Surface charge
Encyclopedia
Surface charge is the electric charge
present at an interface. There are many different processes which can lead to a surface being charged, including adsorption of ions, protonation/deprotonation, and the application of an external electric field. Surface charge causes a particle to emit an electric field, which causes particle repulsions and attractions, and is responsible for many colloidal properties.
, a conductor at equilibrium carrying an applied current has no charge on its interior. Instead, the entirety of the charge of the conductor resides on the surface, and can be expressed by the equation:
When a surface is immersed in a solution containing electrolytes, it develops a net surface charge. This is often because of ionic adsorption. Aqueous solutions universally contain positive and negative ions (cations and anions, respectively), which interact with partial charges on the surface, adsorbing to and thus ionizing the surface and creating a net surface charge. This net charge results in a surface potential [L], which causes the surface to be surrounded by a cloud of counter-ions, which extends from the surface into the solution, and also generally results in repulsion between particles. The larger the partial charges in the material, the more ions are adsorbed to the surface, and the larger the cloud of counter-ions. A solution with a higher concentration of electolytes also increases the size of the counter-ion cloud. This ion/counterion layer is known as the electric double layer
.
A solution's pH can also greatly effect surface charge because functional groups present on the surface of particles can often contain oxygen or nitrogen, two atoms which can be protonated or deprotonated to become charged. Thus, as the concentration of hydrogen ions changes, so does the surface charge of the particles. At a given pH, the average surface charge will be equal to zero; this is known as the point of zero charge (PCZ). A list of common substances and their associated PCZs is shown to the right.
, or charge, is the result of an object's capacity to be moved in an electric field. An interfacial potential is thus defined as a charge located at the common boundary between two phases (for example, an amino acid such as glutamate
on the surface of a protein can have its side chain carboxylic acid deprotonated in environments with pH
greater than 4.1 to produce a charged amino acid at the surface, which would create an interfacial potential). Interfacial potential is responsible for the formation of the electric double layer, which has a broad range of applications in what is termed electrokinetic phenomena
. The development of the theory of the electric double layer is described below.
. It assumes that a solution is only composed of electrolytes, no reactions occur near the electrode which could transfer electrons, and that the only Van der Waals interactions
are present between the ions in solution and the electrode. These interactions arise only due to the charge density associated with the electrode which arises from either an excess or deficiency of electrons at the electrode's surface. To maintain electrical neutrality the charge of the electrode will be balanced by a redistribution of ions close to it's surface. The attracted ions thus form a layer balancing the electrode's charge. The closest distance an ion can come to the electrode will be limited to the radius of the ion plus a single solvation sphere around an individual ion. Overall, two layers of charge and a potential drop from the electrode to the edge of the outer layer (outer Helmholtz Plane) are observed.
Given the above description, the Helmholtz model is equivalent in nature to an electrical capacitor
with two separated plates of charge, for which a linear potential drop is observed at increasing distance from the plates.
The Helmholtz model, while a good foundation for the description of the interface does not take into account several important factors: diffusion/mixing in solution, the possibility of adsorption on to the surface and the interaction between solvent dipole moments and the electrode.
, the counter ion concentration in the external solution, is the Boltzmann factor:
and assuming that, at an infinitely great distance, the potential gradient
is equal to 0, the Grahame equation is obtained :
. Although zeta potential is an intermediate value, it is sometimes considered to be more significant than surface potential as far as electrostatic repulsion is concerned.
.
refers to a variety of effects resulting from an electrical double layer
. A noteworthy example is electrophoresis
, where a charged particle suspended in a media will move as a result of an applied electrical field. Electrophoresis
is widely used in biochemistry to distinguish molecules, such as proteins, based on size and charge. Other examples include electro-osmosis
, sedimentation potential
, and streaming potential.
must have the right surface charge in order to be able to bind a specific substrate.
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
present at an interface. There are many different processes which can lead to a surface being charged, including adsorption of ions, protonation/deprotonation, and the application of an external electric field. Surface charge causes a particle to emit an electric field, which causes particle repulsions and attractions, and is responsible for many colloidal properties.
