Surfactants in suspension systems

This short chapter deals primarily with the effect of surfactants on the properties of suspensions. Suspensions can be prepared without surfactants, there being an increasing use of polymers for stabilizing dispersions of solids in liquids. We will restrict discussion here to the effects of conventional surface-active agents but will deal also with the use of surface-active polymers such as the poloxamers, as these bridge the gap between two very large subjects. The emphasis of the chapter is towards pharmaceutical suspensions. 

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Surfactants in suspension systems of Ka the equation can be written in the form... 

Until recently, only three chlorofluorocarbon (CFC) propellants, namely CFCs 11, 12 and 114 (Table 1), had been approved worldwide for use in medical MDIs. Their widespread acceptance was due to their ability to substantially meet the ideal propellant properties. All the CFC MDIs that are currently marketed employ CFC 12 as the major constituent mixed with either CFC 11 or with a mixture of CFC 11 and CFC 114. These mixtures of propellants closely obey Raoult s law and therefore the blend selected can be used to give a defined vapor pressure (Table 1). The inclusion of CFC 11 in the formulation also offered advantages in that it increased the solvency of most propellant systems, thereby facilitating the dissolution of surfactants in suspension formulations. By virtue of it being a liquid below 24° C, it was used as the primary dispersion medium for either suspending or dissolving the drug. 

The FRRPP can also be implemented in suspension and emulsion polymerization processes. Its analysis in suspension system has turned out to be straightforward, because the suspension size scale (mm sizes) does not interfere with the reaction mechanism, even if one includes mass and thermal transport effects. In emulsion polymerization systems, the submicron size scale of emulsion particles interfere with thermal and probably mass transport effects in the system. Also, the hydrophobic portions of surfactant molecules could affect the phase equilibrium aspects of the FRRPP system. 

The surfactant in suspension formulations might minimize adhesional deposition by solubilization. Precipitation of chloramphenicol palmitate from solutions containing polysorbate 80 has been studied by Moes [50]. Low concentrations of the surfactant give coarse particles and large compact aggregates which on a macroscale have low sedimentation volumes. Systems with low concentrations of polysorbate 80 have higher apparent viscosities because of the aggregation of the particles. 

A cosolvent, typically ethanol, may be used to bring drug into solution. A small number of surfactants (sorbitan trioleate, oleic acid, and lecithin) may be dispersed in propellant systems and can aid in suspension stability and in valve lubrication. 

Similar surfactant-stabilized colloidal systems have been reported by Albach and Jautelat, who prepared aqueous suspensions of Ru, Rh, Pd, Ni nanoparticles and bimetallic mixtures stabilized by dodecyldimethylammonium propane-sulfonate [103]. Benzene, cumene and isopropylbenzene were reduced in biphasic conditions under various conditions at 100-150 °C and 60 bar H2, and TTO up to 250 were obtained. 

Similarly to the solubility of active drugs, the solubility of surfactants that were used in CFC systems has significantly changed. Surfactant solubility in HFA 134a ranges from 0.005% to 0.02% w/v, much lower than the concentration required to stabilize suspensions (0.1-2.0% w/v) (24,42). The surfactants can be solubilized with the addition of cosolvents such as ethanol. However, it is most likely that cosolvents will be incompatible with suspension formulations because drug solubility will also be promoted and crystal growth will occur. 

To provide stabilization or suspension of nonionic surfactants in built liquid systems of low viscosity. 

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