Theoretical Study of Renewable Ionic Liquids in ... - ACS Publications

Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES

Article

Theoretical Study of Renewable Ionic Liquids in Pure State and with Graphene and Carbon Nanotubes Gregorio Garcia, Mert Atilhan, and Santiago Aparicio J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b03809 • Publication Date (Web): 25 Aug 2015 Downloaded from http://pubs.acs.org on August 27, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Theoretical Study of Renewable Ionic Liquids in Pure State and with Graphene and Carbon Nanotubes Gregorio García,a Mert Atilhan,b and Santiago Aparicioa* a b

Department of Chemistry, University of Burgos, 09001 Burgos, Spain

Department of Chemical Engineering, Qatar University, P.O. Box 2713, Doha, Qatar

*

Corresponding author: phone: +34 947 258 062, E-mail: [email protected]

ABSTRACT: The N-ethyl-N-(furan-2-ylmethyl)ethanaminium dihydrogen phosphate ionic liquid was studied as a model of ionic liquids which can be produced from totally renewable sources. A computational study using both Molecular Dynamics and Density Functional Theory a methods was carried out. The properties, structuring, and intermolecular interactions (hydrogen bonding) of this fluid in pure state were studied as a function of pressure and temperature. Likewise, the adsorption on graphene and the confinement between graphene sheets was also studied. The solvation of single walled carbon nanotubes in the selected ionic liquid was analyzed together with the behavior of ions confined inside these nanotubes. The reported results show remarkable properties for this fluid, which show that many of the most relevant properties of ionic liquids and their ability to interact with carbon nanosystems may be maintained and even improved using new families of renewable compounds instead of classis types of ionic liquids with worse environmental, toxicological and economical profiles. KEYWORDS: ionic liquids, renewable, graphene, nanotubes, DFT, molecular dynamics.

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TABLE OF CONTENTS GRAPHIC

2


Page 2 of 33

Page 3 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION

The research area on ionic liquids, IL, evolved in the last decade from a chemical curiosity to a field with possible applications in a wide range of technologies such as electrochemistry,1 separation operations,2,3 carbon capture and gas separation,4 pharmaceutical,5 synthesis,6,7 catalysis,8 lubrication,9 fuel cells,10 batteries,11 solar cells,12 biodiesel,13 energy,14 hybrid materials,15 graphene16 and carbon nanomaterials functionalization.17 The massive amount of available studies (6278 papers published in 2014 year containing the term 'ionic liquid' according to Scopus database) have allowed to acquire a deep knowledge on the properties, characteristics, strengths and weaknesses of these fluids. Most of the applied research on IL have proven these fluids to be suitable alternatives to traditional materials or solvents in many applications leading to a wide interest both in industry and academia. Nevertheless, a relevant number of problems have also risen for use of these materials, mainly for large scale applications.18 The most relevant doubts about the possible evolution of IL from lab-scale to industrial-scale rise from i) economic problems (too high cost),19 ii) environmental problems because of the poor biodegradability20 or the remarkable toxicity21 of some of classic IL such as imidazolium ones, and iii) the fact that many IL have to be synthesized from nonrenewable resources, such as those derived from petroleum or related raw materials.22,23 These important problems have led to the increasing doubt between the scientific community on the real viability of IL for industrial application,24,25 which have also shifted the interest of many researchers toward other related fields such as deep eutectic solvents.26 Nevertheless, many of the most relevant problems remarked for IL may be solved if new types of ions are studied shifting the community interest from classic groups such as imidazolium cations or fluorinated anions to most suitable alternatives.27,28,29,30 Therefore, the objective for new IL studies should be to explore fluids rising from ions with good biodegradability, low toxicity, low cost, and that may be synthesized from renewable sources. Several ions have been proposed for developing new types of IL with most suitable properties from environmental, toxicological and economic viewpoints. Therefore, studies considering IL with cholinium,22 ammonium,31 lactate,32 or aminoacid,30,33,34 among others,35,36 ions have proven to be suitable for several application but avoiding most of the problems from the use of classic types of IL. In a recent work, Socha et al.37 developed a new type of ionic liquids derived from renewable resources (lignin and hemicellulose) as a replacement of imidazolium IL for biomass treatment avoiding the use of petroleum derived materials. Likewise, the cost estimates of these new IL are in the $12-15 / kg although it is estimated to be reduced up to $4 / kg, which is one or two orders of magnitude lower than 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

common IL, and in the same range of the reduced number of low cost IL.19 Among these new IL proposed by Socha et al.,37 N-ethyl-N-(furan-2-ylmethyl)ethanaminium dihydrogen phosphate ([FUET2NH][H2PO4], Figure 1a) was selected as a model of this new family of compounds to be studied in the present work.

Figure 1. (a) Molecular structure of ions forming the ionic liquids studied in this work. Atom labeling used along this work is also reported; (b) Optimized geometry of isolated [FUET2NH][H2PO4] ionic pair. Cationanion interaction energy (∆EIL) as well as interionic charge transfer (CTIL) calculated at B3LYP-D3/6-31G* level and the most important intermolecular interactions (dotted lines) are also displayed. Bond lengths are in Ǻ.

The development of technological applications for new types of IL requires a detailed knowledge of liquid structure at the nanoscopic level and its structure with intermolecular interactions, physicochemical properties and fluid's dynamics. This valuable information may be obtained using computational chemistry tools, which have been used successfully for IL characterization.38,39,40,41 Therefore, the properties of [FUET2NH][H2PO4] IL were studied in this work using a theoretical approach based both in Density Functional Theory (DFT) and classic molecular dynamics (MD) methods. The effect of pressure and temperature on IL structuring, hydrogen bonding, energetic and dynamic properties was studied for the pure 4


Page 4 of 33

Page 5 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[FUET2NH][H2PO4]. Likewise, one of the most relevant recent applications of IL is their use combined

with ,47,48

nanotubes17

carbon

(fullerenes,42,43

nanomaterials

graphene,17,44,45,46

and

), and thus, the behavior of [FUET2NH][H2PO4] adsorbed on graphene

sheets, confined between two graphene sheets, solvating carbon single walled nanotubes (SWNT) and confined inside these SWNT, was also characterized. These results allow to infer the mechanism of interaction for the selected renewable IL with relevant nanomaterials, and the behavior of IL under confinement,49 which should led to dramatic changes both in IL and nanomaterials properties.50,51 The results reported in this work comes from a purely theoretical study with the objective being to analyze the most relevant molecular - level features of this new type of ionic liquids. [FUET2NH][H2PO4] is a new type of protic IL, which main advantage, in comparison with traditional protic IL, rises from the possibility of producing it from totally renewable sources at remarkably low cost, allowing scaling up its applications to industrial level. Cevasco and Chiappe18 showed in a recent work that one of the main concerns on the suitability of IL - based technologies is the requirement of large amounts of IL required for the treatment of billions of tons of raw materials in the most relevant technological applications. Therefore, only low cost IL produced from natural sources, fully renewable, can be suitable alternatives to traditional methods. The main properties and applications of protic IL have been reviewed in the literature,52,53 but this knowledge needs to be extended to new types of IL such as [FUET2NH][H2PO4], which is analyzed in this work both in pure state and regarding to carbon - based nanostructures. It should be remarked that in spite of being a purely theoretical study, computational chemistry is considered in the scientific community as a powerful tool for the development of new materials, for accelerating discover and innovation, which is specially relevant for the case of IL considering the larger number of possible anion-cation combinations.54 Therefore, simulation based approaches for the rational design of materials (IL), as done in this work, allow to explore the properties of these new materials instead of using the costly trial and error traditional experimental methods. The properties of the material reported in this work are very remarkable,37 and thus, they would stimulate further studies, including experimental ones. Modern computational chemistry is a pivotal role for the development of new materials, the large amount of possible new systems hinders the development of systematic trial-and-error costly experimental studies, and thus, screening of material through in silico analysis is a valuable tool for detecting new possibilities. Hence, computational chemistry serves to accelerate discovery and innovation

5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

for materials science (IL) and engineering and because of its predictive ability may help to drive materials innovation (IL) in the right direction.

