Propylammonium Nitrate Research Articles

Contrasting the solvation properties of protic ionic liquids with different nanoscale structure

The limiting activity coefficients of several hydrocarbons and aliphatic alcohols in protic ionic liquids: 2-hydroxyethylammonium nitrate (EOAN), propylammonium nitrate (PAN), butylammonium nitrate (BAN), and butylammonium thiocyanate (BASCN), were determined at 298K using gas chromatographic headspace analysis. The activity coefficients of hydrocarbons in EOAN turned out to be much higher than in BASCN or alkylammonium nitrates. The methylene group increments for the Gibbs free energies of solvation of various homological series in EOAN are also much higher than in BASCN, BAN, PAN as well as in aprotic ionic liquids or molecular solvents except water. Water-like solvation properties of EOAN are linked with the presence of a three-dimensional network of hydrogen bonds, high concentration of ion pairs, and the absence of nanoscale domain structure that exists in liquid alkylammonium salts and favors solvation of hydrocarbons in apolar domains. The sensitivity of solvation properties of PILs to their nanostructure suggests that the presence of apolar and polar domains in a liquid can be probed with the studies of solvation thermodynamics.

Solubilization and recovery of heat-aggregated cytochrome c using alkylammonium nitrate

We investigated the solubilization and recovery of heat-aggregated cytochrome c using alkylammonium nitrates (RAN-ILs), which are a group of protic ionic liquids (ILs), including methylammonium nitrate (MAN), ethylammonium nitrate (EAN), and propylammonium nitrate (PAN), in a wide range of concentrations (X = 0–40 mol%IL for MAN and 0–100 mol%IL for EAN and PAN) using absorption and circular dichroism spectroscopy to obtain basic information on solubilization/recovery solvents for heat-aggregated proteins using ILs. On the whole, the increase of absorbance at the soret band (~405 nm) of cytochrome c indicated that solubilization ability was observed in aqueous solutions with MAN and EAN (X > 10) and PAN (X > 5). These results indicate that the three RAN-ILs studied have solubilization ability for heat-aggregated cytochrome c, and increasing the alkyl-chain length of the cation causes an enhancement of the solubilization ability for heat-aggregated cytochrome c. The activity, secondary structure and heme coordination state (Fe–His18/Met80) of the solubilized cytochrome c in aqueous PAN solutions, which have the highest solubilization ability among the three RAN-ILs, were recovered up to X = 30 after removing PAN by dialysis. The present findings indicate that aqueous solutions with RAN-ILs have a high potential as recovery agents for heat-aggregated proteins.

DTAB micelle formation in ionic liquid/water mixtures is determined by ionic liquid cation structure

HypothesisThe high CMCs and low aggregation numbers of ionic micelles in the extreme electrolyte environment of ionic liquids (ILs) seem to be at odds with the effect of dilute aqueous electrolytes, which lower CMCs and promote elongated micelles. We hypothesise that the driving force for micellisation in ILs is determined by their underlying amphiphilic nanostructure, and that this can be controlled by mixing with water. ExperimentsCMCs and micelle sizes of dodecyltrimethylammonium bromide (DTAB) are determined in mixed solvents comprising water and the ionic liquids ethylammonium nitrate (EAN), ethanolammonium nitrate (EtAN), and propylammonium nitrate (PAN) over a wide composition range. Their behaviour is compared with aqueous electrolytes up to their solubility limit. CMCs are determined by a variety of techniques, and their relative strengths critically evaluated. Micelle morphology is determined by small-angle neutron scattering. FindingsIn water-rich mixtures, ILs do behave like simple electrolytes. Counterion binding dominates, both lowering the aqueous CMC and favouring a sphere-rod transition. However, even at modest concentrations, IL cations become incorporated into the micelle, causing the CMC to pass through a minimum, and arresting the sphere-rod transition. The efficiency of the cation depends on its amphiphilicity. As the IL content increases further, its role as a component of the bulk solvent becomes dominant: Only here does IL nanostructure influence micellization, as it increases alkyl chain solubility (EAN, PAN) and hence raises the CMC.

