In nature, the majority of gases, liquids, and solids are not charged (in the neutral form). Ion exchange, where free ions are exchanged for different ions, occurs when there is an open network structure to carry the ions through it. There are many natural and man-made mediums that are ion exchangers, including solids, liquids, and gases. The medium needs to be in contact with the ion exchanger and these two entities exchange some of its ions for similarly charged ions. The medium is often a solid ion exchanger in contact with an aqueous solution or gas. If you recall from chemistry, there are two types of ions:
Cation: A positively charged ion (examples include Na+ and Fe3+)
Anion: A negatively charged ion (examples include Br- and SO42-)
The ionic charge of representative elements (groups IA, IIA, IIIA, IVA, VA, VIA, and VIIA on the periodic table) dictate the charge on the ion. The transition elements (the groups in the center of the periodic table) typically have one or two known charges associated with those elements. There are also polyatomic ions (i.e., SO42-), which are groups of elements that have a known charge. Polyatomic ions can be either cations or anions.
We can represent an ionic exchanger as A+X-, where A+ is the soluble ion. If AX is placed into a solution that contains salt BY, in the solution it ionizes to give the ions B+ and Y-, the exchange reaction can be written as follows:
This represents a simple displacement reaction between two salts AX and BY. Since the Y is not taking part in the reaction, the equation can be simplified to:
In this example, cations are exchanged; however, anions can be exchanged in the same manner. If you recall from chemistry, when looking at chemical reactions, two ions that have the same charge does not necessarily mean that they can swap places – it also depends upon the relative activity of the ion. There are charts in chemistry called “Activity Series” that you can use to predict whether one ion will replace another ion.
The simplest form of ion exchange reactions are in the liquid phase, which is what occurs inside many battery systems. If we take two solutions, one containing Zn(CN)2 and the other containing Hg(NO3)2 and mix these two solutions, an ion exchange reaction occurs:
Due to the much higher stability of Hg(CN)2 the mercury ions replace the zinc in this chemical equation. The process of ion exchange resembles “sorption,” a physical and chemical process where one substance becomes attached to another. The difference between ion exchange and sorption is that every ion is removed from solution and replaced by an equivalent charged ion. Sorption takes up ions without directly replacing an equal amount with the same charge.
Organic Ion Exchangers
Ion exchangers are insoluble materials that carry a certain number of fixed ions. These ions can be exchanged with other ions that have the same sign in stoichiometric proportion. A conventional class of ion exchangers are functional polymers with a long polymer chain that makes up the majority of the molecule with reactive groups (functional group - charged acidic, basic, or chelating groups) at certain locations throughout the molecule. Acids that have long hydrocarbon chains (i.e., aliphatic acids H3C – (CH2)n – COOH, with n >16) are solid at ambient conditions and have one carboxylic functional group per molecule. When immersed in an aqueous solution, the functional group can exchange ions with ions in the surrounding environment. However, one functional group per molecule is not usually sufficient for rapid ion exchange. Therefore, there are often multiple functional groups per molecule to facilitate fast ion exchange.
In a solid, only groups near the surface (inter-phase boundary) can reach ions dissolved in the aqueous phase. The ion exchange of functional groups at the surface can be written as:
where A and B are the ions and the double bar indicates the surface location. Due to the hydrophilic characteristic of the charged groups, these polymeric molecules are water soluble. Therefore, to prevent these molecules from dissolving in water, the polymer has to be cross-linked to form a 3-D polymeric structure. Polymers that are cross-linked are not soluble in water, but they can swell with a high degree of water content. This allows water molecules and ions to travel throughout the water-filled polymer network and participate in the ion exchange reactions.
Figure 1. Cross-linked polymer with carboxylic groups.
Cross-linking density: The density of cross-links between polymeric chains is termed cross-linking density. The number of crosslinks can make the polymer structure rigid or gel-like – it influences the elasticity, swelling ability, and mobility of the ions traveling through the matrix. Materials that have high cross-linking densities are rigid and often more stable. However, diffusion of ions can be slow. Materials with low crosslinking densities can be softer and more jellylike and enable faster ion interactions. Therefore, selecting the appropriate material will depend upon stability and ion mobility rates required.
Anion and cation exchangers: Ion exchange materials are classified into cation exchangers and anion exchangers. Cation exchangers have negatively charged functional groups and carry exchangeable cations. Anion exchangers carry anions due to the positive charge of their functional groups.
Figure 2. Pictorial example of polymeric ion exchangers: (a) cross-linked cation exchange material, and (b) anion exchange material (1 – Polymeric chain; 2 – cross-link; 3 – physical knot; 4 – negatively charged cation exchange group; 5 – positively charged anion exchange group; 6 – counterion; 7 – water).
