Preparation and characterization of disodium fluorescein powders in association with lauric and capric acids.

Hygroscopic growth of pharmaceutical aerosol powders is a factor governing their deposition, absorption, and pharmacologic effect in the respiratory tract. Surface association of hydrophobic molecules may reduce this effect. Disodium fluorescein was coated with the hydrophobic materials lauric and capric acids. The nature and extent of the association was investigated by a variety of techniques, with the notable use of X-ray photoelectron spectroscopy to examine the powder surface directly. It was concluded that the fatty acids associate at the phenol--sodium bond of the disodium fluorescein, and that the extent of the bulk interaction was governed by a fatty acid-, solute-, and concentration-dependent expanded surface area effect which resulted in much higher surface concentrations than might be expected by simple adsorption.