Hubbry Logo
Point of zero chargePoint of zero chargeMain
Open search
Point of zero charge
Community hub
Point of zero charge
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Point of zero charge
Point of zero charge
Point of zero charge
View on Wikipedia
from Wikipedia
Electrical double layer around a negatively charged particle in suspension in water.

The point of zero charge (pzc) is generally described as the pH at which the net electrical charge of the particle surface (i.e. adsorbent's surface) is equal to zero. This concept has been introduced in the studies dealing with colloidal flocculation to explain why pH is affecting the phenomenon.[1]

A related concept in electrochemistry is the electrode potential at the point of zero charge. Generally, the pzc in electrochemistry is the value of the negative decimal logarithm of the activity of the potential-determining ion in the bulk fluid.[2] The pzc is of fundamental importance in surface science[3]. 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. Therefore, the pzc value has been examined in many application of adsorption to the environmental science.[4][5] The pzc value is typically obtained by titrations and several titration methods have been developed.[6][7] Related values associated with the soil characteristics exist along with the pzc value, including zero point of charge (zpc), point of zero net charge (pznc), etc.[8]

Term definition of point of zero charge

[edit]

The point of zero charge is the pH value for which the net surface charge of adsorbent is equal to zero. This concept has been introduced by an increase of interest in the pH of the solution during adsorption experiments.[1] The reason is that the adsorption of some substances is very dependent on pH. The pzc value is determined by the characteristics of an adsorbent. For example, the surface charge of adsorbent is described by the ion that lies on the surface of the particle (adsorbent) structure like image. At a lower pH, hydrogen ions (protons, H+) would be more adsorbed than other cations (adsorbate) so that the other cations would be less adsorbed than in the case of the negatively charged particle. On the other hand, if the surface is positively charged and pH is increased, anions will be less adsorbed as pH increases. 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.

For example, the electrical charge on the surface of silver iodide (AgI) crystals can be determined by the concentration of iodide ions present in the solution above the crystals. Then, the pzc value of the AgI surface will be described by a function of the concentration of I− in the solution (or by the negative decimal logarithm of this concentration, -log10 [I–] = pI−).

Relation of pzc to isoelectric point

[edit]

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[clarification needed].[9] This is often the case for pure ("pristine surface") oxides in suspension in water. In the presence of specific adsorption, pzc and isoelectric point generally have different values.

Method of experimental determination

[edit]

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 supporting electrolytes (including varying the electrolyte ionic strength). Once satisfactory curves 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.

[edit]

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)[10] and point of zero surface tension (pzst[11] or pzs[12]).

Application in electrochemistry

[edit]

In electrochemistry, the electrode-electrolyte interface is generally charged. If the electrode is polarizable, then its surface charge depends on the electrode potential.

IUPAC defines[2] the potential at the point of zero charge as the potential of an electrode (against a defined reference electrode) at which one of the charges defined is zero.

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

where:

  • Epzc is the electrode potential difference with respect to the point of zero charge, Eσ=0
  • E is the potential of the same electrode against a defined reference electrode in volts
  • Eσ=0 is 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, in volts

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.[13]

At pzc, the colloidal system exhibits zero zeta potential (that is, the particles remain stationary in an electric field), minimum stability (exhibits maximum coagulation or flocculation rate), maximum solubility of the solid phase, maximum viscosity of the dispersion, and other peculiarities. [citation needed]

Application in environmental geochemistry

[edit]

In the field of environmental science, adsorption is involved in many techniques that can eliminate pollutants and governs the concentration of chemicals in soils and/or atmosphere. When studying pollutant degradation or a sorption process, it is important to examine the pzc value related to adsorption. For example, natural and organic substrates including wood ash, sawdust, etc. are used as an adsorbent by eliminating harmful heavy metals like arsenic, cobalt, mercury ion and so forth in contaminated neutral drainage (CND), which is a passive reactor that could possible metal adsorption with low-cost materials. Therefore, the pzc values of the organic substrates were evaluated to optimize the selection of materials in CND.[4] Another example is that the emission of nitrous acid, which controls the atmosphere's oxidative capacity. Different soil pH leads to the different surface charges of minerals so the emission of nitrous acid would be varied, further impacting on the biological cycle involved in the nitrous acid species.[5]

