Investigation of the stability of surface modification by polyelectrolyte complexes - influence of polyelectrolyte complex components and of substrates and media

Silicate surfaces were modified by adsorption of polyelectrolyte complexes (PEC) and parameters that influence the permanence of this kind of surface modification were investigated; for example, different polyelectrolyte (PEL) components, substrate surface charge density and interacting media (pH, salt). The PEL tested were poly(ethyleneimine)s differing in the molar mass, poly(diallyldimethylammonium chloride) and a copolymer of maleic acid and propene. For sensitive detection of the polyanion concentration the poly(maleic acid-co-propene) was labelled with a fluorescent dye. To obtain more comprehensive information about the performance of the surface charge modification the results of PEL titration, electrophoretic mobility investigations, scanning force microscopy (SFM) and fluorescence measurements have been compared. Non-stoichiometric PECs were able to recharge the negatively charged powders and to generate cationic surface charges. However, the stability of the modification layer has been influenced by rinsing the PEC-modified substrate (MS-P/Polymin P) with 10(-2) mole/l NaOH that led to an increase in the surface charge density and the zeta potential due to desorption of the polyanion from the surface of the polymer layer. The adsorbed amounts were found to be dependent on the NaOH concentration. The addition of 10-2 mole/l HCl partly causes the release of the PEC and a negative surface charge density has been observed, whereas the rinsing with NaOH has always been more effective than rinsing with HCl. Substituting the weak Polymin P by PDADMAC leads to decreasing desorbed amounts. The decrease of the substrate's surface charge density and the addition of low molecular weight salts strongly reduce the stability of the PEC surface layer. (C) 1998 Elsevier Science B.V.