- Surface charge
Surface charge is the
electric chargepresent at an interface, for instance on the surfaceof a semiconductormaterial, or for example, on the surface of a proteinin water.
There are multiple factors generating surface charge. First of all, surface charge appears practically always on an object
surfacewhen it is placed into a fluid. All fluids contain ions, positive ( cations) and negative ( anions). These ions interact with the object surface. This interaction might lead to adsorptionsome of them on the surface. If number of adsorbed cations exceeds number of adsorbed anions, surface would gain total positive electric charge. This mechanism is important for colloidsand other fluid based heterogeneoussystems.
There is another possible mechanism leading to surface charging in fluids. It is
dissociationor Differential Solubility of the surface chemical group. The two ionic components of crystals like CaCO3, AgBr, BaSO4, and CaC2O4 always obey the bulk solubility equilibrium. E.g. for AgIKsp = [Ag+] [I-]
The thermodynamic parameter used to describe charged surfaces is the surface potential.The surface potential,y0 , of an ionic crystal is related to the bulk concentration of a potential-determining ion by
where z is the valence (including sign) of the potential-determining ion i, whose concentration is cpzc at the point of zero charge , c is the concentration of the potential determining ion, R is the gas constant,F is the Faraday constant and T is the temperature.
This equation relates the (chemical potentials) concentrations in the bulk, electrically neutral solution to that in the crystal.
i.e. AgI(s) can be precipitated by mixing aqueous solutions of AgNO3 and KI in any ratio. The equilibrium concentrations of [Ag+] and [I-] need not be equal. For mre details of the electric surfface charge description and its relation to the
surface chemistryare given by Lyklema in "Fundamentals of Interface and Colloid Science" [ Lyklema, J. “Fundamentals of Interface and Colloid Science”, vol.2, page.3.208, 1995] ,
Other important example is charging of
semiconductors, which originates from a disrupted latticestructure. The presence of surface charge also alters the distribution of charge in the near-surface region of the semiconductor.
In conductors of uniform resistivity at equilibrium, there can be no free charges in the bulk, instead all the
charge densityis on the surface.
In the case of conducting macroscopic bodies surface charge can be measured using electrostatic fieldmeters or
voltmeters can also be used.
In the case of
colloidsand similar heterogeneousfluid based systems, direct measurement of the surface charge is impossible due to small sizes of the objects. Instead, zeta potentialmeasurement yields information for calculation surface charge. Another method is titrationwith appropriate surface active chemical. Some details are given in the book [ Dukhin, A.S. and Goetz, P.J "Ultrasound for characterizing colloids", Elsevier, 2002 ]
The average surface charge density is given by
where is the net amount of charge and is the surface area of the interface.
The measurement of surface charge density has applications in:
Interface and Colloid Scienceas a part of Double Layer, structure that is responsible for Electrokinetic phenomenaand Electroacoustic phenomena
Electrical engineering. Electrostatic potentials and fields for systems of conducting electrodes may be obtained from the surface charge density.
Biology. Surface charge of proteinsand other bio- macromoleculesallows their separation in gel or capillary electrophoresis
* [http://www.semiconductorglossary.com/default.asp?searchterm=surface+charge Semiconductor glossary] .
* [http://scienceworld.wolfram.com/physics/SurfaceChargeDensity.html Surface charge density] .
* [http://galaxy.cofc.edu/pubs/tpt99/node1.html Why Surface Charges] .
* [http://www.dispersion.com/ Dispersion Technology]
Wikimedia Foundation. 2010.