Room-temperature Ionic Liquid Membrane Research Articles

Amperometric gas sensors based on screen printed electrodes with porous ceramic substrates

In the present study, an amperometric gas sensor based on room temperature ionic liquid (RTIL) electrolyte and screen-printed electrodes (SPE) was fabricated. In order to compose the sensor device, a porous alumina ceramic was used as gas diffusion barrier and substrate of SPE, simultaneously. While RTIL electrolytes were also solidified with casting method to form a solid film and attached onto porous ceramics equipped with printed electrodes. It was found that with this design, the amperometric sensor could become smaller and easier to intergrade. Additionally, the porous ceramics as the diffusion barrier as substrate enable gas molecules diffuse into the three-phase (gas/electrode/electrolyte) interface directly without passing through RTIL membranes. For demonstration, an amperometric nitrogen dioxide (NO2) sensor was composed with our proposed structure and the sensing performance were characterized in terms of sensitivity, selectivity and stability. It was indicated that indeed our NO2 sensor show a very rapid response speed, only 20 s and 10 s for responding and recovering, respectively. This is 4 times shorter than the traditional sensor structure based on RTIL. In addition, the sensor has a good sensitivity (33 nA / ppm) and limit of detection (LOD) around 0.02 ppm with a signal noise ratio (SNR) around 3. It was also detected that the sensor has a wide operating temperature range (-20 to 40 °C), suggesting our sensor devices could work in extreme environments.

Wall-jet ion sensor based on ion transfer processes at a polarized room-temperature ionic liquid membrane

Room-temperature ionic liquid (RTIL) membrane is used to construct a wall-jet ion sensor based on the ion transfer processes at a polarized interface between RTIL and an aqueous electrolyte solution. The stream of the aqueous phase with an injected sample plug containing an electroactive ion is led against the RTIL membrane through a silver tube, which is covered with an AgCl layer, and which serves simultaneously as one of two reference electrodes in a four-electrode cell. The effects of the applied potential, the ion concentration, the volume flow rate of the mobile aqueous phase, and the injected sample volume on the chronoamperometric response of the sensor to the test samples containing tetraethylammonium cation (TEA+) as a model ion are demonstrated. The applicability of the available theory of the wall-jet electrode against which a vertical stream is flowing that is much smaller in diameter than the electrode is confirmed for this sensor. Analytical characteristics for the flow injection analysis of the studied model ion are estimated including excellent repeatability characterized by the relative standard deviation (1.64%) of the average current at the ion concentration (0.5 μmol dm−3), a low limit of detection (22.5 nmol dm−3), and the wide linear dynamic range exceeding three orders of magnitude (1 × 10−7–2 × 10−4 mol dm−3).

Detection of antimuscarinic agents tolterodine and fesoterodine and their metabolite 5-hydroxymethyl tolterodine by ion transfer voltammetry at a polarized room-temperature ionic liquid membrane

Ion transfer voltammetry at a polarized hydrophobic room temperature ionic liquid (RTIL) membrane was used for evaluation of the diffusion coefficients and the standard Gibbs energies of ion transfer of the protonated antimuscarinic agents tolterodine (TOL), fesoterodine (FES), and their common metabolite 5-hydroxymethyl tolterodine (5-HMT), as well as for their determination in the aqueous samples and urine. An analysis of the pH effect provided the parameters characterizing their lipophilicity both in their ionic and neutral forms confirming a remarkably low lipophilicity of 5-HMT. The application of the ion transfer voltammetry for a monitoring of the enzymatic hydrolysis of FES to 5-HMT was demonstrated. For determination of protonated TOL, FES, and 5-HMT in the aqueous samples, linear calibration dependences were plotted in the range 2.0–12.5 μmol L−1 with the mean limit of detection (LOD) 0.43 μmol L−1. Analogous linear calibration dependences were constructed for 10times diluted urine spiked with 2.0–12.5 μmol L−1 TOL, FES, and 5-HMT with the mean LOD 0.65 μmol L−1. Unfortunately, determination of 5-HMT in diluted urine samples was interfered by an unknown ionic urine component, which somewhat increased its particular LOD.