METHODS Molecular Dynamics. MDynaMix v.5.2 molecular modeling package55 was used for all the MD simulations reported in this work. For the study of properties of pure [FUET2NH][H2PO4] in liquid state, simulations in the NPT ensemble were carried out, for pressures 1, 100, 300 and 500 bar, for temperatures in the 298 to 358 K range (at 10 K steps) for 1 bar, whereas for the remaining pressures simulations were done only at 328 K. Pure IL was simulated considering 400 ion pairs in cubic boxes. Low density boxes (0.2 g×cm-3) were built using Packmol program,56 and used as starting points. These initial boxes were heated to 500 K and quenched to 298 K in several cycles to assure equilibration before production runs, which was confirmed through the constancy of system potential energy. Simulations for production purposes with 10 ns total duration were carried out after equilibration steps. The forcefield parameterization for the ions studied in this work are reported in Table S1 (Supporting Information). DFT calculations showed a strong affinity by [H2PO4]- anion for the H9 atom in [FUET2NH], in such a way that the N1 - H9 distance increases to 1.55 Å, and thus, the charge of both ions is very different to +/-1. Therefore, this effect was included in the MD simulations by using the ChelpG57 charges obtained for the [FUET2NH] [H2PO4] ion pair from DFT calculations, at B3LYP/6-311++g(d,p), which are reported in Table S1 (Supporting Information). The total charges for the ions considered in MD simulations are +/- 0.4477. Simulations for [FUET2NH][H2PO4] adsorbed on one graphene sheet or confined between two graphene sheets were carried out using 400 ion pairs. For the case of confinement, two graphene sheets separated 64 Å were considered. Previously equilibrated IL layers were placed on top of graphene or confined between both sheets and used as starting configurations. These simulations were carried out in the NVT ensemble at 328 K, with graphene layers fixed. Forcefield parameterization for graphene was reported in a previous work.44 Simulations for 10 ns were carried out for all graphene containing systems. The behavior of SWNT was analyzed using nanotubes in the armchair configuration for SWNT(10,10), diameter 13.6 Å, and SWNT(15,15), diameter 20.3 Å. SWNTs were hydrogen terminated, maintained fixed along the simulation, with forcefield parameterization reported in a previous work.44 Initial cages for simulations were built considering a previously equilibrated IL box, in which ions from a central cylindrical region of size suitable to fit the 6


Page 6 of 33

Page 7 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SWNT were removed, then SWNT was placed in this region and simulations in the NPT ensemble at 328 K and 1 bar were started. The evolution of SWNT - containing systems was studied for 10 ns. All the simulations were carried out using Ewald method,58 with cut-off radius of 15 Å, was used for the treatment of coulombic interactions, whereas Lorentz-Berthelot mixing rules were applied for Lennard-Jones contributions. Equations of motion were solved with Tuckerman–Berne double time step algorithm,59 for 1 and 0.1 fs long and short time steps, respectively. Temperature and pressure were controlled using Nose-Hoover method. Density Functional Theory. DFT simulations were carried out aimed at obtaining a deeper knowledge on the nature of the interactions between the selected ionic pair and carbonnanostructures. All DFT calculations were done using Gaussian 09 (Revision D.01) package.60 For the study of isolated [FUET2NH][H2PO4] ionic pair, geometry optimizations were carried out with B3LYP61,62,63 method in conjunction with 6-31G* basis set. Due to the large number of atoms of ionic liquids + carbon-nanostructure (IL-CN) systems, only systems composed by one ionic pair adsorbed on the surface of the graphene or SWNT as well as one ionic pair confined in a SWNT were studied. Graphene sheet was modeled using a hexagonal plate of 37 rings, and a SWNT(10,10) was considered with a length able to fully adsorb or encapsulate the ionic pair. Both carbon-nanostructures were hydrogen terminated.

The

geometries of IL-CN were fully optimized in the context of the two-layer ONIOM model.64 This method allows an effective separation of the system into two subunits (IL and carbonnanostructure), each of them modeled with different accuracy, usually called high and low levels. For the IL-CNT systems, the ionic liquid (high level) was studied at B3LYP/6-31G*, while the carbon-nanostructure (low level) was studied at PBE/STO-3G.65 Due to the nature of the systems under study, the interactions of [FUET2NH][H2PO4] ionic pair with selected CNs would involve a large component of dispersive interactions. Thus, the so-called Grimme’s D2 dispersion correction term (PBE-D2),66 as implemented in Gaussian 09 (Revision D.01), has been also considered for the low level, which has allowed an improved description on the dispersion interactions. PBE-D2 has proven to perform well for larger systems for predicting geometries and for describing van der Waals and hydrogen bonding interactions with a moderate computational cost.63,67,68 Based on these optimized structures for the isolated ionic pair at B3LYP/6-31G* and IL - CNs at B3LYP/6-31G*:PBE-D2/STO3G* theoretical levels, single point calculations were done at B3LYP-D3/6-31G* level were performed and molecular properties computed at this level were used for the discussion. B3LYP has been selected since it has showed a suitable performance over a wide range of 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 33

systems.69 Dispersion corrections were described through D3 version of Grimme’s dispersion with the Becke-Johnson (BJ) damping function as implemented in Gaussian 09 (Revision D.01) program.70 DFT-D3 was recently found to provide results with quality close of CCSD(T) methods for various prototypic interactions involving IL.71 For the sake of computational cost, larger basis set than 6-31G* were not considered. Charge transfer process (between both ions as well as between the ionic pair and the CNs) were studied through ChelpG atomic charges. Intermolecular interactions were characterized through the estimation of the binding energies as well as the analysis of the reduced density gradient (RGD) at low densities.72 The interaction energy for the ionic pair (ΔEIL) was defined as: (1)

ΔEIL = EIL - (Ecat + Eani)

where Ecat, Eani and EIL stand for the total energy of the cation, anion and ionic pair, respectively. Likewise, the interaction energies for the IL+CN systems were estimated as:

ΔEIL-G = EIL-G - (EG + EIL)

(2a)

ΔEIL@SWNT = EIL@SWNT - (ESWNT + EIL)

(2b)

ΔEIL-SWNT = EIL-SWNT - (ESWNT + EIL)

(2c)

where EIL, EG / ESWNT and EIL-G / EIL@SWNT / EIL-SWNT are the total energy of the ionic pair, the graphene sheet / SWNT(10,10) nanotube and IL – graphene / IL (inside) – SWNT / IL outside) – SWNT systems. RGD analysis is able to find noncovalent interactions based on the peaks appearing at low densities, and thus, the visualization of RGD iso-surfaces for these peaks allows the visualization of weak interactions. The strength and the nature of the interactions is determined through the sign of the second density Hessian eigenvalue. RGD analysis were carried out using MultiWFN code.73 Total and Partial Density of States (DOS and PDOS, respectively) were extracted by using GaussSum code.74 All these properties were obtained on the wavefunctions previously generated at B3LYP-D3/6-31G* theoretical level.

RESULTS AND DISCUSSION Properties of Pure IL. The optimized structure of [FUET2NH][H2PO4] ionic pair is drawn in Figure 1b. DFT estimated binding energy of the ionic pair studied in this work is equal to 493.08 kJ×mol-1, mainly rising from an important charge transfer between both ions. As seen in Figure 1b, there are several intermolecular H-bonds between both ions, being the interaction between the hydrogen atom in [FUET2NH]+ (H9 atom in Figure 1a) and oxygen atoms in [H2PO4]- the strongest one. There is an important elongation of the N1-H9 bond

8


Page 9 of 33

(1.583 Ǻ), whereas H9-O (in [H2PO4]-) shows a distance typical of intramolecular O-H bond (1.042 Ǻ). Both factors point out to a high the charge transfer between both ions (CTIL = 0.48 e-). These results are in agreement with previously data reported by Socha et al.37 Molecular dynamics simulations would provide a detailed picture of the structuring for the considered IL not only for short range effects but also considering the whole solvation and interaction behavior. Radial distribution functions, g(r), are suitable tools for the analysis of solvation spheres around a central molecule, and thus, Figure 2 shows center-of-mass g(r) for the corresponding ions. 6

A-C A-A C-C

4 g(r)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

0 0

4

8 r/Å

12

Figure 2. Center-of-mass radial distribution functions, g(r), in pure [FUET2NH][H2PO4] at 298 K and 1 bar. C stands for [FUET2NH]+ and A for [H2PO4]-.

Anion - cation interactions are characterized by a first narrow and sharp peak at 4.35 Å followed by a shoulder at 4.80 Å in g(r), which is characteristic of the strong interionic interaction, and the second and third peaks at 9.17 and 13.73 Å indicate remarkable fluid ordering beyond the first solvation shell. Results for anion-anion interactions show a broad band rising from the overlapping of two peaks at 4.35 and 5.14 Å, although the amplitude of this band is lower than the narrow first peak for anion-cation interactions it shows that intermolecular interactions are also developed between anions because [H2PO4]- contains both hydrogen bond donor and acceptor groups. Running integrals of g(r) reported in Figure 2 up to the first minima (6 Å and 7.3 Å for anion-cation and anion-anion, respectively), which correspond to the first solvation shell are 3.2 and 3.1 for anion-cation and anion-anion, 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

respectively, which show that the first solvation shell around [H2PO4]-, in which hydrogen bonding may be developed, is composed both of counterions and ions with the same sign. Regarding g(r) for cation-cation interactions results show a peak at 6.8 Å, which on one side can be justified considering the large size of [FUET2NH] and on the other side with the poor trend for developing self association between cations. Amore detailed picture on the type and extension of intermolecular forces in liquid [FUET2NH][H2PO4] may be inferred from the site-site g(r) reported in Figure 3.

