Substrate is treated in 5% solution of
3-aminopropyldi-methylethoxysilane and washed in organic
solvents. The first step of anionic compound adsorption is
carried out in an acidic solution in order to protonate the
aminogroups on the surface of the substrate (20 min in a
solution containing anionic compound in a mixture of HCl, DMSO,
and water at 0°C). Then substrate is washed with water and used
for the adsorption of cationic compound.