Ionic liquids as electrolytes for nonaqueous capillary electrophoresis

Acetonitrile is a well-suited medium for nonaqueous capillary electroseparations and enables extending the range of applications of capillary electrophoresis (CE) techniques to more hydrophobic species. In this study, the dialkylimidazolium-based low temperature melting organic salts know as "ionic liquids" (ILs) are used as electrolytes. At room temperature these liquids are miscible with acetonitrile which makes it easy to use them for adjustment of analyte mobility and separation. The anionic part as well as the concentration of an IL influence the general electrophoretic mobility of the buffer system. The separation of different analytes is achieved because they become charged in the presence of ILs in separation media. There is also a possibility for a complex formation between the solute and the electrolyte which alters the mobility of the solute. A selected application of separations of phenols and aromatic acids will be discussed.     

Spreading kinetics of liquids on liquids

A simple theory is presented for the spreading rate of film forming organic liquids on thick water layers. This theory is an extension of that of Ahmad and Hansen for thin water layers. The theory is compared with experimental data, and fair agreement is obtained. The analogy to longitudinal waves is discussed.     

Columnar nanostructured polymer films containing ionic liquids in supramolecular one-dimensional nanochannels

Ionic liquids have attracted a considerable attention as the next generation electrolytes for energy devices. We have developed new free-standing and nanostructured polymer films in which ionic liquids are confined into one-dimensionally ordered nanochannels. These polymer films have been obtained by photopolymerization of hydrogen-bonded supramolecular columnar liquid-crystalline self-assemblies of an imidazolium-based ionic liquid and a wedge-shaped diol compound containing polymerizable groups. The macroscopically parallel alignment of the columnar structures on a glass substrate has been achieved by the application of mechanical shearing, and subsequently fixed into polymer films by UV irradiation. This ionic liquid-containing polymer film exhibits higher ionic conductivity than that of the previously reported one-dimensional polymer film obtained by in situ photopolymerization of a covalent-type columnar liquid-crystalline imidazolium salt. The noncovalent supramolecular approach to one-dimensionally ion-conductive polymer films has led to improvement on conductive properties. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 366–371     

Determination of preservatives in soft drinks by capillary electrophoresis with ionic liquids as the electrolyte additives

A capillary electrophoresis method for separating preservatives with various ionic liquids as the electrolyte additives has been developed. The performances for separation of the preservatives using five ionic liquids with different anions and different substituted group numbers on imidazole ring were studied. After investigating the influence of the key parameters on the separation (the concentration of ionic liquids, pH, and the concentration of borax), it has been found that the separation efficiency could be improved obviously using the ionic liquids as the electrolyte additives and tested preservatives were baseline separated. The proposed capillary electrophoresis method exhibited favorable quantitative analysis property of the preservatives with good linearity (r² = 0.998), repeatability (relative standard deviations ≤ 3.3%) and high recovery (79.4–117.5%). Furthermore, this feasible and efficient capillary electrophoresis method was applied in detecting the preservatives in soft drinks, introducing a new way for assaying the preservatives in food products.     

Evaluation of chiral ionic liquids as additives to cyclodextrins for enantiomeric separations by capillary electrophoresis

A great interest has been drawn these last years towards ionic liquids in analytical chemistry, especially for separation methods. Recent synthesis of chiral ILs opened the way of the evaluation of new potential selectors for enantiomeric separations. This work focused on the evaluation of two chiral ILs (ethyl- and phenylcholine of bis(trifluoromethylsulfonyl)imide) by CE. Particular selectivities are awaited by exploiting unique ion-ion or ion-dipole interactions and by tailoring the nature of the cation and the anion. To evaluate such phenomena, a study was carried out with anti-inflammatory drugs 2-arylpropionic acids as model compounds. The results show that these chiral ILs did not present direct enantioselectivity with regard to these model analytes. The influence of chiral ILs in the electrolytes was then studied in the presence of classical chiral selectors (di- or trimethyl-beta-cyclodextrin). Although no general trend could be established, an increase in separation selectivity and resolution was observed in some cases, suggesting synergistic effects. The complementary determination of apparent inclusion constant values of these IL cations in the used cyclodextrins by affinity CE provided support to the understanding of the phenomena involved.     

