Ammonium Counterions Research Articles

From Dy-chain to Dy-dimer: Significant structural variation induced only by counter cations and slow magnetic relaxation behaviors

Herein we report a family of Dy complexes with 1,1,1,5,5,5-hexafluoropentane-2,4‑dione (Hhfac), [Et2MeNH][Dy(hfac)3(OAc)] (1), [Et3NH][Dy(hfac)3(OAc)] (2) and [(n-Pr)3NH]2[Dy2(hfac)6(OAc)2] (3), among which a rare structural variation has been realized by the change of organic ammonium counter cations. These complexes are all constructed from mononuclear [Dy(hfac)3] building units and OAc− linkages, while employing different protonated tertiary amines as the counter cations. The Namine-H∙∙∙Oacetate hydrogen bonds, which are formed between counter cations and OAc− bridges in all complexes, make each counter cations just wedge into the gaps between adjacent [Dy(hfac)3] building units. The hfac‒ groups of each [Dy(hfac)3] unit thus inversed to other side along with the enlarging of the steric hindrance of counter cations, resulting in a significant structural variation from infinite chain topologies of 1 and 2 to discrete dimer structure of 3. For such significant structural variation induced only by the counter cations, to our knowledge, it is the first time in the whole lanthanide complex family. The magnetic studies revealed that 1–3 all show field-induced single-molecule magnets (SMMs) behavior, with the efficient energy barriers (Ueff) of 23.9, 8.5, and 11.2 K, respectively.

Novel phenolate salts of bioactive agents: Cannabidiol phenolate salts

Bioactive phenolic compounds are commonly found in medications, with examples including apomorphine, estrone, thymol, estradiol, propofol, o-phenylphenol, l-Dopa, doxorubicin, tetrahydrocannabinol (THC), and cannabidiol (CBD). This study is the first to explore the creation and assessment of metal and ammonium phenolate salts using CBD as an example. CBD is used in medicine to treat anxiety, insomnia, chronic pain, and inflammation, but its bioavailability is limited due to poor water solubility. In this study exploit a synthetic route to convert CBD into anionic CBD-salts to enhance water solubility. Various CBD-salts with metal and ammonium counterions such as lithium (Li+), sodium (Na+), potassium (K+), choline hydroxide ([(CH3)3NCH2CH2OH]+), and tetrabutylammonium ([N(C4H9)4]+) have been synthesized and characterized. These salts are obtained in high yields, ranging from 74 % to 88 %, through a straightforward dehydration reaction between CBD and alkali metal hydroxides (LiOH, NaOH, KOH) or ammonium hydroxides (choline hydroxide, tetrabutylammonium hydroxide). These reactions are conducted in either ethanol, methanol, or a methanol:water mixture, maintaining a 1:1 molar ratio between the reactants. Comprehensive characterization using Fourier-Transform Infrared Spectroscopy (FT-IR), Nuclear Magnetic Resonance (NMR) spectroscopy, and elemental (CHN) analysis confirms the formation of CBD-salts, as evidenced by the absence of aromatic hydroxyl resonances or stretching frequencies. The molecular formulas of CBD salts were determined based on CHN analysis, and CBD quantification from acid regeneration experiments. Characterization data confirms that each CBD phenolate in a specific CBD salt was electrostatically stabilized by one of the either alkali metal or ammonium ion. The CBD-salts are highly susceptible to acidic conditions, readily reverting back to the original CBD. The percentage and purity of CBD in the CBD-metal/ammonium salts have been studied using High-Performance Liquid Chromatography (HPLC) analysis. Solubility studies indicate that the conversion of CBD into CBD salts significantly enhances its solubility in water, ranging from 110 to 1606 folds greater than pure CBD. Furthermore, the pharmacokinetic evaluation of oral administration of CBD-salts compared to CBD were determined in rats.

