Depletion Attraction versus Steric Repulsion in a System of Weakly Adsorbing PolymerEffects of Concentration and Adsorption Conditions

Interaction forces were measured between surfactant-coated mica surfaces in tetradecane containing polyethylene−propylene. Two molecular weights, M w = 5800 and 33 000 g/mol, were studied on two different surfactant layers at 15, 25, and 35 °C. At low polymer concentration, the force curves exhibited two maxima and two minima. On approach, a very weak repulsion at a separation of D = 6−10 R g was followed by an attractive regime down to a minimum at D ≈ R g, where R g is the radius of gyration. At smaller separations we found a second repulsive maximum, followed by a second attractive minimum at a separation of a few polymer segment widths. At the overlap concentration, a similar force profile, but with an additional depletion attraction, was measured under partial adsorption conditions, while equilibrium adsorption gave a monotonic steric repulsion for both M w . Only monotonic repulsion was measured in tetradecane containing a polyethylene−propylene with an amino end group, which appeared to have bound to the underlying substrate by penetrating through the physisorbed surfactant layer. The effects of time, temperature, and water content were also investigated.