Cation Alkyl Side Chain Length Research Articles

Enhanced bacterial uptake of 131I -labeled antimicrobial imidazolium bromide salts using fluorescent carbon nanodots

Imidazolium bromide salts have been shown as potent antibiotic molecules that show structure-based bioactivity related to their cation alkyl side chain length. To enhance the bioavailability of lipophilic alkyl side chains herein, a 1,8-naphthalimide group containing imidazolium bromide salts bearing different lengths of alkyl chains (NIM1, 2, and 3) are coupled with fluorescent carbon dots (C-NIMs) through electrostatic and pi-pi interactions. Further, obtained nanocarriers were radio-labeled with iodine-131 (131I) to track the bacterial uptake of them by Staphylococcus aureus and Escherichia coli. Antibacterial activities were also investigated by the microdilution method. Comparison studies showed that both radiolabeling efficiency and lipophilicity increased when NIMs were integrated onto the CDots. More importantly, CDots resulted in 4-fold enhanced uptake of NIM1 by S. aureus bacterium as compared to pristine imidazolium bromide salts while at a higher number of alkyl chain lengths enhancement was 2-fold.

Insights into the effect of imidazolium-based ionic liquids on chemical structure and hydrolytic activity of microbial lipase.

This work studied the effect of the cation alkyl chain length of 1-alkyl-n-methylimidazolium chloride ([Cnmim]Cl)-based ILs on the activity of Aspergillus niger lipase. First, the lipase activity in the presence of different ILs concentration over time was determined. ILs with shorter cation alkyl side chain length, namely [C4mim]Cl and [C6mim]Cl, promoted an increase of lipase activity; while, [C8mim]Cl, depending on its concentration, maintained or decreased the enzyme activity. In the presence of ILs with longer cation alkyl chain length, i.e., [C10mim]Cl and [C12mim]Cl, the lipase relative activity was reduced with 0.1 (%v/v) and until suppressed ([C12mim]Cl at 0.3 (%v/v)) as a result of irreversible changes in its secondary structure. Fluorescence and circular dichroism spectroscopy analysis confirmed the results achieved. These findings show that [Cnmim]Cl-based ILs can exert different behavior on the lipase' activity (enhance, maintain or even inhibit) and structural conformation, depending on the cation alkyl chain length and their relative concentration.

Odd-even effect on the formation of aqueous biphasic systems formed by 1-alkyl-3-methylimidazolium chloride ionic liquids and salts.

This work provides a comprehensive evaluation of the effect of the cation alkyl side chain length of the 1-alkyl-3-methylimidazolium chloride series ([C n C1im]Cl, n = 2-14) of ionic liquids (ILs) on their capability to form aqueous biphasic systems (ABSs) with salts and self-aggregation derived properties. The liquid-liquid phase behavior of ternary systems composed of [C n C1im]Cl, water, and K3PO4 or K2CO3 and the respective Setschenow salting-out coefficients (ks ), a quantitative measure of the two-phase formation ability, were determined. An odd-even effect in the ks values along the number of methylene groups of the longest IL cation alkyl side chain was identified for the ABS formed by K2CO3, a weaker salting-out agent where the phenomenon is clearly identified. In general, cations with even alkyl side chains, being likely to display higher molar volumes, are more easily salted-out and thus more prone to undergo phase separation. The odd-even effect in the ks values is, however, more significant in ILs up to n = 6, where the nanostructuration/nanosegregation of ILs plays a less relevant role. Still, with the [C n C1im]Cl (n = 7-14) series of ILs, an odd-even effect was also identified in the ILs' ionization degree, molar conductivity, and conductivity at infinite dilution. In summary, it is shown here that the ILs' odd-even effect occurs in IL aqueous solutions and not just in neat ILs, an already well-established phenomenon occurring in a series of ILs' properties described as a result of the orientation of the terminal methyl groups to the imidazolium ring cation and consequent effect in the ILs' cohesive energy.

Aqueous biphasic systems composed of ionic liquids and polypropylene glycol: insights into their liquid-liquid demixing mechanisms.