A methanol and protic ionic liquid Ugi multicomponent reaction path to cytotoxic α-phenylacetamido amides

The Ugi four component reaction of an aldehyde, amine, isocyanide and an ethanoic acid was effected smoothly in protic ionic liquids ethylammonium nitrate (EAN) and propylammonium nitrate (PAN) to afford analogues of α-phenylacetamido amides in good to excellent isolated yields. The corresponding reactions in [BMIM][PF6] and the protic ionic liquid ethanolammonium nitrate (ETAN) failed. Microwave irradiation in EAN facilitated rapid access to three focused libraries, based on the parent isocyanide: cyclohexyl isocyanide, benzyl isocyanide and ethyl isocyanoacetate. Analysis of the structure activity relationship data suggested the presence of a bulky moiety originating from the isocyanide (cyclohexyl and benzyl) enhanced cytotoxicity. Removal of the acetylenic H-atom from the ethanoic acid moiety was detrimental to cytotoxicity. The most active analogues produced, N-(2-cyclohexylamino)-1-(4-methoxyphenyl)-2-oxoethyl-N-(3,5-dimethoxyphenyl)propiolamide, returned average GI50 values of ≤1 μM across the cancer cell lines evaluated. Combined, these data suggest that analogues of this nature are interesting potential anti-cancer development leads.

Recovery and cryopreservation of insulin amyloid using ionic liquids

Amyloid aggregates containing a high content of β-sheet structure are one of several water-insoluble protein aggregates. In this study, we assessed the use of aqueous ionic liquid (IL) solutions as part of a simple preservation/refolding technique for amyloid aggregates. We investigated the dissolution and refolding of cryopreserved bovine insulin amyloid in concentrated aqueous solutions (X = 20 mol%IL) of six different ionic liquids: three 1-butyl-3-methylimidazolium-based ILs ([bmim][X]; X = SCN, NO3, or Cl), 1-ethyl-3-methylimidazolium nitrate ([emim][NO3]), ethylammonium nitrate (EAN), and propylammonium nitrate (PAN). Dimethyl sulfoxide (DMSO) was used as the reference standard for spectroscopy. The six ILs all exhibited the ability to dissolve insulin amyloid, with imidazolium-based ILs exhibiting increased dissolution capacity compared with ammonium-based ILs or DMSO. Remarkably, >80% of the secondary structure of the folded insulin monomer was recovered from insulin amyloid dissolved in aqueous IL solutions following cryopreservation and IL removal by dialysis. These results indicate that concentrated aqueous IL solutions have potential as a new and simple agent for the dissolution, cryopreservation, and refolding of amyloid aggregates.

Dissolution of Amyloid Aggregates in Aqueous Ionic Liquid Solutions: A Case Study of Insulin Amyloid

Dissolution of amyloid aggregates with high β-sheet content is required for the correct refolding of ordered protein aggregates. The dissolution of bovine insulin amyloid aggregates in five different ionic liquids (ILs) is investigated. These were comprised of three 1-butyl-3-methylimidazolium ([bmim])-based ILs, containing either SCN−, NO3−, or Cl− anions, and two alkylammonium nitrate-based ILs, ethyl- and propylammonium nitrate (EAN and PAN). A broad IL concentration range (x=0–30mol-% IL) was analysed using FTIR spectroscopy combined with the Congo red assay. On the whole, the [bmim]-based ILs showed a higher dissolution ability than EAN and PAN for all concentrations of x. It is notable that the dissolution ability of dilute aqueous IL solutions (x<15) for insulin amyloid was different to that of concentrated aqueous IL solutions (x>15). The former condition for insulin amyloid may affect dissolution based on the denaturant effect of cations and anions in the ILs. The latter condition may affect this dissolution based on the hydrogen-bonding ability (α and β values) of the ILs, as described by the Kamlet–Taft parameters. Moreover, the difference between these α and β values (α–β) was found to be a good indicator of the dissolution ability of ILs for insulin amyloid aggregates in concentrated conditions above x=20 (α–β<0, strong dissolution ability; α–β>0, weak dissolution ability). These findings may assist the future design of aqueous IL-based dissolution agents for ordered aggregated proteins.

