Influence of Surfactants on the Properties of Organic Drug Powder Dispersions

Metered‐dose inhalers consist typically of organic particles dispersed in nonpolar chlorofluoro‐carbon solvents. The future ban on using ozone‐depleting substances has raised the need for the pharmaceutical industry to seek alternative dispersion media. Hence, to aid the reformulation work the influence of oleic acid, soybean lecithin and Span 85 on salbutamol base in some model solvents has been investigated. The electrophoretic mobility and the particle size distribution of the drug dispersed in cyclohexane, carbon tetrachloride and liquid chlorofluorocarbon (trichloromonofluoromethane) were measured. The acid‐base properties of the drug were characterized by the use of different Hammett acid‐base indicators indicating mainly basic surface sites. A clearly different surface charge could then be observed for the different surfactants used. A positive surface charge was obtained with oleic acid surfactant and a negative surface charge with soybean lecithin. Span 85 did not significantly alter the surface charge. The electrophoretic mobility gives information on the acid‐base interaction between the drug and the surfactants in nonpolar solvents, but cannot be correlated with the dispersion stability observed. 1995 Royal Pharmaceutical Society of Great Britain