Lipophilicity of acetylcholine and related ions examined by ion transfer voltammetry at a polarized room-temperature ionic liquid membrane

Ion transfer voltammetry at a polarized room-temperature ionic liquid (IL) membrane was used to evaluate the standard Gibbs energy of ion transfer from water to IL. This quantity was considered to be a measure of the ion lipophilicity, which is one of the factors playing a role in the extraction and transport processes in the two-phase liquid and liquid membrane systems. On this basis, the lipophilicity of several biologically active ions was compared, namely of neurotransmitter acetylcholine (ACH+) and several related ions including choline (CH+, precursor for ACH+), muscarine (MUS+, agonist of the muscarinic ACH+ receptors), protonated atropine (ATH+, antagonist of the muscarinic ACH+ receptors), protonated scopolamine (SAH+, antagonist of the muscarinic ACH+), and the tetramethylammonium ion (TMA+) representing their charged moiety. Cyclic voltammetric measurements were carried out using a 4-electrode cell with the IL membrane composed of highly hydrophobic tridodecylmethylammonium tetrakis[3,5-bis(trifluoromethyl)phenyl] borate. Analysis of the voltammetric data provided the values of the standard Gibbs energy of ion transfer (in kJ mol−1 in parentheses), which follow the order of ions TMA+ (14.6) < ACH+ (16.2) ~ ATH+ (16.4) < MUS+ (20.3) ~ SAH+ (20.4) < CH+ (24.1) indicating their rather weak and similar lipophilicity. An analysis of the effect of pH on the voltammetric behavior of ATH+ and SAH+ suggested that the partition coefficient for the neutral bases AT and SA is likely to be fairly small and, hence, that the lipophilicity of these neutral bases is also rather weak. Comparable theoretical values of the electrostatic contribution to the theoretical standard Gibbs energy of ion transfer were obtained by the density functional theory calculations, which accounted for the solvent effect by using the polarizable continuum model (PCM). On the other hand, an estimate of the (neutral) solvophobic contributions to the standard Gibbs energy of ion transfer that is based on the empirical Uhlig formula predicts a significant lipophilic effect increasing with the ion size. This effect is likely to be compensated by the strong dipole-dipole and specific (e.g., H-bond) interactions stabilizing most of the studied ions in the water environment. These conclusions are supported by an analysis based on the voltammetric data reported in literature for the ion transfer across the water/nitrobenzene interface, and the DFT calculations performed in the present study.

Voltammetric and capillary electrophoretic study of scavenger kinetics of methylglyoxal by antidiabetic biguanide drugs

A novel electroanalytical approach is used to get insight into scavenger kinetics and mechanism of methylglyoxal (MG) by biguanides metformin, phenformin and 1-phenylbiguanide under the physiological conditions (pH7.41 and 37°C). The approach is based on monitoring the time profiles of concentrations of the protonated biguanides by ion transfer voltammetry (ITV) at the polarizable room-temperature ionic liquid membrane. Effects of the reactant's concentrations and pH lead us to propose a mechanism including an acid-base reaction of biguanide, which separates the parallel reactions of the protonated and deprotonated biguanide with methylglyoxal. Reaction rate increases considerably in the sequence metformin<phenformin<1-phenylbiguanide. Kinetic data confirm the previous finding that scavenging of methylglyoxal by metformin is a relatively slow process. Optimization of the background electrolyte enables to use capillary electrophoresis (CE) to detect simultaneously the protonated biguanide and the MG anion in the reaction mixture. CE analysis then yields the mole-to-mole ratios of the reacted biguanides to MG in the range 0.6–0.7 pointing to the simultaneous formation of a 1:1 and a 1:2 reaction products. Usefulness of ion voltammetry or amperometry for further kinetic studies of biguanides, as well as for their assays, can be envisaged.

Supported UV Polymerized Ionic Liquid Membranes with Block Copolymer

In this study, the use of UV polymerizable Room Temperature Ionic Liquid (RTIL) with a block copolymer has been investigated for the generation of composite membranes for potential separation of acidic gases from natural gas streams. Use of RTILs is known to provide significant mass transportation enhancement but with significant shortcomings such as leaching. In order to avoid this limitation, UV polymerized RTIL membranes with poly(ether-bamide) have been generated with superior mechanical and thermal properties. Two different synthetic approaches were utilized in the current study to obtain the supported membranes with different structural aspects, thus, confirming the possibility to control the structure and resulting properties of the membranes.

How do polymerized room-temperature ionic liquid membranes plasticize during high pressure CO 2 permeation?

