Ion-association
Encyclopedia
Ion-association is a chemical reaction
whereby ion
s of opposite electrical charge come together in solution
to form a distinct chemical entity. Ion-associates are classified according to the number of ions that associate with each other, and the nature of the interaction. The most important factor to determine the extent of ion-association is the dielectric constant
of the solvent
. Ion-associates have been characterized by means of vibrational spectroscopy.
There are three distinct types of ion-pair, depending on the extent of solvation
of the two ions.
In the schematic representation above, the circles represent spheres. The sizes are arbitrary and not necessarily similar as illustrated. The cation is coloured red and the anion is coloured blue. The green area represents solvent molecules in a primary solvation shell. For the purposes of this presentation secondary solvation is ignored. When both ions have a complete primary solvation sphere the ion-pair may be termed fully solvated. When there is about one solvent molecule between cation and anion, the ion-pair may be termed solvent-shared. Lastly when the ions are in contact with each other the ion-pair is termed a contact ion-pair. Even in a contact ion-pair, however, the ions retain most of their solvation shell. The nature of this solvation shell is generally not known with any certainty. In aqueous solution and in other donor solvents metal cations are surrounded by between four and nine solvent molecules in the primary solvation shell
, but the nature of solvation of anions is mostly unknown.
An alternative name for a solvent-shared ion-pair is an outer-sphere complex. This usage is common in co-ordination
chemistry and denotes a complex between a solvated metal cation and an anion. Similarly a contact ion-pair may be termed an inner-sphere complex. The essential difference between the three types is the closeness with which the ions approach each other fully solvated > solvent-shared > contact. With fully solvated and solvent shared ion-pairs the interaction is primarily electrostatic, but in a contact ion-pair there will also be some covalent character in the bond between cation and anion.
An Ion-triplet may be formed from one cation and two anions, or from one anion and two cations. Higher aggregates, such as a tetramer, (AB)4 may be formed.
Ternary ion-associates involve the association of three species. Another type, named intrusion ion-pair has also been characterized.
F is the force of attraction, q1 and q2 are the magnitudes of the electrical charges, ε is the dielectric constant
of the medium and r is the distance between the ions. For ions in solution this is an approximation because the ions exert a polarizing effect on the solvent molecules that surround them which attenuates the electric field somewhat. Nevertheless some general conclusions can be inferred.
The equilibrium constant for ion-pair formation, K, like all equilibrium constants, is related to the standard free energy change.
R is the gas constant
and T is the absolute temperature
. Free energy is made up of an enthalpy
term and an entropy
term.
The coulombic energy released when ions associate contributes to the enthalpy
term, ΔH
. In the case of contact ion-pairs, the covalent interaction energy also contributes to the enthalpy, as does the energy of displacing a solvent molecule from the solvation shell of the cation or anion. The tendency to associate is opposed by the entropy
term which results from the fact that the solution containing un-associated ions is more disordered than a solution containing associates. The entropy term is similar for electrolytes of the same type, with minor differences due to solvation effects. Therefore it is the magnitude of the enthalpy term that most determines the extent of ion-association for a given electrolyte type. This explains the general rules given above.
is the most important factor in determining the occurrence of ion-association. The following table gives some typical values.
Water has a relatively high dielectric constant at 25°C, so in aqueous solutions at ambient temperatures 1:1 electrolytes such as NaCl do not form ion-pairs to an appreciable extent except when the solution is very concentrated. 2:2 electrolytes (q1=2, q2=2) form ion-pairs more readily. Indeed the solvent shared ion-pair [Mg(H2O)6]2+SO42- was famously discovered to be present in seawater, in equilibrium with the contact ion-pair [Mg(H2O)5(SO4)] Trivalent ions such as Al3+, Fe3+ and lanthanide
ions form weak complexes with monovalent anions.
The dielectric constant of water decreases with increasing temperature to ca. 55 at 100 °C and ca. 5 at the critical temperature, 217.7 °C. Thus ion-pairing will become more significant in superheated water
.
Solvents with a dielectric constant in the range, roughly, 20-40, show extensive ion-pair formation. For example, in acetonitrile both contact and solvent-shared io pairs of Li(NCS) have been observed. In methanol the 2:1 electrolyte Mg(NCS)2 is partially dissociated into a contact ion-pair, [Mg(NCS)]+ and the thiocyanate ion.