Further reading

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Point of zero charge

View on Grokipedia
from Grokipedia
The point of zero charge (PZC) is the value at which the net total charge on the surface of a solid material dispersed in an aqueous is zero, corresponding to a surface of zero. This condition arises when the activities of charge-determining ions (such as H⁺ and OH⁻) in the bulk solution result in balanced and of surface sites, eliminating any net electrostatic potential difference across the interface. Although often used interchangeably in practice, the PZC is distinct from the isoelectric point (IEP), which is the pH at which the electrophoretic mobility or of particles in suspension is zero. The PZC reflects the total charge on the entire surface (including inner and diffuse layers), whereas the IEP primarily indicates the charge at the shear plane, and discrepancies between the two can occur due to selective adsorption of ions like or that penetrate the electrical double layer. For many metal (hydr)oxides and simple systems without specific ion effects, the PZC and IEP coincide, but in complex materials like activated carbons or soils, the IEP may be lower than the PZC. The PZC is a critical in surface and chemistry, governing the electrostatic interactions that influence , dispersion stability, and the adsorption of ions or molecules onto surfaces. In and environmental applications, it determines the affinity of minerals (e.g., TiO₂ with PZC around 6–7.5) for pollutants, facilitating processes like and soil remediation. Similarly, in and , the PZC affects the binding of reactants to surfaces, enabling selective degradation of contaminants. Recent advancements highlight its role in , where tuning the PZC of materials like activated carbons optimizes charge storage mechanisms in supercapacitors and batteries operating in aqueous electrolytes. Common methods to determine the PZC include , which tracks changes upon acid or base addition to suspensions, and the salt addition method, which identifies the where added does not alter the suspension's equilibrium charge. These techniques, refined over decades since early reviews in the , provide values that vary widely by material—typically 7–10 for many metal oxides, 2–5 for carbons, and 2–5 for silicates—underscoring the PZC's sensitivity to surface composition and preparation.

Definitions and Terminology

Definition of Point of Zero Charge

The point of zero charge (PZC), denoted as pHpzc, is defined as the pH value at which the net surface charge of a solid material immersed in an aqueous solution is zero, arising from the balance between positively and negatively charged surface sites. This condition occurs when the concentrations of H+ and OH- ions adsorbed or dissociated from the surface sites equalize the overall charge, rendering the surface electrically neutral in the absence of specific ion adsorption. The concept of the point of zero charge originated in the early 20th century within colloid and surface science, building on foundational work in the electrical double layer theory. Key contributions came from Louis Georges Gouy in 1910 and David Leonard Chapman in 1913, who described the diffuse distribution of charges at interfaces, laying the groundwork for understanding surface neutrality conditions. The term itself gained prominence in studies of oxide surfaces during the mid-20th century, particularly through experimental investigations of charge balance in aqueous dispersions. Mathematically, at the PZC, the surface charge density σ=0\sigma = 0, where σ\sigma represents the net charge per unit area on the solid-liquid interface. At the PZC, the minimization of electrostatic repulsion between particles promotes aggregation and influences key surface interactions, such as the adsorption of ions or molecules and the stability of colloidal suspensions. This neutrality point is crucial for predicting how surfaces respond to variations, affecting phenomena like in or catalyst performance. For common metal oxides, representative PZC values include silica at approximately pH 2–3 and alumina at pH 8–9, reflecting differences in surface site acidity. The isoelectric point (IEP) is defined as the pH value at which a molecule, such as a protein or dispersed particle, exhibits zero net charge, commonly applied in biochemistry to characterize the charge state of macromolecules like proteins where positive and negative charges balance. In contrast, the point of zero charge (PZC) specifically refers to the pH at which the net surface charge density on a solid material, typically inorganic oxides or minerals, is zero, independent of specific adsorbates and focusing on the intrinsic surface properties without the influence of particle dispersion effects. A key distinction arises in their sensitivity to environmental factors: while the PZC remains relatively fixed for a given surface unless structurally altered, the IEP can shift due to adsorption of ions or other species onto particles, reflecting changes in the effective charge of the entire colloidal system rather than the bare surface alone. Another related concept is the point of zero salt effect (PZSE), which denotes the at which the influence of concentration on the surface charge or pH-dependent behavior becomes negligible, often coinciding with the PZC but differing in cases where ion-specific effects are prominent. Unlike the , which describes the suppression of by added ions of the same type in solution equilibria, the PZSE and PZC emphasize surface neutrality in the context of variations, without altering the fundamental dissociation constants of surface groups. These concepts find distinct applications across scientific fields: the PZC is predominantly used in and to predict adsorption on surfaces, such as in or interfaces, whereas the IEP is central in and science for assessing protein solubility or in suspensions. For instance, in the (α-FeOOH), the PZC is approximately 7.5–8.0, determined from surface-specific methods like , highlighting its role in fixed solid interfaces; in comparison, the IEP for particles in suspension ranges from 7 to 9, varying with measurement techniques like due to potential adsorption influences.

