Preparation and comparison of three zwitterionic stationary phases for hydrophilic interaction liquid chromatography

Surface‐bonded zwitterionic stationary phases have shown highlighted performances in separation of polar and hydrophilic compounds under hydrophilic interaction chromatography mode. So, it would be helpful to evaluate the characteristics of zwitterionic stationary phases with different arranged charged groups. The present work involved the preparation and comparison of three zwitterionic stationary phases. An imidazolium ionic liquid was designed and synthesized, and the cationic and anionic moieties respectively possessed positively charged imidazolium ring and negatively charged sulfonic groups. Then, the prepared ionic liquid, phosphorylcholine and an imidazolium‐based zwitterionic selector were bonded on the surface of silica to obtain three zwitterionic stationary phases. The selectivity properties were characterized and compared through the relative retention of selected solute pairs, and different kinds of hydrophilic solutes mixtures were used to evaluate the chromatographic performances. Moreover, the zwitterionic stationary phases were further characterized by the modified linear solvation energy relationship model to probe the multiple interactions. All the results indicated that the types and arrangement of charged groups in zwitterionic stationary phases mainly affect the retention and separation of ionic or ionizable compounds, and for interaction characteristics the contribution from n and π electrons and electrostatic interactions displayed certain differences.     

The zwitterion of <Emphasis Type="Italic">N</Emphasis>-(2-carboxyphenyl)-4-dimethylaminebenzylideneimine

The crystal and molecular structures of N-(2-carboxyphenyl)-4-dimethylaminebenzylideneimine and its protonated form have been determined. The Schiff base exists as the zwitterion, which is stabilized by the intramolecular ionic hydrogen bond. According to quantum-mechanical calculation results this tautomeric form is energetically unfavorable but in the solid and liquid state the intermolecular interactions support zwitterionic form.     

Capillary zone electrophoresis on chemically bonded imidazolium based salts

In the present paper, fused-silica capillaries were chemically modified with an analogue of the imidazole-based ionic liquid and zwitterionic salt. The coated capillaries were examined for the behavior of the electroosmotic flow in both aqueous and non-aqueous electrolytes. The electroosmotic flow in the capillary coated with an ionic liquid analogue was found to be anodic (reversed) and dependent on the pH of the separation buffer. In the case of a zwitterionic capillary, the electroosmotic flow was cathodic and its velocity remained almost constant in the pH range of 4-7. The zeta-potentials of the modified surfaces were also calculated. The effectiveness of coating was investigated by comparing a separation of five inorganic ions and seven alkylphosphonic acids/monoesters in the modified and uncoated capillaries. All separations were successfully carried out in simple buffers and completed during a short analysis time. Finally, the run-to-run and day-to-day reproducibility of the coated capillaries in terms of the migration time of a neutral marker was determined.     

Remarkably stable gold nanoparticles functionalized with a zwitterionic liquid based on imidazolium sulfonate in a high concentration of aqueous electrolyte and ionic liquid

A new type of gold nanoparticle with a zwitterionic liquid function (zwitter-Au) has been prepared using an imidazolium sulfonate-terminated thiol as a capping agent. Zwitter-Au was found to be remarkably stable in high concentrations of aqueous electrolytes, ionic liquids, and protein.     

Aqueous ionic and complementary zwitterionic soluble surfactants: molecular dynamics simulations and sum frequency generation spectroscopy of the surfaces

An aqueous ionic surfactant, 1-dodecyl-4-(dimethylamino)pyridinium (DMP) bromide, and the corresponding zwitterion 2-[4-(dimethylamino)pyridinio]dodecanoate (DPN) were explored by means of molecular dynamics (MD) simulations and, for the ionic system, by infrared-visible sum frequency generation (IR-vis SFG). The molecular structure of the interfacial layer was investigated for the ionic and zwitterionic systems as a function of surfactant concentration, both in water and in salt (KF or KBr) solutions, by MD simulations in a slab geometry. The buildup of the surface monolayer and a sublayer was monitored, and density and orientational profiles of the surfactants were evaluated. The difference between the ionic and zwitterionic systems and the effect of the added salt were analyzed at the molecular level. The results of MD simulations were compared to those of nonlinear optical spectroscopy measurements. IR-vis SFG was employed to study the DMP ionic surfactant in water and upon addition of simple salts. The influence of added salts on the different molecular moieties at the interface was quantified in detail experimentally.     

