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Interfacial Double layer (DL)

A double layer is a spontaneous ionic structure of two parallel layers of charge on the surface of an immersed object.

Stern layer : condensed counter-ions, stationary, adsorbed on the particle via chemical interactions
Slipping plane (diffuse layer) : more mobile co-counter-ions attached via the Coulomb force, electrically screening the first layer, still under the influence of thermal motion

Diagram_of_zeta_potential_and_slipping_planeV2.svg.png

These structures are particularly relevant substances with high surface to volume ratios, such as colloidal solutions. Milk for example exists only because fat droplets are covered with a DL that prevents their coagulation into butter.

Key point: a high zeta potential $\Rightarrow$ strong repulsion $\Rightarrow$ high stability with respect to aggregation at the bottom of the solution (and not suspension)
Zeta potential : electric potential in the slipping plane, also called electrokinetic potential and often denoted as $ζ$ -potential, it is used to estimate DL charge, usually in the order of tens of mV

Point of zero charge (pzc)

The pzc is the pH of net zero electrical charge of the particle surface, equivalent to $ζ = 0$ . Generally, the pzc in electrochemistry is $- \log a_{M^{Z \pm}}$ , $M^{Z \pm}$ being the potential-determining ion in the bulk fluid.

From the view of the adsorbent, if the pH of the solution is below the pzc value, the surface charge of the adsorbent would become positive so that the anions can be adsorbed. Conversely, if the pH is above the pzc value, the surface charge would be negative so that the cations can be adsorbed.

Experimentally, curves are obtained of acid/base amount = f(pH), and pH=f( $ζ$ ), and the pzc is established as the common intersection point (cip) of the lines. Therefore, pzc is also sometimes referred to as cip.

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