Novel ternary phase diagrams of aqueous biphasic systems (ABSs) composed of polypropylene glycol with an average molecular weight of 400 g mol(-1) (PPG-400) and a vast number of ionic liquids (ILs) were determined. The large array of selected ILs allowed us to evaluate their tuneable structural features, namely the effect of the anion nature, cation core and cation alkyl side chain length on the phase behaviour. Additional evidence on the molecular-level mechanisms which rule the phase splitting was obtained by (1)H NMR (Nuclear Magnetic Resonance) spectroscopy and by COSMO-RS (Conductor-like Screening Model for Real Solvents). Some systems, for which the IL-PPG-400 pairs are completely miscible, revealed to be of type "0". All data collected suggest that the formation of PPG-IL-based ABSs is controlled by the interactions established between the IL and PPG, contrarily to previous reports where a "salting-out" phenomenon exerted by the IL over the polymer in aqueous media was proposed as the dominant effect in ABS formation. The influence of temperature on the liquid-liquid demixing was also evaluated. In general, an increase in temperature favours the formation of an ABS in agreement with the lower critical solution temperature (LCST) phase behaviour usually observed in polymer-IL binary mixtures. Partition results of a dye (chloroanilic acid, in its neutral form) further confirm the possibility of tailoring the phases' polarities of IL-PPG-based ABSs.

Comprehensive study on the impact of the cation alkyl side chain length on the solubility of water in ionic liquids

A comprehensive study on the phase behaviour of two sets of ionic liquids (ILs) and their interactions with water is here presented through combining experimental and theoretical approaches. The impact of the alkyl side chain length and the cation symmetry on the water solubility in the asymmetric [CN−1C1im][NTf2] and symmetric [CN−1CN−1im][NTf2] series of ILs (N up to 22), from 288.15K to 318.15K and at atmospheric pressure, was studied. The experimental data reveal that the solubility of water in ILs with an asymmetric cation is higher than in those with the symmetric isomer. Several trend shifts on the water solubility as a function of the alkyl side chain length were identified, namely at [C6C1im][NTf2] for asymmetric ILs and at [C4C4im][NTf2] and [C7C7im][NTf2] for the symmetric ILs. To complement the experimental data and to further investigate the molecular-level mechanisms behind the dissolution process, density functional theory calculations, using the Conductor-like Screening Model for Real Solvents (COSMO-RS) and the electrostatic potential-derived CHelpG, were performed. The COSMO-RS model is able to qualitatively predict water solubility as a function of temperature and alkyl chain lengths of both symmetric and asymmetric cations. Furthermore, the model is also capable to predict the somewhat higher water solubility in the asymmetric cation, as well as the trend shift as a function of alkyl chain lengths experimentally observed. Both COSMO-RS and the electrostatic potential-derived CHelpG show that the interactions of water and the IL cation take place on the IL polar region, namely on the aromatic head and adjacent methylene groups that explains the differences in water solubility observed for cations with different chain lengths. Furthermore, the CHelpG calculations for the isolated cations in the gas phase indicates that the trend shift of water solubility as a function of alkyl chain lengths and the difference of water solubility in symmetric may also result from the partial positive charge distribution/contribution of the cation.

Can the Cation Alkyl Side Chain Length Affect the Thermal and Transport Properties of Pyrrolidinium-TFSI Ionic Liquid Mixtures

There is growing interest in replacing the volatile and hazardous organic solvents, currently used in electrochemical devices, with ionic liquids (ILs), this pushing them far beyond the initially envisaged role of green solvents. However, the rational design of novel systems based on ILs requires a deep understanding of the intermolecular interactions and transport properties occurring in the ionic liquid bulk. The possibility of fine tuning the physicochemical properties of ionic liquids by forming IL blends is a very appealing characteristic [1]. For instance, the association and ordering of the ions in a mixture is more complicated than in the pure components, thus affecting the short-range aggregation motives and, consequently, the blend physicochemical properties. Ionic liquids are highly structured fluids with short-range aggregation motives, strongly correlated to the alkyl side chain length bounded to the nitrogen group of the cation [1].Our basic idea is to investigate the length effect of the cation main aliphatic side chain on the thermal and transport properties of binary ionic liquid mixtures formed by pyrrolidinium materials. Three sets of IL mixtures were prepared: i) PYR11TFSI-PYR14TFSI, ii) PYR11TFSI-PYR18TFSI and iii) PYR14TFSI-PYR18TFSI. The present study was performed by DSC, conductivity and self-diffusion coefficients (NMR measurements).The DSC heating trace of the ionic liquid blends is shown in Figure 1. The PYR11TFSI-PYR14TFSI samples (panel A) exhibit a well-defined melting peak. The progressive enrichment in PYR11TFSI was seen to hinder the crystallization process of the IL mixtures, as a result of the unfavorable ion packing likely ascribable to the different size of the (PYR11)+ and (PYR14)+ cations. Apart the x = 0.75 sample of panel B, the PYR11TFSI-PYR18TFSI (panel B) and PYR14TFSI-PYR18TFSI (panel C) mixtures do not display any feature, with exception of the glass transition one (Tg ), indicating that these IL blends are not able to be crystallized. This suggests that the “ionic confusion”, resulting from the contemporaneous presence of different symmetry pyrrolidinium cations, is able to prevent the crystallization of the mixtures which, instead, solidify in a glassy state at very low temperatures.Figure 1. DSC trace of PYR11TFSI-PYR14TFSI (panel A), PYR11TFSI-PYR18TFSI (panel B) and PYR14TFSI-PYR18TFSI (panel C) ionic liquid binary mixtures.In Figure 2 is reported the ionic conductivity vs. PYR14TFSI mole fraction dependence for (x)PYR14TFSI-(1-x)PYR18TFSI. At a first sight it is seen that the ionic conductivity decreases with the progressive impoverishment of the highest conductive material (PYR14TFSI) fraction in the binary mixture. However, a more careful observation shows that this trend is true only at temperatures above the melting point of PYR14FSI, e.g., -7 °C. At lower temperatures, the ionic conductivity for PYR14TFSI mole fractions ranging from 0.25 to 0.75, is from two to four orders of magnitude higher than the neat IL materials.Figure 2. Ionic conductivity vs. mole fraction behavior of (x)PYR14TFSI-(1-x)PYR18TFSI mixtures.AcknowledgementsJ.S.M. and G.B.A. thank the Italian Institute of Technology (IIT) for the financial support within the SEED Project “REALIST”.

