Specific Chemical Interaction Affecting the Stability of Dispersed Systems

1) Sorbable species may destabilize colloids at much lower concentrations than nonsorbable ions. The VODL double layer model neglects the dominating role that chemical forces play in causing adsorption and is restricted in its application to lyophilic colloids and simple electrolytes. 2) The distribution of ions in an oxide-electrolyte interface can be evaluated from alkalimetric and acidimetric titration curves of aqueous dispersions of these oxides. 3) A comparison of the differential capacity of the interface at an oxide-electrolyte interface with that of Hg or Ag! shows much larger capacitance values for the hydrophilic strongly aquated oxide surface than for the more hydrophobic surface of Hg and Ag!. The difference is caused primarily by the strongly structured, extensively hydrogen-bonded and chemisorbed water layer immediately adjacent to the solid oxide surface. Ions tend strongly to penetrate (specific sorption) into the compact part of the double layer adjoining the oxide surface, and may thus exert a marked effect on the surface chemical properties beyond those observed by a mere compaction of the diffuse part of the double layer. 4) Association of oxide surfaces with H+, and other cations can, similar as with polyelectrolytes, be characterized by acidity and stability constants. The latter constants can be expressed as intrinsic constants if they are cor rected to a hypothetically chargeless surface. The specificity of the interaction with H+ and cations can be understood by considering simple electrostatic models. This association of oxide surfaces with cations can be used to explain the effect of cations such as Ca2+ on the stability of hydrous oxide colloids, and on the deposition of Mn02 particles on sand surfa ces. The extent to which a coagulant species is specifically adsorbed is reflected in the c. c. c. necessary to produce a ggregation. When the specifically adsorbed species and the colloid are of opposite charge, the sorbed species act to reduce the surface charge of the colloid. The destabilizing agent can, in some cases, even reverse the colloid charge and restabilization will occur. 5) Specific cation interactions as described here represent a basis of related ion specific processes, such as the behavior of ion selective glass or membrane electrodes; the selective ion permeability of cell membranes and potential generating mechanisms in the living cell.