Characterization of Surfactant Effect on Aggregates in Model Aerosol Propellent Suspensions

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Abstract
A novel technique of rheometry combined with image analysis to study model aerosol suspensions used in inhalation technology has been used. The role of surfactants in the stability and aggregation of such suspensions was also investigated. Addition of increasing concentrations of sorbitan monooleate (Span 80) to suspensions of micronized polar solids in model CFC suspensions caused marked changes in aggregate morphology and suspension stability. The aggregate behaviour was characterized in a modified constant stress rheometer by use of image analysis techniques, allowing the measurement of particle size, elongation and fractal dimension as a function of shear stress. This method was used to study the effect of Span 80 on suspensions of lactose and salbutamol sulphate in P113. Lactose suspensions showed a gradual reduction in aggregate size, and more spherical structure, with increasing surfactant concentration. Salbutamol sulphate suspensions displayed a more prominent transition from an aggregated state to a weakly aggregated state at 0.02% w/w surfactant. Both suspensions were disaggregated by shear, the resulting fractal properties of the aggregates in shear demonstrating the increased particle mobility caused by the surfactant.