Anisotropic dynamics of dipolar liquids in narrow slit pores

We report molecular dynamics simulation results for Stockmayer fluids confined to narrow slitlike pores with structureless, nonconducting walls. The translational and rotational dynamics of the dipolar particles have been investigated by calculating autocorrelation functions, diffusion coefficients, and relaxation times for various pore widths (five or less particle diameters) and directions parallel and perpendicular to the walls. The dynamic properties of the confined systems are compared to bulk properties, where corresponding bulk and pore states at the same temperature and chemical potential are determined in parallel grand canonical Monte Carlo simulations. We find that the dynamic behavior inside the pore depends on the distance from the walls and can be strongly anisotropic even in globally isotropic systems. This concerns especially the particles in the surface layers close to the walls, where the single particle and collective dipolar relaxation resemble that of true two-dimensional dipolar fluids with different in-plane and out-of-plane relaxations. On the other hand, bulklike relaxation is observed in the pore center of sufficiently wide pores.     

Dielectric response of polar liquids in narrow slit pores

Based on molecular dynamics (MD) simulations and a simple (Stockmayer) model we investigate the static and dynamic dielectric response of polar liquids confined to narrow slit pores. The MD simulations are used to calculate the time-dependent polarization fluctuations along directions parallel and perpendicular to the walls, from which the components of the frequency-dependent dielectric tensor can be derived via linear response theory. Our numerical results reveal that the system's response is strongly anisotropic. The parallel dielectric function, epsilonparallel(omega), has Debye-like character very similar to the corresponding isotropic bulk function, epsilonbulk(omega), at the same chemical potential. Indeed, the main confinement effect on epsilonparallel(omega) consists in a shift toward smaller values relative to the bulk function. On the other hand, in the perpendicular direction we observe a characteristic peak in the absorption part of the dielectric function, epsilonperpendicular(omega). This peak is absent in the bulk system and reflects strongly pronounced, damped oscillations in the polarization fluctuations normal to the walls. We discuss two possible origins of the oscillations (and the resulting absorption peak), that is collective oscillations of dipoles in clusters formed parallel to the walls, and the existence of a "dipolaron mode" previously observed in MD simulations of bulk polar fluids.     

Novel guanidinium-based ionic liquids as stationary phases for capillary gas chromatography

<正>The present study describes guanidinium-based ionic liquids(GBILs) as stationary phases for capillary gas chromatography (CGC) and to the best of our knowledge,no related reports are available up to now.In this study,a hexaalkylguanidinium ionic liquid(DOTMG-NTf_2) was synthesized and coated statically onto capillary columns.Selectivity of the stationary phase was evaluated by separating Grob test mixture,test mixture,alcohols mixture,and fatty acid methyl esters mixture,and thermal stability was investigated as well.The present study demonstrates that GBILs as CGC stationary phases exhibit satisfactory selectivity and thermal stability and have a great potential as new candidates for CGC stationary phases.     

Ionic liquids as additives for separation of benzoic acid and chlorophenoxy acid herbicides by capillary electrophoresis

Ionic liquids (ILs) were tested as additives to phosphate-acetate buffer for the separation of chlorophenoxy and benzoic herbicide acids. The effects of buffer concentration, buffer pH, IL concentration, and concentration of organic solvent were investigated. It was found that in the presence of 40 mM phosphate-acetate containing 10% acetonitrile at pH 4.5, addition of 10 mM 1-butyl-3-methylimidazoium could reverse EOF. The shoulder-merged peaks of two herbicide acids, 2,4-dichlorobenzoic acid and 3,5-dichlorobenzoic acid, were successfully resolved by the addition of IL cation. Apart from these, results showed different IL cations had different influences on the migration behavior of some of the analytes, while IL anions did not lead to obvious difference on the separation.     

