Electrostatic interaction salt-free medium

Electrostatic Interaction Between Ion-Penetrable Membranes in a Salt-Free Medium... 

Electric behaviors of colloidal particles in a salt-free medium containing counterions only are quite different from those in electrolyte solutions, as shown in Chapter 6. In this chapter, we consider the electrostatic interaction between two ion-penetrable membranes (i.e., porous plates) in a salt-free medium [1]. 

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 (i/>o) potential at the actual surface, using Eq. 5.54. The electrostatic free energy of interaction, AG, between two surfaces, 1, reaches a value of about -M.O mJ/m at V o 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 V o-potentials that are between 50 and 90 mV, in which case AG, while not dominant, is no longer negligible. For instance for a contact between two platey clay particle surfaces over about 100 nm (= 10 ° cm ) an attraction of 1 mJ/m still corresponds to w 2,500 kT. Thus, it is always wise to measure -potentials, from which the actual surface, or V o-potential can be derived. 

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