title: Adsorption and chemical stability of a cationic aggregating ester - propantheline bromide - on silica surfaces in aqueous dispersions.
abstract: The adsorption behavior of cationic aggregating substances such as antimicrobial quats or phenothiazine derivatives on silica surfaces in aqueous media has been extensively investigated. However, the chemical stability of adsorbates in such systems was unknown. Propantheline bromide (PPBr) was selected as a model to investigate the stability of hydrolyzable substances in silica-containing aqueous dispersions or in adsorbates on silica carriers used for solid drugs. The quaternary ester PPBr showed an appreciable adsorption on the silica surface, the extent of which was increased by raising the pH of the aqueous phase or by the addition of neutral salts such as NaNO3. In parallel to the adsorption process, hydrolysis of PPBr occurs in these aqueous silica dispersions to yield xanthene carbonic acid and a quaternary alcohol component. Adsorption and hydrolysis were found to be mutually influencing reactions. Because of the adsorption of PPBr, the rate of ester decomposition was enhanced in these silica dispersions when compared to aqueous solutions of PPBr at the same pH. Simultaneously, an increase in PPBr adsorption is observed, as well as adsorption of the decomposition product xanthene carbonic acid. This result can be attributed to ion-pair adsorption of the latter with PPBr. The rate constants of PPBr decomposition were found to depend directly on the silica content of the dispersion, although at higher concentrations a decreased catalytic effect was observed. These phenomena are discussed on the basis of the adsorbate structure and exchange processes.
authors.affiliation: Institut für Pharmazie, Lehrstuhl für Pharmazeutische Technologic, Universitätsstr., D-8400, Regensburg, FRG.
authors.forename: R
authors.initials: R
authors.lastname: Daniels
authors.forename: H
authors.initials: H
authors.lastname: Rupprecht