The Organic Ammonium Counterion Effect on Slow Magnetic Relaxation of the [Er(hfac)4]− Complexes

The first mononuclear anionic erbium complex [Er(hfac)4]− (hfac = hexafluoroacetylacetone) with an organic ammonium cation [(CH3)4N+] as the counterion was synthesized and structurally and magnetically characterized. The coordination geometries around the Er ions are square antiprisms with pseudo-D4d symmetry. The complex shows distinct field-induced slow magnetization relaxation, which is described by a combination of Orbach (Ueff/kB~28.54(8) K.) and direct mechanisms. Quantum chemical calculations were performed to analyze the magnetic properties of the complex under consideration.

Two-Dimensional Robust Ferromagnetic Semiconductors via Assembly of Magnetic Superatoms [Fe6S8(CN)6]5.

Cluster-assembled materials are of considerable interest owing to their unique properties and extensive application prospects. Nevertheless, the majority of cluster-assembled materials developed to date are nonmagnetic, limiting their applications in spintronics. Thus, two-dimensional (2D) cluster-assembled sheets with intrinsic ferromagnetism are very desirable. Here, via first-principles calculations, utilizing the recently synthesized magnetic superatomic cluster [Fe6S8(CN)6]5- as a building block, we design a series of thermodynamically stable 2D nanosheets [NH4]3[Fe6S8(CN)6]TM (TM = Cr, Mn, Fe, Co) with robust ferromagnetic ordering (Curie temperatures (Tc) up to 130 K), medium band gaps (from 1.96 to 2.01 eV), and sizable magnetic anisotropy energy (up to 0.58 meV per unit cell). Among these nanosheets, the [NH4]3[Fe6S8(CN)6]Cr is a bipolar magnetic semiconductor, whereas the other three ([NH4]3[Fe6S8(CN)6]TM (TM = Mn, Fe, Co) are half semiconductors. Additionally, the electronic and magnetic properties of [NH4]3[Fe6S8(CN)6]TM (TM = Cr, Mn, Fe, Co) nanosheets can be easily modulated by electron and hole doping via simply controlling the number of ammonium counterions. Furthermore, the Curie temperatures of the 2D nanosheets can be improved to 225 and 327 K by choosing 4d/5d transition metals TM = Ru and Os, respectively.

Toughening Ionic Polymer Using Bulky Alkylammonium Counterions and Comb Architecture.

Ionic interactions in ionic polymers, such as ionomers, polyelectrolytes, and polyampholytes, contribute to toughness in systems with high mobility and active ion dynamics, such as hydrogels and elastomers. However, it remains challenging to toughen rigid polymers through ionic interactions without lowering their elastic modulus through plasticization. Here, we present a strategy for toughening without sacrificing the elastic modulus by combining a comb polymer with bulky ammonium counterions. We designed and synthesized ionic comb polymers with oligoethylene glycol side chains and carboxylic acids in each monomer unit of the polynorbornene backbone, neutralized by trialkylamines, ranging from ethyl to octyl. The counterion size in ionic comb polymers influenced the mechanical properties of tensile testing─not the elongation at break and the elastic modulus but the ultimate strength and toughness. The ionic comb polymer containing heptylammonium counterions displayed the highest toughness of 77 MJ m-3. Tensile studies at various strain rates demonstrated a rate-dependent difference between heptyl- and octylammonium counterions. This result suggests that the heptylammonium counterion acted as a sacrificial bond by providing a moderate dissociation rate that was slightly slower than that of the octylammonium counterion, leading to toughening.

