ADSORPTION OF ORGANIC IONS AT THE SOLID-ELECTROLYTE INTERFACE - INTERPRETATION OF COMMON INTERSECTION POINTS

The interfacial thermodynamics of the adsorption of ions on electrically charged surfaces is discussed in relation to three kinds of common intersection points (c.i.p.). A well-known c.i.p. is the point of zero charge in a series of surface charge curves measured at different ionic strengths. A second type of c.i.p. has been reported for the charge-potential curves of AgI-organic adsorbate systems, it relates to the maximum adsorption of the organic compound. Thermodynamic analysis shows that a third c.i.p. might occur for adsorption isotherms of organic ions at different electrolyte concentrations. This c.i.p. corresponds with the isolelectric point (i.e.p.). Experimental verification of this c.i.p. has been made for tetrabutylammonium ions on silver iodide, for an anionic surfactant on positively charged rutile (TiO2) and for two cationic surfactants on the clay mineral kaolinite. Model considerations indicate that a sharp c.i.p. coinciding with the i.e.p. is to be expected under ideal conditions. © 1990.