Synthesis of multi-hydroxyl and sulfonyl dual-functionalized room temperature ionic liquids

Starting from the hydroxylamine (dimethyl amino ethanol, triethanolamine) and 1,3-propane sultone, a series of hydroxyl and sulfonyl dual-functionalized zwitterionic salts and corresponding acidic room temperature ionic liquids have been synthesized. The hydroxyl groups of the synthesized substances were confirmed by the 1H NMR measurement. These zwitterionic salts and ionic liquids may be used for synthesizing other functionalized ionic liquids or ionic liquid-polymer (polyelectrolyte).     

Determination of acid-base dissociation constants of very weak zwitterionic heterocyclic bases by capillary zone electrophoresis

Thermodynamic acid-base dissociation (ionization) constants (pK(a)) of seven zwitterionic heterocyclic bases, first representatives of new heterocyclic family (2,3,5,7,8,9-hexahydro-1H-diimidazo[1,2-c:2',1'-f][1,3,2]diazaphosphinin-4-ium-5-olate 5-oxides), originally designed as chiral Lewis base catalysts for enantioselective reactions, were determined by capillary zone electrophoresis (CZE). The pK(a) values of the above very weak zwitterionic bases were determined from the dependence of their effective electrophoretic mobility on pH in strongly acidic background electrolytes (pH 0.85-2.80). Prior to pK(a) calculation by non-linear regression analysis, the CZE measured effective mobilities were corrected to reference temperature, 25°C, and constant ionic strength, 25 mM. Thermodynamic pK(a) values of the analyzed zwitterionic heterocyclic bases were found to be particularly low, in the range 0.04-0.32. Moreover, from the pH dependence of effective mobility of the bases, some other relevant characteristics, such as actual and absolute ionic mobilities and hydrodynamic radii of the acidic cationic forms of the bases were determined.     

Zwitterionic stationary phase with covalently bonded phosphorylcholine type polymer grafts and its applicability to separation of peptides in the hydrophilic interaction liquid chromatography mode

A novel phosphorylcholine type zwitterionic stationary phase was synthesized by graft polymerization of 2-methacryloyloxyethyl phosphorylcholine onto the surface of porous silica particles. The resulting material possesses both negatively charged phosphoric acid and positively charged quaternary ammonium groups, which renders it a low net charge over a wide pH range. The composition of the surface grafts were determined by elemental analysis and solid state NMR, and the surface charge (zeta-potential) in different buffer solutions were measured using photon correlation spectroscopy. Separation of several peptides was investigated on packed columns in the hydrophilic interaction liquid chromatography (HILIC) separation mode. It was shown that small peptides can be separated based on hydrophilic interaction and ionic interaction between the stationary phase and analyte. The organic solvent composition, the pH and the salt concentration of the eluent have strong effects on the retention time. Compared to native silica before grafting, the newly synthesized zwitterionic material gave more stable retention times for basic peptides over pH range 3-7 due to elimination of the dissociation of silanol groups.     

Photoisomerization of cyanine derivatives in 1-butyl-3-methylimidazolium hexafluorophosphate and aqueous glycerol: influence of specific interactions

Photoisomerization of two cyanine derivatives, 3,3(')-diethyloxadicarbocyanine iodide (DODCI) and merocyanine 540 (MC 540), has been investigated in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate and aqueous glycerol (93 wt % glycerol +7 wt % water) by measuring fluorescence lifetimes and quantum yields. The aim of this work is to understand how the rates of photoisomerization of DODCI and MC 540 are influenced by specific solute-solvent interactions besides the viscosity of the medium. For DODCI, it has been observed that the nonradiative rate constants, which represent the rates of photoisomerization, are almost identical in the ionic liquid and aqueous glycerol at given temperature, indicating that viscosity is the sole parameter that governs the rate of photoisomerization. In contrast, the photoisomerization rate constants of MC 540 have been found to be a factor of 2 higher in aqueous glycerol compared to the ionic liquid. The observed behavior is due to the zwitterionic character of MC 540, a consequence of which, the twisted state gets stabilized by the solute-solvent hydrogen bonding interactions in aqueous glycerol, thus lowering the barrier for isomerization.     