Cation Alkyl Side Chain Length and Symmetry Effects on the Surface Tension of Ionic Liquids

Aiming at providing a comprehensive study of the influence of the cation symmetry and alkyl side chain length on the surface tension and surface organization of ionic liquids (ILs), this work addresses the experimental measurements of the surface tension of two extended series of ILs, namely R,R'-dialkylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C(n)C(n)im][NTf2]) and R-alkyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C(n)C(1)im][NTf2]), and their dependence with temperature (from 298 to 343 K). For both series of ILs the surface tension decreases with an increase in the cation side alkyl chain length up to aliphatic chains no longer than hexyl, here labeled as critical alkyl chain length (CACL). For ILs with aliphatic moieties longer than CACL the surface tension displays an almost constant value up to [C12C12im][NTf2] or [C16C1im][NTf2]. These constant values further converge to the surface tension of long chain n-alkanes, indicating that, for sufficiently long alkyl side chains, the surface ordering is strongly dominated by the aliphatic tails present in the IL. The enthalpies and entropies of surface were also derived and the critical temperatures were estimated from the experimental data. The trend of the derived thermodynamic properties highlights the effect of the structural organization of the IL at the surface with visible trend shifts occurring at a well-defined CACL in both symmetric and asymmetric series of ILs. Finally, the structure of a long-alkyl side chain IL at the vacuum-liquid interface was also explored using Molecular Dynamics simulations. In general, it was found that for the symmetric series of ILs, at the outermost polar layers, more cations point one of their aliphatic tails outward and the other inward, relative to the surface, than cations pointing both tails outward. The number of the former, while being the preferred conformation, exceeds the latter by around 75%.

Impact of the cation symmetry on the mutual solubilities between water and imidazolium-based ionic liquids

Aiming at the evaluation of the impact of the ionic liquids (ILs) cation symmetry on their phase behaviour, in this work, novel mutual solubilities with water of the symmetric series of [CnCnim][NTf2] (with n=1–5) were determined and compared with their isomeric forms of the asymmetric [CnC1im][NTf2] group. While the solubility of isomeric ILs in water was found to be similar, the solubility of water in ILs follows the same trend up to a maximum cation alkyl side chain length. For n≥4 in [CnCnim][NTf2] the solubility of water in the asymmetric ILs is slightly higher than that observed in the symmetric counterparts. The thermodynamic properties of solution and solvation derived from the experimental solubility data of ILs in water at infinite dilution, namely the Gibbs energy, enthalpy and entropy were used to evaluate the cation symmetry effect on the ILs solvation. It is shown that the solubility of ILs in water is entropically driven and highly influenced by the cation size. Accordingly, it was found that the ILs solubility in water of both symmetric and asymmetric series depends on their molecular volume. Based on these findings, a linear correlation between the logarithm of the solubility of ILs in water and their molar volume is here proposed for the [NTf2]-based ILs at a fixed temperature.

