Tetraalkylammonium Ions Research Articles

Thermochemistry of ammonium based ionic liquids: Tetra-alkyl ammonium nitrates – Experiments and computations

In this work data of the molar enthalpies of formation of the ionic liquids [R4N][NO3] were measured by means of combustion calorimetry. The thermal behavior of [R4N][NO3] was studied using differential scanning calorimetry. First-principles calculations of the enthalpy of formation in the gaseous phase have been performed for the ionic species using the G3(MP2) theory. Enthalpies of formation of aqueous tetra-alkyl ammonium ions ΔfHm° ([R4N]+(aq)) derived in this work are useful for prediction thermochemical properties of the ammonium based ionic liquids.

Hydroxide based decomposition pathways of alkyltrimethylammonium cations

A systematic study that altered the number of β-hydrogen atoms susceptible to Hofmann elimination and introduced increased steric hindrance of substituted (ethyl, n-propyl, isobutyl, and neopentyl) alkyltrimethylammonium cations was performed. The mechanism of the thermal decomposition of these four ammonium cations in deuteroxide form was studied using evolved gas analysis (EGA) because of their potential importance in alkaline membrane fuel cells or electrolyzers. The products of the decomposition reactions are in many cases the expected Hofmann elimination products (trimethylamine and olefins), however, as the number of β-hydrogen atoms decrease or they become more sterically encumbered (from the addition of adjacent methyl groups), nucleophilic attack of hydroxide on the methyl groups increases in relative importance. The use of deuterated water and deuteroxide in our study shows that deprotonation of the tetraalkylammonium ions establishes a rapid equilibrium between the nitrogen ylide species that is formed by methyl group deprotonation and water that scrambles deuterium into the methyl groups of the amine. The results of this work show that at high temperature and low water content tetraalkylammonium hydroxide salts are relatively unstable in membranes.

Temperature and Length Scale Dependence of Tetraalkylammonium Ion Solvation in Water, Formamide, and Ethylene Glycol

This paper focuses on the temperature and length scale dependence of solvation of familiar hydrophobic solutes: tetraalkylammonium ions in highly associated solvents with H-bond networks. Standard enthalpies and heat capacities of solution of symmetrical tetraalkylammonium bromides in formamide (FA) and ethylene glycol (C(2)H(4)(OH)(2)) have been computed using experimental ΔH(m)(sol) values and compared with previously reported enthalpies and heat capacities in pure water. Heat capacities of hydration are found to be large, positive, and increase almost linearly up to tetrapentylammonium bromide. Hydration of tetrahexyl- and tetraheptylammonium bromides is accompanied by much smaller heat capacity changes than would be expected from the length scale dependence observed. The same behavior reveals enthalpies and heat capacities of solute transfer from FA and C(2)H(4)(OH)(2) to water. The results obtained rather indicate that solvation of large tetraalkylammonium ions containing more than twenty carbon atoms is accompanied by much less water orientational ordering than for solutes of a moderate size. It results in smaller fluctuations in the water-water pair energy in a hydration shell and consequently reduces the heat capacity of hydration. The comparison of tetraalkylammonium ion behavior in water and the nonaqueous solvents studied shows that the length scale dependence of solvophobic solvation is strongly pronounced in water but not in nonaqueous media.

Effects of the Spaces Available for Cations in Strongly Acidic Cation-Exchange Resins on the Exchange Equilibria by Quaternary Ammonium Ions and on the Hydration States of Metal Ions

The spaces (voids) available for cations in the five exchange resins with varying exchange capacities and cross-linking degrees were estimated, on the basis of the additivity of molar volumes of the constituents. Tetraalkylammonium ions (NR(4)(+); R: Me, Et, Pr) may completely exchange potassium ion on the resin having a larger void radius. In contrast, the ratio of saturated adsorption capacity to exchange capacity of the resin having a smaller void radius decreased with an increase in size of NR(4)(+) ions, due to the interionic contacts. Alkali metal ions could be exchanged quantitatively. While the hydration numbers of K(+), Rb(+), and Cs(+) were independent of the void radius, those of Li(+) and Na(+), especially Na(+), decreased with a decrease in void radius. Interionic contacts between the hydrated ions enhance the dehydration. Multivalent metal ions have the hydration numbers, comparable to or rather greater than those in water. A greater void volume available due to exchange stoichiometry released the interionic contacts and occasionally promoted the involvement of water molecules other than directly bound molecules. The close proximity between ions in the conventional ion-exchange resins having higher exchange capacities may induce varying interactions.

