Characterisation of surface modified salbutamol sulphate-alkylpolyglycoside microparticles prepared by spray drying

There were three aims of this work: (1). to study the suitability of spray drying to prepare surface modified microparticles coated with alkylpolyglycoside surfactants (for potential use in metered dose inhalation systems, although their use is not reported here); (2). to assess the utility of inverse phase gas chromatography (IGC) as a means of assessing the surface properties of modified microparticles; and (3). to attempt to relate dynamic surface tension measurements with the ability for a molecule to diffuse to a surface during spray drying. Microparticles of salbutamol sulphate-alkylpolyglycosides were prepared by spray drying from solution and then characterised using scanning electron microscopy, particle size analysis (laser diffraction) and inverse gas chromatography. Further to this, the critical micelle concentration (CMC) and the dynamic surface tension of alkylpolyglycosides were measured. Spray drying a solution of salbutamol sulphate with alkylpolyglycosides produced spherical amorphous microparticles with a diameter of less than 10 microm. The analysis of the surface energies of spray dried salbutamol sulphate showed that the addition of alkylpolyglycosides, at concentrations below and above their CMC, decreases substantially the basic component of the surface energy. This demonstrates that it is possible to sequentially modify the surface energy of the particles. Dynamic surface tension measurements of the alkylpolyglycosides above their CMC showed that the surfactant that has the least effect on the surface energy of the particles, presents the slowest diffusion in water. This may indicate that the diffusion of this particular molecule in water may be too slow to allow the surfactant to migrate to the surface of the microparticle during the drying process. IGC can be useful to analyse the surface energy of the particles after spray drying in order to assess the presence of the surfactant on the surface of the microparticles.