2. Biphasic Suspensions: Stokes' Law, Flocculated vs Deflocculated Systems
A pharmaceutical suspension is a coarse, heterogeneous dispersion where insoluble solid drug particles (dispersed phase, size 0.5 to 5.0 μm) are uniformly distributed throughout a liquid vehicle (continuous phase). The sedimentation velocity of particles is governed by Stokes' Law ($v = \frac{2r^2(\rho_s - \rho_0)g}{9\eta}$). Suspensions are classified into Flocculated systems (particles form loose, porous aggregates that settle rapidly but redisperse instantly upon shaking without caking) and Deflocculated systems (particles settle slowly as individual units, but form an irreversible, hard, compact cementitious 'cake' at the container bottom).
Formula for settling velocity: $v = \frac{d^2(\rho_s - \rho_0)g}{18\eta}$
| Property | Flocculated Suspension | Deflocculated Suspension |
|---|---|---|
| Particle Association | Loose aggregates (flocs) with porous network | Individual, separate particles |
| Rate of Sedimentation | Rapid (flocs settle quickly) | Slow (individual tiny particles settle gradually) |
| Nature of Sediment | Loose, porous, easily redispersible with shaking | Hard, compact 'cake' that CANNOT be redispersed |
| Supernatant Liquid | Clear (particles settle into flocs together) | Turbid / cloudy (fine particles remain suspended) |
| Elegance / Appearance | Less elegant initially due to quick clear layer | Pleasing, uniform appearance for a longer duration |
| Preferred Clinical Choice | IDEAL formulation for commercial shelf-life | Avoided due to risk of irreversible caking |
- Sedimentation Volume ($F$): $F = \frac{V_u}{V_0}$ (Ratio of ultimate settled volume $V_u$ to original suspension volume $V_0$. For an ideal flocculated suspension, $F \approx 1.0$).
- Degree of Flocculation ($\beta$): $\beta = \frac{F}{F_\infty} = \frac{(V_u / V_0)_{\text{flocculated}}}{(V_u / V_0)_{\text{deflocculated}}}$.
- Suspending Agents: Tragacanth (0.2-1.0%), Sodium CMC (0.5-2.0%), Veegum (Magnesium Aluminum Silicate).
सस्पेंशन में दवा के अघुलनशील कण द्रव में तैरते हैं। Flocculated सस्पेंशन सबसे अच्छा होता है क्योंकि इसके कण नीचे बैठने के बाद भी बोतल हिलाने पर आसानी से फिर से घुल (Redisperse) जाते हैं और कड़ा केक नहीं बनाते।