Oligoether Carboxylates: Task-Specific Room-Temperature Ionic Liquids

Recently, a new family of ionic liquids based on oligoether carboxylates was introduced. 2,5,8,11-Tetraoxatridecan-13-oate (TOTO) was shown to form room-temperature ionic liquids (RTILs) even with small alkali ions such as lithium and sodium. However, the alkali TOTO salts suffer from their extremely high viscosities and relatively low conductivities. Therefore, we replaced the alkali cations by tetraalkylammonium (TAA) ions and studied the TOTO salts of tetraethyl- (TEA), tetrapropyl- (TPA), and tetrabutylammonium (TBA). In addition, the environmentally benign quaternary ammonium ion choline (Ch) was included in the series. All salts were found to be ionic liquids at ambient temperatures with a glass transition typically at around -60 °C. Viscosities, conductivities, solvent polarities, and Kamlet-Taft parameters were determined as a function of temperature. When using quaternary ammonium ions, the viscosities of the resulting TOTO ionic liquids are >600 times lower, whereas conductivities increase by a factor of up to 1000 compared with their alkali counterparts. Solvent polarities further reveal that choline and TAA cations yield TOTO ionic liquids that are more polar than those obtained with the, per se, highly polar sodium ion. Results are discussed in terms of ion-pairing and structure-breaking concepts with regard to a possible complexation ability of the TOTO anion.

Electric conductivities of 1:1 electrolytes in high-temperature ethanol along the liquid–vapor coexistence curve. II. Tetraalkylammonium bromides

The molar electrolyte conductivities of dilute solutions of the tetramethyl, tetraethyl, tetra-n-propyl, and tetra-n-butylammonium bromides were measured in ethanol along the liquid-vapor coexistence curve up to 160 °C. The limiting molar electrolyte conductivities and the molar association constants were obtained from the analysis of the concentration dependence of the conductivity. The ionic friction coefficients were estimated from the electrolyte conductivities. On the basis of the present data together with the literature ones at higher densities (lower temperatures) and comparisons with the continuum dielectric friction theory, the density (temperature) dependence of the translational friction coefficients of the tetraalkylammonium ions were discussed in the range of 0.810 ≥ ρ ≥ 0.634 g cm(-3) (-5 °C ≤ t ≤ 160 °C). The dielectric friction effect was important for the tetramethylammonium ion in the whole range studied. The tetraethylammonium ion showed a relatively small friction coefficient in ambient condition indicating the structure-loosening effect around the ion, while the dielectric friction effect became more important as the density reduces and the temperature raises. For the tetra-n-butylammonium ion, the friction coefficients were determined mainly by the bulky size effect. The tetra-n-propylammonium ion showed an intermediate tendency between the tetraethylammonium and tetra-n-butylammonium ions.

Huge Seebeck coefficients in nonaqueous electrolytes

The Seebeck coefficients of the nonaqueous electrolytes tetrabutylammonium nitrate, tetraoctylphosphonium bromide, and tetradodecylammonium nitrate in 1-octanol, 1-dodecanol, and ethylene-glycol are measured in a temperature range from T = 30 °C to T = 45 °C. The Seebeck coefficient is generally of the order of a few hundreds of microvolts per Kelvin for aqueous solution of inorganic ions. Here we report huge values of 7 mV/K at 0.1 M concentration for tetrabutylammonium nitrate in 1-dodecanol. These striking results open the question of unexpectedly large kosmotrope or "structure making" effects of tetraalkylammonium ions on the structure of alcohols.

Electrical Mobility Spectrometer Using a Diethylene Glycol Condensation Particle Counter for Measurement of Aerosol Size Distributions Down to 1 nm