Alkylimidazolium Based Ionic Liquids: Impact of Cation Symmetry on Their Nanoscale Structural Organization

Aiming at evaluating the impact of the cation symmetry on the nanostructuration of ionic liquids (ILs), in this work, densities and viscosities as a function of temperature and small-wide angle X-ray scattering (SWAXS) patterns at ambient conditions were determined and analyzed for 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (asymmetric) and 1,3-dialkylimidazolium bis(trifluoromethylsulfonyl)imide (symmetric) series of ionic liquids. The symmetric IL series, [CN/2CN/2im][NTf2], presents lower viscosities than the asymmetric [CN-1C1im][NTf2] counterparts. For ionic liquids from [C1C1im][NTf2] to [C6C6im][NTf2], an odd-even effect in the viscosity along the cation alkyl side chain length was observed, in contrast with a linear increase found for the ones ranging between [C6C6im][NTf2] and [C10C10im][NTf2]. The analysis of the viscosity data along the alkyl side chain length reveals a trend shift that occurs at [C6C1im][NTf2] for the asymmetric series and at [C6C6im][NTf2] for the symmetric series. These results are further supported by SWAXS measurements at ambient conditions. The gathered data indicate that both asymmetric and symmetric members are characterized by the occurrence of a distinct degree of mesoscopic structural organization above a given threshold in the side alkyl chain length, regardless the cation symmetry. The data also highlight a difference in the alkyl chain dependence of the mesoscopic cluster sizes for symmetric and asymmetric cations, reflecting a different degree of interdigitation of the aliphatic tails in the two families. The trend shift found in this work is related to the structural segregation in the liquid after a critical alkyl length size (CALS) is attained and has particular relevance in the cation structural isomerism with higher symmetry.

The impact of self-aggregation on the extraction of biomolecules in ionic-liquid-based aqueous two-phase systems

On aqueous two-phase systems (ATPS) composed of ionic liquids (ILs) it is often observed that the increase in the cation alkyl side chain length enhances the biomolecules extraction for the IL-rich phase. However, mounting evidences suggest that very long alkyl side chains are unfavorable to the separation. This change on the extraction trends was previously suggested to be due to micelle formation; yet, no evidence was provided and an evaluation of the effect of the pH and of the hydrophobicity of the biomolecules on the partition coefficients for a homologous family of ILs was never carried out. In this work a systematic study of the cation alkyl side chain length effect on the partitioning of a series of alkaloids of variable hydrophobicity in ATPS constituted by 1-alkyl-3-methylimidazolium chloride ILs ([Cnmim]Cl) and potassium citrate (at controlled pH) was conducted. The results here reported show that the alkaloids partition coefficients increase with the cation alkyl chain length until the formation of micelles on the systems with cations larger than [C6mim]Cl. Cations with longer aliphatic chains are not favorable for the extraction of alkaloids, inducing a trendshift on the system by decreasing the partition coefficients or the preferential migration for the IL-rich phase. The pH of the media, and consequently the charged/non-charged state of the alkaloids, as well as the hydrophobicity of the molecules, do not alter the extraction pattern observed. These results indicate that the IL ability to form micelles in ATPS is the interfacial phenomenon that creates the trendshift observed in the partition coefficients.

Enhanced extraction of caffeine from guaraná seeds using aqueous solutions of ionic liquids

The extraction of caffeine from bioresources using more benign and cost-effective processes is of fundamental relevance towards the finding of alternative (bio)pesticides. Classically, the best attempts to extract caffeine from biomass have resulted in low efficiency and in a large consumption of hazardous organic solvents and/or energy requirements. Here, we report an enhanced and selective extraction of caffeine from guarana (Paullinia cupana, Sapindaceae) seeds using aqueous solutions of ionic liquids. Several ionic liquids composed of imidazolium or pyrrolidinium cations combined with the chloride, acetate and tosylate anions were investigated. Furthermore, the effect of the cation alkyl side chain length and the presence of functionalized groups were also addressed. Additional conditions such as the ionic liquid concentration, the contact time, the solid–liquid ratio and temperature were further optimized by a response surface methodology. Outstanding extraction yields (up to 9 wt% of caffeine per guarana dry weight) were obtained at a moderate temperature and in a short-time. The recyclability and reusability of the ionic liquids were also confirmed. For the first time it is shown that aqueous solutions of ionic liquids are superior alternatives for the solid–liquid extraction of caffeine from biomass samples and, as a result, the development of an ionic-liquid-based process is straightforwardly envisaged.

