Electrokinetics

Type of physical science: Chemistry

Field of study: Chemical processes

Electrokinetics is the study of chemical reactions and molecular motion that can cause (or result from) the division of material into parts with properties that can induce an electrical current between those parts. Such studies enable scientists to improve the design of electrochemical devices (such as batteries) and processes (such as electrosynthesis).

Overview

Electrokinetics is the study of the motion of particles and chemical transformations 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 exists between two points if these points have different amounts of electric charge. There are two types of electrical charges: negative charges and positive charges. The charge on a molecule or an atom 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 occur as a result of the fact that particles with like charges repel one another and those with unlike charges are attracted to 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 (the person can also become positively 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 electrical 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 negative charge.

The movement of charged particles in solution is important. For example, the oceans contain 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 solution move in a random fashion. The fact that electrolyte solutions contain ions allows an ordering of the molecular motion. It is known that electrolyte solutions conduct electricity. The electricity is the charges carried by the 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 toward the positive end of the potential difference, and the cations move toward the negative end of the potential difference. The velocity of the ions and, hence, the electrical 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 composition 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. An area of chemistry in which this situation is common is electrochemistry. Thus, electrokinetics is also the study of electrochemical reactions, that is, reactions that occur at electrically charged surfaces.

Electrochemical reactions take 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 an 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 by means of 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, and here electrons are transferred to molecules through chemical reactions. The fundamental nature of electrokinetic phenomena at a single electrode is independent of the processes occurring at other electrodes, and therefore the discussions here will focus on processes at one charged surface. Unlike the studies 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 potential 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 electrical 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 by using a relationship established by Michael Faraday in the nineteenth 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.

In order to describe the effect of the electrical potential difference between a charged metal surface and the surrounding electrolyte on chemical reactions, a general example of electrode reactions will first be discussed. In this example, a chemical species, R, is transformed at the surface of an electrode into the species O, and at the same time electrons are transferred to the electrode. The reverse of this reaction can also occur at the electrode, that is, O is transformed into the species R. These reactions are represented by the following chemical equation R → O + ne, where n denotes the number of electrons (e) that are transferred across the electrochemical interface for each molecule that reacts. When the forward reaction occurs (R → O + ne), n electrons are transferred across the electrochemical interface from a molecule of R to the electrode, and R is transformed into the molecule O. The molecule R is said to be oxidized by the reaction, and O is called the oxidized species. During the reverse reaction O + ne → R, n electrons are transferred from the electrode to each molecule of O that reacts, and O is transformed into R. The molecule O is said to be reduced by the reaction (because it gains negative charge), and R is called the reduced species.

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

A useful principle to apply in order to anticipate what happens when an overpotential is applied is that opposite charges attract, and that the greater the difference in the amount of charge, the greater the attraction. Increasing the potential difference between the electrode and the solution causes the electrode to appear more positive to the molecules in solution.

Consequently, changing the potential to values that are greater than the equilibrium potential increases the rate of the transfer of electrons, which are negatively charged, to the electrode.

Hence, the rate of the oxidation of R, that is, the rate of the forward reaction, increases. On the other hand, when a change in electrical potential difference makes it easier for electrons to be transferred in one direction, this change makes it more difficult to transfer electrons in the opposite direction. Hence, the rate of the reduction of O, that is, the rate of the reverse reaction, decreases. Consequently, there is a net flow of electrons across the electrochemical interface to the electrode.

Decreasing the potential to values that are less than the equilibrium value makes the electrode appear negative to the surrounding solution, and thus causes the net flow of electrons across the interface to be from the electrode into the electrolyte.

The effect of the electrical potential on the rate of reaction is important. By controlling the potential, one can control the rate of a reaction. Also, the potential can often be set at a value for which a desired reaction occurs at a reasonable rate, and for which other, undesired reactions occur at slow rates.

Almost all electrode processes involve more than one elementary reaction. 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 converted to an intermediate; the intermediate may then be converted to the final product or to another intermediate, which subsequently undergoes reaction. Some of the elementary reactions may not depend on the electrode potential, and some may not even occur at the electrode.

Returning to the motion of molecules in electrolyte solutions, the next process to consider is the transport of molecules from the bulk of the electrolyte to a charged surface. The molecular motion in the immediate vicinity of the charged surface is different from the motion of molecules at large distances from the 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, as the distance from the positively charged surface increases. At sufficiently large distances from the charged surface, 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 toward 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 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 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.

Applications

That electrical potential differences can occur spontaneously in nature is evidenced by corrosion. Corrosion is the deterioration of metals and occurs through chemical reactions in which there is electron transfer. The chemical reactions convert the metal to molecules known as metal oxides, which consist of metal atoms and oxygen atoms. Potential differences arise in different parts of the metal. Parts of the metal behave as if they were charged positively and receive electrons from other metal atoms. The latter metal atoms become metal cations.

Neighboring parts of the metal that receive electrons become negatively charged. Reaction of gases, such as oxygen, and liquids, such as water, produce anions at these parts of the metal. The final step in corrosion is reaction of the metal cations with the anions. Rust is an example of a product of the final step that occurs during the corrosion of iron and steel. The electrokinetics of corrosion is studied in laboratories by using the metal as an electrode. By adjusting the potential across the electrochemical interface, one can subject the metal to severe conditions that accelerate corrosion. These studies have led to the development of films and inhibitors that help prevent the corrosion of metals.

Potential differences occur naturally in biological systems. To understand their formation, first consider the consequence of a potential difference applied across a membrane separating two electrolyte solutions. Ions, which are permeable to the membrane, will flow from one side to the other until one side will eventually have an excess of negative charges and the other side will have an excess of positive charges. A potential difference will arise naturally if a membrane separates two solutions in which the amount of charge is already different in the two solutions. This is how potential differences form across biological membranes. Such potential differences can be used to perform work. A biological cell can use the potential difference to aid the transport of charged nutrients into the cell.