Cluster ion beam generation from a wetted needle emitter for organic secondary ion mass spectrometry (organic SIMS) using a protic ionic liquid, propylammonium nitrate.

Propylammonium nitrate (PAN, [C3 H7 NH3 ][NO3 ]), a protic ionic liquid, has an active proton in its molecular structure, so that it can promote protonation. In addition, PAN has high aggregability, so that it tends to form large aggregated (or cluster) ions. These features will be desirable for cluster ion beams in secondary ion mass spectrometry (SIMS) of organic materials. The aggregability may enable us to generate a cluster ion beam from a needle tip wetted with PAN by electrospray in vacuum. For these reasons, cluster ion beam generation was investigated using an externally wetted needle emitter. A sharpened glass rod was used as a needle emitter. PAN was electrosprayed in vacuum using the needle emitter to generate a cluster ion beam. Beam characteristics were investigated with an apparatus for measuring transient responses of a beam current. SIMS experiments were also performed using the cluster ion beam as a primary ion beam; arginine and polyethylene glycol (PEG300) were analyzed. A stable cluster ion beam was generated from the needle emitter wetted with PAN. The ion beam consisted of mixed cluster ions whose m/z ranged from about 180 to 5000 or higher. The cluster ion beam successfully produced protonated molecules [M + H]+ (M denotes arginine and PEG molecules) with relatively little fragmentation. Adduct ions [M + C3 H7 NH3 ]+ formed by propylammonium-attachment reaction were also detected for PEG. It has been demonstrated that a needle emitter wetted with PAN can generate a cluster ion beam that includes massive cluster ions. The cluster ion beam proved to be helpful in producing molecular secondary ions and suitable for a primary ion beam in organic SIMS.

Effect of halides on the solvation of poly(ethylene oxide) in the ionic liquid propylammonium nitrate

HypothesisThe solvation characteristics of poly(ethylene oxide) (PEO) in nanostructured protic ionic liquids (PILs) are driven by polymer-solvent interactions in the polar domains of the PIL. This work hypothesises that the nanostructure of a PIL can be altered via halide addition, directly affecting the solvation of PEO. ExperimentsSmall angle neutron scattering (SANS) is used to explore the conformation of 38 kDa PEO dissolved in the PIL propylammonium nitrate (PAN), a mol fraction of 10% propylammonium chloride (PACl) in PAN, and a mole fraction of 10% propylammonium bromide (PABr) in PAN. FindingsEach of these solutions are shown to behave as a good solvent for PEO, as determined by their Flory exponents and Zimm plot analysis. The quality of solvation is reduced by the addition of the halide salt, with the order of solvation as follows: PAN > Br− addition > Cl− addition. Our experimental observations are consistent with the recently reported solvation structure of PEO in these solutions (Stefanovic et al., 2018). The increased charge density from NO3− to Br− to Cl− results in greater net ionic interaction between the ionic charge centres. As PEO interacts with PAN primarily through the ammonium hydrogens of the cation, this increased ionic interaction effectively displaces the PEO, resulting in poorer solvation.

Multiple crystal pathways and crystal polymorphs in protic ionic liquids

Multiple crystal pathways and crystal polymorphs were observed in protic ionic liquids (pILs), which are liquid or solid at room temperature. The pILs studied were methylammonium nitrate (MAN), dimethylammonium nitrate (DAN), ethylammonium nitrate (EAN), and propylammonium nitrate (PAN). MAN and DAN solidified as plastic crystals at room temperature, whereas EAN and PAN are liquids at room temperature. Solid MAN and DAN indicated crystal polymorphs. In spite of its simple molecular structure, complicated phase behaviors such as multiple crystal pathways and crystal polymorphs were observed in liquid EAN at low temperature [Abe et al., J. Mol. Liq. 241 (2017) 301]. Liquid PAN also possesses low-temperature anomalies in analogy with liquid EAN. Proton fluctuations, which disturb the crystal nucleation of liquid pILs at room temperature, cause multiple crystal pathways via a non-equilibrium state.