Surface charge density
Surface charge density is defined as the amount of electric charge, q, that is present on a surface of given area, A:
Conductors
According to Gauss’s lawGauss's law
In physics, Gauss's law, also known as Gauss's flux theorem, is a law relating the distribution of electric charge to the resulting electric field. Gauss's law states that:...
, a conductor at equilibrium carrying an applied current has no charge on its interior. Instead, the entirety of the charge of the conductor resides on the surface, and can be expressed by the equation:
Where E is the electric fieldElectric fieldIn physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
caused by the current in the conductor and 
is the permittivity of the free space. It’s worth noting that this equation is only strictly accurate for conductors with infinitely large area, but it provides a good approximation if E is measured at the surface of the conductor.
Colloids and immersed objects
| Compound | Chemical Formula | Point of Zero Charge |
|---|---|---|
| tungsten(VI) oxide | WO3 | 0.2-0.5 |
| silicon carbide Silicon carbide Silicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive... (alpha) |
SiC | 2-3.5 |
| manganese(IV) oxide Manganese(IV) oxide Manganese oxide is the inorganic compound with the formula . This blackish or brown solid occurs naturally as the mineral pyrolusite, which is the main ore of manganese and a component of manganese nodules. The principal use for MnO2 is for dry-cell batteries, such as the alkaline battery and the... |
MnO2 | 4-5 |
| silicon nitride Silicon nitride Silicon nitride is a chemical compound of silicon and nitrogen. If powdered silicon is heated between 1300° and 1400°C in an atmosphere of nitrogen, trisilicon tetranitride, Si3N4, is formed. The silicon sample weight increases progressively due to the chemical combination of silicon and nitrogen... |
Si3N4 | 6-7 |
| thallium(I) oxide | Tl2O | 8 |
| copper(II) oxide Copper(II) oxide Copper oxide or cupric oxide is the higher oxide of copper. As a mineral, it is known as tenorite.-Chemistry:It is a black solid with an ionic structure which melts above 1200 °C with some loss of oxygen... |
CuO | 9.5 |
| nickel(II) oxide Nickel(II) oxide Nickel oxide is the chemical compound with the formula NiO. It is notable as being the only well characterized oxide of nickel . The mineralogical form of NiO, bunsenite, is very rare. It is classified as a basic metal oxide... |
NiO | 10-11 |
When a surface is immersed in a solution containing electrolytes, it develops a net surface charge. This is often because of ionic adsorption. Aqueous solutions universally contain positive and negative ions (cations and anions, respectively), which interact with partial charges on the surface, adsorbing to and thus ionizing the surface and creating a net surface charge. This net charge results in a surface potential [L], which causes the surface to be surrounded by a cloud of counter-ions, which extends from the surface into the solution, and also generally results in repulsion between particles. The larger the partial charges in the material, the more ions are adsorbed to the surface, and the larger the cloud of counter-ions. A solution with a higher concentration of electolytes also increases the size of the counter-ion cloud. This ion/counterion layer is known as the electric double layer
Double layer (interfacial)
A double layer is a structure that appears on the surface of an object when it is placed into a liquid. The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. The DL refers to two parallel layers of charge surrounding the object...
.
A solution's pH can also greatly effect surface charge because functional groups present on the surface of particles can often contain oxygen or nitrogen, two atoms which can be protonated or deprotonated to become charged. Thus, as the concentration of hydrogen ions changes, so does the surface charge of the particles. At a given pH, the average surface charge will be equal to zero; this is known as the point of zero charge (PCZ). A list of common substances and their associated PCZs is shown to the right.
Interfacial potential
An interface is defined as the common boundary formed between two different phases, such as between a solid and gas. Electric potentialElectric potential
In classical electromagnetism, the electric potential at a point within a defined space is equal to the electric potential energy at that location divided by the charge there...
