The distribution of oleic acid between salbutamol base drug and different propellant blends

The distribution of oleic acid between Salbutamol base drug and the solvent in metered-dose inhalers (MDI's) has been investigated. The equilibrium surfactant concentration in the drug dispersions has been determined using a colorimetric method. The samples examined contained Salbutamol base drug particles and oleic acid dispersed in different propellant blends of freon 11 and 12. The maximum equilibrium concentration observed depended on the propellant blend used. The propellant blend and the distribution of the surfactant affected the dispersion stability. The effect of the surfactant is illustrated by the adsorption isotherm for oleic acid onto the Salbutamol particles. The results are correlated with zeta-potentials and particle size measurements made on similar systems in order to characterize the properties of surfactant stabilized MDI's.