Figure 3. Site-site radial distribution functions in [FUET2NH][H2PO4] at 298 K and 1 bar. C stands for [FUET2NH]+ and A for [H2PO4]-. Intra stands for peaks assigned to intramolecular contributions. Panels (a) show relevant anion - anion (A-A), (b) cation-cation (C-C), and (c) cation-anion (C-A) interactions.

Results for anion-anion g(r), Figure 3a, show a very intense and sharp peak at 1.73 Å, which confirm the development of hydrogen bonding between anions through the hydroxyl 10


Page 10 of 33

Page 11 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


groups as donors and any of the oxygen atoms in [H2PO4]- as acceptors. Regarding the possibility of cation-cation hydrogen bonding, results in Figure3b discard this possibility because the presence of the bulky groups bonded to the central nitrogen atom seems to hinder this possibility. The anion-cation interaction is characterized by strong hydrogen bonding between the hydrogen atom bonded to nitrogen in [FUET2NH]+ (H9, Figure 1a) and all the oxygen atoms in [H2PO4]-, Figure 3c. In [H2PO4]- two types of hydrogen bond acceptors are present, oxygen atoms without hydrogen (O2 and O4, Figure 1a) and hydroxyl oxygens (O3 and O5, Figure 1a), g(r) for both type of oxygens show very similar patterns with a first peak at 2.1 and 2.3 Å for O2/O4 and O3/O5, respectively. Therefore, anion-cation hydrogen bonding is developed through all oxygen atoms in [H2PO4]- as acceptors but the intensity of the g(r) peaks show a larger number of interactions through the non-hydroxylic sites. Likewise, the development of a second and even a third peak in g(r) confirms the high degree of structuring by hydrogen bonding between both counterions. The available literature on protic IL showed the existence of heterogeneities in these fluids because of amphiphilicity,75 with large effect on the network of hydrogen bonds.76 In the case of [FUET2NH][H2PO4], the most relevant features raised from the strong interaction through the H9 site in the cation, which led to very large charge transfer between ions, together with the presence of the furan group in the cation. The bulky structure of [FUET2NH]+ should condition the mechanism of interaction with [H2PO4]- for hydrogen bonding development, and thus some type of cation structural rearrangement should appear in liquid state to allow anion-cation efficient interaction. Therefore, the most relevant torsional angles around the -NH hydrogen bond donor group in [FUET2NH]+ were calculated showing that the gauche arrangement (ϕ =0º) is the most probable

conformation

regarding

to

the

furan

group,

Figure

S1

(Supporting

Information),which is in contrast with DFT results in gas phase which lead to values of both torsional angles around 64º. This cationic structural rearrangement lead to a more efficient packing with [H2PO4]-, and thus, to the interionic hydrogen bonding reported in Figure 3c. The extension of hydrogen bonding was quantified from molecular dynamics results. This quantification requires to develop a proper definition of hydrogen bonding based on donor-acceptor distance and angle. The distance cutoff criterion is 3.0 Å along this work, which is the common option in the literature, nevertheless the donor-acceptor angle, αmax, deserves a more detailed analysis. The αmax effect on the calculated number of hydrogen bond is analyzed in Figure S2 (Supporting Information) for H9-O2 anion-cation hydrogen bonding (analogous results were obtained for other interacting pairs) showing that an asymptotic value 11



is reached for αmax > 60º, and thus, αmax = 60º was used along this work. The analysis of hydrogen bonds was carried out first for all the possible donor-acceptor combinations, Table S2 (Supporting Information). Anion-anion hydrogen bonding is developed mainly (92 % of total anion-anion hydrogen bonds) with hydroxyl groups in [H2PO4]- acting as donors and non-hydroxyl oxygen (O2 and O4) as acceptors. Likewise, anion-cation hydrogen bonding is developed between H9 atom in [FUET2NH]+ and non-hydroxyl oxygen in [H2PO4]- (79 % of total anion-cation hydrogen bonds). Although O2 and O4 positions in [H2PO4]- are equivalent, results in Table S2 (Supporting Information) show that a larger number of anionanion hydrogen bonds is developed through O4 site in comparison with O2, whereas the number of anion-cation hydrogen bonds is larger through the O2 position. Therefore, these results show that O2 and O4 non-hydroxyl sites in [H2PO4]- allow an efficient interaction both with neighbor anions and cations but with a preferential occupation of each site depending on the type of ion, and thus maximizing the hydrogen bonding ability. Nevertheless, the number of total anion-anion hydrogen bonds is an order of magnitude larger than anion-cation ones, which in spite of the strong anion-cation coulombic interaction because of being charged entities show the pivotal role of hydrogen bonding in the structuring of this fluid in particular considering the large trend of anion for developing self-association, Figure 4. 650

A-A A-C

600 550 NH-bonds

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 33

500 60 50 40 30 290

310

330 T/K

350

370

Figure 4. Number of hydrogen bonds, NH-bonds, in [FUET2NH][H2PO4] at 298 K and 1 bar. C stands for [FUET2NH]+ and A for [H2PO4]-. NH-bonds for A-C were calculated as the sum of H-bonds between H9 atoms in [FUET2NH]+ and O2, O3, O4 and O5 atoms in [H2PO4]-; NH-bonds for A-A were calculated as the sum of Hbonds between HP and HQ atoms with O2, O3, O4 and O5 atoms in [H2PO4]-. Values calculated with 3.0 Å cutoff distance between donor and acceptor and 60º for the corresponding angle for a simulation box containing 400 ion pairs. C stands for [FUET2NH]+ and A for [H2PO4]-. Atom labeling as in Figure 1a. 12


Page 13 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Likewise, this hydrogen bonding mechanisms do not change in the studied 298 to 358 K range, the total number of hydrogen bonds decreases a 20 % and 32 % for anion-anion and anion-cation interactions, respectively, which show the strength of these intermolecular hydrogen bonding with a high degree of interionic interaction even at high temperatures. Likewise, the changes of the total number of hydrogen bonds with temperature for anionanion and anion-cation interactions

reported in Figure 4 show a linear decrease with

increasing temperature with parallel trends, thus confirming the cross-linking of anion-anion and anion-cation interactions, in agreement with g(r) results reported in Figures 2 and 3. Considering the pivotal role of [H2PO4]- in the fluid structuring through hydrogen bonding the spatial distribution, SDF, of hydrogen atoms, both from anions and cations, around it is reported in Figure 5a and 5b. SDF results show the high density caps around nonhydroxyl oxygen atoms, with larger densities for hydrogen atoms belonging to neighbor anions but at the same time interacting also with cations in such a way that all the hydrogen bond donor/acceptor sites are occupied both with anions and cations. Likewise, the appearance of further caps in Figure 5b, show that ordering extends further than the same solvation sphere, in agreement with radial distribution functions behavior reported in Figure 3c.

Figure 5. Spatial distribution function of (a) H9 atoms in [FUET2NH]+ around [H2PO4]- (gray surface) and (b) HP/HQ atoms in [H2PO4]- around [H2PO4]- (yellow surface), for [FUET2NH][H2PO4] at 298 K and 1 bar. Isosurfaces show 5-times bulk density.

The strength of interionic interactions is quantified through the intermolecular interaction energies, Einter, reported in Figure 6 split for all the ion-ion contributions. Result show positive Einter contributions both for cation - cation and anion - anion pairs, which rises from the large coulombic repulsive contributions because of the same sign of ions in each 13



case. Likewise, Einter for anion-anion is larger than for cation-cation, which is justified considering the strong tendency of [H2PO4]- anions for developing hydrogen bonding reported in previous paragraphs, and thus, leading to closer distances between anions than between cations, Figure 2, which led to larger repulsive coulombic contributions to Einter. It should be remarked that the Lennard-Jones contribution to the total Einter is remarkably larger for the case of cation-cation (-34.1±0.8 kJ×mol-1 in the 298 to 358 K range) than for anionanion (-2.6±0.2 kJ×mol-1 in the 298 to 358 K range) in spite of the absence of cation-cation hydrogen bonds and the large number of anion-anion ones, Table S2 (Supporting Information). 200

A-A C-C A-C

180 Einter / kJ×mol-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 33

160 -540 -560 -580 290

310

330 T/K

350

370

Figure 6. Intermolecular interaction energies, Einter, as the sum of coulombic and Lennard-Jones contributions, in [FUET2NH][H2PO4] at 1 bar as a function of temperature. C stands for [FUET2NH]+ and A for [H2PO4]-.