, or charge, is the result of an object's capacity to be moved in an electric field. An interfacial potential is thus defined as a charge located at the common boundary between two phases (for example, an amino acid such as glutamate
Glutamic acid
Glutamic acid is one of the 20 proteinogenic amino acids, and its codons are GAA and GAG. It is a non-essential amino acid. The carboxylate anions and salts of glutamic acid are known as glutamates...
on the surface of a protein can have its side chain carboxylic acid deprotonated in environments with pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
greater than 4.1 to produce a charged amino acid at the surface, which would create an interfacial potential). Interfacial potential is responsible for the formation of the electric double layer, which has a broad range of applications in what is termed electrokinetic phenomena
Electrokinetic phenomena
Electrokinetic phenomena are a family of several different effects that occur in heterogeneous fluids or in porous bodies filled with fluid. The term heterogeneous here means a fluid containing particles...
. The development of the theory of the electric double layer is described below.
Helmholtz
The model dubbed the 'electric double layer' was first introduced by Hermann von HelmholtzHermann von Helmholtz
Hermann Ludwig Ferdinand von Helmholtz was a German physician and physicist who made significant contributions to several widely varied areas of modern science...
. It assumes that a solution is only composed of electrolytes, no reactions occur near the electrode which could transfer electrons, and that the only Van der Waals interactions
Van der Waals force
In physical chemistry, the van der Waals force , named after Dutch scientist Johannes Diderik van der Waals, is the sum of the attractive or repulsive forces between molecules other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral...
are present between the ions in solution and the electrode. These interactions arise only due to the charge density associated with the electrode which arises from either an excess or deficiency of electrons at the electrode's surface. To maintain electrical neutrality the charge of the electrode will be balanced by a redistribution of ions close to it's surface. The attracted ions thus form a layer balancing the electrode's charge. The closest distance an ion can come to the electrode will be limited to the radius of the ion plus a single solvation sphere around an individual ion. Overall, two layers of charge and a potential drop from the electrode to the edge of the outer layer (outer Helmholtz Plane) are observed.
Given the above description, the Helmholtz model is equivalent in nature to an electrical capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
with two separated plates of charge, for which a linear potential drop is observed at increasing distance from the plates.
The Helmholtz model, while a good foundation for the description of the interface does not take into account several important factors: diffusion/mixing in solution, the possibility of adsorption on to the surface and the interaction between solvent dipole moments and the electrode.
Gouy-Chapman
Gouy-Chapman theory describes the effect of a static surface charge on a surface's potential. "Gouy suggested that interfacial potential at the charged surface could be attributed to the presence of a number of ions of given charge attached to its surface, and to an equal number of ions of opposite charge in the solution." A positive surface charge will form a double layer, since negative ions in solution tend to balance the positive surface charge. Counter ions are not rigidly held, but tend to diffuse into the liquid phase until the counter potential set up by their departure restricts this tendency. The kinetic energy of the counter ions will, in part, affect the thickness of the resulting diffuse double layer. The relation between C, the counter ion concentration at the surface, and, the counter ion concentration in the external solution, is the Boltzmann factor:
Where z is the charge on the ion, e is the charge of a proton, k is the Boltzmann constant and ψ is the potential of the charged surface.
This however is inaccurate close to the surface, because it assumes that molar concentration is equal to activity. It also assumes that ions were modeled as point charges and was later modified. An improvement of this theory, known as the modified Gouy-Chapman theory, included the finite size of the ions with respect to their interaction with the surface in the form of a plane of closest approach.
Surface charge & surface potential
The relation between surface charge and surface potential can be expressed by the Grahame equation, derived from the Gouy-Chapman theory by assuming the electroneutrality condition, which states that the total charge of the double layer must be equal to the negative of the surface charge. Using the one dimensional Poisson equationPoisson's equation
In mathematics, Poisson's equation is a partial differential equation of elliptic type with broad utility in electrostatics, mechanical engineering and theoretical physics...
and assuming that, at an infinitely great distance, the potential gradient
Potential gradient
A potential gradient is the local space rate of change of the potential with respect to displacement.In electrostatics then, it is the local space rate of change of the electric potential:Units are volts per meter...
is equal to 0, the Grahame equation is obtained :
For the case of lower potentials, 
can be expanded to 
= 
, and 
is defined as the Debye lengthDebye lengthIn plasma physics, the Debye length , named after the Dutch physicist and physical chemist Peter Debye, is the scale over which mobile charge carriers screen out electric fields in plasmas and other conductors. In other words, the Debye length is the distance over which significant charge...
