Electrical Potential in Ion Exchange Processes

Electrical Potential in Ion Exchange Processes

Conventional ion exchange processes use chemical reactants in solution for the ion exchange process. However, ion exchange processes are not just chemically driven, are also electrically driven. An example of an electrically driven ion exchange process is electrodialysis, (also known as electrodeionization), where ionizable species are removed from liquids using electrically active media and the electrical potential as a driving force for ion transport. Electrodeionization can also be used for water treatment, separation of electrolytes from non-electrolytes, concentrating or depletion of ionic substances, and exchange of ions between solutions. The end-use applications include food production, treatment of effluents from nuclear industry equipment, separation of amino acids, splitting of salts to produce acids and alkalis, and synthesis of new ionic compounds. The key to successfully using ion exchange membranes (IEMs) in these processes is to understand the ion transport mechanisms that occur.



Here are some important definitions to help understand the some of the terminology used in this post:

  1. Anion Exchange Membranes (AEM): These are materials with positively charged fixed groups; therefore, they exchange anions.
  2. Cation Exchange Membranes (CEM): These are materials with negatively charged fixed groups; therefore, they exchange cations.
  3. Co-ions: These are ions that have the same charge as the functional groups in the ion exchange material. Therefore, they are repelled by the material instead of attracted to it.
  4. Functional groups (ion exchange group) in ion exchange material: Total number of groups that are available to exchange ions in the material.
  5. Counterions (or sometimes I just call this ions): These are the exchangeable groups in the ion exchange material.
  6. Transport number: The fraction of the electric current carried by the species. The transport number is positive for all ions, independent of their charge and zero for electrically neutral species.
  7. Transference number: The number of moles of ionic species transferred by 1 Faraday of electricity through a stationary cross-section in the direction of positive current.

We will also introduce a concept called “Donnan Equilibria” to obtain a basic understanding of the science occurring in ion exchange systems.


Donnan Equilibria

Ion exchange materials can sorb not only ions, but whole chemical entities (i.e., whole salts from surrounding solutions). This phenomenon is usually reversible because the chemical entity can be removed using a pure solvent. Sorption behavior varies depending upon whether the material is a strong or weak electrolyte. Essentially, IEMs can absorb a certain amount of whole chemical molecules without violating the electroneutrality principle. Certain whole molecules and IEMs may be subject to electrostatic forces arising from the presence of charged functional groups and counterions inside the material.

Most ion exchange materials restrict the co-ions that can enter the material. This restriction is not due to the electroneutrality law because theoretically co-ions could transfer into the material; however, this often does not occur. A concept that has been developed to explain this phenomenon is the Donnan potential. The Donnan potential explains why one kind of ion (counterions) are free to migrate through the IEM and into the surrounding solution and another kind of ions (co-ions) cannot penetrate into the ion exchange material.

The Donnan model says that only a few ions migrating out of a neutral phase are enough to create a high electric potential between the IEM and the solution. This electric potential is enough to prevent diffusion-driven migrations of oppositely charged ions. The Donnan potential pulls counterions back into the IEM and repels co-ions back into the solution. Therefore, the entire IEM has an overall charge. An equilibrium is established to level out the concentration difference and this results in an electric field. Although electric potential is the driving force, there is no deviation from electroneutrality between the two phases (membrane and solution). Therefore, the difference between electric potentials of the two phases is the Donnan potential. The Donnan potential affects ionic distribution at the interface and thus affects the interface transfer processes. Figure I illustrates the Donnan exclusion concept.

Figure 1. Donnan exclusion in CEM: The Donnan potential is illustrated when anions try to diffuse into the CEM. The electrical charge of each phase does not allow a considerable amount of anions into the CEM.


Electrical Current in Ion Exchange Mediums

Like any homogenous liquid ionic mixture, ion exchange materials filled with water are electrically conductive. The fixed ionic groups attached to a polymeric matrix allows the mobile ions in the liquid phase to jump from one functional group to another to transfer the ions through the material. The electrical conductivity of the ion exchange membrane (IEM) is determined by the concentrations and mobilities of charge carriers. The concentration of ion exchange groups is usually around 1 molarity (1 M), which makes the electrical conductivity considerably high. However, the electrical conductivity is less than in aqueous solutions of the same concentrations. In IEMs, the diffusion is slower than in ionic solutions due to the reduced amount of water in the material and the presence of the bulky polymer. The conductivity of IEMs in influenced by:

  • Large concentration of counterions in the solution, which can provide excess counterions in the material
  • Low-degree of cross-linking, which promotes faster diffusion
  • Counterions of small size and low valence, to travel quickly through the material
  • Elevated temperatures, to enhance the diffusion process

