Ammonium-based Protic Ionic Liquids Research Articles

Investigating the Wettability, Rheological, and Tribological Properties of Ammonium-Based Protic Ionic Liquids as Neat Lubricants for Steel–Steel and Steel–Aluminium Contacts

This study aims to evaluate the tribological properties of two protic ionic liquids (PILs) under different tribological conditions as a sustainable alternative for mineral oil-based neat lubricants. The synthesis of PILs in this study uses a relatively simple and less expensive method. The Fourier transform infrared (FTIR) spectroscopy results help validate the synthesised PILs’ formation. Further, their physicochemical and tribological properties were investigated. The PILs as neat lubricants were tested on a ball-on-plate reciprocating tribometer using bearing steel–bearing steel and bearing steel–aluminium alloy friction pairs at 30 °C and 80 °C. The results show that the investigated PILs significantly reduced the coefficient of friction and wear. The dodecylamine-based PILs performed better in friction and wear reduction than the other investigated lubricants. The formation of the adsorption layer on the friction pairs was assumed to be the dominant friction and wear reduction mechanism.

Molecular simulations of understanding the Na+ ion structure, dynamic and thermodynamic behavior in ionic liquids: Butyl ammonium hydrogen bisulfate and tri-butyl ammonium hydrogen bisulfate

This manuscript presents the all-atom molecular dynamics simulations to investigate intermolecular structure and solvation thermodynamics of Na+ ion in two different ammonium-based protic ionic liquids (1) Butyl Ammonium hydrogen bisulfate [BA+][HSO4−], (2) Tri-butyl ammonium hydrogen bisulfate [TBA+][HSO4−]. The ionic liquid [BA+][HSO4−] show a more coordinated behavior when compared to [TBA+][HSO4−], which is observed over the temperature range from 278 K to 348 K. Hydrogens of the cations show a hydrogen bonding interaction with oxygens of anions. The cationic [TBA+] molecules show more solvation behavior with anions when compared to the [BA+]. The Na+ ion show a strong coordination structure with [HSO4−] in [TBA+][HSO4−] when compared to the [BA+][HSO4−]. We further calculate the detailed solvation free energy (ΔG) calculations using thermodynamic integration. We found that the ΔG of Na+ is more favorable in [TBA+][HSO4−] when compared to [BA+][HSO4−] in the temperature range varying from 278 K to 348 K. With the temperature rise, we observe the more favorable solvation of Na+ in both ionic liquids. On the other hand, the solvation of Cl− becomes less favorable. Overall, this manuscript provides detailed molecular level structural and thermodynamic origins of Na+ in protic ionic liquids useful for designing and developing sustainable electrolytes for Na+ battery applications.

Synthesis and characterization of ammonium-based protic ionic liquids for carbon dioxide absorption.

A series of ammonium-based protic ionic liquids (APILs) namely ethanolammonium pentanoate [ETOHA][C5], ethanolammonium heptanoate [ETOHA][C7], triethanolammonium pentanoate [TRIETOHA][C5], triethanolammonium heptanoate [TRIETOHA][C7], tributylammonium pentanoate [TBA][C5] and tributylammonium heptanoate [TBA][C7] was synthesized via proton transfer. Their structural confirmation and physiochemical properties namely thermal stability, phase transition, density, heat capacity (Cp) and refractive index (RI) have been determined. Specifically, [TRIETOHA] APILs have crystallization peaks ranging from -31.67 to -1.00 °C, owing to their large density values. A comparison study revealed the low Cp values of APILs in comparison to monoethanolamine (MEA) which could be advantageous for APILs to be used in CO2 separation during recyclability processes. Additionally, the performance of APILs toward CO2 absorption was investigated by using a pressure drop technique under a pressure range of 1-20 bar at 298.15 K. It was observed that [TBA][C7] recorded the highest CO2 absorption capacity with the value of 0.74 mole fraction at 20 bar. Additionally, the regeneration of [TBA][C7] for CO2 absorption was studied. Analysis of the measured CO2 absorption data showed marginal reduction in the mole fraction of CO2 absorbed between fresh and recycled [TBA][C7] thus proving the promising potential of APILs as good liquid absorbents for CO2 removal.

