Steric Stabilization of Disperse Systems

To maintain colloid stability over a long period of time (i.e. 2-3 years), one needs to ensure the following conditions High zeta potential by ensuring adequate coverage of the particles or droplets by ionic surfactant low electrolyte concentration low valency of the electrolyte (multivalent ions should be avoided). 

Many polymers are used for the preparation of disperse systems (suspensions and emulsions) in pharmaceutical formulations. An example of such systems is the A-B-A block copolymer of poly(ethylene oxide)-poly(propylene oxide)-poly(ethyl-ene oxide), PEO-PPO-PEO, commercially available as Poloxamers (ICI), Pluronics (BASF) and Synperonic PE (ICI). On hydrophobic drug particles or oil droplets, the polymer adsorbs with the B hydrophobic chain (PPO) close to the surface, leaving the two hydrophilic A chains dangling in solution. 

These nonionic polymers provide stabilisation against flocculation and/or coalescence by a mechanism usually referred to as steric stabilisation (discussed in detail in Chapters 6 and 7). 

To understand the principles of steric stabilisation, one must first consider the adsorption and conformation of the polymer at the solid/liquid or liquid/liquid interface. This is discussed in detail in Chapter 5, and only a summary is given here. 

---