. Which leads to the simple expression:
Stern
The Stern model of the double layer is essentially a combination of Helmholtz and Gouy-Chapman theories. His theory states that ions do have finite size, so cannot approach the surface closer than a few nanometers. Through a distance known as the Stern Layer, ions can be adsorbed onto the surface up to a point referred to as the slipping plane, where the ions adsorbed meet the bulk liquid. At the slipping plane the potential, Ψ, has decreased to what is known as the zeta potentialZeta potential
Zeta potential is a scientific term for electrokinetic potential in colloidal systems. In the colloidal chemistry literature, it is usually denoted using the Greek letter zeta, hence ζ-potential...
. Although zeta potential is an intermediate value, it is sometimes considered to be more significant than surface potential as far as electrostatic repulsion is concerned.
Applications of surface charge
Charged surfaces are extremely important and are used in many applications. For example, solutions of large colloidal particles depend almost entirely on repulsion due to surface charge in order to stay dispersed. If these repulsive forces were to be disrupted, perhaps by the addition of a salt or a polymer, the colloidal particles would no longer be able to sustain suspension and would subsequently flocculateFlocculation
Flocculation, in the field of chemistry, is a process wherein colloids come out of suspension in the form of floc or flakes by the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually...
.
Electrokinetic phenomena
Electrokinetic phenomenaElectrokinetic phenomena
Electrokinetic phenomena are a family of several different effects that occur in heterogeneous fluids or in porous bodies filled with fluid. The term heterogeneous here means a fluid containing particles...
refers to a variety of effects resulting from an electrical double layer
Double layer (interfacial)
A double layer is a structure that appears on the surface of an object when it is placed into a liquid. The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. The DL refers to two parallel layers of charge surrounding the object...
. A noteworthy example is electrophoresis
Electrophoresis
Electrophoresis, also called cataphoresis, is the motion of dispersed particles relative to a fluid under the influence of a spatially uniform electric field. This electrokinetic phenomenon was observed for the first time in 1807 by Reuss , who noticed that the application of a constant electric...
, where a charged particle suspended in a media will move as a result of an applied electrical field. Electrophoresis
Electrophoresis
Electrophoresis, also called cataphoresis, is the motion of dispersed particles relative to a fluid under the influence of a spatially uniform electric field. This electrokinetic phenomenon was observed for the first time in 1807 by Reuss , who noticed that the application of a constant electric...
is widely used in biochemistry to distinguish molecules, such as proteins, based on size and charge. Other examples include electro-osmosis
Electro-osmosis
Electroosmotic flow is the motion of liquid induced by an applied potential across a porous material, capillary tube, membrane, microchannel, or any other fluid conduit...
, sedimentation potential
Sedimentation potential
Sedimentation potential occurs due to dispersed particles motion relative to the fluid under influence of either gravity or centrifugation. This motion disrupts equilibrium symmetry of the particles double layer. Enveloping viscous flow around the particles drags ions of the diffuse layer away from...
, and streaming potential.
Proteins
Proteins often have groups present on their surfaces that can be ionized or deionized depending on pH, making it relatively easy to change the surface charge of a protein. This has particularly important ramifications on the activity of proteins that function as enzymes or membrane channels, mainly, that the protein's active siteActive site
In biology the active site is part of an enzyme where substrates bind and undergo a chemical reaction. The majority of enzymes are proteins but RNA enzymes called ribozymes also exist. The active site of an enzyme is usually found in a cleft or pocket that is lined by amino acid residues that...
must have the right surface charge in order to be able to bind a specific substrate.