Experimental Investigation on Thermophysical Properties of Ammonium-Based Protic Ionic Liquids and Their Potential Ability towards CO2 Capture.

Ionic liquids, which are extensively known as low-melting-point salts, have received significant attention as the promising solvent for CO2 capture. This work presents the synthesis, thermophysical properties and the CO2 absorption of a series of ammonium cations coupled with carboxylate anions producing ammonium-based protic ionic liquids (PILs), namely 2-ethylhexylammonium pentanoate ([EHA][C5]), 2-ethylhexylammonium hexanoate ([EHA][C6]), 2-ethylhexylammonium heptanoate ([EHA][C7]), bis-(2-ethylhexyl)ammonium pentanoate ([BEHA][C5]), bis-(2-ethylhexyl)ammonium hexanoate ([BEHA][C6]) and bis-(2-ethylhexyl)ammonium heptanoate ([BEHA][C7]). The chemical structures of the PILs were confirmed by using Nuclear Magnetic Resonance (NMR) spectroscopy while the density (ρ) and the dynamic viscosity (η) of the PILs were determined and analyzed in a range from 293.15K up to 363.15K. The refractive index (nD) was also measured at T = (293.15 to 333.15) K. Thermal analyses conducted via a thermogravimetric analyzer (TGA) and differential scanning calorimeter (DSC) indicated that all PILs have the thermal decomposition temperature, Td of greater than 416K and the presence of glass transition, Tg was detected in each PIL. The CO2 absorption of the PILs was studied up to 29 bar at 298.15 K and the experimental results showed that [BEHA][C7] had the highest CO2 absorption with 0.78 mol at 29 bar. The CO2 absorption values increase in the order of [C5] < [C6] < [C7] anion regardless of the nature of the cation.

Antiproliferative and Antimicrobial Effects of Tris(2-hydroxyethyl)ammonium-Based Protic Ionic Liquids with Some Fatty Acids

Protic ionic liquids (PILs) that emerge as promising compounds are of great interest in industry and academia due to their easy synthesis and unique physical and chemical properties. This study aims to evaluate the antimicrobial activities against various microorganisms and the antiproliferative effects of four PILs namely, the PILs tris(2-hydroxyethyl)ammonium dodecanoate (TALA), tris(2-hydroxyethyl)ammonium tetradecanoate (TAMA), tris(2-hydroxyethyl)ammonium palmitate (TAPA) and tris(2-hydroxyethyl)ammonium stearate (TASA). Antiproliferative effects of PILs were investigated in vitro on breast cancer cell line (MDA-MB 231), colon cancer cell line (HT29) and prostate cancer cell line (PC3). Furthermore, minimum inhibitory concentrations (MIC) were established for PILs tested against rods, cocci and fungi. The antimicrobial activities of the PILs are strongly related to the alkyl chain length of the anion.

Thermophysical Properties of Newly Synthesized Ammonium-Based Protic Ionic Liquids: Effect of Temperature, Anion and Alkyl Chain Length

Ionic liquids which are often classified as low melting point salts have received significant attention from research groups and industries to be used in a wide range of applications. Many of these applications require thorough knowledge on the thermophysical properties of the ionic liquids before utilizing their full potentials in various fields. In this work, a series of alkylammonium cation and carboxylate anion-based room temperature protic ionic liquids (PILs) were synthesized by varying length of alkyl chain of the cation from diethyl to dibutyl combined with pentanoate, hexanoate and heptanoate anions. These ammonium-based PILs named as diethylammonium pentanoate [DEA][C5], diethylammonium hexanoate [DEA][C6], diethylammonium heptanoate [DEA][C7], dibutylammonium pentanoate [DBA][C5], dibutylammonium hexanoate [DBA][C6] and dibutylammonium heptanoate [DBA][C7] were characterized using Nuclear Magnetic Resonance (NMR) spectroscopy. The thermophysical properties of the PILs namely density, dynamic viscosity and refractive index were measured and analyzed. Density, ρ and dynamic viscosity, η were determined at T = (293.15 to 363.15) K and refractive index, nD was measured at T = (293.15 to 333.15) K. The fitting parameters are proposed for the empirical correlations of density, dynamic viscosity and refractive index. The values of thermal expansion coefficient, αp, molecular volume, Vm, standard entropy, S° and lattice potential energy, Upot also have been calculated by using the specified equations. The thermal decomposition temperature, Td was also determined using a thermogravimetric analyzer (TGA) while the differential scanning calorimetry (DSC) technique provided the glass transition, Tg, melting point, Tm and crystallization, Tc temperatures of the PILs. The experimental results revealed that the dependency of the experimental values namely the ρ, η, nD, and Td on the alkyl chain of the anion, size of the cations and the temperature of measurement.

