Monday, November 1, 2010

Formulation considerations:

It is difficult to formulate a stable suspension in the inherent thermodynamic aspect. In order to prepare a thermodynamically stable suspension, the interfacial tension is minimized by the use of surface active agents. Let consider the following equation –

∆G = Ys/1. ∆A …………………….. (i)
Where,
∆G = change in surface free energy
Ys/1 = interfacial tension
∆A = change in surface area
Eq (i) illustrate that as the interfacial tension and /or surface area approaches zero, the surface free energy is minimized. Further -

d2 (ρ – ρ0) g
s = …………………. (ii)
18η




Where,
s = sedimentation rate
d = particle diameter
ρ = density of the dispersed phase (solid phase)
ρ0 = density of the dispersion phase (liquid phase)
g = acceleration due to gravity
η = viscosity of dispersion phase

According to the Eq (ii) sedimentation are will decreases with the decreasing of particle size. It also indicates that an increase in the viscosity of the liquid phase and / or minimal difference between the densities of the solid / liquid phases will minimize the sedimentation rate enhance physical stability.

As it is difficult to formulate an injectable suspension, the pre-formulation data is useful in developing a suspension and should include various physical and chemical properties and interactions between the active ingredient (s), excipients and packaging components.

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