Impact of Self-Aggregation on the Formation of Ionic-Liquid-Based Aqueous Biphasic Systems

This work reports on the systematic investigation of the influence of the cation alkyl side-chain length of 1-alkyl-3-methylimidazolium chloride ionic liquids ([C(n)C(1)im]Cl, with n = 1-14), as well as the substitution of the most acidic hydrogen in the imidazolium core by a methyl group, in the formation of aqueous biphasic systems. Ternary phase diagrams, tie-lines, tie-line slopes, tie-line lengths, and critical points for the several systems (ionic liquid + water + K(3)PO(4)) were determined and reported at 298 K and atmospheric pressure. It is shown that the increase of the cation alkyl chain length enhances the formation of aqueous biphasic systems if alkyl chain lengths until the hexyl are considered. The results for longer alkyl side chains show, nevertheless, that the phenomenon is more complex than previously admitted and that the capacity of the ionic liquid to self-aggregate also governs its ability to phase separate. The effect of the alkyl side-chain length on the phase-forming ability of the studied systems was quantitatively evaluated based on their salting-out coefficients derived from a Setschenow-type behavior. The aptitude of each ionic liquid for liquid-liquid demixing as a function of the cation alkyl side-chain length clearly follows three different patterns. The results obtained for the trisubstituted cation indicate that the hydrogen-bonding interactions between the ionic liquid cation and water are not a relevant issue in the formation of aqueous two-phase systems. In general, for the [C(n)C(1)im]Cl series, a multifaceted ratio between entropic contributions and the ability of each ionic liquid to self-aggregate in aqueous media control the phase behavior.

Viscosity of (C2–C14) 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ionic liquids in an extended temperature range

Dynamic viscosities of several members of the 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide, [Cnmim][NTf2], ionic liquids family, with the cation alkyl side-chain length varying from 2 to 14 carbon atoms, have been measured in the 278.15K to 393.15K temperature range using two different apparatuses. To the best of our knowledge, it is the first time that such an extensive assessment of viscosity – in terms of the number of compounds of a single ionic liquid homologous series, of the broad range of temperature covered, and the use of two different experimental techniques – is reported. The use of two different instruments, using different methodologies, provides information about the uncertainties in the measurement of viscosity of ionic liquids, including its dependence on the presence of traces of water and other impurities. An extensive critical analysis of the deviations between the data measured in this work and those reported in literature has been carried out.

Solubility of non-aromatic ionic liquids in water and correlation using a QSPR approach

The solubility of ionic liquids (ILs) in water is of significant relevance for both process design and evaluation of their environmental impact. In this work, the solubilities of the non-aromatic piperidinium- and pyrrolidinium-based ILs in water, combined with the anion bis(trifluoromethylsulfonyl)imide, were determined in the temperature range from (288.15 to 318.15) K. Electrospray ionization mass spectrometry (ESI-MS) was used as the analytical method after a proper validation of the experimental results obtained. The effect of the ILs structural combinations, such as cation family and alkyl side chain length, in their solubility in water, is analyzed and discussed. From the ILs solubility dependence on temperature, the standard molar thermodynamic functions of solution, namely Gibbs energy, enthalpy and entropy at infinite dilution, were determined. The results indicate that the ILs dissolution in water is an endothermic process and entropically driven. To predict the mutual solubilities between ILs and water, a quantitative structure–property relationship (QSPR) model is also proposed. It is shown that it can provide a good description of the mutual solubilities at 303.15 K with deviations smaller than 10%.

An overview of the mutual solubilities of water–imidazolium-based ionic liquids systems

Ionic liquids (ILs) are a novel class of chemical compounds with interesting properties that are driving a lot of research in several fields. For ionic liquids to be effectively used as solvents in liquid–liquid extractions, the knowledge of the mutual solubilities between ILs and the second liquid phase is fundamental. Furthermore, while ILs cannot volatilise and lead to air pollution, even the most hydrophobic ones present some miscibility with water posturing secondary aquatic environmental risks. Despite the importance of the ILs and water systems, there are few extensive reports on their phase equilibria. The aim of this work is to discuss and understand the mutual solubilities of water and imidazolium-based ILs. The results indicate that these mutual solubilities are primarily defined by the anion followed by the cation alkyl side chain length. It was also found that the solubility of the studied ILs in water is more dependent on the ILs structural modifications than the solubility of water in those ILs. The substitution of the most acidic hydrogen in the imidazolium cation by a methyl group leads to different behaviours in both rich-phases, where the solubility of water in ILs showed to be more hydrogen bonding dependent. From the solubility results it can be concluded that the hydrophobicity of the anions increases in the order [BF 4] − < [CH 3(C 2H 4O) 2SO 4] − < [C(CN) 3] − < [PF 6] − < [N(SO 2CF 3) 2] − while the hydrophobicity of the cations increases, as expected, with the alkyl chain length increase.