We report a new scanning mobility particle spectrometer (SMPS) for measuring number size distributions of particles down to ∼1 nm mobility diameter. This SMPS includes an aerosol charger, a TSI 3085 nano differential mobility analyzer (nanoDMA), an ultrafine condensation particle counter (UCPC) using diethylene glycol (DEG) as the working fluid, and a conventional butanol CPC (the “booster”) to detect the small droplets leaving the DEG UCPC. The response of the DEG UCPC to negatively charged sodium chloride particles with mobility diameters ranging from 1–6 nm was measured. The sensitivity of the DEG UCPC to particle composition was also studied by comparing its response to positively charged 1.47 and 1.70 nm tetra-alkyl ammonium ions, sodium chloride, and silver particles. A high resolution differential mobility analyzer was used to generate the test particles. These results show that the response of this UCPC to sub-2 nm particles is sensitive to particle composition. The applicability of the new SMPS for atmospheric measurement was demonstrated during the Nucleation and Cloud Condensation Nuclei (NCCN) field campaign (Atlanta, Georgia, summer 2009). We operated the instrument at saturator and condenser temperatures that allowed the efficient detection of sodium chloride particles but not of air ions having the same mobility. We found that particles as small as 1 nm were detected during nucleation events but not at other times. Factors affecting size distribution measurements, including aerosol charging in the 1–10 nm size range, are discussed. For the charger used in this study, bipolar charging was found to be more effective for sub-2 nm particles than unipolar charging. No ion induced nucleation inside the charger was observed during the NCCN campaign.

Transfer Functions and Penetrations of Five Differential Mobility Analyzers for Sub-2 nm Particle Classification

The transfer functions and penetrations of five differential mobility analyzers (DMAs) for sub-2 nm particle classification were evaluated in this study. These DMAs include the TSI nanoDMA, the Caltech radial DMA (RDMA) and nanoRDMA, the Grimm nanoDMA, and the Karlsruhe-Vienna DMA. Measurements were done using tetra-alkyl ammonium ion standards with mobility diameters of 1.16, 1.47, and 1.70 nm. These monomobile ions were generated by electrospray followed by high resolution mobility classification. Measurements were focused at an aerosol-to-sheath flow ratio of 0.1. A data inversion routine was developed to obtain the true transfer function for each test DMA, and these measured transfer functions were compared with theory. DMA penetration efficiencies were also measured. An approximate model for diffusional deposition, based on the modified Gormley and Kennedy equation using an effective length, is given for each test DMA. These results quantitatively characterize the performance of the test DMAs in classifying sub-2 nm particles and can be readily used for DMA data inversion.

THF hydrate crystal growth inhibition with small anionic organic compounds and their synergistic properties with the kinetic hydrate inhibitor poly(N-vinylcaprolactam)

Small, cationic tetraalkylammonium ions (particularly for alkyl=butyl or pentyl) are known to inhibit tetrahydrofuran (THF) and natural gas hydrate crystal growth and have been used as synergists for commercial kinetic hydrate inhibitor polymers (KHIs), such as N-vinylcaprolactam polymers, for a number of years. The ability for small, organic anionic molecules to inhibit (THF) hydrate crystal growth and their potential as KHI synergists in blends with poly(N-vinylcaprolactam) have been investigated. Several series of sodium alkyl carboxylates, sulphates and sulphonates were synthesised. It was found that none of these molecules were capable of inhibiting THF hydrate crystal growth as well as the best tetraalkylammonium salts. Alkyl carboxylates appeared to be more effective as inhibitors than the sulphonates or sulphates. The most effective anionic THF hydrate crystal growth inhibitors had butyl or pentyl groups, with alkyl branching at the tail (i.e. iso- rather than n-isomers) being advantageous. Anionic carboxylate molecules, particularly with isopentyl or isobutyl groups, showed some kinetic inhibition synergy with poly(N-vinylcaprolactam) lowering the onset and catastrophic hydrate formation temperatures in high pressure (78 bar) constant cooling experiments with Structure II hydrates by 1–2 °C when dosed at 2500 ppm compared with using 2500 ppm polymer alone. This synergism was however less than the best tetraalkylammonium salts (alkyl=n-butyl or n-pentyl) at the same test conditions. Sodium butyl sulphonate and sodium 4-methylpentanoate did not prevent hydrate agglomeration with 3.6% brine and decane at 25% water cut in stirred sapphire cells when dosed at 20,000 ppm based on the aqueous phase, whereas 10,000–20,000 ppm active material of several commercially available anti-agglomerants gave fine transportable slurries and no hydrate deposits at the same conditions.

ChemInform Abstract: Strong Medium and Counterion Effects Upon the Redox Potential of the 12-Tungstocobaltate(III/II) Couple.