Room-temperature ionic liquids (RTILs) are a class of organic solvents that have been explored as novel media for CO 2 separations. Polymerized RTILs (poly(RTILs)) can be synthesized from RTIL monomers to form dense, solid gas selective membranes. It is of interest to understand the permeation properties under single gas and mixed gas conditions at elevated pressure of CO 2. The ionic nature of the polymers may result in tight arrangements between the oppositely charged ionic domains in the poly(RTIL) eventually preventing the membrane from excessive swelling and deterioration of its performance at increased pressure and/or temperature. In this work we characterize the permeation behaviour of three different poly(RTILs) at single and mixed gas pressures up to 40 bar and over a temperature range from T = 10–40 °C. We find that CO 2 is an equally strong plasticizer in single gas as well as mixed gas experiments: the permeability of CO 2 increases by more than 60% over a pressure range of 40 bar. Methane does not plasticize the poly(RTIL) by itself in single gas experiments, however the presence CO 2 accelerates its transport by more than 250%. The plasticization effect of CO 2 is fully reversible on the time scale of the diffusional processes. Even though the poly(RTILs) are glassy in nature, the plasticization behaviour is distinctly different from regular glassy polymers such as polyimides, polysulfones or polycarbonates due to the reversibility of swelling extent. Tailoring the length of the side chain of poly(RTIL) indicates that short side chains suppress plasticization and acceleration of the methane permeability up to a pressure of 10 bars CO 2: longer side chains however do not. This suggests the interpretation that a molecular energy balance of ionic attraction and swelling-induced relaxation exists: however, the swelling-induced relaxations overrule the ionic interaction.

Determination of the upper limits, benchmarks, and critical properties for gas separations using stabilized room temperature ionic liquid membranes (SILMs) for the purpose of guiding future research

The literature reports that supported ionic liquid membranes (SILMs) outperform standard polymers for the separations of CO 2/N 2 and CO 2/CH 4, even under continuous flow mixed gas conditions. Before the expenditure of more resources to develop new room temperature ionic liquids (RTILs) and SILMs, it is time to consider what benchmarks for SILM performance exist and if upper limits could be projected based on the physical chemistry of RTILs. At this juncture, we should ask if the current research efforts are properly focused based on the successes and failures in the literature. We summarize literature data, along with adding new data, on the SILM permeabilities and selectivities for the following gas pairs: CO 2/N 2, CO 2/CH 4, O 2/N 2, ethylene/ethane, propylene/propane, 1-butene/butane, and 1,3-butadiene/butane. The analysis predicts a maximum CO 2-permeability for SILMs and an upper bound for permeability selectivity vs. CO 2-permeability with respect to the CO 2/N 2 and CO 2/CH 4 separations. Also summarized are the representative successes and failures for improving the separation performance of SILMs via functionalization and facilitated transport in the context of the CO 2/N 2, CO 2/CH 4, and olefin/paraffin separations. In the context of the CO 2-separations, the analysis recommends a number of future research foci including research into SILMs cast from RTILs with smaller molar volumes. In the context of olefin/paraffin separations, the preliminary data is encouraging when considering the use of facilitated transport via silver carriers. Since RTIL-solvent/solvent interactions dominate in terminating the overall SILM performance, past attempts at enhancing solute/solvent interactions via the addition of functional groups to the RTILs have not produced SILMs with better separation performance compared to the unfunctionalized RTILs. Future research into functionalized RTILs needs to consider the changes to the dominant solvent/solvent interactions and not just the solute/solvent interactions.

Voltammetry of Ion Transfer across a Polarized Room-Temperature Ionic Liquid Membrane Facilitated by Valinomycin: Theoretical Aspects and Application

Cyclic voltammetry is used to investigate the transfer of alkali-metal cations, protons, and ammonium ions facilitated by the complex formation with valinomycin at the interface between an aqueous electrolyte solution and a room-temperature ionic liquid (RTIL) membrane. The membrane is made of a thin (approximately 112 microm) microporous filter impregnated with an RTIL that is composed of tridodecylmethylammonium cations and tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anions. An extension of the existing theory of voltammetry of ion transfer across polarized liquid membranes makes it possible to evaluate the standard ion-transfer potentials for the hydrophilic cations studied, as well as the stability constants (K(i)) of their 1:1 complexes with valinomycin, as log K(i) = 9.0 (H(+)), 11.1 (Li(+)), 12.8 (Na(+)), 17.2 (K(+)), 15.7 (Rb(+)), 15.1 (Cs(+)), and 14.7 (NH(4)(+)). These data point to the remarkably enhanced stability of the valinomycin complexes within RTIL, and to the enhanced selectivity of valinomycin for K(+) over all other univalent ions studied, compared to the conventional K(+) ion-selective liquid-membrane electrodes. Selective complex formation allows one to resolve voltammetric responses of K(+) and Na(+) in the presence of an excess of Mg(2+) or Ca(2+), which is demonstrated by determination of K(+) and Na(+) in the table and tap water samples.