Scientists have learned to reap benefits by imposing potential differences at electrodes.

Such potential differences provide a driving force for chemical synthesis of important compounds. In this type of synthesis, electrical current is employed to bring about chemical change; the general name for this process is electrolysis. Almost all chlorine gas is made through electrolysis. Chlorine is used as a reactant in the production of pesticides and plastics, and, in addition, it can be used in the treatment of water and sewage. Thus, the production of chlorine though electrolysis is an important industrial process. Chlorine gas is produced at a graphite electrode immersed in a salt brine (salt dissolved in water). Pure, solid aluminum is also produced through electrolysis. Molten aluminum can be formed at a graphite electrode in an electrolyte consisting of aluminum ore heated to around 1,000 degrees Celsius. Aluminum is produced at the cathode, and oxygen gas is produced at the anode. Many organic molecules are also synthesized through electrolysis. The study of the electrokinetic mechanism, as well as the effects of potential and electrolyte solutions, has led to improved efficiencies in chemical processes that use electrolysis.

The double layer, or the ionic atmosphere, that forms around a central ion also has practical consequences. Large aggregates of small molecules that are highly insoluble in solutions are called lyophobic colloids. A colloid dispersion can be used in soaps and detergents.

These colloids remain stable as a result of the formation of a double layer. The outer sphere is of opposite charge to the inner part of the colloid. Without the double layer, two colloids can come close enough that their interaction will pull them apart. The repulsion of the like charges belonging to the outer part of the two double layers keeps any two given colloid particles separated.

The fact that electrolytes can support an electrical current has many practical consequences. Measurement of the current that passes through a solution can be used to determine the number of ions in that solution. These measurements have practical applications in testing water purity and the concentration of ions in body fluids. Different ions and molecules move at different rates in electric fields. Consequently, electrical separation through electrophoresis is used to break down mixtures or large molecules into components. Hospitals use electrophoresis to measure the major proteins in blood plasma, and electrophoresis is also employed in biochemistry to separate proteins into their components (amino acids).

Context

Electrokinetics evolved from studies on electricity. In the eighteenth century, Luigi Galvani, although he was not completely aware of it, studied transport across cell membranes possessing a potential difference. Galvani discovered that muscle in the legs of frogs twitched in the presence of a source of electrical charge or when the muscle was touched by a metal.

Alessandro Volta was the first to interpret correctly Galvani's results by pointing out that the twitch was a result of a change in external potential differences. Volta showed that a potential difference could arise between two different metals immersed in an electrolyte solution, and by the beginning of the nineteenth 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 nineteenth 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 electrical current. Arrhenius proposed that molecules dissociate to a greater or lesser extent into ions. An equilibrium is established between the neutral molecules and their ions. Arrhenius' theory proved to be invalid for molecules that completely dissociate and that consequently produce strong electrolyte solutions. In 1923, a satisfactory theory for strong electrolytes was formulated by Peter Debye and Erich Huckel. An important consequence of their theory is the 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 the large number of particles whose motion must be followed, these theoretical studies involve sophisticated modeling using computers. The fundamental cause of the behavior in these systems, however, remains the same: molecules with unlike charges attract one another, whereas like charges repel one another.

Principal terms

ANION: an ion that possesses negative charge

CATION: an ion that possesses positive charge

CHARGE: a fundamental property of subatomic particles; it is of two types, negative and positive; unlike charged particles attract one another, and like charges repel one another

CURRENT: a term representing a flow of charge, that is, the number of electric charges that pass through a cross section of material per unit time

ELECTRIC POTENTIAL: the amount of work required to move a unit positive charge between two points

ELECTROLYTE: a solution that contains ions

ELECTRON: a subatomic particle with a unit negative charge

ELECTROPHORESIS: the movement of electrically charged particles in an electric field

IONS: molecules or atoms that possess an electric charge

OXIDATION-REDUCTION REACTIONS: chemical reactions in which electrons are transferred; molecules or atoms that lose electrons during the reaction are said to be oxidized, and those that gain electrons are said to be reduced

PROTON: a subatomic particle with a unit positive charge

Bibliography

Bard, Allen J., and L. R. Faulkner. ELECTROCHEMICAL METHODS: FUNDAMENTALS AND APPLICATIONS. New York: John Wiley & Sons, 1980. An advanced and comprehensive book that covers the fundamentals of electrode reactions and behavior in electrolyte solutions.

Feynman, Richard P., Robert B. Leighton, and Matthew Sands. THE FEYNMAN LECTURES ON PHYSICS. Vol. 2. Reading, Mass.: Addison-Wesley, 1963. Although written on electromagnetism for physics students, this book has several sections that provide excellent information regarding the properties of charges, electricity, electric potential, and current, in particular, section 1-1 and chapter 9.

Fuoss, Raymond M., and Filippo Accascina. ELECTROLYTIC CONDUCTANCE. New York: Interscience, 1959. Detailed descriptions of both electrokinetic behavior in electrolyte solutions and applications.

Ostwald, Wilhelm. ELECTROCHEMISTRY: HISTORY AND THEORY. New Delhi: Amerind Publishing Co., 1980. The history of electrokinetic phenomena is traced from the time that the scientific fields of electricity and chemical reactions were first combined in experiments to the time that Arrhenius proposed that molecules dissociate into ions. Details are presented of the events that led to both discoveries and misconceptions. Here, the theory of electrolytes and of electrode processes can be followed without any mathematical background.

Smedley, I. Stuart. THE INTERPRETATION OF IONIC CONDUCTIVITY IN LIQUIDS. New York: Plenum Press, 1982. A useful source for the interested reader.