Point of zero charge
The point of zero charge (pzc), in physical chemistry, is a concept relating to the phenomenon of adsorption, and it describes the condition when the electrical charge density on a surface is zero. It is usually determined in relation to an electrolyte's pH, and the pzc value is assigned to a given substrate or colloidal particle. For example, the pzc of solid FeOOH is 9. In other words, pzc is (usually) the pH value at which a solid submerged in an electrolyte exhibits zero net electrical charge on the surface.
A related concept in electrochemistry is the electrode potential at the point of zero charge.
The value of pH is used to describe pzc only for systems in which H+/OH− are the potential-determining ions (which is the common case). Generally, pzc is the value of the negative decimal logarithm of the activity of the potential-determining ion in the bulk fluid. For example, the charge on the surface of silver iodide crystals may be determined by the concentration of iodide ions in the solution above the crystals. Then, the pzc value of the AgI surface will be described by the concentration of I− in the solution (or negative decimal logarithm of this concentration, pI−).
When the pH is lower than the pzc value, the system is said to be "below the pzc." Below the pzc, the acidic water donates more protons than hydroxide groups, and so the adsorbent surface is positively charged (attracting anions). Conversely, above pzc the surface is negatively charged (attracting cations/repelling anions).
Point of zero charge is of fundamental importance in surface science. For example, in the field of environmental science, it determines how easily a substrate is able to adsorb potentially harmful ions. It also has countless applications in technology of colloids, e.g., flotation of minerals.
At pzc, the colloidal system exhibits zero zeta potential (i.e., the particles remain stationary in an electric field), minimum stability (i.e., exhibits maximum coagulation/flocculation rate), maximum solubility of the solid phase, maximum viscosity of the dispersion, and other peculiarities.
Relation of pzc to isoelectric point
The pzc is the same as the isoelectric point (iep) if there is no adsorption of other ions than the potential determining H+/OH− at the surface. This is often the case for pure ("pristine surface") oxides in water. In the presence of specific adsorption, pzc and isoelectric point generally have different values.
Method of experimental determination
The pzc is typically obtained by acid-base titrations of colloidal dispersions while monitoring the electrophoretic mobility of the particles and the pH of the suspension. Several titrations are required to distinguish pzc from iep, using different electrolytes (including varying the electrolyte ionic strength). Once satisfactory graphs are obtained (acid/base amount—pH, and pH—zeta potential), the pzc is established as the common intersection point (cip) of the lines. Therefore, pzc is also sometimes referred to as cip.
Besides pzc, iep, and cip, there are also numerous other terms used in the literature, usually expressed as initialisms, with identical or (confusingly) near-identical meaning: zero point of charge (zpc), point of zero net charge (pznc), point of zero net proton charge (pznpc), pristine point of zero charge (ppzc), point of zero salt effect (pzse), zero point of titration (zpt) of colloidal dispersion, and isoelectric point of the solid (ieps) and point of zero surface tension (pzst or pzs).
Application in electrochemistry
In electrochemistry, the electrode-electrolyte interface is generally charged. If the electrode is polarizable, then its surface charge depends on the electrode potential.
The potential of zero charge is used for determination of the absolute electrode potential in a given electrolyte.
IUPAC also defines the potential difference with respect to the potential of zero charge as:
- Epzc = E - Eσ=0
- Epzc - the electrode potential difference with respect to the point of zero charge,Eσ=0
- E - the potential of the same electrode against a defined reference electrode, V
- Eσ=0 - the potential of the same electrode when the surface charge is zero, in the absence of specific adsorption other than that of the solvent, against the reference electrode as used above, V
The structure of electrolyte at the electrode surface can also depend on the surface charge, with a change around the pzc potential. For example, on a platinum electrode, water molecules have been reported to be weakly hydrogen-bonded with "oxygen-up" orientation on negatively charged surfaces, and strongly hydrogen-bonded with nearly flat orientation at positively charged surfaces.
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