Functional Groups: There is a wide variety of functional groups that can attach to polymer chains for ion exchange. Cation exchange materials contain negatively charged groups, such as phosphate, sulfate, and carboxylate, attached to the polymer backbone material. Anion exchange material contains positively charged groups such as amino groups and alkyl substituted sulfides attached to the polymer backbone. Common cation exchangers are strongly acidic resins with sulphonic acid groups (SO3-) and weak acid polymers with carboxylic acid groups (COO-). Ion exchangers with the same functional groups can have vastly different properties to the type of hydrocarbon backbone. There are many other materials that have been developed to match a wide variety of requirements, such as phosphonic, phosphinic, arsonic, selenonic acid groups. Most anion exchangers have functional groups containing nitrogen as the proton accepting atom. There are both weak and strong base anion exchangers along with quaternary phosphonium and tertiary sulphonium ion exchange groups.
Inorganic Ion Exchangers
Besides polymeric ion exchangers, there is also inorganic ion exchangers. Polymer ion exchangers are often used for room-temperature applications because of their mechanical and chemical stability, but inorganic materials have the unique ability to have high stability at elevated temperatures and in the presence of radiation. They have unique applications for nuclear waste, catalytic applications, and high temperature fuel cells. The chemical composition of inorganic ion exchange materials can be diverse, and a list of some commonly used materials are zeolites, silicotitanates, tin(IV) compounds, manganese compounds, hexacyanoferrate compounds, hydroxides, and oxides. Ion exchange of these materials occur through layers or cavities, with exchangeable cations on the internal surfaces.
One of the most common ion exchangers are zeolites, which are synthesized in a wide diversity of variations. Zeolites are inorganic polymers built from tetrahedral MeO4 where Me is Si4+ or Al3+ ion. The chemical composition of zeolites is usually represented by the following empirical formula:
where A is the counterion with valence Z, y accounts for the SiO2/Al2O3 ratio and W is the water content in the hydrated form of the zeolite. The value of y is usually between 2 and 10 but can be up to 100 in “high-silica” zeolites. A pictorial illustration of a zeolite with general formula (MeO4)m is shown in Figure 3. In (a), the structure consists of SiO2 units (silica). The positive charges of Si4+ ions are balanced by the negative charges from the O2-. In (b), there is a partial replacement of Si4+ with Al3+ ions. The ion replacement results in excess negative charge compensated by the presence of counterions that are not included in the oxide units. Counterions are exchangeable ions carried by ion exchangers that can freely move within the framework but their movement is compensated by counter-movements of other ions of the same charge to fulfil the electroneutrality principle. The counterions are held in the crystalline pores and are not shown in 3(b) for simplicity.
Figure 3. Pictorial illustration of a zeolite with general formula (MeO4)m (a) Non-charged structure of silica and (b) charged structure of a zeolite.
Ion Exchange Capacity
Ion exchange capacity is a significant characteristic of ion exchange materials. An ion exchange material can be thought of as a material containing a certain amount of fixed charge sites available to counterions. This statement can be expressed as the number of counterion equivalents in a specified amount of material. It is difficult to compare ion exchange capacities of different materials because the availability of the functional groups for exchange reactions never achieved 100%. Different fractions of the functional groups participate in the process depending upon properties such as swelling degree, ion size, and the contact boundary between the phase of the ion exchanger and surrounding media. Therefore, sometimes a conditional capacity (capacity exhibited under certain particular conditions) is used to describe and compare ion exchangers.
The most precise method of estimating the ion exchange capacity is using a theoretical capacity, which is the number of functional groups per unit weight of the dry ion exchanger. This can be obtained by directly analyzing the material. For example, each atom of sulfur corresponds to one functional group in a sulphonated polymer; therefore, the number of functional groups can be derived from the sulfur content. The theoretical capacity can be expressed as:
Where ms is the mass of sulfur, mr is the mass of the polymer, W is the water fraction in the sample, and 32 is the atomic weight of sulfur. There are other ways of estimating QTheor also, such as from the synthesis conditions. The theoretical capacity does not have practical use, but it allows a comparison of membranes under ideal (theoretical) conditions.
There are a number of chemical and physical properties that can be used to describe and compare ion exchange materials. Some of the chemical properties are (1) the type of matrix, (2) cross-linking degree, (3) functional group types, (4) ion exchange capacity, and (5) ionic form. Relevant physical properties include (1) physical structure and morphology, (2) surface area, (3) pore size, and (4) volume or thickness in the swollen state. However, the actual material characteristics are dependent upon the type of functional groups and matrix, cross-linking degree, ion exchange capacity, surface contact area, available ions, water amount, and composition of the surrounding media among other variables.