ABSTRACT
While the electrokinetic surface, or ^-potentials, originate from the surface or interfacial properties of solid materials, they are actually situated about 0.3 to 0.5 nm outside a material's surface and have to be extrapolated inward to the (^o) potential at the actual surface, using Eq. 5.54. The electrostatic free energy of interaction, A (7 , between two surfaces, 1, reaches a value of about +1.0 mJ/m2 at ipo « 75 mV, in an aqueous medium with a 100 mM salt content of a mono-mono-salt; see Table 5.1. Now various clay and other mineral particles can have 7/>o-potentials that are between 50 and 90 mV, in
EL which case A(7 , while not dominant, is no longer negligible. For instance for a contact between two platey clay particle surfaces over about 100 nm2 (= 10~10 cm2) an attraction of 1 mJ/m2 still corresponds to AGEL w 2,500 kT. Thus, it is always wise to measure (^-potentials, from which the actual surface, or ^0-potential can be derived.