Long-term, continuous mixed-gas dry fed CO 2/CH 4 and CO 2/N 2 separation performance and selectivities for room temperature ionic liquid membranes

Previously, we reported on using room temperature ionic liquids (RTILs) in place of traditional solvents in liquid membranes and showed that stabilized RTIL-membranes outperformed standard polymers for the separations of CO 2/CH 4 and CO 2/N 2 (considering ideal gas permeabilities). Here, we report on mixed-gas permeances and selectivities for the gas pairs CO 2/CH 4 and CO 2/N 2 using continuous flows of the mixed gases at various carbon dioxide concentrations (up to 2 bars of CO 2 partial pressure). Under mixed-gas test conditions, three of the tested membranes still operated with commercially attractive mixed-gas selectivity combined with CO 2-permeability for CO 2/CH 4 separations. In addition, one of the tested membranes is, potentially, economically viable for CO 2 capture from flue gas. We answer three objections to reduction-to-practice of RTIL-membranes for gas separations; namely, mixed-gas operations did not reduced the gas selectivities, membranes give advantageous performance even under dry gas feed conditions, and we achieved long-term stability in continuous operation, up to 106 days, without performance degradation. Furthermore, the RTIL-membranes operated under CO 2-partial pressures of at least 207 kPa without decrease in separation ability. The RTIL-membranes tested include [emim][BF 4], [emim][dca], [emim][CF 3SO 3], [emim][Tf 2N], and [bmim][BETI].

Fabrication, Characterization, and Application of ‘Sandwich‐Type’ Electrode Based on Single‐Walled Carbon Nanotubes and Room Temperature Ionic Liquid

AbstractThe much‐enhanced electrochemical responses of potassium ferricyanide and methylene blue (MB) were firstly explored at the glassy carbon electrode modified with single‐walled carbon nanotubes (SWNT/GCE), indicating the distinct electrochemical activity of SWNTs towards electroactive molecules. A hydrophobic room temperature ionic liquid (RTIL), 1‐butyl‐3‐methylimidazolium hexafluorophosphate (BMIMPF6), was used as electrode modification material, which presented wide electrochemical windows, proton permeation and selective extraction ability. In consideration with the advantages of SWNTs and RTIL in detecting target molecules (TMs), a novel strategy of ‘sandwich–type’ electrode was established with TMs confined by RTIL between the SWNT/GCE and the RTIL membrane. The strategy was used for electrochemical detection of ascorbic acid (AA) and dopamine (DA), and detection limits of 400 and 80 fmol could be obtained, respectively. The selective detection of DA in the presence of high amount of AA could also be realized. This protocol presented many attractive advantages towards voltammetric detection of TMs, such as low sample demand, low cost, high sensitivity, and good stability.

Cyclic voltammetry of ion transfer across a room temperature ionic liquid membrane supported by a microporous filter

Cyclic voltammetry is used to study the transfer of a series of cations and anions across a room-temperature ionic liquid (RTIL) membrane composed of tridodecylmethylammonium cation (TDMA +) and tetrakis(pentafluorophenyl)borate anion (TPFPB −), and supported by a thin (∼112 μm) microporous filter. Essential advantage of the thin membrane system is the substantial reduction of the ohmic potential drop, which is compensated in voltammetric measurements. Reversible partition of TPFPB − allows fixing the potential difference at one membrane interface and polarizing the other membrane interface in a defined way. It is shown that the polarized potential window for the interface between an aqueous electrolyte solution and RTIL attains the value of ca. 0.7 V at the ambient temperature of 25 ± 2 °C. Tetraphenylarsonium tetraphenylborate hypothesis is used for the first time to estimate the standard Gibbs energies of ion transfer from water to RTIL and to establish the scale of the absolute potential differences. A linear Gibbs energy relationship is found for the ion transfer from water to RTIL and o-dichlorobenzene.