Static magnetic field alters properties of confined alkylammonium nitrate ionic liquids

Ethylammonium nitrate (EAN) and propylammonium nitrate (PAN) ionic liquids confined between polar glass plates and exposed to a strong magnetic field of 9.4 T demonstrate gradually slowing diffusivity, a process that can be reversed by removing the sample from the magnetic field. The process can be described well by the Avrami equation, which is typical for autocatalytic (particularly, nucleation controlled) processes. The transition can be stopped by freezing the sample. Cooling and heating investigations showed differences in the freezing and melting behavior of the sample depending on whether it had been exposed to the magnetic field. After exposure to the magnetic field, the sample demonstrated decrease in the 1H NMR signal of residual water. 1H NMR spectroscopy with presaturation demonstrates that the most probable mechanism of the decrease of the bulk water signal is adsorption of water on polar surfaces of glass plates. Generally, our findings confirm our previous suggestion that alteration of the dynamic properties of confined alkylammonium nitrate ionic liquids exposed to a magnetic field is related to the alteration of real physical-chemical phases.

Structure and dynamics of propylammonium nitrate-acetonitrile mixtures: An intricate multi-scale system probed with experimental and theoretical techniques.

In this article, we report the study of structural and dynamical properties for a series of acetonitrile/propylammonium nitrate mixtures as a function of their composition. These systems display an unusual increase in intensity in their X-ray diffraction patterns in the low-q regime, and their 1H-NMR diffusion-ordered NMR spectroscopy (DOSY) spectra display unusual diffusivities. However, the magnitude of both phenomena for mixtures of propylammonium nitrate is smaller than those observed for ethylammonium nitrate mixtures with the same cosolvent, suggesting that the cation alkyl tail plays an important role in these observations. The experimental X-ray scattering data are compared with the results of molecular dynamics simulations, including both ab initio studies used to interpret short-range interactions and classical simulations to describe longer range interactions. The higher level calculations highlight the presence of a strong hydrogen bond network within the ionic liquid, only slightly perturbed even at high acetonitrile concentration. These strong interactions lead to the symmetry breaking of the NO3- vibrations, with a splitting of about 88 cm-1 in the ν3 antisymmetric stretch. The classical force field simulations use a greater number of ion pairs, but are not capable of fully describing the longest range interactions, although they do successfully account for the observed concentration trend, and the analysis of the models confirms the nano-inhomogeneity of these kinds of samples.

Nanostructure of propylammonium nitrate in the presence of poly(ethylene oxide) and halide salts.

Nanoscale structure of protic ionic liquids is critical to their utility as molecular electrochemical solvents since it determines the capacity to dissolve salts and polymers such as poly(ethylene oxide) (PEO). Here we use quantum chemical molecular dynamics simulations to investigate the impact of dissolved halide anions on the nanostructure of an archetypal nanostructured protic ionic liquid, propylammonium nitrate (PAN), and how this impacts the solvation of a model PEO polymer. At the molecular level, PAN is nanostructured, consisting of charged/polar and uncharged/nonpolar domains. The charged domain consists of the cation/anion charge groups, and is formed by their electrostatic interaction. This domain solvophobically excludes the propyl chains on the cation, which form a distinct, self-assembled nonpolar domain within the liquid. Our simulations demonstrate that the addition of Cl- and Br- anions to PAN disrupts the structure within the PAN charged domain due to competition between nitrate and halide anions for the ammonium charge centre. This disruption increases with halide concentration (up to 10 mol. %). However, at these concentrations, halide addition has little effect on the structure of the PAN nonpolar domain. Addition of PEO to pure PAN also disrupts the structure within the charged domain of the liquid due to hydrogen bonding between the charge groups and the terminal PEO hydroxyl groups. There is little other association between the PEO structure and the surrounding ionic liquid solvent, with strong PEO self-interaction yielding a compact, coiled polymer morphology. Halide addition results in greater association between the ionic liquid charge centres and the ethylene oxide components of the PEO structure, resulting in reduced conformational flexibility, compared to that observed in pure PAN. Similarly, PEO self-interactions increase in the presence of Cl- and Br- anions, compared to PAN, indicating that the addition of halide salts to PAN decreases its utility as a molecular solvent for polymers such as PEO.