The dielectric constant of liquid ammonia decreases from 26 at its freezing point
, −80 °C to 17 at 20 °C (under pressure). Many simple 1:1 electrolytes form contact ion-pairs at ambient temperatures. The extent of ion-pairing decreases as temperature decreases. With lithium salts there is evidence to show that both inner-sphere and outer-sphere complexes exist in liquid ammonia solutions.
Of the solvents with dielectric constant of 10 or less, tetrahydrofuran is particularly relevant in this context as it solvates cations strongly with the result that simple electrolytes have sufficient solubility to make the study of ion-association possible. In this solvent ion association is the rule rather than the exception. Indeed, higher associates such as tetramers are often formed. Triple cations and triple anions have also been characterized in THF solutions.
Ion-association is an important factor in phase-transfer catalysis since a species such as R4P+Cl− is formally neutral and so can dissolve easily in a non-polar solvent of low dielectric constant. In this case it also helps that the surface of the cation is hydrophobic.
In SN1 reactions the carbocation intermediate may form an ion-pair with an anion, particularly in solvents of low dielectric constant, such as diethylether. This can affect both the kinetics of the reaction and the stereochemistry of the reaction products.
and Raman spectroscopy
have been used. Anions containing a CN group, such as cyanide
, cyanate
and thiocyanide have a vibration frequency a little above 2000 cm−1 which can be easily observed as the spectra of most solvents (other than nitrile
s) are weak in this region. The anion vibration frequency is "shifted" on formation of ion-pairs and other associates and the extent of the shift gives information about the nature of the species. Other monovalent
anions which have been studied include nitrate
, nitrite
and azide
. Ion-pairs of monatomic anions, such as halide
ions, cannot be studied by this technique. NMR spectroscopy
is not very useful as association/dissociation reactions tend to be fast on the NMR time scale, giving time-averaged signals of the cation and/or anion.
Nearly the same shift of vibration frequency is observed for solvent-shared ion-pairs of LiCN, Be(CN)2 and Al(CN)3 in liquid ammonia. The extent of this type of ion-pairing decreases as the size of the cation increases. Thus, solvent-shared ion-pairs are characterized by a rather small shift of vibration frequency with respect to the "free" solvated anion and the value of the shift is not strongly dependent on the nature of the cation. The shift for contact ion-pairs is, by contrast, strongly dependent on the nature of the cation and decreases linearly with cations's charge:radius squared ratio in the two series Cs+ > Rb+ > K+ > Na+ > Li+, and Ba2+ > Sr2+ > Ca2+.
The extent of contact ion-pairing can be estimated from the relative intensities of the bands due to the ion-pair and free ion. It is greater with the larger cations . This is counter to the trend expected if coulombic energy were the determining factor. Instead, the formation of a contact ion-pair is seen to depend more on the energy needed to displace a solvent molecule from the primary solvation sphere
of the cation. This energy decreases with the size of the cation, making ion-pairing occur to a greater extent with the larger cations. The trend may be different in other solvents.
Higher ion-aggregates, sometimes triplets M+X-M+, sometimes dimers
of ion pairs (M+X-)2, or even larger species can be identified in
the Raman spectra of some liquid ammonia solutions of Na+ salts by the presence of bands which cannot be attributed to either contact- or solvent-shared ion-pairs.
Evidence for the existence of fully solvated ion-pairs in solution is mostly indirect as the spectroscopic properties of such ion-pairs are indistinguishable from those of the individual ions. Much of the evidence is based on the interpretation of conductivity
measurements.
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
whereby ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s of opposite electrical charge come together in solution
Solution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
to form a distinct chemical entity. Ion-associates are classified according to the number of ions that associate with each other, and the nature of the interaction. The most important factor to determine the extent of ion-association is the dielectric constant
Dielectric constant
The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...
of the solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
. Ion-associates have been characterized by means of vibrational spectroscopy.
Classification
Ion-pairs are formed when a cation and anion come together.- An+ + Bm- AB(n-m)+
There are three distinct types of ion-pair, depending on the extent of solvation
Solvation
Solvation, also sometimes called dissolution, is the process of attraction and association of molecules of a solvent with molecules or ions of a solute...
of the two ions.