Dependency of the mobility of carboxymethylated cellulosic polyions on the nature of their counterions

In this paper, we propose a new approach in order to interpret the dependency of the conductibility of carboxymethylated cellulosic polyelectrolytes XCMC1, XCMC2, XCMC3 in water at 25 °C, with their alkali counterions (X ≡ Li+, Na+ and K+) or ammonium counterions (X ≡ Et4N+ and Bu4N+).The 750 ring-blocks of each CMCi chain are randomly substituted by CH2COO groups, of electric charge (eαX), where αX is the degree of dissociation of the counterions X. It is found that for a given CX ≤ 7 10−3 mol·l−1, the experimental equivalent conductivity λP,X of the polyion CMCi decreases as follows: λP, K+ > λP, Na+ ≥ λP,Li+ > λP,Et4N+ > λP,Bu4N+. This behavior cannot be explained either by a specific ionic condensation, or in terms of hydrodynamic friction or ionic friction undergone by the polyion, or in terms of change of configuration of the polyions (ellipsoidal configuration). However, calculations taking into account the dielectric friction effect according to our new general formula, recently published, giving the dielectric friction coefficient, βPdf, for pearl-necklace-like polyelectrolyte chains, allowed to obtain good convergence between experimental and theoretical results. This new approach improves the interpretations given by Manning and Vink in terms of electrophoretic effect only.

High-Field Magnetoelectric and Spin-Phonon Coupling in Multiferroic (NH4)2[FeCl5·(H2O)

We combine high field polarization, magneto-infrared spectroscopy, and lattice dynamics calculations with prior magnetization to explore the properties of (NH4)2[FeCl5·(H2O)]─a type II molecular multiferroic in which the mixing between charge, structure, and magnetism is controlled by intermolecular hydrogen and halogen bonds. Electric polarization is sensitive to the series of field-induced spin reorientations, increasing linearly with the field and reaching a maximum before collapsing to zero across the quasi-collinear to collinear-sinusoidal reorientation due to the restoration of inversion symmetry. Magnetoelectric coupling is on the order of 1.2 ps/m for the P∥c, H∥c configuration between 5 and 25 T at 1.5 K. In this range, the coupling takes place via an orbital hybridization mechanism. Other forms of mixing are active in (NH4)2[FeCl5·(H2O)] as well. Magneto-infrared spectroscopy reveals that all of the vibrational modes below 600 cm-1 are sensitive to the field-induced transition to the fully saturated magnetic state at 30 T. We analyze these local lattice distortions and use frequency shifts to extract spin-phonon coupling constants for the Fe-O stretch, Fe-OH2 rock, and NH4+ libration. Inspection also reveals subtle symmetry breaking of the ammonium counterions across the ferroelectric transition. The coexistence of such varied mixing processes in a platform with intermolecular hydrogen- and halogen-bonding opens the door to greater understanding of multiferroics and magnetoelectrics governed by through-space interactions.

The influence of α-coordinating groups of aldehydes on E/Z-selectivity and the use of quaternary ammonium counter ions for enhanced E-selectivity in the Julia-Kocienski reaction.

Modified reaction conditions for improved E-selectivity of olefins in the Julia-Kocienski reaction of aldehydes having α-coordinating substituents are demonstrated. The chelating groups in aldehydes are expected to stabilize the syn-transition state with metal ions, whereas the weakly coordinating quaternary ammonium ions are devoid of all possible chelating interactions to enhance E-selectivity. A systematic investigation is presented to study the size of the neighbouring protecting groups of aldehydes and their chelation effect on E/Z-selectivity in the Julia-Kocienski reaction.

Uranyl Ion Coordination by Benzene‐1,2,3‐tricarboxylate: Building Chains and Networks from Binuclear Bricks