Dynamical heterogeneities of rotational motion in room temperature ionic liquids evidenced by molecular dynamics simulations.

Room temperature ionic liquids (RTILs) have been shown to exhibit spatial heterogeneity or structural heterogeneity in the sense that they form hydrophobic and ionic domains. Yet studies of the relationship between this structural heterogeneity and the ∼picosecond motion of the molecular constituents remain limited. In order to obtain insight into the time scales relevant to this structural heterogeneity, we perform molecular dynamics simulations of a series of RTILs. To investigate the relationship between the structures, i.e., the presence of hydrophobic and ionic domains, and the dynamics, we gradually increase the size of the hydrophobic part of the cation from ethylammonium nitrate (EAN), via propylammonium nitrate (PAN), to butylammonium nitrate (BAN). The two ends of the organic cation, namely, the charged Nhead-H group and the hydrophobic Ctail-H group, exhibit rotational dynamics on different time scales, evidencing dynamical heterogeneity. The dynamics of the Nhead-H group is slower because of the strong coulombic interaction with the nitrate counter-ionic anions, while the dynamics of the Ctail-H group is faster because of the weaker van der Waals interaction with the surrounding atoms. In particular, the rotation of the Nhead-H group slows down with increasing cationic chain length, while the rotation of the Ctail-H group shows little dependence on the cationic chain length, manifesting that the dynamical heterogeneity is enhanced with a longer cationic chain. The slowdown of the Nhead-H group with increasing cationic chain length is associated with a lower number of nitrate anions near the Nhead-H group, which presumably results in the increase of the energy barrier for the rotation. The sensitivity of the Nhead-H rotation to the number of surrounding nitrate anions, in conjunction with the varying number of nitrate anions, gives rise to a broad distribution of Nhead-H reorientation times. Our results suggest that the asymmetry of the cations and the larger excluded volume for longer cationic chain are important for both the structural heterogeneity and the dynamical heterogeneities. The observed dynamical heterogeneities may affect the rates of chemical reactions depending on where the reactants are solvated in ionic liquids and provide an additional guideline for the design of RTILs as solvents.

Time-of-flight secondary ion mass spectrometry using a new primary ion beam generated by vacuum electrospray of a protic ionic liquid, propylammonium nitrate.

Protic ionic liquids have the potential to be useful materials for primary ion beams in terms of protonation, since they have active protons. Selecting protic ionic liquids suitable for primary ion beams is of great importance to increase molecular secondary ion yields. Propylammonium nitrate ([C3 H7 NH3 ][NO3 ]) seems promising in view of its proton affinity. It is likely that [C3 H7 NH3 ]+ cations can act as proton donors, and [NO3 ]- anions can work as proton acceptors. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) experiments have been performed to verify the usefulness of [C3 H7 NH3 ][NO3 ]. A primary propylammonium nitrate cluster ion beam was generated by vacuum electrospray, and then used to analyze amino acids (arginine, glutamic acid, aspartic acid), angiotensin II and polyethylene glycol. Positive and negative secondary ion mass spectra were obtained to study both protonation and deprotonation. The propylammonium nitrate cluster ion beam successfully generated protonated molecules [M+H]+ of all the analytes in positive ion mode. The primary ion beam also generated deprotonated molecules [M-H]- of glutamic acid, aspartic acid and angiotensin II in negative ion mode. Additionally, adduct ions related to [C3 H7 NH3 ][NO3 ] were detected in the case of arginine and polyethylene glycol. The TOF-SIMS experiments confirmed that the propylammonium nitrate cluster ion beam was useful in generating molecular secondary ions, demonstrating that it is well suited for a primary ion beam in TOF-SIMS.

