Electrokinetics: Overview, History, and Theory

Electrokinetics is the study of the motion of particles and chemical transformation that result from or produce an electric potential difference.

(An electric potential difference can be thought of as a measure of the increase or decrease in the work that is required to move a particle between two points in space when that particle possesses a unit of electric charge). An electric potential difference always exist between two points if these two points have different amount of electric charge.

There are two types of charges: negative and positive charges. The charge of the molecule corresponds to the difference between the number of electrons the chemical species possesses and the number it possesses when it is not charged.

An increase in the number of electrons corresponds to a negative charge, and a decrease in the number of electrons corresponds to a positive charge. All electrokinetic phenomena take place as a result of the fact that particles with like charges repel one another and those with unlike charges are attracted to one another.

Electrokinetics evolved from studies of electricity. In the 18th century Louigi Galvani, although he was not completely aware of it, studied transport across cell membrane possessing a potential difference. Galvanic discovered that muscle in the legs of frog twitched in the presence of a source of electrical charge or when the muscle was touched by a metal.

Allessandro Volta was the first to interpret correctly Galvani's result by pointing out that the twitch was a result of a change in external potential differences. He showed that a potential difference could arise between two different metals immersed in an electrolyte solution, and by the beginning of the 19th century; he had constructed the first battery.

It was early in that century that Michael Faraday was able to show the relationship between the amount of electricity produced and the amount of chemicals that react at an electrode.

By the late part of the 19th century, it was well-known that solutions could conduct electricity. It was at this time that Svante August Arrhenius proposed the concept of ions as electrically charged molecules or atoms, and that it was these species that were responsible for a solution's ability to support an electric current.

Arrhenius proposed that molecules dissociate to a greater or lesser extent into ions. An equilibrium is established between the neutral molecules and the ions. Arrhenius's theory proved to be invalid for molecules that completely dissociate and that consequently produce strong electrolyte solutions.

In 1923, Peter Debye and Erich Huckel formulated a satisfactory theory for a strong electrolyte. An important consequence of their theory is their idea of an ionic atmosphere. The Debye-Huckel theory is valid only for dilute ionic solutions.

Theoretical research since the time they developed their theory has focused on the behavior of more concentrated solutions of ions. Because of the complicated nature of the problem, and large number of particles whose motions must be followed, these theoretical studies involve sophisticated modeling using computers.

The fundamental cause of the behavior of these systems, however, remains the same; molecules with unlike charges attract one another, whereas ones with like charges repel one another.

The effects of charge and potential difference are often experienced by people who first walk on a rug and then touch a doorknob. Excess electrons accumulate on the rug walker, which causes the person to be negatively charged and consequently a potential difference exists between the walker and the doorknob.

The potential is the underlying cause for the flow of electrons from the person to the doorknob, that is, for the electric shock. A potential that becomes more positive in a given direction impedes the movement in that direction of particles with a positive charge and accelerates the movement of particles with a negative charge.

The movement of charged particles in a solution is important. For example, the ocean contains charged atoms and molecules. Electrically charged molecules or atoms are called ions. Ions with negative charge are called anions, and ions with positive charge are called cations. A solution containing ions is called an electrolyte. To make an electrolyte is as simple as dissolving salt in pure water. Some of the salt dissociates into ions.

Usually, molecules in a solution move in a random fashion. The fact that electrolyte solutions contain ions allows an ordering of the molecular motion. It is known that an electrolyte solution conducts electricity. Electricity is the charges carried by ions in the presence of an electric field.

This field can be thought of as imposing a potential difference across the solution, which places a type of order on the ionic motion, in the sense that the ions move along the direction of the field. The anions move towards the positive end of the potential difference, and the cations move towards the negative of the potential difference.

The velocity of the ions, and hence, the electric current, increase with the strength of the electric field.

An electric potential difference is commonly found in situations in which two different phases of different chemical compositions are in contact.

An interface separates the two phases and the electric potential difference can be accomplished through a charge separation such that one side of the interface is positively charged and the other side is negatively charged; one phase could be a metal and the other could be an aqueous solution containing both positively and negatively charged molecules.