In the schematic representation above, the circles represent spheres. The sizes are arbitrary and not necessarily similar as illustrated. The cation is coloured red and the anion is coloured blue. The green area represents solvent molecules in a primary solvation shell. For the purposes of this presentation secondary solvation is ignored. When both ions have a complete primary solvation sphere the ion-pair may be termed fully solvated. When there is about one solvent molecule between cation and anion, the ion-pair may be termed solvent-shared. Lastly when the ions are in contact with each other the ion-pair is termed a contact ion-pair. Even in a contact ion-pair, however, the ions retain most of their solvation shell. The nature of this solvation shell is generally not known with any certainty. In aqueous solution and in other donor solvents metal cations are surrounded by between four and nine solvent molecules in the primary solvation shell
Solvation shell
A Solvation shell is a shell of any chemical species acting as a solvent, surrounding a solute species. When the solvent is water it is often referred to as a hydration shell or hydration sphere....
, but the nature of solvation of anions is mostly unknown.
An alternative name for a solvent-shared ion-pair is an outer-sphere complex. This usage is common in co-ordination
Coordinate covalent bond
A dipolar bond, also known as dative covalent bond or coordinate bond is a kind of 2-centre, 2-electron covalent bond in which the two electrons derive from the same atom. Typically, a dipolar bond is formed when a Lewis base donates a pair of electrons to a Lewis acid. This description of bonding...
chemistry and denotes a complex between a solvated metal cation and an anion. Similarly a contact ion-pair may be termed an inner-sphere complex. The essential difference between the three types is the closeness with which the ions approach each other fully solvated > solvent-shared > contact. With fully solvated and solvent shared ion-pairs the interaction is primarily electrostatic, but in a contact ion-pair there will also be some covalent character in the bond between cation and anion.
An Ion-triplet may be formed from one cation and two anions, or from one anion and two cations. Higher aggregates, such as a tetramer, (AB)4 may be formed.
Ternary ion-associates involve the association of three species. Another type, named intrusion ion-pair has also been characterized.
Theory
Ions of opposite charge are naturally attracted to each other by the electrostatic force. This is described by Coulomb's law.F is the force of attraction, q1 and q2 are the magnitudes of the electrical charges, ε is the dielectric constant
Dielectric constant
The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...
of the medium and r is the distance between the ions. For ions in solution this is an approximation because the ions exert a polarizing effect on the solvent molecules that surround them which attenuates the electric field somewhat. Nevertheless some general conclusions can be inferred.
- Ion association will increase as
- the magnitude(s) of the electrical charge(s) q1 and q2 increase
- the magnitude of the dielectric constant, ε decreases
- the size of the ions decreases so that the distance between cation and anion, r, decreases
The equilibrium constant for ion-pair formation, K, like all equilibrium constants, is related to the standard free energy change.
- ΔG = -RT lneK.
R is the gas constant
Gas constant
The gas constant is a physical constant which is featured in many fundamental equations in the physical sciences, such as the ideal gas law and the Nernst equation. It is equivalent to the Boltzmann constant, but expressed in units of energy The gas constant (also known as the molar, universal,...
and T is the absolute temperature
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
. Free energy is made up of an enthalpy
Enthalpy
Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.Enthalpy is a...
term and an entropy
Entropy
Entropy is a thermodynamic property that can be used to determine the energy available for useful work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when...
term.
- ΔG = ΔH − TΔS
The coulombic energy released when ions associate contributes to the enthalpy
Enthalpy
Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.Enthalpy is a...
term, ΔH
. In the case of contact ion-pairs, the covalent interaction energy also contributes to the enthalpy, as does the energy of displacing a solvent molecule from the solvation shell of the cation or anion. The tendency to associate is opposed by the entropyEntropy
Entropy is a thermodynamic property that can be used to determine the energy available for useful work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when...
term which results from the fact that the solution containing un-associated ions is more disordered than a solution containing associates. The entropy term is similar for electrolytes of the same type, with minor differences due to solvation effects. Therefore it is the magnitude of the enthalpy term that most determines the extent of ion-association for a given electrolyte type. This explains the general rules given above.
Occurrence
Dielectric constantDielectric constant
The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...
is the most important factor in determining the occurrence of ion-association. The following table gives some typical values.