Thermophysical Properties and CO2 Absorption of Ammonium-Based Protic Ionic Liquids Containing Acetate and Butyrate Anions

Ionic liquids, which are classified as new solvents, have been identified to be potential solvents in the application of CO2 capture. In this work, six ammonium-based protic ionic liquids, containing ethanolammonium [EtOHA], tributylammonium [TBA], bis(2-ethylhexyl)ammonium [BEHA] cations, and acetate [AC] and butyrate [BA] anions, were synthesized and characterized. The thermophysical properties of the ammonium-based protic ionic liquids were measured. Density, , and dynamic viscosity, , were determined at temperatures between 293.15 K and 363.15 K. The density and viscosity values were correlated using empirical correlations and the thermal coefficient expansion, p, and molecular volume, Vm, were estimated using density values. The thermal stability of the ammonium-based protic ionic liquids was investigated using thermogravimetric analyzer (TGA) at a heating rate of 10 C.min‒1. The CO2 absorption of the ammonium-based ionic liquids were measured up to 20 bar at 298.15 K. From the experimental results, [BEHA][BA] had the highest affinity towards CO2 with the mol fraction of CO2 absorbed approaching 0.5 at 20 bar. Generally, ionic liquids with butyrate anions have better CO2 absorption than that of acetate anions while [BEHA] ionic liquids have higher affinity towards CO2 followed by [TBA] and [EtOHA] ionic liquids.

Aggregation Behavior of Pyrrolidinium Ionic Liquid Surfactants in −OH-Functionalized Ammonium-Based Protic Ionic Liquids

Aggregation of surfactants in ionic liquids (ILs) is poorly understood in spite of their great potential for applications in many fields. In this paper, the aggregation behavior of pyrrolidinium IL...

Changes in microstructure of two ammonium-based protic ionic liquids proved by in situ variable-temperature 1 H NMR spectroscopy: influence of anion.

In this work, changes in microstructure of two protic ionic liquids (PILs), namely n-butylammonium acetate (N4Ac) and n-butylammonium nitrate (N4NO3 ), are proved by in situ variable-temperature 1 H NMR spectroscopy at the temperature range from 25 to 115°C, and the influence of the nature of anion is discussed accordingly. The results demonstrate that 1 H NMR chemical shifts of alkyl protons of both N4Ac and N4NO3 are almost not changed with the increasing of temperature, due to the absence of hydrogen bond interaction between alkyl protons with anions. Whereas those of + N-H of cation decrease linearly with the temperature increasing, indicating that the hydrogen bond interaction between + N-H and anion weakens gradually. In addition, the strength of hydrogen bond interaction between + N-H and NO3- is stronger than that between + N-H and Ac- , suggesting that anions have a significant influence on microstructure due to the acidity of a Brønsted acid. Consequently, the proton transfer from cation to anion is much easier in N4Ac compared to N4NO3 . Further analyses of 1 H NMR chemical shifts of + N-H in N4Ac at the temperature range from 100 to 115°C suggest that the splitting of + N-H peak may be attributed to obvious evidence of the existence of the proton transfer from + N-H to Ac- , which leads to dissociate the contact ion-pair in N4Ac to form the neutral ion-pair 'molecule'. The results will help us to extensively understand the behavior of proton transfer and offer us some valuable information for the design of PILs. Copyright © 2017 John Wiley & Sons, Ltd.