Therefore, although cation-cation interactions are characterized by the larger separation between the center of masses of interacting cations, Figure 2, in comparison with anion-anion ones, the largest number of atoms in [FUET2NH]+ and the shape of the cation allow a more efficient cation-cation interaction through Lennard-Jones terms, whereas in the case of anion-anion ones the strong trend for developing hydrogen bonding through the oxygen sites in the anion, which is highly directional, hinders the development of further contacts between the remaining atoms in the anion, and thus, Lennard-Jones term is remarkably low. The large Einter for anion - cation pairs rises from the attractive coulombic interaction between both ions but also a remarkable Lennard-Jones contribution (-48.8±1.0 kJ×mol-1 in the 298 to 358 K range, 8.8 % of total anion-cation Einter) is inferred, which is also remarkably larger than the Lennard-Jones contribution to anion-anion Einter interaction in spite 14


Page 15 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of the lower number of hydrogen bonds in the case of anion-cation contacts than for anionanion ones. This behavior may be also justified considering that the shape of [FUET2NH]+ allows to develop not only hydrogen bonds with oxygen atoms in [H2PO4]- but also the presence of alkylic chains and furan ring lead to large Lennard-Jones interactions, Figure S3 (Supporting Information). The extensive hydrogen bonding in liquid [FUET2NH][H2PO4] should be characterized not only from the strength or the geometrical properties of the interactions but also from the dynamic properties, that is to say the lifetime of ion-ion hydrogen bonding. Therefore, the residence time of hydrogen bond acceptor atoms around donor ones is reported in Figure 7 and was calculated from the decay of conditional probability of remaining inside a sphere of radius 3.0 Å (first minima in radial distribution functions reported in Figure 3) around the corresponding central atom. These results would allow to infer local motions of ions, which cause hydrogen bonding breaking and formation. Skarmoutsos et al.77 divided the dynamic of hydrogen bonding in ionic liquids in two types: continuous and intermittent. From the viewpoint of the method used to obtain residence times reported in Figure 7, continuous dynamics stands exactly for those values, that is to say when one atom crosses the boundary of the sphere defined around a central atom the hydrogen bonding is considered as broken. Intermittent hydrogen bonding would stand for those cases in which once an atom crosses the boundary of the limiting sphere defining the hydrogen bond, it stays in a region close to that sphere limits because of the limits rising form the ions caging because of strong coulombic interactions and low diffusion rates, and thus, it may recross the boundary and reform the hydrogen

bonding.

Skarmoutsos

al.77

et

reported

that

in

[1-butyl-3-

methylimidazolium][Chloride], the strong hydrogen bonding between the most acidic imidazolium hydrogen atom and chloride anion leads to a continuous residence time of 119.9 ps (6-times larger than the values obtained in this work for [FUET2NH][H2PO4], Figure 7) whereas the intermittent residence time is 5528 ps, both at 353.15 K. The larger continuous residence

times

obtained

by

Skarmoutsos

et

al.77

for

[1-butyl-3-

methylimidazolium][Chloride] may be mainly justified considering that the cut off distance used by these authors (4.13 Å) is larger than the one used in this work (3.0 Å).

15



40

O atoms in A around H atoms in A O atoms in A around H9 in C

36 32 tres / ps

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 33

28 24 20 16 290

310

330 T/K

350

370

Figure 7. Residence time, tres, for the reported atoms around a central one in [FUET2NH][H2PO4] at 1 bar as a function of temperature. tres was calculated from the exponential decay of conditional probability, P, for each atom to remain within a sphere of radius R+δr around a given central one, entering the sphere of radius R-δr at time t=0 with R = 3.0 Å and δr = 0.25 Å. C stands for [FUET2NH]+ and A for [H2PO4]-. O atoms in A around H atoms in A, shows the residence time of oxygen atoms in [H2PO4]- (O2, O3, O4 and O5) around hydrogen atoms (HP and HQ) in the same anion. ]-. O atoms in A around H9 atoms in C, shows the residence times of oxygen atoms (O2, O3, O4 and O5) in [H2PO4]- around H9 atom in [FUET2NH]+ Atoms labeling as in Figure 1a.

The behavior of residence times (continuous) reported in Figure 7 follow different trends for anion-anion and anion-cation hydrogen bonds. First, residence times are larger for anion-anion than for anion-cation hydrogen bonds for temperatures lower than 330 K, whereas for higher temperatures they are almost the same decreasing in a linear way with increasing temperature. Second, residence time for anion-anion hydrogen bonds show two well defined regions for its variation with temperature, both with linear decrease of residence time with temperature, whereas for temperatures lower than 330 K the decreasing rate of residence time is -0.41 ps×K-1, for higher temperatures it is -0.19 ps×K-1 (almost equal to that for anion-cation in the full temperature range being -0.19 ps×K-1). Therefore, the dynamics of anion-anion hydrogen bonding is more affected by the temperature than that for anion-cation interactions, which may be justified considering that because of the larger number of anionanion hydrogen bonds, Figure 4, the disruption of effect of increasing temperature is larger. Likewise, the larger size of [FUET2NH]+ together with its shape leads to an effect in which cation rearrangements break the hydrogen bonds without the possibility of further reorganization for rebonding. 16


Page 17 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The structuring of [FUET2NH][H2PO4] should be the main effect controlling the macroscopic properties in the liquid state, and thus, some of the most relevant thermophysical properties were evaluated from molecular dynamics simulations. Figure S4a (Supporting Information) show density predictions for [FUET2NH][H2PO4] in the liquid state as a function of temperature. Up to our knowledge, there are no experimental data reported in the literature for this

ionic liquid, and thus, comparison with predictions from molecular

dynamics was not possible. Nevertheless, density was also calculated using COSMOtherm method,78 which gives suitable predictions for most of IL,79 according to the method previously reported.80 Density values from COSMOtherm are lower than the ones obtained from molecular dynamics, although this difference is reasonable and decreases with increasing temperature. [FUET2NH][H2PO4] is a dense fluid in spite of the voluminous [FUET2NH]+ cation, which could hinder packing and decrease density, nevertheless the extension of hydrogen bonding and the efficient arrangement reported in Figure S3 (Supporting Information) should justify these density data. Calculated thermal expansion coefficient, αp, from molecular dynamics simulation is 0.560×10-3×K-1 at 298 K increasing in a linear way

to 0.580×10-3×K-1 for 358 K. This slight increase of

αp with increasing

temperature is in agreement with reported behavior for other types of IL. Nevertheless, although αp for [FUET2NH][H2PO4] is in the same range of other IL such as imidazolium,81 phosphonium,81 ammonium and hydroxylammonium,82 it is remarkably lower than for other IL such as those based on lactam cations (αp roughly 0.8×10-3×K-1 at 298 K),83 which rises from the strength of interionic hydrogen bonding reported in previous sections leading to poorly compressible fluids. The compressibility of [FUET2NH][H2PO4] should be related with the distribution of the size of cavities, which were calculated according to the procedure reported by Margulis.,84 in which the smallest distance between a network of points in the simulation box and all the atoms is calculated. It should be remarked that in this work, the van der Waals radium of each atoms was considered for measuring these distances, Figure S5 (Supporting Information). Cavity size distribution reported in Figure S5 (Supporting Information) shows the availability of small cavities, mainly because of the shape of [FUET2NH]+, in agreement with the moderate αp. This distribution of cavities size is very similar to that in [1-butyl-3-methylimidazolium][PF6] reported by Huang et al.85 The increase in temperature leads to a decrease in the population of small cavities and an increase in larger cavities, although this effect is weak in agreement with the weak temperature effect on the properties reported in Figures 6 and S3 (Supporting Information).

17



The dynamics of pure [FUET2NH][H2PO4] was characterized through the selfdiffusion coefficients, D, calculated from the Einstein's equation and mean square displacements, msd. The use of fully diffusive regime for the calculation of D was assured by calculating the so-called β parameter, defined as the slope of log-log plots of msd vs. simulation time, which should be the unity when this regime is obtained.86 β parameter was in the 0.97 to 1.00 for the 298 to 358 K, respectively, range used in the calculation of D reported in Figure 8. 25

[FUET2NH]+ [H2PO4]-

20 1012× D / m2×s-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 33

15 10 5 0 290

310

330 T/K

350

370

Figure 8. Self-diffusion coefficients, D, calculated for the center-of-mass of [FUET2NH]+ and [H2PO4]- in [FUET2NH][H2PO4] at 1 bar as a function of temperature. D values calculated from means square displacements and Einstein's relationship. Continuous lines show Vogel-Fulcher-Tamman fits.