The 12-tungstocobaltate (III/II) redox couple, both oxidation states being highly negatively charged (5-/6-), is strongly influenced by the nature of the counterion present. In particular, tetraalkylammonium ions decrease E o [Co(III)/(II)] significantly, relative to that measured in the presence of alkali-metal ions in an organic solvent such as acetonitrile, thus causing large decreases in redox reactivity with respect to oxidation

Using a buffer gas modifier to change separation selectivity in ion mobility spectrometry

The mobilities of a set of common α-amino acids, four tetraalkylammonium ions, 2,4-dimethyl pyridine (2,4-lutidine), 2,6-di-tert-butyl pyridine (DTBP), and valinol were determined using electrospray ionization-ion mobility spectrometry-quadrupole mass spectrometry (ESI-IMS–QMS) while introducing 2-butanol into the buffer gas. The mobilities of the test compounds decreased by varying extents with 2-butanol concentration in the mobility spectrometer. When the concentration of 2-butanol increased from 0.0 to 6.8 mmol m −3 (2.5 × 10 2 ppmv), percentage reductions in mobilities were: 13.6% (serine), 12.2% (threonine), 10.4% (methionine), 10.3% (tyrosine), 9.8% (valinol), 9.2% (phenylalanine), 7.8% (tryptophan), 5.6% (2,4-lutidine), 2.2% (DTBP), 1.0% (tetramethylammonium ion, TMA, and tetraethylammonium ion, TEA), 0.0% (tetrapropylammonium ion, TPA), and 0.3% (tetrabutylammonium ion, TBA). These variations in mobility depended on the size and steric hindrance on the charge of the ions, and were due to the formation of large ion-2-butanol clusters. This selective variation in mobilities was applied to the resolution of a mixture of compounds with similar reduced mobilities such as serine and valinol, which overlapped in N 2-only buffer gas in the IMS spectrum. The relative insensitivity of tetraalkylammonium ions and DTBP to the introduction of 2-butanol into the buffer gas was explained by steric hindrance of the four alkyl substituents in tetraalkylammonium ions and the two tert-butyl groups in DTBP, which shielded the positive charge of the ion from the attachment of 2-butanol molecules. Low buffer gas temperatures (100 °C) produced the largest reductions in mobilities by increasing ion-2-butanol interactions and formation of clusters; high temperatures (250 °C) prevented the formation of clusters, and no reduction in ion mobility was obtained with the introduction of 2-butanol into the buffer gas. Low temperatures and high concentrations of 2-butanol produced a series of ion clusters with one to three 2-butanol molecules in compounds without steric hindrance. Clusters of two and three molecules of 2-butanol were also visible. Ligand-saturation on the positive ions with 2-butanol molecules occurred at high concentrations of modifier (6.8 mmol m −3 at 150 °C); when saturated, no further reduction in mobility occurred when 2-butanol was introduced into the buffer gas.

Behaviour of tetraalkylammonium ions in high‐field asymmetric waveform ion mobility spectrometry

High-field asymmetric waveform ion mobility spectrometry (FAIMS) is an ion-filtering technique recently adapted for use with liquid chromatography/mass spectrometry (LC/MS) to remove interferences during analysis of complex matrices. This is the first systematic study of a series of singly charged tetraalkylammonium ions by FAIMS-MS. The compensation voltage (CV) is the DC offset of the waveform which permits the ion to emerge from FAIMS and it was determined for each member of the series under various conditions. The electrospray ionization conditions explored included spray voltage, vaporizer temperature, and sheath and auxiliary gas pressure. The FAIMS conditions explored included carrier gas flow rate, electrode temperature and composition of the carrier gas. Optimum desolvation was achieved using sufficient carrier gas (flow rate > or = 2 L/min) to ensure stable response. Low-mass ions (m/z 100-200) are more susceptible to changes in electrode temperature and gas composition than high mass ions (m/z 200-700). As a result of this study, ions are reliably analyzed using standard FAIMS conditions (dispersion voltage -5000 V, carrier gas flow rate 3 L/min, 50% helium/50%nitrogen, inner electrode temperature 70 degrees C and outer electrode temperature 90 degrees C). Variation of FAIMS conditions may be of great use for the separation of very low mass tetraalkylammonium (TAA) ions from other TAA ions. The FAIMS conditions do not appear to have a major effect on higher mass ions.