Protic Ionic Liquids Used as Metal-Forming Green Lubricants for Aluminum: Effect of Anion Chain Length

Among the applications for protic ionic liquids (PILs), lubrication is one of the newest and the most promising. In this work, ammonium-based protic ionic liquids were tested as lubricant fluids for aluminum-steel contacts. PILs were synthesized with 2-hydroxyethylamine (2HEA) and a carboxylic acid (formic and pentanoic), aiming to understand the effect of two different anion chain lengths on the lubricant behavior. The synthesized PILs were characterized by RMN, FTIR and TGA. Wear tests, conducted using a ball-on-plate configuration, showed that the increase of the anion carbon chain length in the PIL structure reduced significantly the coefficient of friction value. Besides, after the wear tests, the PILs structural integrity was not affected. In the same way, bending under tension (BUT) tests evidenced that the performance for stamping conditions of the PIL with the longest anion carbon chain was similar to that of the commercial lubricant. Since, both formed a uniform tribofilm, developed the same lubrication regime and the drawing forces values were close and constant. Hence, the ionic liquid obtained with 2HEA and pentanoic acid (2HEAPe) is as suitable as the commercial lubricant for metal forming processes.

Structural and Transport Properties of Tertiary Ammonium Triflate Ionic Liquids: A Molecular Dynamics Study

Ammonium-based protic ionic liquids (PILs) have shown promising physicochemical properties as proton conductors in polymer membrane fuel cells. In this work, molecular dynamics simulations are used to study the structural, dynamic, and transport properties of a series of tertiary ammonium triflate PILs. Nonpolarizable all-atom force fields were used, including two different models for the triflate anion. Previous simulation studies of these PILs have yielded poor results for transport properties due to unrealistically slow dynamics. To improve performance, polarization and charge-transfer effects were approximately accounted for by scaling all atomic partial charges by a uniform factor, γ. The optimum scaling factor γ = 0.69 was determined by comparing simulation results with available experimental data and found to be transferable between different ammonium cations and insensitive to both the temperature and choice of experimental data used for comparison. Simulations performed using optimized charge scaling showed that the transport properties significantly improved over previous studies. Both the self-diffusion coefficients and viscosity were in good agreement with experiment over the whole range of systems and temperatures studied. Simulated PIL densities were also in good agreement with experiment, although the thermal expansivity was underestimated. Structural analysis revealed a strong directionality in interionic interactions. Triflate anions preferentially approach the ammonium cation so as to form strong hydrogen bonds between sulfonate oxygen atoms and the acidic ammonium hydrogen. The ionic conductivity was somewhat underestimated, especially at lower temperatures. Analysis of conductivity data suggests that there is significant correlated motion of oppositely charged ions in these PILs, especially at short times. These results overall indicate that the transport properties of this class of PILs are accurately modeled by these force fields if charge scaling is used and properly calibrated against selected experimental data.

Proof of ion-pair structures in ammonium-based protic ionic liquids using combined NMR and DFT/PCM-based chemical shift calculations.

The self-assembly of triethylammonium bis(trifluoromethylsulfonyl)imide, i.e. [(C2H5)3NH][TFSI], in chloroform and aqueous solutions has been investigated using 1H NMR spectroscopy and computational (DFT/PCM prediction) methods. We have examined a number of ion pairs formed between the [(C2H5)3NH]+ cation with different conformations of alkyl substituents as well as various dispositions of the multi-site [TFSI]- anion. Based on the agreement between the calculated (DFT) and observed 1H NMR chemical shifts, [(C2H5)3NH][TFSI] in chloroform formed lipophilic complexes with effective N+-HN or N+-HO hydrogen bonding, whereas hydrophilic complexes with Cα-HO and Cα-HF hydrogen bonding are found in aqueous solutions. This study provides a new insight into the self-aggregation of ammonium PILs incorporating the widely used [TFSI]- anion and demonstrates the importance of solvent effects on chemical shifts. The simulations with explicit and implicit dielectric continuum solvents are found to be the most realistic method, yielding a representative ensemble of structures.

Thermophysical properties of two ammonium-based protic ionic liquids.

Experimental data for density, viscosity, refractive index and surface tension are reported, for the first time, in the temperature range between 288.15 K and 353.15 K and at atmospheric pressure for two protic ionic liquids, namely 2-(dimethylamino)-N,N-dimethylethan-1-ammonium acetate, [N11{2(N11)}H][CH3CO2], and N-ethyl-N,N-dimethylammonium phenylacetate, [N112H][C7H7CO2]. The effect of the anion aromaticity and the cation's aliphatic tails on the studied properties is discussed. From the measured properties temperature dependency the derived properties, such as the isobaric thermal expansion coefficient, the surface entropy and enthalpy, and the critical temperature, were estimated.

