Use of nonionic ethylene oxide surfactants as phase-transfer catalysts for poly(acrylic acid) adsorption to silica against an electrostatic repulsion

Abstract

Using a combination of optical reflectometry, atomic force microscopy, and streaming current measurements, we show that complexation with nonionic ethoxylated surfactants enables poly(acrylic acid) (PAA) to adsorb to negatively charged silica surfaces despite its unfavorable electrostatic interaction with the surface. In the absence of surfactant, PAA does not adsorb to silica to any extent that is measurable by our methods. We used optical reflectometry to measure the adsorption isotherm for t-octyl-phenoxypolyethoxyethanol (Triton X-100) surfactants on silica. When PAA is added to the surfactant solution, optical reflectometry reveals a significant enhancement of the total adsorbed mass below the surfactant critical micelle concentration. Atomic force microscopy confirms the presence of PAA in the mixed adsorbed layer. At lower surfactant concentrations, the presence of PAA in the adsorbed layer is manifested in the form of long-range bridging adhesion between opposing surfaces. Removing surfactant by thoroughly rinsing the mixed layer leaves an irreversibly adsorbed, “deposited” PAA layer. The force between silica surfaces that display the deposited PAA layers is dominated by a double-exponential repulsion that is of considerably longer range than the electrostatic double-layer repulsion, indicating a steric interaction between extended PAA chains. Streaming current measurements show that the magnitude of the ζ potential is less after rinsing a mixed adsorbed layer than it was before adsorption. This further supports the occurrence of surfactant-mediated deposition of PAA layers on silica. Our results suggest intriguing applications for surfactants as phase-transfer catalysts for polymers in surface-modification processes, enabling the attainment of single-component adsorbed polymer states that are inaccessible from single-component polymer solutions.