ELECTRICAL DOUBLE-LAYER STRUCTURE REVISITED VIA A SURFACE FORCE APPARATUS - MICA INTERFACES IN LITHIUM-NITRATE SOLUTIONS

The force as a function of separation is measured between two mica surfaces immersed in lithium nitrate aqueous solutions over wide concentration and pH ranges. In most cases the force-distance profiles are in good agreement with DLVO theory. However, at short separations deviations from DLVO prediction are observed. Their possible sources, and in particular the so-called hydration force, are discussed. The exponentially decaying parts of force profiles, yielding information about the ψd potentials, are analyzed using a conventional electrical double layer (e.d.l.)/charge formation model (triple-layer model). By assuming a shift of the diffuse layer boundary on the order of a diameter of one hydrated ion away from the mica-mica contact surface, the obtained ψd-potentials can be described together with ion adsorption data reported previously by P.M. Claesson et al. (J. Colloid Interface Sci.109, 31 (1986)). A set of parameters reflecting the charging properties of the mica/water interface and the spacial organization of the e.d.l. have been estimated. Contact adhesion measurements allow the assumption that in primary contact, the mica surfaces are separated by a single layer of hydrated ions, which share their hydration water with the mica surfaces (“bridging effect”). © 1993 by Academic Press, Inc.