Interface mineral/water, electric double layer

The Electrical Double Layer at Mineral/Water Interfaces... 

In many flotation systems, the electrical nature of the mineral/water interface controls the adsorption of collectors. The flotation behavior of insoluble oxide minerals, for example, is best understood in terms of electrical double-layer phenomena. A very useful tool for the study of these phenomena in mineral/water systems is the measurement of electrokinetic potential, which results from the interrelation between mechanical fluid dynamic forces and interfacial potentials. Two methods most commonly used in flotation chemistry research for evaluation of the electrokinetic potential are electrophoresis and streaming potential. 

Figure 3. Highly schematic view of the electrical double layer (EDL) at a metal oxide/aqueous solution interface showing (1) hydrated cations specifically adsorbed as inner-sphere complexes on the negatively charged mineral surface (pH > pHpzc of the metal oxide) (2) hydrated anions specifically and non-specifically adsorbed as outer-sphere complexes (3) the various planes associated with the Gouy-Chapman-Grahame-Stem model of the EDL and (4) the variation in water structure and dielectric constant (s) of water as a function of distance from the interface, (from Brown and Parks 2001, with permission)...

Ito M (2008) Structures of water at electrified interfaces microscopic understanding of electrode potential in electric double layers on electrode surfaces. Surf Sci Rep 63 329-389 Janczuk B, Bialopiotroeicz T (1988) Components of surface free energy of some clay minerals. Qays Clay Min 36 243-248... 

The solvation of cation and electrical double layer structure near clay surface was studied by neutron diffraction methods (156-158). The intaplay between molecular simulations and neutron diffraction techniques also has been also applied to this clay mineral-water-cation interface system. Park and Sposito (112) simulated the total radial distribution function (TRDF) of interlayer water from Na-, Li-, and K-montmorillonite hydrates as a physical quantity from molecular simulations. They obtained TRDF values from Monte Carlo simulations and directly compared with previously obtained H/ D isotopic difference neutron diffraction results (9,10). 

Thus, the formation of powerful electric double layer on the surfaces of floating air bubbles, covered by adsorbed surfactants, makes the removal of ions released from dissociated mineral impurities possible. The selected surfactants should be strong fatty acids, strong fatty bases, or other ionizable suitable compounds with high affinity to the air-water interface. 

Consider a simple experiment in which a clean solid surface (free of adsorbed liquid and vapour impurities) is immersed in an excess of pure liquid (Path 1 in Fig. 6.5). If thermal effects arising from absorption, solubility, and swelling of a solid may be eliminated, the whole enthalpy change on immersion is ascribed only to the interface. Sometimes the immersion of a solid in a liquid is accompanied by the formation of an electrical double layer. For mineral oxide-water systems [51, 52], the double-layer effects (i.e., generation of surface charge by protonation or deprotonation of some surface hydroxyl groups, and adsorption of counterions in the Stern or/and diffuse layers) are clearly secondary in comparison with the basic wetting (this contribution is 10-15 % of the total heat effect, at the most). 

However, as we noted early in this chapter, numerous assumptions are employed in the field applications of surface complexation models. Davis et al. (1998) noted that surface complexation models are mainly developed from well-controlled laboratory experiments. It is unclear how the models can be applied to soil and sediments where the double layers of the heterogenous particles may interact and the competitive adsorption of many different ions can cause significant changes in the electrical properties of mineral-water interfaces. 

Figure 2.10 Structure of the double electrical layer as function of distance (x) vs. interface between mineral and water.

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