Synthesis of polymers having zwitterionic structure via the radical polymerization of 4-vinylphenyl isothiocyanate/cyclic amidine adduct

Polymers having a zwitterionic pendant group have found a wide range of applications such as fungicides, fire-retardants, lubricating oil additives, emulsifiers, cryoprotectants, and electronic materials. Their properties depend not only on the polymer main chains but also on the zwitterionic structures. In this article, the synthesis of novel polymers having zwitterionic structures is described. A zwitterionic monomer (4-vinylphenyl isothiocyanate [VPI]-1,2-dimetyl-1,4,5,6,-tetrahydropirimidine [DMTHP]) was synthesized by the reaction of VPI and a cyclic amidine DMTHP. A radical polymerization of VPI-DMTHP was carried out using 2,2′-azobis(2,4-dimethyl-4-methoxy)valeronitrile (V-70) as an initiator to give poly(VPI-DMTHP). The obtained poly(VPI-DMTHP) was not soluble in any of the investigated organic solvents, probably because it had a networked structure resulting from the ionic interactions. Copolymers of VPI-DMTHP and other vinyl monomers such as styrene, methyl methacrylate, t-butyl methacrylate, and N-vinylpyrrolidone were synthesized. The obtained copolymers were also insoluble in all of the examined common organic solvents even when the VPI-DMTHP content was low (5 mol %), indicating the existence of strong ionic interactions in the zwitterionic moieties. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2303–2309     

Novel approaches for the preparation of silica-based zwitterionic hybrid copolymers

Two novel routes for the preparation of silica-based zwitterionic hybrid copolymers were proposed. A series of zwitterionic hybrid copolymers were prepared by the sulfonation of phenyl groups and the quaternary amination of tertiary amine groups alternately. Both FT-IR and ¹H NMR spectra confirm the step products. TGA and DrTGA analyses indicate that the thermal stability of these zwitterionic hybrid copolymers is higher than 400 °C. The determination of sulfonation degree reveals that the zwitterionic hybrid copolymer (c) from sulfonation-quarteramination (Route I) has the minimal value; meanwhile the anion-exchange capacity exhibits that the zwitterionic hybrid copolymer (e) from quarteramination-sulfonation (Route II) has the minimal value. These findings demonstrate the impact of electrostatic effect on the charge content of ionic groups. MALDI-TOF mass spectra exhibit that the decrease in the stability of the charged hybrid copolymers can be ascribed to the electrostatic effect between the molecular chains. The surface SEM images demonstrate that the surface of zwitterionic hybrid copolymer (e) from quarteramination-sulfonation (Route II) has some aggregated particles and form clusters regions in the hybrid matrix, which can also be attributed to the ionic interactions between those charged groups.     

Vesicles in ionic liquids

The formation of vesicles from 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) in several room-temperature ionic liquids, namely, 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF(4)), 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF(6)), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmimNTf(2)), and N-benzylpyridinium bis(trifluoromethylsulfonyl)imide (BnPyNTf(2)), as well as in a water/BmimBF(4) mixture, was investigated. In pure ionic liquids, observations by staining transmission electron microscopy demonstrated clearly the formation of spherical structures with diameters of 200-400 nm. The morphological characteristics of these vesicles in ionic liquids, in particular, the membrane thicknesses, were first investigated by small-angle neutron scattering measurements. The mean bilayer thickness was found to be ～63 ± 1 ? in a deuterated ionic liquid (BnPyNTf(2)-d). This value was similar to that observed in water. The effect of ILs on the modification of the phase physical properties of multilamellar vesicles (MLVs) was then investigated by differential scanning calorimetry. In pure IL as in water, DPPC exhibited an endothermic pretransition followed by the main transition. These transition temperatures and the associated enthalpies in ILs were higher than those in water because of a reduction of the electrostatic repulsion between zwitterionic head groups. To better understand the effect of ionic liquid on the formation of multilamellar vesicles, mixtures of BmimBF(4) and water, which are miscible in all proportions, were analyzed (BmimBF(4)/water ratio from 0% to 100%). SANS and DSC experiments demonstrated that the bilayer structure and stability were strongly modified by the IL content. Moreover, matching SANS experiments showed that BmimBF(4) molecules prefer to be located inside the DPPC membrane rather than in water.     

