Forces between polymer-covered surfaces: a colloidal probe study.

An atomic force microscope was used to measure the interaction forces between a polymer-covered silica sphere and a polymer-covered silica plate at various pH values and electrolyte concentrations and for different polymer chain lengths. The polymer used was poly(ethylene oxide) (PEO). The force measurements were performed in aqueous solution without dissolved polymer, at scan rates corresponding to the velocity of Brownian collisions between dispersed colloidal particles. In all cases the repulsion on approach was found to be electrostatic in nature: although the PEO adsorption layers are saturated, there is no sign of steric repulsion before the distance of closest approach between the silica surfaces is reached. At pH 4 the approach curves show, for separations smaller than 20 nm, an attractive component which partly compensates the electrostatic repulsion. On retraction a strong adhesion is observed, which is attributed to bridging. At pH 8 and low electrolyte concentration (10−3 M NaCl) the interaction is repulsive on approach and on retraction: no adhesion by bridging takes place. However, upon increasing the NaCl concentration a weak adhesion is induced. At neutral pH (~6.5) the adhesion on separation depends on the force with which the surfaces have been pressed together (10−3 M NaCl). The pH dependence of the interaction curves is discussed in terms of the segmental adsorption energy, which is known to decrease with increasing pH. Measurements at pH 4 show a strong dependence of the adhesion force on the chain length. A linear relationship between the adhesion force and the surface coverage (in mass per unit area) is found.