The molar conductivity of IEMs are more sensitive to ionic charge than ions in solution because of stronger electrostatic and hydrogen interactions between the counterions and functional groups of the polymer. If more than one counterion is present with the same charge, the molar conductivity will:

  1. Increase with the increase of ionic size in a non-hydrated state
  2. Decrease with increase of ionic size in the hydrated state

This effect is stronger in cation exchange materials (CEMs) than in anion exchange materials (AEMs). Due to the effect of ionic size, the fastest current carriers in CEMs are H+ ions and OH- ions in AEMs. As we discussed previously, the conductivity of the IEM is also dependent upon the concentration and composition of the surrounding solution. If the concentration is at a certain level, the Donnan exclusion does not prevent co-ions from entering the IEM. Due to this, the ion carriers increase and, thus, electrical conductivity of the material increases. If an electrical field is applied to an ion exchanger, the ions move in the direction of the oppositely charged electrode, and this creates an electrical current. In a very short time, an equilibrium is established between the friction of the surrounding medium and the accelerating force of the electrical field. Therefore, there is a constant transfer of ions and electrical current throughout the system.

There are fundamental differences between species transfer in solutions and ion exchange materials. In solutions, the concentrations of the cations and anions are equivalent; which means that the ratio of transference numbers is determined by the ratio of ionic mobilities. When an electric field is applied to an ion exchange medium, the transport number of counterions is almost unity. Hence, both cations and anions are exposed to an equal overall force from the electric field and there is an equal frictional force from the cations and anions, despite their valence charge and rate of migration. Due to this, the current transport in liquids is rather stationary. The friction momentum applied by the ions are defined solely by the different strengths of interactions between the charge carriers and the solvent.

In the case of ion exchange materials, the mobility of one type of ion participating in the reaction is zero because the functional group is attached to the polymer matrix. Only the ions that are moving apply the frictional momentum to the internal solvent and carry the solvent in the same direction. Therefore, the charge carriers move faster than in stationary solutions. This increase in conductivity is caused by solvent transfer and is called convection conductivity. If there are co-ions in the IEM, there will be two-directional movement of charge carriers and reduced conductivity. It is obvious that the presence of co-ions in the exchanger phase (reduced Donnan exclusion) causes two-directional movement of charge carriers, thus reducing the convection conductivity.


Ion Exchange Membranes

Ion exchange membranes are an extremely thin polymer matrix layer, often measured in microns. Depending upon the ion exchange process, the layer may be cut or formed into different sizes and shapes. Ion exchange membranes often serves as a separator between two different liquids, but they can also separate between different solid material layers like in fuel cells. In both the liquid and solid states, the ions are free to pass through the membrane. Therefore, they can easily move between compartments, complying with the concept of electroneutrality.

Due to the Donnan principle, co-ions act differently in a membrane compared with a solution because they have difficulty passing through the membrane. However, if the membrane is exposed to a solution with a high concentration, the permselectivity is suppressed, and the transfer of co-ions through the membrane is promoted. The membrane system usually obtains equilibrium with the ions in the liquids or solids on either side, while the co-ions remain separated. In addition to the different permeability (depending upon the charge of the ions), the permselectivity causes a difference in electrical potential between the two solutions/solids.

All ion exchange membranes have a high electrical conductivity and a high ionic permeability. The electrical conductivity can be increased by increasing the ionic charge density, but sometimes this can also lead to extremely high and undesirable swelling. Therefore, the material must be sufficiently cross-linked to prevent excess swelling, which can affect the material properties. The basic characteristics of good ion exchange are high permselectivity, high electrical conductivity, moderate swelling, and high mechanical strength. Electrical conductivity is a characteristic of IEMs, but it is not constant and is dependent upon the number of functional groups, ionic composition, and amount of water content. Therefore, the membrane conductivity values provided on the manufacturer specification sheets are based upon certain conditions.



Ion exchange processes are both chemically and electrically driven. The water and many of the products that we use on a daily basis were partially derived from membrane-based processes. The key to successfully selecting and using ion exchange membranes (IEMs) in these processes is to understand the ion transport mechanisms that occur.

Dr. Colleen Spiegel Posted by Dr. Colleen Spiegel

Dr. Colleen Spiegel is a mathematical modeling and technical writing consultant (President of SEMSCIO) and Professor holding a Ph.D. and an MSc degree in Engineering. She has seventeen years of experience in engineering, statistics, data science, research & technical writing work for many companies as a consultant, employee, and independent business owner. She is the author of ‘Designing and Building Fuel Cells’ (McGraw-Hill, 2007) and ‘PEM Fuel Cell Modeling and Simulation Using MATLAB’ (Elsevier Science, 2008). She previously owned Clean Fuel Cell Energy, LLC, which was a fuel cell organization that served scientists, engineers, and professors world-wide.

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