ABSTRACT
II. FUNCTION OF POLYMERS IN DISPERSE SYSTEMS Polymeric excipients function to physically stabilize disperse systems, which can be described as one or more discontinuous phases homogeneously distributed in a continuous phase. The size of the dispersed phase (s) is used as a nonabsolute criterion in the classification of these systems. The sizes of dispersed phases are as follows:
• Suspensions, > 1000 nm • Emulsions, 50-1,000 nm • Liposomes, 10-500 nm • Nanoparticles, 10-100 nm • Microemulsions, 10-75 nm
Disperse systems are metastable due to their nonhomogeneity and tend to equilibrate to a decreased energy state with time (e.g., destabilize via sedimentation, aggregation, or coalescence). Polymers can be used to stabilize these systems to increase their shelf life, consumer appeal, and effectiveness. They stabilize these systems (often in conjunction with polyelectrolytes to regulate zeta potential) by adsorbing at the interface of the dispersed phase, where they modify surface properties and reduce surface free energy. At the surface they prevent the close approach of dispersed phase particles, which can result in physical instability. Achieving particle separation of the dispersed phase is accomplished by electrical effects (zeta potential) and/or steric hindrance. A third stabilization mechanism uses the viscosity increasing effects of polymers in the continuous phase. The reader is referred to other chapters of this text series for a more complete and in depth discussion of this topic.
