Time dependence of forces between mica surfaces in water and its relation to the release of surface ions

A surface force balance has been used to investigate the time-dependent behavior of the normal forces between two smooth mica surfaces immersed in salt-free (conductivity) water. A long-ranged repulsion on initial approach to adhesive contact indicates an equilibrium surface-charge density sigma=sigma(0) due to the loss of K+ ions from the mica surface into the water. Subsequent force measurements at times tau following separation of the surfaces reveal that sigma drops markedly following contact and separation, but then relaxes back to sigma(0) with a characteristic time tau(r)=11+/-2 min. This behavior is attributed to condensation of the counterions into the surfaces when they come into contact, and to their subsequent slow release into the water. The energy barrier associated with the counterion release was estimated from the value of tau(r) to be 33+/-4 k(B)T. (C) 2002 American Institute of Physics.