Low D values are obtained both for the anion and cation, which is in agreement with a highly viscous fluid, with cation and anion self-diffusion almost equal for temperatures lower than 320 K and higher diffusion rates for the cation at larger temperatures. This is in agreement with large number of anion-anion hydrogen bonds even at the highest temperatures, which is in contrast with the null self-association of cations through hydrogen bonding and the low number of anion-cation hydrogen bonds specially at high temperature. Therefore, ions mobility is strongly correlated with the changes in hydrogen bonding with increasing temperature. Likewise, ions diffusion follows a non-Arrhenius trend with temperature, which can be properly described according to a Vogel-Fulcher-Tamman model in agreement with a glassy like behavior at the lowest temperatures. Dynamic viscosity was also predicted from molecular dynamics simulations and Green-Kubo method, Figure S4b (Supporting Information), showing a highly viscous fluid for the lowest temperatures in 18


Page 19 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


agreement with the behavior of self-diffusion coefficients, which led to large errors in the determination of viscosity from molecular dynamics simulations (see large error bars for the lowest temperatures in Figure S4, Supporting Information). Nevertheless, viscosity follows a non-Arrhenius behavior in the studied temperature ranges. The pressure effect on the liquid structuring of [FUET2NH][H2PO4] is summarized in Figure S6 (Supporting Information). Results in Figure S6a (Supporting Information) show that a 500 bar increase in pressure does not lead to remarkable changes in the fluid structural properties, hydrogen bonding

remains without

remarkable changes, with a moderate

rearrangement in the second solvation shells, which would lead to the increase in density reported in Figure S6b (Supporting Information). Likewise, this also leads to an 4 % increase, in absolute value, of the total interaction energy on going to 300 bar and then remaining almost constant for higher pressure. Therefore, increasing pressure only leads to a weak rearrangement in the solvation shells, increasing interaction between ions but once a limit pressure is reached (roughly 300 bar) these effects vanish at least for the studied pressure range.

Adsorption on Graphene and Confinement. The changes in [FUET2NH][H2PO4] upon adsorption on graphene sheets and confinement between two parallel graphene sheets will be analyzed in this section. Previous studies on several types of ionic liquids have showed a strong reorganization of IL in the first layers close to graphene surface.17,44,45,87,88 The structuring of adsorbed IL was firstly analyzed by using number density profiles along the direction perpendicular to the graphene surface, z-coordinate, Figure 9, which show a strong densification in the vicinity in the regions close to graphene, with densities 3.7 and 5.7 times larger than in bulk IL for anion and cation respectively. Results in Figure 9a show that the first layer of adsorbed [HPO4]- is 0.85 Å closer to the graphene than [FUET2NH]+, although the density of anions is 50 % lower than the one for the cation, which may be justified considering that a second high density peak for anion appears at 7.60 Å, in contrast with the only peak for the cation. Therefore, results show a high density adsorbed layer for [FUET2NH]+ surrounded by two layers of [H2PO4]-, one closer to the graphene and the other one pointing to the bulk IL. The arrangement of [FUET2NH]+ in the adsorbed layer on graphene is analyzed from the number density profiles reported for relevant atoms in Figures 9b and 9c, which show that carbon atoms in alkylic chains and furan ring are placed at almost the same distance of the graphene plane whereas nitrogen atom in cation is placed further (roughly 1.2 Å) of the surface. Likewise, results in Figure 9c show that furan ring is almost parallel to the graphene surface. Therefore, both alkylic chains and furan ring are extended on 19



the graphene surface. This structuring of the strongly adsorbed IL layer is confirmed by the charge density profile reported in Figure S7 (Supporting Information), which shows a negatively charged region in the vicinity of graphene layer (with minimum at 3.5 Å) followed by a positive region (with maxima at 4.6 Å), followed by several charge peaks that show that ordering extends at least to a second weakly adsorbed layer. 0.02

[FUET2NH]+ [H2PO4]-

(a) 4.70 Å

0.016

ρ / Å-3

0.015 3.85 Å

0.01

0.005

0.016

C9 N1 O1 C7

(b)

0.012 0.008

0 0

10

20 30 z/Å

40

50

(c)

0.008

0.004

0.004

0

C1 O1 C4 C3 C2

0.012 ρ / Å-3

0.02

ρ / Å-3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 33

0 0

2

4 6 z/Å

8

10

0

1

2 3 z/Å

4

5

Figure 9. Number density, ρ, along the z direction for (a) ions center of mass and (b,c) relevant atoms in [FUET2NH]+ cation calculated for [FUET2NH][H2PO4] on top of graphene sheet at 328 K. Graphene sheet is placed at z = 0. Atoms labeling as in Figure 1a. Data inside panel (a) shows the position of the first maxima.

The strength of IL - graphene interactions is quantified by the intermolecular interaction energies reported in Table S3 (Supporting Information). First, it should be remarked the changes in ion-ion interaction energies upon IL adsorption. Anion-anion and cation-cation interaction energies decreases a 6 and 21 % respectively, whereas anion-cation decreases (in absolute value) a 10 %, which rises from the rearrangement of ions upon adsorption showed in Figure 9. Likewise, results in Table S3 (Supporting Information) show that [H2PO4]- - graphene interaction is stronger than [H2PO4]- - graphene one, in spite of the closer distance between the anion and graphene in comparison with cation, but ion - graphene interactions are of Lennard-Jones type (graphene is not charged in these simulations), and thus, the larger number of atoms in the cation together with the parallel arrangement of alkyl chains and furan rings with regard to graphene surface will lead to a larger number of interacting atoms for the cation adsorbed layer in comparison with anion. The adsorption of ions on top of graphene sheet should lead to a change in the hydrogen bonding characteristics of the IL in the firstly adsorbed layer in comparison with the bulk fluid, Table S4 (Supporting Information). The strong anion-anion hydrogen bonding is 20


Page 21 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


maintained for ions in the first adsorbed layer and for those ions in the remaining fluid in comparison with IL not adsorbed on graphene (Tables S2 and S4, Supporting Information). Nevertheless, if the number of hydrogen bonds are normalized with regard to the number of ions in each layer, the total number of anion-cation hydrogen bonds is 0.2, 0.1 and 0.1 for ions in the first adsorbed layer (ads in Table S4, Supporting Information), for ions in regions farther from the graphene surface and for pure IL (not adsorbed on graphene, Table S2, Supporting Information), whereas the normalized anion-anion hydrogen bonds are 1.8, 1.7 and 1.4 for the same regions. Therefore, adsorption on graphene leads to a first layer in which anion-anion hydrogen bonding is reinforced in comparison with bulk fluid. Moreover, the regions not directly in contact with graphene are also affected in their hydrogen bonding structuring leading to a larger number of interactions for anion-anion contacts, which may be justified with the presence of a second strong adsorption peak for anions reported in Figure 9a. This would justify the decrease in anion-anion interaction energies, which are repulsive (positive) because of the same sign of ions for the coulombic contribution but the increase in Lennard-Jones interactions rising from the largest number of hydrogen bonds upon adsorption on graphene. The dynamics of adsorbed layer is inferred from self-diffusion coefficients reported in Table S5 (Supporting Information). The diffusion coefficient tool for VMD by Giorgino89 was used for all the one-dimensional and two-dimensional D calculations, from the trajectories obtained in this work. The calculation of one-dimensional and two-dimensional D values was carried out ignoring motions along the remaining coordinates. All the D values were obtained for the corresponding center-of-mass of the studied ions. Diffusion along the direction perpendicular to the graphene sheets for ions in the first strongly adsorbed layer is two orders of magnitude slower than in the graphene plane, which lead to very large residence times of ions in the adsorbed layer. Likewise, anions remain longer times in the adsorbed layer than cations, which is in agreement with the graphene-ion interaction energies reported in Table S3 (Supporting Information) and the fact that cations tend to interact with the sheet with alkylic chains and furan rings. Diffusion of ions in the graphene plane is faster than for ions in the bulk regions, which should rise from the lost of ion-ion interactions upon adsorption, Table S3 (Supporting Information). Ionic diffusion in bulk layer is also slightly larger than those in pure IL (Figure 8). Confinement of [FUET2NH][H2PO4] between rigid graphene sheets is analyzed in Figure 10. Density profiles show equivalent features of ions in the vicinity of both graphene sheets resembling those when only one graphene sheet is considered (Figures 9a and 10a) and 21



analogously for charge density profile, Figure 10b. Interaction energies reported in Table S3 (Supporting Information) show that confinement leads to a further decrease of ion-interaction energies in comparison with systems adsorbed on a single sheet. Likewise, anion-graphene interaction energy is roughly twice that for one graphene sheet whereas cation-graphene is lower than twice the energy for the system containing a single sheet. 0.02

0.02

[FUET2NH]+ [H2PO4]-

(a)

0.015

(b)

0.01 ρc / e×Å-3

ρ / Å-3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 33

0.01

0

-0.01

0.005

-0.02

0 0

20 40 z/Å

0

60

20 40 z/Å

60

Figure 10. (a) Number density, ρ, for ions center of mass and (b) charge density, ρc, along the z direction calculated for [FUET2NH][H2PO4] confined between two graphene sheets at 328 K. Dashed lines show the position of graphene sheets.

