Forces between mica surfaces bearing adsorbed macromolecules in liquid media

Abstract

The forces between two molecularly smooth curved mica surfaces a distance D apart immersed in a liquid have been measured, both in the absence and in the presence of macromolecules adsorbed from the liquid. The forces between bare mica surfaces in 0.2 mol dm^–3 KNO₃ aqueous solution are repulsive for D≲ 5 nm and correspond reasonably well to electrostatic double-layer (DLVO) theory. Following adsorption of soluble monomeric collagen onto the surfaces from the solution, repulsion commences at D≲ 600–700 nm and increases monotonically with decreasing D, suggesting that the collagen monomers are adsorbed in an upright configuration normal to each surface. The forces between bare mica surfaces in cyclohexane are monotonically attractive for D≲ 10 nm, and appear to obey a van der Waals-like potential acting across the non-polar medium. Following adsorption of polystyrene (of two molecular weights) onto the mica from the cyclohexane, interactions between the surfaces are found to commence at D≲ 2.5 R_g(unperturbed radius of gyration of the respective polymers) and are initially attractive. For D≲R_g the attraction decreases, and on further reducing D strong repulsion between the surfaces is observed. These results may be understood in terms of the phase equilibrium of the polystyrene–cyclohexane system (which under the experimental conditions is below its critical temperature), and are in good accord with the predictions of a recent theory for interaction between adsorbed polymer layers in a poor solvent.