Measurement of forces between two mica surfaces in aqueous electrolyte solutions in the range 0-100 nm

Abstract

The main results and conclusions of experimental measurements of the forces between molecularly smooth mica surfaces in aqueous electrolyte solutions are as follows: (1) The attractive van der Waals forces in the range 1-15 nm are largely independent of the type and concentration of the aqueous electrolyte solution. From ∼1 to ∼6.5 nm the forces are non retarded with a Hamaker constant of (2.2 ± 0.3) × 10-20 J. Above ∼6.5 nm retardation effects set in and the forces decay more rapidly with increasing separation. The refractive indices of water and aqueous solutions between two mica surfaces are within 1 % of bulk values for surface separations in the range 2-100 nm. (2) In KNO3 solutions (10-4-10-1 mol dm-3) the measured double-layer repulsive forces are well described by non-linear (exact) solutions to the Poisson-Boltzmann equation for two double-layers interacting at constant surface potential. The effective surface potentials remain constant both as the surfaces approach each other and as the concentration is changed, and are independent of pH in the range 5.5-7. In concentrated KNO3 solutions (10-2-10-1 mol dm-3) the double-layer repulsions still decay roughly exponentially with distance but the mean exponential decay lengths are ∼25 % higher than the theoretical values. (3) In Ca(NO3)2 and BaCl 2 solutions the double-layer forces are much reduced from those in KNO3 solutions, and are poorly described by theory. The mean exponential decay lengths are much lower, by 20-45%, than the theoretical Debye lengths even in dilute (10-4-10-3 mol dm-3) solutions. (4) The surface or boundary from which the double-layer forces arise (the Outer Helmholtz Plane) is not always at the mica-water interface but may initially be up to 2.5 nm farther out from each surface. The existence and extent of such boundaries varies from mica to mica. These boundaries are irreversibly shifted towards the mica-water interfaces as the two surfaces approach each other, and thus give rise to hysteresis effects. (5) Apart from the normal van der Waals and double-layer forces there is also an additional repulsive force. This force is an additional force and not a modification of the double-layer force, since it is independent of the type and concentration of electrolyte. Its magnitude varies from mica to mica; in all cases where it has been observed it is roughly exponential, having a characteristic decay length of 0.95 ± 0.20 nm. (6) At small separations (below ∼5 nm) the attractive van der Waals forces often exceed the repulsive forces, and the surfaces then fall into strong adhesive contact at a separation of 0.0 ± 0.4 nm relative to contact in uncleaved mica. The adhesion energies of mica surfaces in contact are complex, and are not given by extrapolating the long-range van der Waals and double-layer interaction energies down to separations of the order of interatomic spacings.