Solvation of SWNTs and Confinement. The interaction of [FUET2NH][H2PO4] with SWNTs is analyzed in this work using SWNT(10,10) and SWNT(15,15). In the molecular dynamics simulations, empty SWNTs were placed in simulation boxes containing IL, and thus, the nanotube filling rate could be inferred, Figure S8 (Supporting Information). Regarding ions penetrating inside SWNT(15,15), rate for [FUET2NH]+ is larger than for [H2PO4]-, which rises from the fact that the SWNT used in these simulations are hydrogen terminated, with these terminal hydrogen atoms being positively charged (+0.1 per hydrogen atom), and thus, anions approaching the entrance of nanotube should remain longer times in this region because of electrostatic attraction, whereas the opposite effect should be inferred for cations. This leads to a maximum number of 21 anions and cations confined inside SWNT(15,15), corresponding to a number density of 0.0619 Å-3, which is 35 % lower than the value for bulk fluid (0.0948 Å-3). Therefore the size and shape of [FUET2NH]+ leads to a certain steric hindrance for its confinement inside of the nanotube, which lead to a decrease in 22


Page 23 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


density in comparison with bulk fluid. The structuring of confined IL is reported in Figure 11, ions adopt a well defined two-shells structure, with a layer close to the SWNT inner surface composed by anions and cations (but richer in cations) and an internal layer, around the nanotube longitudinal axis, with larger concentration of anions, Figures 11a and 11b. In the external confined shell anions are closer to the nanotube surface than cations, in agreement with results for adsorption on graphene reported in the previous section. Number density along the nanotube longitudinal axis show alternating anion and cation peaks, Figure 11c and 14d. Regarding the distribution in SWNT(10,10), Figure S9 (Supporting Information), reported results show a larger number of cations (10) than anions (4) confined inside this narrow nanotube, and thus, at least in the studied timeframe then confined fluid would be positively charged. Nevertheless, the arrangement of confined ions for both nanotubes is compared in Figure 12, which show in both cases anions being closer to the nanotube internal wall, and thus, also stronger anionic affinity for the surface as for graphene.

Figure 11. Results for [FUET2NH][H2PO4] confined inside SWNT_15_15, showing snapshots and density profiles, ρ, along the SWNT_15_15 (a,b) radius and (c,d) long axis. In panels (a.c) only the centers of mass of [FUET2NH]+ (blue balls) and [H2PO4]- (green balls) are reported for the sake of visibility. Values calculated at 328 K.

Interaction energies are reported in Table S3 (Supporting Information), showing that ion-ion interactions are not remarkably perturbed by the presence of the nanotube because 23



these interaction energies are very close to those in pure ionic liquid. Likewise, cationic interaction with the nanotubes is stronger than anionic one. The dynamic properties of ions confined in nanotubes are remarkably different of those in bulk fluids, Chaban and Prezhdo47 showed that diffusion of [1-ethyl-3methylimidazolium][chloride] increases up to 5 times upon confinement in smallest nanotubes in comparison with bulk fluid, and this diffusion approaches to that in the bulk with increasing nanotube diameter. On the contrary, other authors have showed that diffusion for confined ions is slower than in the bulk fluid.44,48 Therefore, self-diffusion coefficients were calculated for [FUET2NH][H2PO4] confined inside nanotubes leading to values reported in Table S6 (Supporting Information), and confirming slower diffusion for confined ions in comparison with bulk fluid and diffusion rates decreasing with decreasing nanotube diameter.

6

SWNT_10_10

[FUET2NH]+ [H2PO4]-

4 g(r)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 33

SWNT_15_15

2

0 0

4

8

12

r/ Å Figure 12. Radial distribution functions for the center of mass of ions in [FUET2NH][H2PO4] confined inside SWNT_10_10 (top) and SWNT_15_15 (bottom) as a function of distance with the center of the nanotube, r. Dashed lines show the position of the nanotubes walls. Values calculated at 328 K.

The hydrogen bonding of ions confined inside SWNT(15,15) was also analyzed, Table S7 (Supporting Information), showing strong anion-anion interactions. The normalized number of hydrogen bonds (divided by 21 ion pairs inside the nanotube) leads to 0.2 and 2.4 anion-cation and anion-anion hydrogen bonds per ion, which shows that anion-cation hydrogen bonding is similar to the adsorbed layer on graphene but anion-anion hydrogen

24


Page 25 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


bonding is remarkably increased upon confinement both in comparison with bulk liquid and with IL adsorbed on graphene. The external solvation of the nanotubes is characterized by a strong first shell followed by additional shells, Figure 13a and 13b, which confirm a strong structuring effect of nanotubes for [FUET2NH][H2PO4] in the regions close to the nanotube external surface.

Figure 13. (a) Snapshot and (b) density profiles of ions solvating SWNT_15_15 in [FUET2NH][H2PO4]. In panel (a) blue sticks stand for [FUET2NH]+ and green ones for [H2PO4]-; blue and green balls stand for the centers of mass of [FUET2NH]+ and [H2PO4]- ions, respectively, in the first solvation sphere around SWNT_15_15. In panel (b) dashed lines stand for SWNT_15_15 wall. Ions confined inside SWNT_15_15 are omitted to improve visibility. Values calculated at 328 K.

Deepening into the nature of the [FUET2NH][H2PO4] – carbon-nanostructures interaction mechanism through DFT. The optimized structures of one ionic pair on top of the graphene surface (IL-G), confined in a SWNT(10,10) nanotube (IL@SWNT) as well as on the outer surface of a SWNT(10,10) nanotube (IL-SWNT) are reported in Figure 14. The strength of cation-anion interactions is quantified by the binding energy as defined in eq. 1 (ΔEIL), which was obtained from single point calculations at B3LYP-D3/6-31G* level on the geometry of the ionic pair taken from IL-G, IL@SWNT or IL-SWNT optimized structures. For the ionic pair adsorbed on top of the graphene surface and confined inside SWNT, ΔEIL values decreases (from |ΔEIL| = 493.08 kJ×mol-1 for the isolated ionic liquid) up to 451.15 kJ×mol-1 and 441.66 kJ×mol-1, respectively. This weakening of the interionic interaction is mainly due to an elongation (equal to 0.854 Ǻ for both IL-G, IL@SWNT systems) of the intermolecular interaction between H9-O in [H2PO4]. There is also a decreasing interionic charge transfer from the anion to the cation (CTIL = 0.35 e- / 0.26 e- for IL-G / IL@SWNT). Due to the curvature of the nanotube surface, the adsorption of the ionic pair on top of the 25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

nanotube surface leads to a separation (length of H9-O intermolecular bond is 2.635 Ǻ), and thus, ΔEIL and CTIL are dramatically affected by the outside interaction with the nanotube (|ΔEIL| = 306.40 kJ×mol-1, CTIL = 0.24 e-). This change on H9-O interaction allows a new intermolecular interaction between H9 and the nanotube surface.

Figure 14. Top (left) and side (right) views for the optimized structure of [FUET2NH][H2PO4] ionic pair on top of the graphene surface (IL-G), confined inside of SWNT_10_10 (IL@SWNT) and outside of SWNT_10_10 nanotube (IL-SWNT). Interaction energies as well as total charges over the cation (q+), the anion (q-), the graphene sheet (qG) and the SWNT (qSWNT) according ChelpG are also displayed. The most important intermolecular interactions are drawn as dotted lines. Bond lengths are in Ǻ.