Thus, electrokinetics is also the study of electrochemical reactions, i.e., reactions that occur at electrically charged surfaces.

Electro chemical reactions takes place in electrochemical cells, which consist of at least two separate conductors of electricity, called electrodes, that are partially immersed in an electrolyte solution. The interface between the electrode and the electrolyte is referred to as an electrochemical interface.

Electrons are transferred from molecules across the electrochemical interface at one of the electrodes due to chemical reactions. This electrode is called an anode. The electrons move from the anode through an external circuit to a second electrode; this electrode is called the cathode. Here electrons are transferred to molecules through chemical reactions.

The fundamental nature of electrokinetic phenomenon at a single electrode is independent of the processes occurring at other electrodes. Unlike the study of electrochemical cells and batteries in which the potential difference between the anode and cathode is important, the focus of electrokinetics is on the changes in behavior of processes that occur at a charged surface with respect to changes in the potential difference between that surface and the surrounding electrolyte.

A quantity whose change with respect to changes in the electrical potentials across the electrochemical interface is important to measure is the amount of electricity produced per unit of time by the electrode reaction. This quantity is equal to the electric current that flows into the external circuit from an electrode.

The overall rate of an electrode reaction can be obtained from the knowledge of the amount of electricity produced using a relationship established by Michael Faraday in the 19th century.

Faraday discovered that the amount of electricity produced by an electrode reaction over a period of time was proportional to the number of molecules that reacted at the electrode during the same period of time.

Since electrical charge is being transferred, the rates at which the forward and reverse reactions occur depend on the electrical potential between the electrodes and the surrounding electrolytes. Chemical equilibrium exists when the forward electrode reaction has the same rate as the reverse reaction.

The potential at which an equilibrium exist is called an equilibrium potential. The difference between the actual value of the electrical potential and the equilibrium potential is called the overpotential, and the value of the overpotential provides a measure of how far the system is from equilibrium.

The sequence of elementary reactions that occur during the conversion of reactants to products is called the reaction mechanism. In processes with several elementary reactions, the reactant is first to an intermediate; the intermediate may then be converted to a final product or to another intermediate, which subsequently undergoes reaction.

The main process to be considered is the transport of molecules from the bulk of the electrolyte to a charged surface. The majority of ions that exist in the neighborhood of a positively charged surface are anions. On the average, the number of anions decreases, and the number of cations increases.

At sufficiently large distances from the charged surfaces, the solution is neutral. The region in which the solution has a net charge is called a double layer. A major effect caused by the double layer is the influence on the transport of molecules towards the charged surface.

For example, the negatively charged double layer will impede the movement of an anion to a positively charged surface.

The charge distributions and potential differences lead to electrokinetic effects in the solution, which are similar to the effect of the double layer. In solutions containing ions, anions will attract cations as well as neutral molecules, which possess a positively charged part.

Also, cations attract anions and molecules with a part that is negatively charged. Consequently, an ion in a solution is surrounded by an atmosphere of opposite charge. This ionic atmosphere can be thought of as a double layer. The motion of the central ion is slowed as a result of the double layer, because the double layer moves with the central ion.

On the other hand, if an electric field is applied across the solution, the central ion will move in one direction and the double layer, since it is of opposite charge, will move in the other direction. This effect is known as electrophoresis.

Now, moving to the application part, the most visible example is evidenced by corrosion in metals. Corrosion is the deterioration of metals and occurs through chemical reactions in which there is chemical reaction.

Biological examples include the mixing of chlorine in treatment of water and sewages and in addition used as a reactant in the production of pesticides and plastics. Hospitals use electrophoresis to measure the major proteins of blood plasma, and electrophoresis is also employed in biochemistry to separate proteins into their components (amino acids).

References
1. The Feynman Lectures in Physics Volume 2 by Feynman, Richard P., Leighton Robert B., and Sands Matthew
2. The Interpretation of Ionic Conductivity in Liquids by Smedley I. Stuart
3. Electrochemistry: History and Theory by Ostwald Wilhelm, Date N. P.