A MICROCALORIMETRIC INVESTIGATION OF THE INTERACTION OF SURFACTANTS WITH CRYSTALLINE AND PARTIALLY CRYSTALLINE SALBUTAMOL SULFATE IN A MODEL INHALATION AEROSOL SYSTEM

Purpose. The purpose of the work is to study the adsorption of Oleic a cid and Span 85 (materials frequently used in aerosols as surfactants) onto partially amorphous and essentially crystalline salbutamol sulph ate, attempting to understand the behaviour of metered dose inhalers ( MDIs) and observing whether there were any differences in adsorption b ehaviour and if this could be related to the surface properties of the powder. Methods. isothermal titration microcalorimetry was the princi pal technique used to measure the adsorption behaviour of surfactants to salbutamol sulphate. A Malvern particle size analyzer was also empl oyed to provide size data on the interactions between the surfactant a nd powder suspensions. Results. The calorimetric data revealed that su rfactant adsorption to the crystalline micronised powder (78% RH and a ged dry sample) produced significant exotherms, whereas adsorption to the partially amorphous micronised powder resulted in small heat respo nses. The differences in adsorption behaviour to the partially crystal line and crystalline surfaces resulted in changes in aggregation behav iour. Conclusions. The stability of MDIs varies depending on the water content, crystallinity and surface composition of the powder. The adv antages of using isothermal titration microcalorimetry to evaluate thi s surface behaviour in such difficult systems was demonstrated.