Surface charge
In chemistry, there are many different processes which can lead to a surface being charged, including adsorption of ions, protonation or deprotonation, and, as discussed above, the application of an external electric field.
Surface charge emits an electric field, which causes particle repulsion and attraction, affecting many colloidal properties.
According to Gauss’s law, 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:
This equation is only strictly accurate for conductors with infinitely large area, but it provides a good approximation if E is measured at an infinitesimally small Euclidean distance from the surface of the conductor.
A solution with a higher concentration of electrolytes also increases the size of the counter-ion cloud.
[10] A solution's pH can also greatly affect 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.
An interface is defined as the common boundary formed between two different phases, such as between a solid and gas.
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 model dubbed the 'electric double layer' was first introduced by Hermann von Helmholtz.
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.
To maintain electrical neutrality the charge of the electrode will be balanced by a redistribution of ions close to its surface.
The attracted ions thus form a layer balancing the electrode's charge.
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 kinetic energy of the counter ions will, in part, affect the thickness of the resulting diffuse double layer.
, the counter ion concentration in the external solution, is the Boltzmann factor:
This however is inaccurate close to the surface, because it assumes that molar concentration is equal to activity.
The Otto 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.
For example, solutions of large colloidal particles depend almost entirely on repulsion due to surface charge in order to stay dispersed.
[15] 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 flocculate.
[16] Electrokinetic phenomena 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.
[17] Electrophoresis is widely used in biochemistry to distinguish molecules, such as proteins, based on size and charge.
This has particularly important ramifications on the activity of proteins that function as enzymes or membrane channels, mainly, that the protein's active site must have the right surface charge in order to be able to bind a specific substrate.
Polymers are very useful in this respect in that they can be functionalized so that they contain ionizable groups, which serve to provide a surface charge when submerged in an aqueous solution.
