Direct measurements of steric interactions between mica surfaces covered with electrostatically bound low-molecular-weight polyethylene oxide

Abstract

The forces between two mica surfaces covered with electrostatically anchored chains of a low-molecularweight (mol wt ∼ 1900 g/mole) polyethylene oxide ester of lysine (PEO-lysine) have been measured directly in aqueous solutions. In dilute solutions of PEO-lysine the long-range forces observed can be accounted for by the DLVO theory. However, at separations below 10 nm a strong repulsive force of steric origin prevents the surfaces from reaching molecular contact. At higher PEO-lysine concentrations ( C > 10 −3 M) a long-range ( D < 50 nm) repulsive force not accounted for by DLVO theory is present. It is suggested that this force, which is stronger on approach than on separation, is caused by PEO-lysine molecules weakly associated with the coated mica surface (for instance as counter ions), but not electrostatically bonded to it. Upon addition of salt or upon increasing the temperature, the range of the repulsive force decreases and the hysteresis disappears. The repulsive force observed under such conditions originates from the interaction between electrostatically bonded polyethylene oxide chains. The water content in a compressed adsorbed PEO-lysine monolayer was found to be substantial. At 23°C the adsorbed layer contains about six water molecules per ethylene oxide unit and at 55°C there is about four.