Atomistic simulations of ammonium-based protic ionic liquids: steric effects on structure, low frequency vibrational modes and electrical conductivity.

Protic ionic liquids (PILs) are of great interest as electrolytes in various energy applications. Molecular dynamics simulations of trialkylammonium (with varying alkyl group such as methyl, ethyl, and n-propyl) triflate PILs are performed to characterize the influence of the alkyl group on the acidic site (N-H) of the ammonium cation. Spatial distribution function of anions over this site on the cation reveals significant influence of the length of alkyl tail on intermolecular structure. Vibrational density of states and normal modes are calculated for bulk liquids to probe atomic displacements in the far infrared region. The observed N-H···O hydrogen bond stretching vibration in 155-165 cm(-1) frequency region agrees well with experiments. Trends in electrical conductivity calculated using Nernst-Einstein and Green-Kubo relation are in qualitative agreement with experiments. The self-diffusion coefficient and the electrical conductivity is highest for N,N-dimethyl-N-ethylammonium triflate ([N112][TfO]) and is lowest for N,N-di-n-propyl-N-methylammonium triflate ([N133][TfO]) IL.

Prevention of insulin self-aggregation by a protic ionic liquid

The self-aggregation and thermal instability of insulin (In) was considerably controlled in the presence of ammonium-based protic ionic liquids (PILs). The thermal stability of In in PILs was observed using fluorescence and absorption spectroscopy of the Tyr environment of the biomolecule. Additionally, from circular dichroism (CD) measurements, we observed the shift in the wavelength towards lower values in the presence of PILs, which indicates the formation of monomers of In, further evidently supported by dynamic light scattering (DLS) measurements. Surprisingly, it is the monomeric form of the In that exists in the active form. For the first time, ammonium-based PILs have been shown to be novel solvents for In, which prevent it from associating into an inactive form and also stabilizes In against thermal influence.

Thermodynamic properties of aqueous solutions of two ammonium-based protic ionic liquids at 298.15 K

Densities (ρ), refractive indices (nD) and electrical conductivities (κ) of ethylammonium acetate (N2Ac) protic ionic liquid+water and propylammonium acetate (N3Ac) protic ionic liquid+water binary mixtures over the entire composition range were measured at 298.15K under atmospheric pressure. The excess molar volumes (VE), apparent molar volumes (Vϕi), partial molar volumes (V¯m,i) and excess partial molar volumes (V¯m,iE) of the binary mixtures were calculated from the density data. The refractive index deviations (ΔnD) of the binary mixtures were determined using the refractive index data. The excess molar volumes and refractive index deviations were correlated with the Redlich–Kister equation. The excess molar volumes of the above-mentioned systems are negative over the whole composition range, while the refractive index deviations are positive. The concentration dependence of electrical conductivity was fitted according to the empirical Casteel–Amis equation. The intermolecular interactions and structural effects were analyzed on the basis of the measured and the derived properties.

Physicochemical properties of phosphonium-based and ammonium-based protic ionic liquids

Trioctylphosphonium triflate, trioctylammonium triflate, triphenylphosphonium triflate and triphenylammonium triflate were synthesized and characterized. It was found that phosphonium-based protic ionic liquids (PILs) exhibit higher thermal stability and ionic conductivity than the corresponding ammonium-based PILs, no matter whether the P and N center atoms are bonded to the electron-donating octyl groups or the electron-withdrawing phenyl groups. The ion conduction behavior of the PILs can be adequately described by the Vogel–Fulcher–Tamman (VFT) equation. The higher ionic conductivity of phosphonium-based PILs may be attributed to their weaker hydrogen bond and Coulombic interactions as well as higher carrier ion concentrations, indicated by infrared analysis, lattice potential energy estimation and VFT fitting results. Interestingly, the stronger hydrogen bonds inside trioctylammonium triflate may lead to a much decreased melting point. Furthermore, compared with electron-withdrawing phenyl, electron-donating octyl enhanced the thermal stability of the PILs.