ADSORPTION OF CODEINE ON HYDROPHILIC SILICA AND SILICA SURFACE-MODIFIED BY HYDROPHOBIC GROUPS IN THE PRESENCE OF ELECTROLYTES

The adsorption of codeine from aqueous solution onto colloidal silica and silica surface-modified with chemiadsorbed octadecyl dimethyl sila ne (ODDMS) or dimethyl silane (DMS) groups was studied in the presence of neutral electrolytes at different pH values. From codeine-hydrochl oride solutions codeine cations are strongly bound to negatively charg ed silica surfaces. Inorganic salts (NaCl, NaNO3) reduce the adsorptio n of the organic cation. On silica modified by ODDMS (10% of surface s ilanol groups are occupied), codeine cations are adsorbed to a higher extent at pH 6, while at pH 8 the adsorbed amounts are lower than on t he bare silica surface. Neutral electrolytes reduce codeine adsorption on the ODDMS modified silica. On the hydrophobic silica, completely c overed by DMS groups, codeine adsorption is considerably lower than on the bare silica, but neutral salts increase the adsorption. The adsor ption of codeine is compared with the adsorption of aggregating surfac tant ions. Common and different features of their interactions with si lica surfaces are outlined.