Surface properties of polyamidines

Aromatic and aliphatic polyamidines of the common structure –(R1–N=C(R2)–NH)n– were investigated by electrokinetic measurements in dependence on the pH value. It could be evidenced, that all polyamidine surfaces show Brønsted basic behaviour. pKA values of the surfaces between 7 and 11 were calculated. Particularly, the aliphatic polyamidines (R1, R2=alkyl) has proven to be strongly basic. The surface properties correlate well with the basic behaviour of the polyamidines in solution. Furthermore, the surface charge density of the different solid substances was determined. It was found that in the case of aromatic polyamidines (R1=aryl) the charge density is dependent on specific interactions of the amidine groups in the solid phase. Especially, in polyformamidines mutual shielding of the amidine groups by hydrogen bonds resulted in reduced charge densities on the surface.