Preferential Site Binding of Monovalent Cations With the Random Coil Form of DNA

Collins and Rogers (Chem. Biol. Interactions 19 197 1977) have suggested that monovalent tetraalkylammonium (TXA+) ions diminish the melting temperature of duplex DNA by preferential binding to the random coil form. We have investigated this suggestion using a known 16 base hairpin as the test oligomer and electrophoretic mobility in free solution as the analytical parameter. Measurements were made in the range 15-60° using background electrolyte solutions (BGE) containing 0.01-0.9 M monovalent cation (M+) and diethylmalonate as the buffering cation at pH 7.3. A 16 base all-T oligonucleotide served as a reference random coil to correct for changes in hairpin mobility contributed by changes in the physical properties of the BGE. Thermal transitions were observed as a decrease in the mobility ratio of the hairpin/coil to 1.00 as the temperature was increased at a fixed [M+]. The Tm of the hairpin increased hyperbolically from 37° as the [M+] increased linearly. The increase in Tm observed with Na+ is exactly that predicted by the DINAMelt algorithm. The span of the increase in Tm has the order Na=Li=K>NH4>TMA>Tris>TEA>TPA>TBA. Cation site binding was analyzed as a change in the mobility ratio with increasing [M+] at 20° where the hairpin conformation predominates. The mobility ratio increases hyperbolically with increasing [M+], consistent with the view that M+ preferentially site binds with the coil form. The magnitude of the increase in mobility ratio from the binding measurements at 20° correlates with the dTm/d(log M+) from the melting measurements, suggesting that the two phenomena are related. We propose that the effect of M+ on DNA melting is the net result of two opposing contributions, preferential stabilization of the counterion cloud about the structured form and preferential site binding with the unstructured random coil form.

On the nature of tetraalkylammonium ions in common electrochemical solvents: General and specific solvation – Quantitative aspects

The free energies of each of 80 tetraalkylammonium ion/solvent complexes [R 4N +/(solv) n ], with R ranging from methyl through butyl and n ranging from 1 through 4, were computed by density functional theory (DFT) in five common electrochemical solvents: dimethylformamide (DMF), dimethylsulfoxide (DMSO), acetonitrile (AN), dichloromethane (DCM), and methanol (MeOH). The energies of the complexes were computed both with and without their solvation energies. Additional computations of the energies of the individual components, both solvated and unsolvated, were also carried out. The resulting data permit construction of a thermodynamic cycle for each R 4N +/solvent pair that in turn allows the determination of the extent of general and specific solvation energies for that pair. An additional series of computations for pentane as solvent were carried out. Since this solvent should not coordinate with tetraalkylammonium ions, these computations provide a test of the validity of the computational method. This work represents a useful new general protocol for assessing the relative importance of general and specific solvation in chemical systems.

Orientation Correlation of Sulfosuccinate-based Room-Temperature Ionic Liquids Studied by Polarization-Resolved Hyper-Rayleigh Scattering

Hyper-Rayleigh scattering (HRS) of sulfosuccinate-based room-temperature ionic liquids (RTILs), tetraalkylammonium salts of bis(2-ethylhexyl)sulfosuccinate (BEHSS(-)) and di(isobutyl)sulfosuccinate (DIBSS(-)), shows strong orientation correlation between sulfosuccinate ions for both BEHSS(-)-based and DIBSS(-)-based RTILs. The hyperpolarizability and polarization resolved HRS, giving information at the dipolar electric order and at the quadrupolar electric order, respectively, have been measured as a function of the distance between sulfosuccinates by changing the size of the tetraalkylammonium ions constituting the ionic liquids. The variations obtained for the dipolar electric and quadrupolar electric components of the HRS intensity show the orientation correlation between sulfosuccinate ions. In particular, the weight parameter eta introduced for molecular assemblies deviates from a pure dipolar electric order that is generally expected for a homogeneous molecular solution. Nonmonotonic variations of eta with increasing the alkyl chain length of tetraalkylammonium ions indicate a transition in the organization of the anions from an homogeneous to an heterogeneous arrangement with increasing the intermolecular distance between sulfosuccinate ions due to the competition between the short- and long-range molecular interactions.