The strength of the interaction between [FUET2NH][H2PO4] ionic pair and the studied carbon-nanostructures has been also assessed through binding energies as defined in

26


Page 26 of 33

Page 27 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


eq. 2 (Figure 14). IL-G / IL-SWNT yield |ΔEIL-G| / |ΔEIL-SWNT| = 79.89 kJ×mol-1 / 77.52 kJ×mol-1, and the confinement of [FUET2NH][H2PO4] leads to |ΔEIL-SWNT| = -182.42 kJ×mol1

. This behavior is in agreement with results from MD simulations. As seen in Figure S8

(Supporting Information), ions quickly penetrate inside the nanotube, due to higher binding energies between confined ionic pair and the nanotube. Therefore, intermolecular interaction energies reported in Table S3 (Supporting Information) for [FUET2NH][H2PO4] with SWNT_10_10 and SWNT would be mainly due to those confined ionic pairs. Thus, these intermolecular interactions energies can be also correlated with binding energies estimated through DFT simulations. Then, similar ΔEIL values for IL-G and IL@SWNT systems are coherent with cation-anion intermolecular energies not remarkably perturbed by the presence of the nanostructure, the larger |ΔEIL@SWNT| value also agrees with higher ion-carbonnanostructure interactions (A-G + C-G). Regarding to intermolecular interaction between the ionic pair and the carbonnanostructure, the green region of the RGD isosurfaces (Figure S10, Supporting Information) located in the intermolecular region points out to van der Waals forces being the main contribution to the interaction between the ionic pair and carbon-nanostructures. Likewise, in agreement with larger C-G intermolecular energies, Table S3, Supporting Information), isosurfaces between the cation and the nanostructure display a larger extension than those ones related with A-G interactions. For the three studied structures, the intermolecular distance between the carbon-nanostructure and the aromatic ring in the cation is roughly 3.35Ǻ for IL-C and IL@SWNT systems and 3.40 Ǻ for IL-SWNT. Regarding anionnanostructure interactions, they are mainly carried out through intermolecular H-bonds with bond length ~ 2.47 Ǻ. Moreover, there is also a small charge transfer process between [FUET2NH][H2PO4] and carbon-nanostructures. For IL-G and IL@SWNT systems, the nanostructure becomes slightly negative charge (qG = -0.05e- and qSWNT = 0.07 e-, Figure 14), because of the small charge transfer from the anion to the nanostructure. However, the nanotube shows a positive charge of 0.06 e- for IL-SWNT, and thus, the charge is transferred from the nanotube to anion. Finally, the electronic properties of the studied systems were also analyzed. Figure S11 (Supporting Information) show the density of states (DOS) of [FUET2NH][H2PO4], IL-G, IL@SWNT and IL-SWNT systems (DOS of pristine graphene and CNT are also displayed as doted lines). The interaction of [FUET2NH][H2PO4] with carbon-nanostructures has not dramatic effect on the electronic structure of the graphene or carbon nanotube. Figure S11

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 33

(Supporting Information) also plots the partial density of states (PDOS) of the ionic liquid, as well as PDOS contributions from both ions. The lowest unoccupied molecular orbital located over the ionic pair (LUMOIL) is always due to the cation with energies of around 0.0 eV. However, different trends are obtained for the highest occupied molecular orbital located over the ionic pair (HOMOIL). For isolated [FUET2NH][H2PO4], HOMOIL is also mainly due to the cation, with a energy of -6.44 eV (with a energy gap ΔEH-G = 6.32 eV), whereas this HOMO mainly comes from contributions of the anions for IL-carbon-nanostructure systems. For IL-G and IL@SWNT,

HOMOIL is destabilized, reaching an energy ~ -5.12 eV, while

ΔEH-G ~ 5.34 eV. The largest charges are noted for the ionic pair adsorbed on the nanotube surface, with HOMOIL energy of -4.52 eV and ΔEH-G = 4.20 eV).

CONCLUSIONS The properties of [FUET2NH][H2PO4] renewable ionic liquid were studied

using a

computational chemistry approach. The reported results show a strongly structured fluid characterized by anion - cation hydrogen bonding but specially by anion-anion hydrogen bonding. The pressure and temperature effect on fluid properties were also analyzed together with the mechanism of ion-ion interactions characterized by the prevailing role of hydroxyl groups in anion and hydrogen attached to nitrogen in cation, which does not suffer remarkable changes in the studied pressure and temperature ranges. Regarding the interaction of this ionic liquid with graphene, a large trend for adsorption is inferred with anions being placed closer to the graphene surface but with cations adopting an extended molecular configuration on the sheet increasing cation-graphene interaction energies. Likewise, confinement between graphene sheets leads to adsorbed layers with symmetric properties. The confinement in carbon nanotubes is characterized by a decrease in density in comparison with bulk fluid, which rises from the size and shape of the cation, but at the same time with a decrease in diffusion rates in comparison with the bulk liquid. Likewise, the external solvation of the nanotubes is also characterized by strongly adsorbed layers with similar characteristics to those on planar graphene sheets. Finally, DFT simulations have shed some information on the interactions mechanism between selected ionic pairs and carbon-nanostructure, being van der Waals interactions the driving force for the adsorption on the graphene or nanotube surface as well as confination inside a nanotube. In addition, important charges have been described for the electronic structure of the isolated ionic liquid due to its interaction with carbonnanostructures.

28


Page 29 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ACKNOWLEDGEMENT This work was made possible by Ministerio de Economía y Competitividad (Spain, project CTQ2013-40476-R) and Junta de Castilla y León (Spain, project BU324U14). Gregorio García acknowledges the funding by Junta de Castilla y León, cofunded by European Social Fund, for a postdoctoral contract. We also acknowledge The Foundation of Supercomputing Center of Castile and León (FCSCL, Spain), Computing and Advanced Technologies Foundation of Extremadura (CénitS, LUSITANIA Supercomputer, Spain), and Consortium of Scientific and Academic Services of Cataluña (CSUC, Spain) for providing supercomputing facilities. The statements made herein are solely the responsibility of the authors. The authors declare no competing financial interests.

ASSOCIATED CONTENT Supporting Information Table S1 (forcefield parameterization for [FUET2NH]+ and [H2PO4]-), Table S2 (number of hydrogen bonds), Table S3 (intermolecular interaction energies), Table S4 (number of hydrogen bonds), Table S5 (self-diffusion coefficients), Table S6 (self-diffusion coefficients), Table S7 (number of hydrogen bonds), Figure S1 (distribution of torsion angles), Figure S2 (number of hydrogen bonds), Figure S3 (snapshot of the structure of the first solvation shell), Figure S4 (density obtained from molecular dynamics simulations), Figure S5 (distribution of cavity sizes), Figure S6 (pressure effects on radial distribution functions), Figure S7 (charge density), Figure S8 (number of ions inside nanotubes), Figure S9 (ions confined inside nanotube), Figure S10 (RGD isosurfaces) and Figure S11 (density of states). This material is available free of charge via the Internet at http://pubs.acs.org.