An efficient approach for the synthesis of spirooxindole derivatives catalyzed by novel sulfated choline based heteropolyanion at room temperature

A new zwitterionic salt, sulfated choline based heteropolyanion, which was reported earlier by our group, has been found to be very effective for the synthesis of spirooxindole derivatives at room temperature. The heteropolyanion based on sulfated ionic liquid (HIL) showed promising features for the reaction, such as shorter reaction time, high product yields (about 90–95%), simple work up, easy removal, and reusability of the catalyst for several times without significant loss in its efficiency. This has made the protocol sustainable and economic.     

Characterization of PVdF(HFP) gel electrolytes based on 1-(2-Hydroxyethyl)-3-methyl imidazolium ionic liquids

Poly(vinylidenefluoride)-hexafluoropropylene (PVdF(HFP))-ionic liquid gel electrolytes were prepared using ionic liquids based on 1-(2-hydroxyethyl)-3-methyl imidazolium tetrafluoroborate and 1-(2-hydroxyethyl)-3-methyl imidazolium hexafluorophosphate. A conventional metathesis reaction was used to prepare these ionic liquids, which have high purity and exhibit a liquid state at room temperature. The prepared polymer-ionic liquid gel proved to be a free-standing and rubbery film in which the degree of transparency differed according to the ratio and type of ionic liquid used. TGA and FTIR analyses confirmed that the solvent, N,N-Dimethylacetamide (DMAC), used for mixing PVdF(HFP) polymer with ionic liquid was almost totally removed during the gelling and drying processes. SEM photographs were taken of the surface structure of the PVdF(HFP)-ionic liquid gel in order to evaluate the morphology of the film's surface according to the mixing ratio and the nature of the ionic liquid. The thermal behaviors of PVdF(HFP)-ionic liquid gels were observed to be similar to those of neat ionic liquids through DSC analysis, and the compatibility between the polymer and ionic liquid was investigated by XRD analysis. The ionic conductivities of all the gels were 10(-3)-10(-5) S cm(-1) in a temperature range of 20-70 degrees C.     

Noncovalent approach to one-dimensional ion conductors: enhancement of ionic conductivities in nanostructured columnar liquid crystals

Noncovalent design of new liquid-crystalline (LC) columnar assemblies based on an ionic liquid has shown to be useful to achieve anisotropic high ionic conductivities. An equimolar mixture of an ionic liquid, 1-butyl-3-methylimidazolium bromide, and 3-[3,4,5-tri(octyloxy)benzoyloxy]propane-1,2-diol, which is partially miscible with the ionic liquid, exhibits an LC hexagonal columnar phase from -4 to 63 degrees C. This columnar supramolecular assembly forming the nanostructures shows the one-dimensional (1D) ionic conductivity of 3.9 x 10(-3) S cm(-1) at 50 degrees C along the column, which is more than 700 times higher than that of the corresponding covalent-type columnar ionic liquid, 1-methyl-3-[3,4,5-tri(octyloxy)benzyl]imidazolium bromide, which is 5.3 x 10(-6) S cm(-1) at 50 degrees C. This significant enhancement of the ionic conductivity is attributed to the increase of the mobility of the ionic part.     

Silver(I)-carbene complexes/ionic liquids: novel N-heterocyclic carbene delivery agents for organocatalytic transformations

[reaction: see text] N-Heterocyclic carbene (NHC) complexes with silver were investigated as sources of unsaturated NHC carbene catalysts via thermal decomposition. The NHC complex (1-ethyl-3-methylimidazol-2-ylidene)silver(I) chloride is an ionic liquid, and was found to catalyze the ring-opening polymerization of lactide at elevated temperatures to give narrowly dispersed polylactide of predictable molecular weight. Silver-carbene complexes can also be used for the catalysis of small molecule transesterification reactions. Thermolysis of the silver complexes in the presence of CS(2) yielded the zwitterionic CS(2) adducts of the carbene, implicating the intermediacy of the free carbene in these reactions.     