Geminate ion-electron recombination kinetics in non-polar liquids by time-resolved fluorescence

The dynamics of geminate ion-electron recombination following laser photoionization of N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) in non-polar solution were observed by time resolving the resulting delayed fluorescence. Geminate recombination was observed for TMPD excited by intense (3μJ) picosecond pulses of 305 nm light in low electron mobility solvents. Preliminary comparisons were made to theoretical models.     

Ionic liquids used in and analyzed by capillary and microchip electrophoresis

This review, covering reports published from 2001 to December 2008, shows how ionic liquids (ILs) have made significant contributions in the improvement of capillary and microchip electrophoresis (CE and μCE) for the separation and detection of analytes such as phenols and aromatic acids, metal ions, medicines, enantiomers, biological materials, etc. Furthermore, CE methods applied in the sensitive and accurate determination of physico-chemical properties of ILs have been summarized. Accordingly, research vacancies and future development trends in these areas are discussed.     

Critical temperature of capillary condensation in narrow cylindrical pores

The dependence of the critical temperature of capillary condensation of an adsorbate in cylindrical pores on the diameter of pores is investigated. The calculation is carried out in the quasi-chemical approximation with a calibration function providing the agreement of results for small pores with exact values obtained by the fragment method. The contribution of the size effect to the calibration function as a function of the diameter of pores is considered in terms of the concept of quasi-one-dimensional behavior of the adsorbate in narrow pores. Various adsorbate-adsorbate potentials are considered.     

Pressure-induced transport of DNA confined in narrow capillary channels

Pressure-induced transport of double-stranded DNA (dsDNA) from 10 base pairs (bp) to 1.9 mega base pairs (Mbp) confined in a 750-nm-radius capillary was studied using a hydrodynamic chromatographic technique and four distinct length regions (rod-like, free-coiled, constant mobility, and transition regions) were observed. The transport behavior varied closely with region changes. The rod-like region consisted of DNA shorter than the persistence length (～150 bp) of dsDNA, and these molecules behaved like polymer rods. Free-coiled region consisted of DNA from ～150 bp to ～2 kilo base pairs (kbp), and the effective hydrodynamic radius R(HD) of these DNA scaled to L(0.5) (L is the DNA length in kbp), a characteristic property of freely coiled polymers. Constant mobility region consisted of DNA longer than ～100 kbp, and these DNA had a constant hydrodynamic mobility and could not be resolved. Transition region existed between free-coiled and constant mobility regions. The transport mechanism of DNA in this region was complicated, and a general empirical equation was established to relate the mobility with DNA length. Understanding of the fundamental principles of DNA transport in narrow capillary channels will be of great interest in the development of "lab-on-chip" technologies and nongel DNA separations.     

Automatic parameterization of force fields for liquids by simplex optimization

In this study we demonstrate an automatic method of force field development for molecular simulations. Parameter tuning is taken as an optimization problem in many dimensions. The parameters are automatically adapted to reproduce known experimental data such as the density and the heat of vaporization. Our method is more systematic than guessing parameters and, at the same time, saves human labor in parameterization. It was applied successfully to several molecular liquids. As a test, force fields for 2-methylpentane, tetrahydrofurane, cyclohexene, and cyclohexane were developed. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1009–1017, 1999     

Atomic force microscopy in viscous ionic liquids

Extracting quantitative information from amplitude-modulation atomic force microscopy (AM-AFM) in viscous ionic liquids is difficult because existing theory requires knowledge of the cantilever natural frequency, which cannot be measured in the absence of a resonance peak. We present a new model that describes cantilever dynamics in an overdamped medium (Q < 0.5) and derive the theory necessary to extract the stiffness and damping in highly viscous liquids. The proposed methodology is used to measure the solvation layers of an ionic liquid at a gold electrode.     