Surface Ordering in Binary Mixtures of Protic Ionic Liquids.

The surface composition of binary mixtures of the protic ionic liquids ethylammonium nitrate and propylammonium nitrate has been investigated using surface tension measurements and the perfectly surface sensitive method metastable induced electron spectroscopy. Given that the latter technique is sensitive only to the outermost layer, it allows for the determination of the surface fraction occupied by a given species. The piecewise linear relationship between surface fraction and surface tension found in this study can be described by a phase separation within the surface layer.

Helix-forming ability of proteins in alkylammonium nitrate

Understanding the helical formation of proteins in concentrated aqueous solutions of alkylammonium-based ionic liquids (RAN-ILs) forming the glassy state is necessary to devise the guidelines for the design of ILs for use in protein cryopreservation. To reveal the effect of the alkyl-chain length of cation on the helix forming ability of RAN-ILs, helix formation in five model proteins (ribonuclease A, β-lactoglobulin, ubiquitin, α-lactalbumin, and cytochrome c) with different secondary structure contents in the solutions of three RAN-ILs, namely methylammonium nitrate (MAN), ethylammonium nitrate (EAN), and propylammonium nitrate (PAN) were investigated over a wide concentration range X (mol%IL) using optical spectroscopy. Overall, although the addition of RAN-ILs to proteins caused protein unfolding, condensed solutions with EAN and PAN (X>10) induced helical formation, which was disrupted by the tertiary structure of proteins, except for ubiquitin and α-lactalbumin. In contrast, aqueous MAN solutions did not induce the non-native helical structure of the five model proteins throughout the studied concentration range, X. The difference in the helix-forming ability between MAN and the other two RAN-ILs was dependent on the low polarity of the solutions. Moreover, although EAN and PAN have helix-forming abilities, these abilities were dependent on protein species. The present finding is that the consideration of solution polarity and dependence of protein species is related to the guidelines for the design of RAN-ILs for the cryopreservation of proteins such as recombinant proteins.

Effect of Imidazolium-Based Surface-Active Ionic Liquids on the Orientation of Liquid Crystals at Various Fluid/Liquid Crystal Interfaces.

A series of imidazolium-based surface-active ionic liquids (IM-SAILs), viz., single-chained IM-SAILs, 1-alkyl-3-methylimidazolium bromide ([Cnmim]Br, n = 12, 14, 16), 1-dodecyl-3-methylimidazolium salicylate ([C12mim]Sal), 1-dodecyl-3-methylimidazolium 3-hydroxy-2-naphthoate ([C12mim]HNC), 1-dodecyl-3-methylimidazolium cinnamate ([C12mim]CA), 1-dodecyl-3-methylimidazolium para-hydroxy-cinnamate ([C12mim]PCA), gemini IM-SAIL, and 1,2-bis(3-dodecylimidazolium-1-yl)ethane bromide ([C12-2-C12im]Br2), along with three short-chained ionic liquids (ILs) [ethylammonium nitrate (EAN), propylammonium nitrate (PAN), and butylammonium nitrate (BAN)] were synthesized and applied to nematic liquid crystal (LC)/fluid interfaces. First, we evaluated the influence of the length and number of aliphatic chains as well as the counterion in the IM-SAIL structures on the anchoring of LCs at the aqueous/LC interface. It was observed that the threshold concentration of [Cnmim]Br (n = 12, 14, 16) decreased with the increase in aliphatic chain length. And double-chained [C12-2-C12im]Br2 has a far lower threshold concentration than single-chained [C12mim]Br. But the alteration of counterions (e.g., Br- and aromatic counterions) scarcely affected the anchoring of LCs at the interface. Second, we investigated the alignment of LCs at the diverse IL/LC interfaces in the presence of IM-SAILs. It is found that the variations in both aliphatic chain length and number can remarkably change the trigger points of the orientational transition of LCs at the EAN/LC interface. Specifically, with a slight increase in the alkyl chain length of short-chained ILs, as the fluid medium, the orientation of LCs varied tremendously at the IL/LC interface. Therefore, the higher threshold concentration of IM-SAILs and the corresponding greater stability in the optical appearance of LCs at the EAN/LC interface compared to that of the aqueous/LC interface can be ascribed to the discrepancy in the microstructure of water and IL. Finally, we verified that the volume ratio of H2O to EAN could more dramatically affect the alignment of LCs than the change in IM-SAIL concentration in aqueous solution. This work first illustrated the impact of SAIL structure on the LCs orientation at the aqueous/LC, IL/LC, and H2O-IL mixture/LC interfaces, which will inspire us to obtain a stabilized molecular alignment of LCs at the IL/LC interfaces and to further design novel functionalized SAIL molecules for various chemical and biological applications.