Abbreviations and Notation

In the literature on surface chemistry and colloid science, several standard abbreviations are employed to denote key concepts related to surface charge neutrality. The point of zero charge is commonly abbreviated as PZC, referring to the condition where the net surface charge density is zero. The isoelectric point, which marks the pH at which a particle's net charge is zero in a dispersion, is abbreviated as IEP. Additional terms include PZSE for the point of zero salt effect, the pH where surface charge remains independent of ionic strength, and PZNPC for the point of zero net proton charge, indicating zero proton-related surface charge. Notation conventions in PZC studies emphasize clarity in distinguishing pH values from other parameters. The lowercase pzc typically denotes the value at the point of zero charge, while the surface at this point is often symbolized as σ0\sigma_0, representing zero net charge per unit area in contexts. Field-specific variations arise due to differing emphases across disciplines. In , particularly for mineral surfaces, PZC is standardly applied to describe charge neutrality in aqueous environments involving oxides and silicates. In , the notation E_pzc is prevalent for the (in volts) at zero charge, highlighting the interfacial potential rather than pH. The terminology has evolved from early 20th-century electrochemical concepts, such as "zero charge potential" introduced by Frumkin around 1930, to the modern standardization of PZC in the mid-20th century. This shift gained prominence post-1950s through influential reviews that unified terms across , promoting PZC for pH-based charge neutrality in colloidal and systems. To maintain consistency, usage guidelines recommend distinguishing pzc (as a metric) from E_pzc (as an ) to prevent misinterpretation in interdisciplinary work, especially when modeling adsorption or interfacial phenomena.

Surface Charge Behavior

pH-Dependent Charging Mechanisms

The pH-dependent charging of solid surfaces, particularly metal oxides and silicates, primarily arises from the and equilibria of amphoteric surface functional groups. These groups, such as groups (>SiOH) on silica surfaces or sites (>MeOH) on oxides like alumina or titania, respond to changes in solution by gaining or losing protons, thereby developing a net surface charge. For instance, at low , dominates, forming positively charged species like >SiOH₂⁺ or >MeOH₂⁺, while at high , prevails, yielding negatively charged >SiO⁻ or >MeO⁻. This behavior is governed by acid-base equilibria, such as >MeOH ⇌ >MeO⁻ + H⁺ (with dissociation constant K_a) and >MeOH + H₂O ⇌ >MeOH₂⁺ + OH⁻ (related to K_b), where the surface sites act as weak acids or bases. The acid-base character of these surfaces influences their charging profiles and the position of the point of zero charge (PZC), defined as the where net surface charge is zero. Acidic surfaces, exemplified by silica with a low PZC around 2–3, tend to deprotonate readily due to weaker Me–O bonds, resulting in negative charge above the PZC. In contrast, basic surfaces like those of metal oxides (e.g., alumina with PZC ~9 or ZnO with PZC ~9.2) protonate more easily, exhibiting positive charge below the PZC owing to stronger coordination of protons to oxygen ligands. These differences stem from the intrinsic constants: for amphoteric sites, the first constant (pK_{a1}) governs the loss of a proton from the neutral site, and the second (pK_{a2}) from the protonated site, with PZC ≈ (pK_{a1} + pK_{a2})/2. For alumina, typical values are pK_{a1} ≈ 7.1 and pK_{a2} ≈ -9.1 (adjusted for intrinsic conditions). The net surface (σ) can be quantitatively expressed as σ = F (Γ_{+} - Γ_{-}), where F is the , and Γ_{+} and Γ_{-} represent the surface densities of positively and negatively charged sites, respectively. These densities depend on through the fractional occupancies Θ of protonated and deprotonated forms, such that σ_{0,H} = F (Θ_{H^+} - Θ_{OH^-}) \times Γ_{total}, with Θ derived from the equilibria and Boltzmann factors accounting for electrostatic effects. plays a crucial role in this process by forming hydration layers on the surface, where its autoionization (H₂O ⇌ H⁺ + OH⁻) supplies protons and ions that participate in the exchange with surface sites, stabilizing the hydroxylated layer essential for charge development. Factors influencing shifts in the PZC distinguish intrinsic charging, driven solely by pH-dependent proton transfer to/from surface groups, from extrinsic charging caused by specific adsorption of ions (e.g., anions or cations) that alter the effective constants. In inert electrolytes like NaCl, intrinsic mechanisms dominate, but specific ion binding—such as on iron oxides—can shift the PZC downward by enhancing negative charge. This separation is critical for understanding charge behavior independent of effects. Seminal work by Parks established foundational distinctions in surface acid-base properties across oxides, while models by James and Healy formalized the equilibria underlying charge development.

Influence of Electrolytes and Ions

The presence of background electrolytes in solution modulates the surface charge at the solid-liquid interface by screening the electrostatic potential through the formation of a diffuse electrical double layer. According to the Gouy-Chapman model, this diffuse layer consists of counterions and co-ions distributed according to Boltzmann statistics, which effectively compresses the electric field extending from the charged surface into the electrolyte. The thickness of this diffuse layer is characterized by the Debye length, κ−1\kappa^{-1}, given by κ−1=εRT2F2I\kappa^{-1} = \sqrt{\frac{\varepsilon R T}{2 F^2 I}}
Add your contribution
Related Hubs
User Avatar
No comments yet.