Adsorption of quarternarised polyvinylpyridine and subsequent counterion binding of perfluorinated anionic surfactants on silica as a function of concentration and pH: a zeta potential study

The adsorption of a cationic polyelectrolyte, poly(2 -vinyl-1-methyl-pyridi nium bromide), P2VP, on colloidal silica (0.15 mum radius) and the subseque nt counterion binding of perfluorinated anionic surfactants, CF3CF2COONa, C F3(CF2)(2)COONa, CF3(CF2)(6)COONa, CF3(CF2)(7)SO3Li and CF3(CF2)(9)COOLi, w ere studied by electrophoresis. The zeta potential (zeta -potential) of sil ica changed its sign from negative to positive with an increase in P2VP con centration at pH 4.0, 6.6, and 9.2. The fractional surface coverage of P2VP on silica (theta) was estimated from the zeta potentials of bare silica (z eta (1)) and silica fully covered with P2VP (zeta (2)), and the Langmuir ad sorption model as a function of P2VP concentration and pH. The negative zet a (1) increased with increasing pH, whereas the positive zeta (2) was const ant at all pH. The higher theta values at high pH suggested that the domina nt interaction of the P2VP adsorption on silica was electrostatic. Surfacta nt anions did not adsorb onto the bare silica surface when pH > isoelectric point, but did adsorb onto a silica surface modified to saturation with P2 VP. Electrokinetic measurements in the presence of 0.1 mM lower alkyl chain -length surfactants (n less than or equal to 3) indicated that they behaved as indifferent electrolytes, with no surfactant adsorption detectable. The adsorption increased with the chain-length for 0.1 mM surfactants with > 6 carbon chains, indicating specific binding. An increase in the concentrati on of CF3(CF2)(6)COONa or CF3(CF2)(7)SO3Li changed the sign of the zeta -po tential of the P2VP modified silica surface from positive to negative. The mechanism of charge reversal was discussed in terms of the excess adsorptio n of the surfactant anions. (C) 2001 Elsevier Science B.V. All rights reser ved.