Thermal and dilute-solution properties of zwitterionic copolymers

We report the synthesis, characterization, thermal and dilute-solution properties of zwitterionic copolymers of the sulfobetaine type, where the proportion of ionic groups is varied over a wide range. Two different structures are considered: poly (2-vinyl pyridine sulfopropyl betaine) and poly (4-vinyl pyridine sulfopropyl betaine). Synthesis is carried out to obtain random copolymers which are further characterized and their ionic content determined. At low proportions of ionic groups, the behavior is similar to that observed in ionomers, with substantial differences between the two structures, Increased interaction among the zwitterionic groups at higher charge densities is evidenced, which leads to the formation of clusters and agglomerates. An increase in the intrinsic viscosity with salt content in aqueous solutions is also observed. This behavior is analogous to that of homopolymers with 100% ionic content, but with particular features of interest.     

Electromembrane extraction of zwitterionic compounds as acid or base: Comparison of extraction behavior at acidic and basic pHs

This study has performed on electromembrane extraction (EME) of some zwitterionic compounds based on their acidic and basic properties. High performance liquid chromatography (HPLC) equipped with UV detection was used for determination of model compounds. Cetirizine (CTZ) and mesalazine (MS) were chosen as model compounds, and each of them was extracted from acidic (as a cation) and basic (as an anion) sample solutions, separately. 1-Octanol and 2-nitrophenyl octylether (NPOE) were used as the common supported liquid membrane (SLM) solvents. EME parameters, such as extraction time, extraction voltage and pH of donor and acceptor solutions were studied in details for cationic and anionic forms of each model compound and obtained results for two ionic forms (cationic and anionic) of each compound were compared together. Results showed that zwitterionic compounds could be extracted in both cationic and anionic forms. Moreover, it was found that the extraction of anionic form of each model compound could be done in low voltages when 1-octanol was used as the SLM solvent. Results showed that charge type was not highly effective on the extraction efficiency of model compounds whereas the position of charge within the molecule was the key parameter. In optimized conditions, enrichment factors (EF) of 27–60 that corresponded to recoveries ranging from 39 to 86% were achieved.     

Influence of water on the surface of the water-miscible ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate: a sum frequency generation analysis

Sum frequency generation spectroscopy (SFG) was used to study the influence of water on the surface of the water-miscible ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate. The orientation of the cation at the gas-liquid interface was analyzed as a function of ionic liquid concentration in water for concentrations from 0 to 1 mole fraction of the ionic liquid. The cation was found to be oriented with the imidazolium ring nearly parallel to the surface plane with a tilt angle > or = 70 degrees when the ionic liquid was dry. Furthermore, no noticeable change in the orientation was observed when high concentrations of water were mixed with the ionic liquid. The cation butyl chain is projecting into the gas phase with a CH(3) tilt angle of 54 +/- 2 degrees when the ionic liquid is dry and 46 +/- 4 degrees when mixed with water. Water is oriented at the surface only for concentration < or = 0.02 mole fraction of the ionic liquid. At higher ionic liquid concentrations (mole fractions > or = 0.05) the gas-liquid interface resembles that of the pure ionic liquid.     

Approaches to separation interfaces: Electrostatic interaction and local structures of ions at zwitterionic interface

The interaction and separation of ions with zwitterionic layers are reviewed principally based on a series of the author's work. An electrostatic model has allowed us to discuss the chromatographic retention of ions on the zwitterionic stationary phase, and has revealed the ionic interaction occurring at the zwitterionic interface. Similar consideration is applicable to the ionic partition into zwitterionic micelles having the spherical dimension. In the electrostatic models, ion association and solvation changes of ions have been assumed to explain the selectivity in ion recognition. Both assumptions are applicable to polarizable large ions, whereas the former cannot account for the results obtained for small and well-hydrated ions (Cl⁻ and Br⁻). A special X-ray absorption finestructure (XAFS) measurement, which allows selective access to ions interacting with surface monolayers, has been developed, and applied to ions attracted by a zwitterionic monolayer. The X-ray absorption spectra suggest that Zn²⁺ attracted by the zwitterionic monolayer is still hydrated. In contrast, the direct interaction of Br⁻ has been confirmed, indicating that the electrostatic model involving either ion association or the solvation change of an ion does not properly explain the observed phenomena but both effects should be taken into consideration.