AbstractBenzene‐1,2,3‐tricarboxylic acid (123btcH3) was used to synthesize 7 uranyl ion complexes under hydro‐solvothermal conditions, in the presence of different structure‐directing agents. [Zn(phen)3][Zn2(123btc)(phen)4][(UO2)2(123btc)2(OH)(CH3CN)] ⋅ 3H2O (1, phen=1,10‐phenanthroline), the only discrete species in the series, crystallizes as a binuclear, cup‐shaped dimer. Both [PPh3Me][UO2(123btc)] (2) and [C(NH2)3][UO2(123btc)] ⋅ 0.5H2O (3) contain anionic, monoperiodic coordination polymers based on quasi‐planar, binuclear (UO2)2(123btc)22− subunits in which two oxygen atoms of each ligand are uncoordinated. Similar chains are found in [UO2(123btc)K(18C6)] ⋅ H2O (4, 18C6=18‐crown‐6), the potassium cations being decorating groups bound to carboxylato and uranyl oxo donors. An analogous decorating role to monoperiodic polymers is played by UO2(DMF)2(H2O)22+ cations in [(UO2)3(123btc)2(DMF)2(H2O)2] (5), while bridging of chains by UO2(NMP)32+ cations in [(UO2)3(123btc)2(NMP)3] (6) yields a diperiodic network with V2O5 topology. [NH4][UO2(123btc)] (7), which does not contain the dimeric subunit found in the other cases, crystallizes as a diperiodic network with sql topology, the layers being associated through hydrogen bonding interactions with the ammonium counterions.

Tetra-ammonium μ-ethyl-enedi-amine-tetra-acetato-1κ3 O,N,O':2κ3 O'',N',O'''-bis-[trioxidotungstate(VI)] tetra-hydrate.

The title compound, (NH4)4[W2(C10H12N2O8)O6]·4H2O, was obtained from a mixture of tungstic acid, ammonia and ethyl-enedi-amine-tetra-acetic acid (H4edta) in a 2:4:1 ratio. The anion of the complex contains two WO3 units and one bridging edta4- ligand. Each central metal atom is tridentately coordinated by nitro-gen and two carboxyl-ate groups of the edta4- ligand, together with the three oxido ligands, producing a distorted octa-hedral coordination environment around each tungsten atom. The center of the carbon-carbon bond of the ethyl-ene bridge represents a crystallographic inversion center. The crystal structure consists of a three-dimensional supra-molecular framework built up by the dinuclear cations, the ammonium counter-ions and the solvent water mol-ecules via hydrogen bonds of the N-H⋯O and O-H⋯O type.

Investigation of the crystal structures and magnetic features of two bis(dithiolato)nickelate salts with flexible organic cations

Two bis(dithiolato)nickel salts with different flexible ammonium counterions, [Et3MeN][Ni(dmit)2] (1) and [(i-Bu)Et3N][Ni(dmit)2] (2) (dmit2− = 2-thioxo-1,3-dithiole-4,5-dithiolate, Et3MeN+ = triethylmethylammonium, (i-Bu)Et3N+ = triethylisobutylammonium), are prepared and identified by powder X-ray diffraction (PXRD) patterns and single-crystal X-ray diffraction. Salt 1 crystallizes in the triclinic space group P − 1 at 293 K. The anions and cations in 1 form alternating layered arrangements along the a-axis direction. Salt 2 belongs to the monoclinic crystal system and space group P21/n. The anions and cations in 2 form separate columnar stacks along bc-plane direction. The neighboring anions are stacked as dimers in the anion columnar stacks of 2. The magnetic features of salts 1 and 2 show 1D alternating spin chain magnetic exchange behavior, and the magnetic experimental data are well fitted through a spin chain magnetic model. The difference in the crystal structures and magnetic properties between the two new salts 1 and 2 fully demonstrates that the magnetic properties are dependent on the alignment of the [Ni(dmit)2]− anions, which are related to the flexible organic cations.

Influence of counter ions of ammonium for nitrogen doping and carbon properties in hydrothermal carbonization: characterization and supercapacitor performance

The ammonium counter ion is shown to influence the morphology, physicochemical properties and electrochemical performance of hydrothermal carbon.

Pyridinyl Amide Ion Pairs as Lewis Base Organocatalysts.

Pyridinyl amide ion pairs carrying various electron-withdrawing substituents were synthesized with selected ammonium or phosphonium counterions. Compared to neutral pyridine-based organocatalysts, these new ion pair Lewis bases display superior catalytic reactivity in the reaction of isocyanates with alcohols and the aza-Morita-Baylis-Hillman reaction of hindered electrophiles. The high catalytic activity of ion pair catalysts appears to be due to their high Lewis basicities toward neutral electrophiles as quantified through quantum chemically calculated affinity data.

