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-pyridinium bromide), P2VP, on colloidal silica (0.15 μm radius) and the subsequent counterion binding of perfluorinated anionic surfactants, CF3CF2COONa, CF3(CF2)2COONa, CF3(CF2)6COONa, CF3(CF2)7SO3Li and CF3(CF2)9COOLi, were studied by electrophoresis. The zeta potential (ζ-potential) of silica changed its sign from negative to positive with an increase in P2VP concentration at pH 4.0, 6.6, and 9.2. The fractional surface coverage of P2VP on silica (θ) was estimated from the zeta potentials of bare silica (ζ1) and silica fully covered with P2VP (ζ2), and the Langmuir adsorption model as a function of P2VP concentration and pH. The negative ζ1 increased with increasing pH, whereas the positive ζ2 was constant at all pH. The higher θ values at high pH suggested that the dominant interaction of the P2VP adsorption on silica was electrostatic. Surfactant anions did not adsorb onto the bare silica surface when pH>isoelectric point, but did adsorb onto a silica surface modified to saturation with P2VP. Electrokinetic measurements in the presence of 0.1 mM lower alkyl chain-length surfactants (n≤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 concentration of CF3(CF2)6COONa or CF3(CF2)7SO3Li changed the sign of the ζ-potential of the P2VP modified silica surface from positive to negative. The mechanism of charge reversal was discussed in terms of the excess adsorption of the surfactant anions.