Capillary electrophoretic study of thiolated α-cyclodextrin-capped gold nanoparticles with tetraalkylammonium ions

Capillary zone electrophoresis (CZE) has been employed to characterize nanometer-sized thiolated α-cyclodextrin-capped gold nanoparticles (α-CD-S-AuNPs). The addition of tetrabutylammonium (Bu 4N +) ions to the run buffer greatly narrows the migration peak of α-CD-S-AuNP. The optimal run buffer was determined to be 10 mM Bu 4N + in 30 mM phosphate buffer at pH 12 and an applied voltage of 15 kV. The effect of various tetraalkylammonium ions on the peak width and electrophoretic mobility ( μ e ) of α-CD-S-AuNP was studied in detail. Bu 4N + ions assist in inter-linking the α-CD-S-AuNPs and narrowing the migration peak in CZE. This observation can be explained by the fact that each Bu 4N + ion can simultaneously interact with several hydrophobic cavities of the surface-attached α-CDs on AuNPs. The TEM images show that α-CD-S-AuNPs with Bu 4N + are linked together but in the absence of Bu 4N +, they are more dispersed. The migration mechanism in CZE is based on the formation of inclusion complexes between Bu 4N + and α-CD-S-AuNPs which induces changes in the charge-to-size ratio of α-CD-S-AuNPs and μ e . An inverse linear relationship ( r 2 > 0.998) exists between the μ e and size of α-CD-S-AuNPs in the core range 1.4–4.1 nm. The CZE analyses are rapid with migration time less than 4 min. A few nanoliters of each of the α-CD-S-AuNP samples were injected hydrodynamically at 0.5 psi for 5 s. Our work confirms that CZE is an efficient tool for characterizing the sizes of α-CD-S-AuNPs using Bu 4N + ions.

Counterion motions and thermal ordering effects in perfluorosulfonate ionomers probed by solid-state NMR

Perfluorosulfonate ionomers (PFSIs) neutralized by tetraalkylammonium ions have been investigated using variable temperature 1H and 13C solid-state NMR (ssNMR) spectroscopy to probe the thermally induced properties of tetraalkylammonium ions at temperatures near the α-relaxations. Tetramethylammonium (TMA+), tetraethylammonium (TEA+), tetrapropylammonium (TPA+) and tetrabutylammonium (TBA+) ions have been incorporated in our study for the systematic control of ionic interactions within PFSIs according to the chain-length of ammonium ions. 13C static ssNMR results show that bulkier TPA+ and TBA+ ions undergo molecular tumbling motions near or above the α-relaxation temperature of the corresponding PFSI ionomers, with jumping rates of κ≈1kHz. Moreover, the results of 1H spin-lattice (T1) relaxation time measurements suggest that smaller TMA+ ions in the PFSI exhibit a thermally induced ordering effect as the sample temperature approaches the α-relaxation temperature of TMA+–PFSI. This ordering phenomenon is also supported by the results from small-angle X-ray scattering.

Holey Gold Nanowires Formed by Photoconversion of Dissipative Nanostructures Emerged at the Aqueous–Organic Interface

Gold nanowires are obtained by photoreduction of linear self-assemblies formed from Au(OH)4− and tetra-alkyl ammonium ions at the water–chloroform interface. They are dissipative nanostructures formed only under non-equilibrium conditions, which require continuous vectorial transport of ammonium ions across the interface. Nanocavities are observed in nanowires at almost regular intervals, which is a salient feature of dissipative structures. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Superheated Water Ion-Exchange Chromatography: An Experimental Approach for Interpretation of Separation Selectivity in Ion-Exchange Processes

Cation-exchange selectivity for alkali and alkaline-earth metal ions and tetraalkylammonium ions on a strongly acidic sulfonic acid cation-exchange resin has been investigated in the temperature range of 40-175 degrees C using superheated water chromatography. Dependence of the distribution coefficient (ln KD) on the reciprocal of temperature (1/T) is not linear for most of the ions studied, and the selectivity coefficient for a pair of alkali metal ions or that of alkaline-earth metal ions approaches unity as temperature increases. On the other hand, the retention order of tetraalkylammonium ions is reversed at 160 degrees C or above when eluted with Na2SO4 aqueous solution and the larger ions are eluted faster than the smaller ones contrary to the retention order obtained at ambient temperature. The change in ion-exchange selectivity with temperature observed with superheated water chromatography has been discussed on the basis of the effect of temperature on hydration of the ions and specific adsorption or distribution of ionic species between the external solution and ion-exchange resin. In superheated water, the electrostatic interaction or association of the ions with the fixed ion becomes a predominant mechanism resulting in different separation selectivity from that obtained at ambient temperature.