Structural characterization of propylammonium nitrate and N-methyl-2-pyrrolidone mixtures

Propylammonium nitrate (PAN) ionic liquid and N-methyl-2-pyrrolidone (NMP) are well know compounds employed for many applications in different industrial fields. The most interesting feature is the possibility of modulating the structural properties of the ionic liquid by mixing it with the molecular solvent. A dynamic characterization of the PAN-NMP mixtures is desirable to understand the significant effects on the structural properties and consequently on the chemical-physical features induced by the formation of such solutions. In order to improve the knowledge of the structure of PAN-NMP solutions, useful to the development of their applications, energy dispersive X-ray diffraction (EDXD), and molecular dynamics (MD) simulations were employed in this work. Our study evidenced a microheterogeneous solvation of PAN by NMP and indicate the presence of strong attracting interactions between the constituents.

Surface and bulk characterisation of mixtures containing alkylammonium nitrates and water or ethanol: Experimental and simulated properties at 298.15 K

This work is a contribution to the knowledge of the surface and bulk behaviour of mixtures containing an ammonium based protic ionic liquid (PIL) and a molecular solvent. To achieve our goal, we have measured the surface tension and the density of the binary mixtures {ethylammonium nitrate, propylammonium nitrate or butylammonium nitrate+water or ethanol} over the whole composition range at 298.15K. The values obtained for both magnitudes depend on the PIL alkyl chain length, the solvent nature and the solvent concentration. The surface tension of the aqueous mixtures rich in water show a deep dependence on the concentration of PIL, whereas the values of the PIL/ethanol mixtures are not influenced by the alkyl chain length of the PIL up to equimolar composition. The results obtained aid us to describe the structure of the surface. Furthermore, we have calculated the excess molar volumes from the densities and the values obtained were negative in all cases. The sign and the sequence of the curves can be explained mainly in terms of the H-bond interactions and steric hindrance, respectively. To provide a microscopic insight into the surface and the bulk of the mixtures studied, we have performed molecular dynamic simulations observing a good relationship between the simulated results and the experimental ones. Finally, we have measured the refractive index of the mixtures in order to verify the validity of the Gladstone-Dale phenomenological model to predict refractive indexes from density.

3-Dimensional Structure of a Prototypical Ionic Liquid–Solid Interface: Ionic Crystal-Like Behavior Induced by Molecule–Substrate Interactions

The molecular structure of the ionic liquid–electrode interface has been recently investigated by atomic force microscopy (AFM) methods focusing either on the vertical structure of the ion layers or on the lateral structure of the innermost layer. Here, we combine high-resolution AFM imaging with atomic force spectroscopy measurements to elucidate the structure of the interface between the ionic liquid propylammonium nitrate (PAN) and highly ordered pyrolytic graphite (HOPG). The lateral structure of the innermost layer of adsorbed molecules (i.e., the Stern layer) is resolved on the molecular scale by means of amplitude modulation atomic force microscopy (AM-AFM). A quasi (4 × 4)R0° overlayer is formed by the ionic liquid molecules on the HOPG surface. Additional dynamic mode force spectroscopy measurements reveal the existence of a layered structure of the ionic liquid normal to the surface plane and allow for a precise determination of the layer spacing. We are able to infer the three-dimensional struc...