| Solvent | dielectric constant | temperature /K |
|---|---|---|
| benzene Benzene Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6.... |
2.3 | 298 |
| diethyl ether Diethyl ether Diethyl ether, also known as ethyl ether, simply ether, or ethoxyethane, is an organic compound in the ether class with the formula . It is a colorless, highly volatile flammable liquid with a characteristic odor... |
4.3 | 293 |
| tetrahydrofuran Tetrahydrofuran Tetrahydrofuran is a colorless, water-miscible organic liquid with low viscosity at standard temperature and pressure. This heterocyclic compound has the chemical formula 4O. As one of the most polar ethers with a wide liquid range, it is a useful solvent. Its main use, however, is as a precursor... (THF) |
7.6 | 298 |
| dichloromethane Dichloromethane Dichloromethane is an organic compound with the formula CH2Cl2. This colorless, volatile liquid with a moderately sweet aroma is widely used as a solvent. Although it is not miscible with water, it is miscible with many organic solvents... |
9.1 | 293 |
| liquid ammonia Ammonia Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or... |
17 | 273 |
| ethanol Ethanol Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a... |
24.3 | 298 |
| methanol Methanol Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol... |
32.7 | 298 |
| nitromethane Nitromethane Nitromethane is an organic compound with the chemical formula . It is the simplest organic nitro compound. It is a slightly viscous, highly polar liquid commonly used as a solvent in a variety of industrial applications such as in extractions, as a reaction medium, and as a cleaning solvent... |
35.9 | 303 |
| dimethyl formamide (DMF) | 36.7 | 298 |
| acetonitrile Acetonitrile Acetonitrile is the chemical compound with formula . This colourless liquid is the simplest organic nitrile. It is produced mainly as a byproduct of acrylonitrile manufacture... |
37.5 | 293 |
| water Water Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a... |
78.7 | 298 |
| formamide Formamide Formamide, also known as methanamide, is an amide derived from formic acid. It is a clear liquid which is miscible with water and has an ammonia-like odor. It is used primarily for manufacturing sulfa drugs and synthesizing vitamins and as a softener for paper and fiber... |
109 | 293 |
Water has a relatively high dielectric constant at 25°C, so in aqueous solutions at ambient temperatures 1:1 electrolytes such as NaCl do not form ion-pairs to an appreciable extent except when the solution is very concentrated. 2:2 electrolytes (q1=2, q2=2) form ion-pairs more readily. Indeed the solvent shared ion-pair [Mg(H2O)6]2+SO42- was famously discovered to be present in seawater, in equilibrium with the contact ion-pair [Mg(H2O)5(SO4)] Trivalent ions such as Al3+, Fe3+ and lanthanide
Lanthanide
The lanthanide or lanthanoid series comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium...
ions form weak complexes with monovalent anions.
The dielectric constant of water decreases with increasing temperature to ca. 55 at 100 °C and ca. 5 at the critical temperature, 217.7 °C. Thus ion-pairing will become more significant in superheated water
Superheated water
Superheated water is liquid water under pressure at temperatures between the usual boiling point and the critical temperature . It is also known as subcritical water and pressurized hot water...
.
Solvents with a dielectric constant in the range, roughly, 20-40, show extensive ion-pair formation. For example, in acetonitrile both contact and solvent-shared io pairs of Li(NCS) have been observed. In methanol the 2:1 electrolyte Mg(NCS)2 is partially dissociated into a contact ion-pair, [Mg(NCS)]+ and the thiocyanate ion.
The dielectric constant of liquid ammonia decreases from 26 at its freezing point
Freezing Point
Freezing Point is a news journal in the People's Republic of China which has been the subject of controversy over its criticism of Communist Party officials and the sympathetic ear it lent to a Chinese historian who had criticized official history textbooks...
, −80 °C to 17 at 20 °C (under pressure). Many simple 1:1 electrolytes form contact ion-pairs at ambient temperatures. The extent of ion-pairing decreases as temperature decreases. With lithium salts there is evidence to show that both inner-sphere and outer-sphere complexes exist in liquid ammonia solutions.
Of the solvents with dielectric constant of 10 or less, tetrahydrofuran is particularly relevant in this context as it solvates cations strongly with the result that simple electrolytes have sufficient solubility to make the study of ion-association possible. In this solvent ion association is the rule rather than the exception. Indeed, higher associates such as tetramers are often formed. Triple cations and triple anions have also been characterized in THF solutions.
Ion-association is an important factor in phase-transfer catalysis since a species such as R4P+Cl− is formally neutral and so can dissolve easily in a non-polar solvent of low dielectric constant. In this case it also helps that the surface of the cation is hydrophobic.
In SN1 reactions the carbocation intermediate may form an ion-pair with an anion, particularly in solvents of low dielectric constant, such as diethylether. This can affect both the kinetics of the reaction and the stereochemistry of the reaction products.
