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Physicochemical properties of spray‐dried agglomerated particles of salicylic acid and sodium salicylate

Aqueous slurries of salicylic acid and sodium salicylate, containing various kinds of binder, namely gum arabic, gelatin, polyvinyl alcohol, carboxymethylcellulose, methylcellulose, polyvinylpyrrolidone, tragacanth and sodium alginate, were spray dried using a centrifugal wheel atomizer. Almost all the spherical dried products were free flowing microgranules, although formulations including gelatin or polyvinyl alcohol proved to be difficult due to agglomeration. Particle properties such as diameter, true density, sphericity and surface area were measured, all of these being strongly influenced by the concentrations of sodium salicylate and binder. The surface topography of the products was studied with a scanning electron microscope. Granules bound with gum arabic and polyvinylpyrrolidone had smooth surfaces without holes or craters. Analysis of the X‐ray diffraction patterns and the infrared absorption spectrum of products showed that these granules still contained both salicylic acid and sodium salicylate, in contrast to granules prepared using other binders, where only sodium salicylate remained, the salicylic acid having sublimed. They are microcapsules containing salicylic acid and sodium salicylate of grain size of about 20–80 and 13–17 nm respectively, and amorphous contents of about 50–80%. The thickness of the encapsulating film was 1–3 μm. The change in appearance of a drying slurry droplet is discussed and two typical drying processes are described.