Chiral ionic liquids for enantioseparation of pharmaceutical products by capillary electrophoresis

A chiral ionic liquid (IL), S-[3-(chloro-2-hydroxypropyl)trimethylammonium] [bis((trifluoromethyl)sulfonyl)amide] (S-[CHTA](+)[Tf(2)N](-)), which can be easily and readily synthesized in a one-step process from commercially available reagents, can be successfully used both as co-electrolyte and as a chiral selector for CE. A variety of pharmaceutical products including atenolol, propranolol, warfarin, indoprofen, ketoprofen, ibuprofen and flurbiprofen, can be successfully and baseline separated with the use of this IL as electrolyte. Interestingly, while S-[CHTA](+)[Tf(2)N](-) can also serve as a chiral selector, enantioseparation cannot be successfully achieved with S-[CHTA](+)[Tf(2)N](-) as the only chiral selector. In the case of ibuprofen, a second chiral selector, namely a chiral anion (sodium cholate), is needed for the chiral separation. For furbiprofen, in addition to S-[CHTA](+)[Tf(2)N](-) and sodium cholate, a third and neutral chiral selector, 1-S-octyl-beta-d-thioglucopyranoside (OTG), is also needed. Due to the fact that the chirality of this chiral IL resides on the cation (i.e., -[CHTA](+)), and that needed additional chiral selector(s) are either chiral anion (i.e., cholate) or chiral neutral compound (OTG), the results obtained seem to suggest that additional chiral selector(s) are needed to provide the three-point interactions needed for chiral separations.     

Polymolecular adsorption and capillary condensation in narrow slit pores

Capillary condensation and polymolecular adsorption in narrow slits has been calculated, where the fields of surface forces overlap one another. The calculations were carried out on the basis of macroscopic theory of dispersion forces and the isotherms of lone adsorption layers at the free surface. It has been shown that under the effect of mutual attraction through a gap, polymolecular adsorption films lose their stability long before their thickness has approached the half-width of a flat slit. This results in hysteresis of the capillary condensation in an ensemble of plane-parallel slits.

In the case of systems having strong adsorbate-adsorbate interaction, there has been detected the existence of the lower limit of sizes of slit pores, wherein the capillary meniscus can coexist with adsorption films. With a slit width smaller than the critical one, the meniscus is likely to form a finite contact angle with “dry” surfaces of a slit. Thus an explanation has been given of the lower limit of the capillary condensation in an ensemble of flat-surface, slit pores. In the case of strong adsorbate-adsorbent interaction, the coexistence of meniscus with adsorption films within the scope of the approach used is possible in slits of any width.

The value of corrections for the surface forces effect to be entered in the calculations of slit pores dimensions has been analyzed on the basis of the capillary condensation data obtained.

In wedge-shaped slits there also exists, besides lower limit the upper limit of capillary hysteresis.     

Electroosmotic flow modulation in capillary electrophoresis by organic cations from ionic liquids

This paper describes the ability of several ionic liquids cations for electroosmotic flow modulation in capillary electrophoresis. Organic salts based on phosphonium, sulfonium, cysteinium, ammonium, and guanidinium cations were selected to study this property. In addition, the synergistic effect of these compounds in cyclodextrin chiral separation was also evaluated. In comparison with most studied imidazolium-based ionic liquids, several of the cations studied, are stronger modifiers in terms of electroosmotic flow (EOF) modulation. Phosphonium-based compounds and tri-octyl methylammonium chloride ([Aliquat]Cl) had the strongest ability to reverse EOF both in acidic and in basic conditions and had the lowest EOF reversal concentrations in the presence of hydroxypropyl-β-cyclodextrin. EOF modulation ability of phosphonium cations also contributed to the improvement of chiral separation of DL-propranolol by hydroxypropyl-β-cyclodextrin at lower concentrations in comparison with most commonly used EOF modulators such as tetrabutylammonium phosphate.