Structure-Directing Effects of Coordinating Solvents, Ammonium and Phosphonium Counterions in Uranyl Ion Complexes with 1,2-, 1,3-, and 1,4-Phenylenediacetates.

The three isomers 1,2-, 1,3-, and 1,4-phenylenediacetic acids (1,2-, 1,3-, and 1,4-pdaH2) have been used to synthesize 16 uranyl ion complexes under solvo-hydrothermal conditions and in the presence of various coligands and organic counterions. The two neutral and homoleptic complexes [UO2(1,2-pda)]·CH3CN (1) and [UO2(1,3-pda)] (2) crystallize as diperiodic assemblies with slightly different coordination modes of the ligands, but the same sql topology. Introduction of the coordinating solvents N-methyl-2-pyrrolidone (NMP) or N,N'-dimethylpropyleneurea (DMPU) in the uranyl coordination sphere produces the four complexes [UO2(1,2-pda)(DMPU)] (3), [UO2(1,3-pda)(NMP)] (4), [UO2(1,4-pda)(NMP)] (5), and [UO2(1,4-pda)(DMPU)] (6), which are either monoperiodic (4) or diperiodic species with the fes (3 and 5) or 3,4L13 (6) topology. The presence of dimethylammonium cations is associated with the formation of ladder-like monoperiodic polymers with the 1,2 isomer in the complexes [H2NMe2]2[(UO2)2(1,2-pda)3]·H2O (7) and [H2NMe2]2[(UO2)2(1,2-pda)3]·3H2O (8), while a conformational change giving the 1,3 and 1,4 isomers a pincer-like geometry favors the formation of dinuclear ring subunits assembled into daisy-chain-like monoperiodic polymers in [H2NMe2]2[(UO2)2(1,3-pda)3]·0.5H2O (9), [H2NMe2]2[(UO2)2(1,4-pda)3] (10), and the mixed-ligand species [H2NMe2]2[(UO2)2(1,2-pda)(1,4-pda)2] (11). The unique complex including guanidinium cations, [C(NH2)3]2[(UO2)2(1,2-pda)3]·0.5H2O·CH3CN (12), crystallizes as a diperiodic polymer with the hcb topology. Due to differences in ligand conformations, the phosphonium-containing complexes [PPh3Me]2[(UO2)2(1,3-pda)3] (13) and [PPh4]2[(UO2)2(1,4-pda)3] (14) contain ladder-like and daisy-chain-like monoperiodic polymers, respectively, while only the latter geometry is found in the mixed-cation complexes [PPh3Me][H2NMe2][(UO2)2(1,4-pda)3]·H2O (15) and [PPh3Me][H2NMe2][(UO2)2(1,2-pda)(1,4-pda)2] (16). The influence of ligand conformation and the structure-directing effects of coligands and counterions throughout the series are discussed. The uranyl emission spectra of 14 of the complexes display the usual vibronic fine structure, the peak positions being dependent on the number of equatorial donors.

Enhancement of Formic Acid Dehydrogenation Selectivity of Pd(111) Single Crystal Model Catalyst Surface via Brønsted Bases

The influence of ammonia (NH3) on the doubly deuterated formic acid (DCOOD, FA) dehydrogenation selectivity for a Pd(111) single crystal model catalyst surface was investigated under ultrahigh vacuum conditions using temperature-programmed desorption and temperature-programmed reaction spectroscopy techniques. NH3 adsorption on Pd(111) revealed reversible, molecular desorption without any significant decomposition products, while DCOOD adsorption on Pd(111) yielded D2, D2O, CO, and CO2 as a result of dehydration and dehydrogenation pathways. Functionalizing the Pd(111) surface with ammonia suppressed the FA dehydration and enhanced the dehydrogenation pathway. The boost in the FA dehydrogenation of Pd(111) in the presence of NH3 can be linked to the ease of FA deprotonation as well as the stabilization of the decomposition intermediate (i.e., formate) due to the presence of ammonium counterions on the surface. In addition, the presence of a H-bonded ammonia network on the Pd(111) surface increased the hydrogen atom mobility and decreased the activation energy for molecular hydrogen desorption. In the presence of NH3, catalytic FA decomposition on Pd(111) also yielded amidation reactions, which further suppressed CO liberation and prevented poisoning of the Pd(111) active sites due to strongly bound CO species.

