Interaction forces and zeta potentials of cationic polyelectrolyte coated silica surfaces in water and in ethanol: Effects of chain length and concentration of perfluorinated anionic surfactants on their binding to the surface

Silica surfaces were premodified by the saturated adsorption of the cationi c polyelectrolyte of poly(2-vinyl-1-methyl-pyridinium bromide), P2VP, in wa ter. The interaction forces between and the zeta potential of the silica su rfaces were then measured in water and in ethanol solutions of perfluorinat ed anionic surfactants as a function of their chain length and concentratio n using the AFM surface force and electrophoresis methods. In water, the el ectrostatic repulsive forces between P2VP-modified silica surfaces in CF3CF 2COONa, CF3(CF2)(6)COONa, and CF3(CF2)(7)SO3Li solutions of 0.1 mM were ide ntical to the force curve in 0.1 mM NaNO3 but greatly decreased in 0.1 mM C F3(CF2)(9)COOLi (critical micelle concentration (cmc), 0.39 mM). The concen tration increase of CF3(CF2)(7)SO3Li (cmc, 6.3 mM) from 0.1 to 1.0 mM cause d the repulsive force curves to weaken and then to strengthen after passing through a zero repulsive force. The surface potentials obtained by the bes t curve fitting of the force curves agreed well with the zeta potentials, w hich indicated a surface charge reversal from positive to negative for a hi gh concentration of CF3(CF2)(7)SO3Li. These observations were explained by the formation of a Stern layer due to specific counterion binding of the su rfactant anions, which increased with the surfactant chain length and conce ntration. In ethanol, CF3(CF2)(7)SO3Li always showed strong repulsive force curves, when CF3(CF2)(7)SO3Li concentrations that were adjusted to give id entical Debye lengths as those in water were used and the surface charge of P2VP-modified silica was the same as that in water. The surface potential obtained by the best curve fitting coincided with the zeta potential of pos itive sign, confirming no charge reversal. This suggested no obviously firm formation of a Stern layer by the surfactant ions. This was not always the case for shorter carbon-chain surfactants, since CF3(CF2)(6)COONa revealed a much weaker repulsive force curve than CF3(CF2)(7)SO3Li for concentratio ns of identical Debye lengths. This was explained in terms of an increased surfactant binding, due to the polarity difference between the solvent and surfactant molecules.