, mixing,
   cooling, distillation etc., viscosity is a parameter which influences heat
   flow and thus the time required to carry out the process.
5. Biologically also viscosity is significant. When a person suffers from fever,
   his temperature rises and the viscosity of blood decreases, giving his vital
   organs better perfusion. The viscosity of blood and other body fluids is used
   in diagnosis of disease states.

Thus viscosity is a property of liquids which is highly significant
pharmaceutically and biologically.