FORCES BETWEEN HYDROPHOBIC SURFACES COATED WITH ETHYL(HYDROXYETHYL)CELLULOSE IN THE PRESENCE OF AN IONIC SURFACTANT

The forces between hydrophobic surfaces across an aqueous solution containing 0.25 wt % ethyl(hydroxyethyl)cellulose (EHEC) and 4 mM SDS have been studied and compared to those appearing in the absence of SDS. A long-range repulsive force (measurable at distances up to 1200 angstrom) is present already after an adsorption time of 30 min. The range of the repulsive force increaes during the first few hours, indicating that the adsorption process is rather slow. After 20 h equilibration, a repulsion was measurable at separations up to 2500 angstrom. The force is rather insensitive to temperature and decays essentially exponentially at large separations, with a decay-length of approximately 300 angstrom. The force measured on compression is always slightly larger than that observed on decompression. Hence, the forces are not measured at true equilibrium. The system behaves strikingly different from the pure EHEC-water system, where less long-range and reversible forces are observed. The adsorbed amount in the presence of SDS was about 2 mg/m2 at adsorption equilibrium, independent of temperature (20-35-degrees-C), which is considerably less than that found in the absence of SDS (5 mg/m2 at 20-degrees-C, and 15 mg/m2 at 37-degrees-C). These effects are discussed in terms of polymer-surfactant aggregation and competitive adsorption. Comparison is made with ellipsometry results on hydrophobized silica surfaces.