Performances of ionic liquid matrices with butyl ammonium counterion for matrix-assisted laser desorption/ionization mass spectrometric detection and analysis of sucralfate

In this work, the performances of two ionic liquid matrices (ILMs) with the same ammonium counterpart for mass spectrometric analysis of the insoluble and soluble sucralfate were compared. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) was performed assisted by the butylammonium salts of α-cyano-hydroxycinnamic acid (CHCAB) and 2,5-dihydroxybenzoic acid (DHBB). CHCAB has a higher IE than DHBB, but better optical properties. CHCAB is more suitable for the analysis of sucralfate, although molecular ions of both compounds were detectable only with low intensities. Thus, optical properties of ILMs are crucial to enhance the sensitivity of MALDI MS detection of polysulfated oligosaccharides.

Durability-Limiting Factor of Anion Exchange Membrane Fuel Cells

Anion exchange membrane fuel cells (AEMFCs) hold the promise of low-cost energy conversion. However, one of the most significant challenges in the development of AEMFCs has been their limited durability, ca. < 1,000 h, when compared to proton exchange membrane fuel cells (PEMFC). The most common explanations for loss in fuel cell performance (AEM degradation, (bi)carbonation, water management, and catalyst aggregation) do not fully explain the irreversible performance degradation that limits the durability of AEMFCs. In our study, we set out to identify the source or causes of that performance loss in AEMFCs. To mitigate known sources of degradation, we applied fully humidified conditions and periodically replenished the cell with a dilute NaOH solution. To resolve the current density dependence of cell degradation we conducted several extended duration experiments at different current densities and observed the lifetimes of those cells. Finally, we conducted a rotating disk electrode (RDE) and 1H NMR spectroscopic experiments to identify the durability-limiting factor. We found that when controlling for the known mechanism of performance loss significant and accelerating performance degradation was present. This pointed to a previously unreported means of performance loss. Lower current densities (higher operating potential) resulted in decreased cell lifetime. These tests led to the electrode rather than membrane degradation as the source for performance loss. Our studies with RDE and 1H NMR experiments identified the generation of phenolic compounds that accumulate at the cathode throughout fuel cell operation. Once produced, the phenolic groups remain tethered to the ionomer, and cannot be removed by replenishment of the cell with a caustic solution. While quick recovery of cell performance occurs after caustic treatment continued to fuel cell operation inevitably leads to more phenol group formation that consumes quaternary ammonium counter ions. Eventually, these compounds reach a critical concentration that results in cell failure. The proposed mechanism is consistent with the degradation behaviors during our long-term test. Finally, we will discuss the mitigation strategy of the observed AEMFC degradation.

Pseudo Halide Chemistry in Ionic Liquids with Decomposable Anions

Several pseudohalide containing ionic liquids with quarternary ammonium counter cations of the general formula [R3MeN]X [R = ethyl (1X), n‐butyl (2X) with X– = CN–, N3–, OCN–, and SCN–] were synthesized by decomposition of the corresponding trialkylammonium methylcarbonate in the reaction with Me3Si–X. We also treated 2CN with OP(OMe)3, yielding [nBu3MeN][O2P(OMe)2] and acetonitrile (Me‐CN). The double salt [nBu3MeN]2{[B(OMe)3(CN)](CN)} was obtained from the reaction of 2CN with B(OMe)3, featuring the formation of the monocyanotrimethoxyborate anion, [B(OMe)3(CN)]–, co‐crystallized with [nBu3MeN]CN. [nBu3MeN]2{[B(OMe)3(CN)](CN)} was fully characterized including structure elucidation.