Characterization
Vibrational spectroscopy provides the most widely used means for characterizing ion-associates. Both infrared spectroscopyInfrared spectroscopy
Infrared spectroscopy is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic...
and Raman spectroscopy
Raman spectroscopy
Raman spectroscopy is a spectroscopic technique used to study vibrational, rotational, and other low-frequency modes in a system.It relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range...
have been used. Anions containing a CN group, such as cyanide
Cyanide
A cyanide is a chemical compound that contains the cyano group, -C≡N, which consists of a carbon atom triple-bonded to a nitrogen atom. Cyanides most commonly refer to salts of the anion CN−. Most cyanides are highly toxic....
, cyanate
Cyanate
The cyanate ion is an anion with the chemical formula written as [OCN]− or [NCO]−. In aqueous solution it acts as a base, forming isocyanic acid, HNCO. The cyanate ion is an ambidentate ligand, forming complexes with a metal ion in which either the nitrogen or oxygen atom may be the electron-pair...
and thiocyanide have a vibration frequency a little above 2000 cm−1 which can be easily observed as the spectra of most solvents (other than nitrile
Nitrile
A nitrile is any organic compound that has a -C≡N functional group. The prefix cyano- is used interchangeably with the term nitrile in industrial literature. Nitriles are found in many useful compounds, one example being super glue .Inorganic compounds containing the -C≡N group are not called...
s) are weak in this region. The anion vibration frequency is "shifted" on formation of ion-pairs and other associates and the extent of the shift gives information about the nature of the species. Other monovalent
Valence (chemistry)
In chemistry, valence, also known as valency or valence number, is a measure of the number of bonds formed by an atom of a given element. "Valence" can be defined as the number of valence bonds...
anions which have been studied include nitrate
Nitrate
The nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
, nitrite
Nitrite
The nitrite ion has the chemical formula NO2−. The anion is symmetric with equal N-O bond lengths and a O-N-O bond angle of ca. 120°. On protonation the unstable weak acid nitrous acid is produced. Nitrite can be oxidised or reduced, with product somewhat dependent on the oxidizing/reducing agent...
and azide
Azide
Azide is the anion with the formula N3−. It is the conjugate base of hydrazoic acid. N3− is a linear anion that is isoelectronic with CO2 and N2O. Per valence bond theory, azide can be described by several resonance structures, an important one being N−=N+=N−...
. Ion-pairs of monatomic anions, such as halide
Halide
A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...
ions, cannot be studied by this technique. NMR spectroscopy
NMR spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...
is not very useful as association/dissociation reactions tend to be fast on the NMR time scale, giving time-averaged signals of the cation and/or anion.
Nearly the same shift of vibration frequency is observed for solvent-shared ion-pairs of LiCN, Be(CN)2 and Al(CN)3 in liquid ammonia. The extent of this type of ion-pairing decreases as the size of the cation increases. Thus, solvent-shared ion-pairs are characterized by a rather small shift of vibration frequency with respect to the "free" solvated anion and the value of the shift is not strongly dependent on the nature of the cation. The shift for contact ion-pairs is, by contrast, strongly dependent on the nature of the cation and decreases linearly with cations's charge:radius squared ratio in the two series Cs+ > Rb+ > K+ > Na+ > Li+, and Ba2+ > Sr2+ > Ca2+.
The extent of contact ion-pairing can be estimated from the relative intensities of the bands due to the ion-pair and free ion. It is greater with the larger cations . This is counter to the trend expected if coulombic energy were the determining factor. Instead, the formation of a contact ion-pair is seen to depend more on the energy needed to displace a solvent molecule from the primary solvation sphere
Metal ions in aqueous solution
A metal ion in aqueous solution is a cation, dissolved in water, of chemical formula [Mn]z+. The solvation number, n, determined by a variety of experimental methods is 4 for Li+ and Be2+ and 6 for elements in rows 3 and 4 of the periodic table. Lanthanide and actinide aqua ions have solvation...
of the cation. This energy decreases with the size of the cation, making ion-pairing occur to a greater extent with the larger cations. The trend may be different in other solvents.
Higher ion-aggregates, sometimes triplets M+X-M+, sometimes dimers
of ion pairs (M+X-)2, or even larger species can be identified in
the Raman spectra of some liquid ammonia solutions of Na+ salts by the presence of bands which cannot be attributed to either contact- or solvent-shared ion-pairs.
Evidence for the existence of fully solvated ion-pairs in solution is mostly indirect as the spectroscopic properties of such ion-pairs are indistinguishable from those of the individual ions. Much of the evidence is based on the interpretation of conductivity
Conductivity (electrolytic)
The conductivity of an electrolyte solution is a measure of its ability to conduct electricity. The SI unit of conductivity is siemens per meter ....
measurements.