Multi-electron reduction of Wells-Dawson polyoxometalate films onto metallic, semiconducting and dielectric substrates.

The investigation of conditions allowing multi-electron reduction and reoxidation of polyoxometalate (POM) films onto solid substrates is considered an issue of critical importance for their successful incorporation in electronic devices, different types of sensors and catalytic systems. In the present paper, the rich multi-electron redox chemistry of films of Wells-Dawson ammonium salts, namely (NH4)6P2Mo18O62 and (NH4)6P2W18O62, on top of metallic (Al), semiconducting (ITO) and dielectric (SiO2) substrates under ambient conditions is investigated. The respective Keggin heteropolyacids, H3PMo12O40 and H3PW12O40, are also investigated for comparison. On Al substrates, the Wells-Dawson ammonium salts are found to be significantly more reduced (4-6e-) compared to the respective Keggin heteropolyacids (∼2e-), in accordance with their deeper lying lowest unoccupied molecular orbital (LUMO) level. Subsequent thermal treatment in air results in reoxidation of the initially highly reduced POM films. Similar behavior is found on ITO substrates, but in initially less reduced (2-4e-) Wells-Dawson POM films. On the other hand, on SiO2 substrates, the thermal reduction of (NH4)6P2Mo18O62 film is observed and attributed to the thermal oxidation of ammonium counterions by [P2Mo18O62]6- anions. Overall, the multi-electron reduction of Wells-Dawson ammonium salts onto metallic and semiconducting substrates (Al, ITO) is determined by the relative position of the LUMO level of POMs in relation to the Fermi level of the substrate (i.e. substrate work function) and affected in a synergistic way by the presence of ammonium counterions. In contrast, on dielectric substrates (SiO2) the reduction of Wells-Dawson POMs ((NH4)6P2Mo18O62) is attributed only to the oxidation of ammonium counterions.

Exploring the Effect of Buffer Strength on the Retention Time of Weak Acids, Neutral and Weak Bases in Hydrophilic Interaction Liquid Chromatography (HILIC) Mode

Background: Hydrophilic Interaction Liquid Chromatography (HILIC) orthogonal to conventional reversed phase High-Performance Liquid Chromatography (HPLC) mode allowing separation of polar compounds. HILIC has been reported to be an alternative to normal phase liquid chromatography, yet the separation mechanism reported in HILIC is much more complicated than that in normal phase liquid chromatography. Objective: To investigate the effect of water layer thickness on silica gel and the amount of ammonium ions present within the buffer on retention mechanism in hydrophilic interaction chromatography. Methodology: A test system was designed which used weak acids, neutrals and weak bases as probes with three different strengths (5, 10 and 20 mM) of ammonium acetate, ammonium formate and ammonium propionate as the counter-ions to compete with the test probes with ionised silanol groups and water present in the stationary phase. A Kromasil 60-5SIL column (150 mm×4.6 mm×4 μm, pore size 60Å) was used as the stationary phase to perform the study. Results: Retention times were examined for the test probes at 90% acetonitrile (ACN) with 10% of 5, 10 and 20 mM of ammonium acetate, ammonium formate and ammonium propionate. As the buffer strength increases, the thickness of the water layer on the surface of the silica gel increases and also the repulsion between ionized silanol groups and acidic test probes will decrease. On the other hand, such increase in buffer strength will increase the competition between the ammonium ions and basic test probes. In addition, the hydration energy of buffer’s counter ions and hydrophilicity may be important in retention mechanism in HILIC mode. Conclusion: At 20 mM buffer strength acidic probes with low log P values retain more due to reduced repulsion by silanol groups, while basic probes retention time will decrease due to increased competition from ammonium counter ions. However, in 5 mM buffer strength basic probes with low logP value will be retained longer, while acidic probes will be eluted earlier due to the repulsion between ionized acids and ionized silanol groups.