29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

REFERENCES (1) Fedorov, M.; Kornyshev, A. Ionic Liquids at Electrified Interfaces. Chem. Rev. 2014, 114, 29783036. (2) Lei, Z.; Dai, C.; Zhu, J. Extractive Distillation with Ionic Liquids: A Review. AIChE J. 2014, 60, 3312-3329. (3) Abro, A.; Abdeltawab, A. A.; Al-Deyab, S.; Yu, G.; Qazi, A. B.; Gao, S.; Chen, X. A Review of Extractive Desulfurization of Fuel Oils Using Ionic Liquids. RSC Adv. 2014, 4, 35302-35317. (4) Lei, Z.; Dai, C.; Chen, B. Gas Solubility in Ionic Liquids. Chem. Rev. 2014, 114, 1289-1326. (5) Marrucho, I. M.; Brnaco, L. C.; Rebelo, L. P. N. Ionic Liquids in Pharmaceutical Applications. Ann. Rev. Chem. Biomol. Eng. 2014, 5, 527-546. (6) Duan, X.; Ma, J.; Lian, J.; Zheng, W. The Art of Using Ionic Liquids in the Synthesis of Inorganic Nanomaterials. Cryst. Eng. Comm. 2014, 16, 2550-2559. (7) Freudenmann, D.; Wolf, S.; Wolff, M.; Feldmann, C. Ionic Liquids: New Perspectives for Inorganic Synthesis? Angew. Chem. Int. Ed. 2011, 50, 11050-11060. (8) Zhang, Q.; Zhang, S.; Deng, Y. Recent Advances in Ionic Liquid Catalysis, Green Chem. 2011, 13, 26191637. (9) Somers, A. E.; Howlett, P. C.; MacFarlane, D. R.; Forsyth, M. A Review of Ionic Liquid Lubricants. Lubricants 2013, 1, 3-21. (10) Marino, M. G.; Kreuer, K. D. Alkaline Stability of Quaternary Ammonium Cations for Alkaline Fuel Cell Membranes and Ionic Liquids. ChemSusChem. 2014, 8, 513-523. (11) Zhang, S.; Sun, J.; Zhang, X.; Xin, J.; Miao, Q.; Wang, J. Ionic Liquid-Based Green Processes for Energy Production. Chem. Soc. Rev. 2014, 43, 7838-7869. (12) Lau, G. P. S.; Tsao, H. N.; Zakeeruddin, S. M.; Grätzel, M.; Dyson, P. Highly Stable Dye-Sensitized Solar Cells Based on Novel 1,2,3-Triazolium Ionic Liquids. ACS Appl. Mater. Interfaces. 2014, 6, 13571-13577. (13) Fauzi, A. H. M.; Amin, N. A. S. An Overview of Ionic Liquids as Solvents in Biodiesel Synthesis. Renew. Sus. Ener. Rev. 2012, 16, 5770-5786. (14) MacFarlane, D. R.; Tachikawa, N.; Forsyth, M.; Pringle, J. M.; Howlett, P. C.; Elliott, G. D., Davis, J. H.; Watanabe, M., Simon, P.; Angell, C. A. Energy Applications of Ionic Liquids. Energy Environ. Sci. 2014, 7, 232-250. (15) Le Bideau, J.; Viau, L.; Vioux, A. Ionogels, Ionic Liquid Based Hybrid Materials. Chem. Soc. Rev. 2011, 40, 907-925. (16) Zhu, C.; Zhai, J.; Dong, S. Ionic Liquid - Induced Three-Dimensional Macroassembly of Graphene and Its Applications in Electrochemical Energy Storage. Nanoscale 2014, 6, 10077-10083. (17) Pensado, A. S.; Malberg, F.; Costa Gomes, M. F.; Pádua, A. A. H.; Fernández, J.; Kirchner, B. Interactions and Structure of Ionic Liquids on Graphene and Carbon Nanotubes Surfaces. RSC Adv. 2014, 4, 1801718024. (18) Cevasco, G.; Chiappe, C. Are Ionic Liquids a Proper Solution to Current Environmental Challenges? Green Chem. 2014, 16, 2375-2385. (19) Chen, L.; Sharifzadeh, M.; MacDowell, N., Welton, T.; Shah, N.; Hallett, J. P. Inexpensive Ionic Liquids: [HSO4]- - Based Solvent Production at Bulk Scale. Green Chem. 2014, 16, 3098-3106. (20) Neumann. J.; Steudte, S.; Cho, C. W.; Thöming, J.; Stolte, S. Biodegradability of 27 Pyrrolidinium, Morpholinium, Piperidinium, Imidazolium and Pyridinium Ionic Liquid Cations Under Aerobic Conditions. Green Chem. 2014, 16, 2174-2184. (21) Egorova, K. S.; Anaikov, V. P. Toxicity of Ionic Liquids: Eco(cyto)activity as Complicated, but Unavoidable Parameter for Tak-Specific Optimization. ChemSusChem. 2014, 7, 336-360. (22) Liu, Q. P.; Hou, X. D.; Li, N.; Zong, M. H. Ionic Liquids from Renewable Biomaterials: Synthesis, Characterization and Application in the Pretreatment of Biomass. Green Chem. 2012, 14, 304-307. (23) Bubalo, M. C., Rdosevic, K., Redovnikovic, I. R.; Halambek, J.; Srcek, V. G. A Brief Overview of the Potential Environmental Hazards of Ionic Liquids. Ecotox. Environ. Safety 2014, 99, 1-12. (24) Daillo, A. O.; Len, C.; Morgan, A. B., Marlair, G. Revisiting Physico-Chemical Hazards of IOnic Liquids. Separ. Purif. Sci. Technol. 2012, 97, 228-234. (25) McCrary, P. D.; Rogers, R. D. 1-Ethyl-3-methylimidazolium Hexafluorophosphate: from Ionic Liquid Prototype to Antitype. Chem. Commun. 2013, 49, 6011-6014. (26) Smith, E. L.; Abbott, A. P.; Ryder, K. S. Deep Eutectic Solvents (DESs) and Their Applications. Chem. Rev. 2014, 114, 11060-11082. (27) Petkovic, M.; Ferguson, J. L.; Gunaratne, N.; Ferreira, R., leitao, M. C., Seddon, K. R.; Rebelo, L. P. N.; Pereira, C. S. Novel Biocompatible Cholinium - Based Ionic Liquids - Toxicity and Biodegradability. Green Chem. 2010, 12, 643-649. 30


Page 30 of 33

Page 31 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(28) Petkovic, M.; Seddon, K. R.; Rebelo, L. P. N.; Pereira, C. S. Ionic Liquids: a Pathway to Acceptability. Chem. Soc. Rev. 2011, 40, 1383-1403. (29) Gouveia, W.; Jorge, T. F.; Martins, J. S.; Meireles, M.; Cruz, M.; Almeida, T. V.; Araújo, M. E. M. Toxicity of Ionic Liquids Prepared from Biomaterials. Chemosphere 2014, 104, 51-66. (30) Song, Z.; Liang, Y.; Fan, M.; Zhou, F., Liu, W. Ionic Liquids from Amino Acids: Fully Green Lubricants for Various Surface Contacts. RSC Adv. 2014, 4, 19396-19402. (31) Zarrougui, R.; Raouafi, N.; Lemordant, D. New Series of Green Cyclic Ammonium-Based Room Temperature Ionic Liquids with Alkylphosphite-Containing Anion: Synthesis and Physicochemical Characterization. J. Chem. Eng. Data 2014, 59, 1193-1201. (32) Ferlin, N.; Courty, M., Gatard, S., Spulak, S.; Quilty, B.; Beadham, I., Ghavre, M.; Hais, A.; Kummerer, K., Gathergood, N.; Bouquillon, S. Biomass Derived Ionic Liquids: Syntheis from Natural Organic Acids, Characterization, Toxicity, Biodegradation and Use as Solvents for Catalytic Hydrogernation Processes. Tetrahedron 2013, 69, 6150-6161. (33) Ohno, H.; Fukumoto, K. Amino Acid Ionic Liquids. Acc. Chem. Res. 2007, 40, 1122-1129. (34) Shaikh, A. R.; Kamio, E.; Takaba, H.; Matsuyama, H. Efects of Water Concentration on the Free Volume of Amino Acid Ionic Liquids Investigated by Molecular Dynamics Simulations. J. Phys. Chem. B 2015, 119, 263-273. (35) Deive, F. J.; Ruivo, D., Rodrigues, J. V.; Gomes, C. M.; Sanroman, M. A.; Rebelo, L. P. N.; Esperanca, J. S. S.; Rodríguez, A. On the Hunt for Truly Biocompatible Ionic Liquids for Lipase-Catalyzed Reactions. RSC Adv.2015, 5, 3386-3389. (36) Costa, A.; Forte, A.; Zalewska, K.; Tiago, G.; Petrovski, Z.; Branco, L. C. Novel Biocompatible Ionic Liquids Based on Gluconate Anion. Green. Chem. Lett. Rev. 2015, 8, 8-12. (37) Socha, A. M., Parthasarathi, R.; Shi, J.; Pattathil, S.; Whyte, D., Bergeron, M.; George, A.; Tran, K.; Stavila, V.; Venkatachalam, S.; Hahn, M. G.; Simmons, B. A.; Singh, S. Efficient Biomass Pretreatment Using Ionic Liquids Derived from Lignin and Hemicellulose. Proc. Nat. Acad. Sci. 2014, 111, E3587E3595. (38) Zhang, Y.; Maginn, E. J. Direct Correlation Between Ionic Liquid Transport Properties and Ion Pair Lifetimes: A Molecular Dynamics Study. J. Phys. Chem. Lett. 2015, 6, 700-705. (39) Schwenzer, B.; Kerisit, S.; Vijayakumar, M. Anion Pairs in Room Temperature Ionic Liquids Predicted by Molecular Dynamic Simulation, Verified by Spectroscopic Characterization. RSC Adv. 2014, 4, 5457-5464. (40) Zahn, S.; Brehm, M.; Brüssel, M.; Holloczki, O.; Kohagen, M.; Lehmann, S.; Malberg, F.; Pensado, A. S.; Schöppke, M.; Weber, M.; Kirchner, B. b. Understanding Ionic Liquids from Theoretical Methods. J. Mol. Liq. 2014, 192, 71-76. (41) Kirchner, B., Holloczki, O.; Canongia Lopes, J. N., Padua, A. G. H. Multiresolution Calculation of Ionic Liquids. WIREs Comput. Mol. Sci. 2015, 5, 202–214. (42) García, G.; Atilhan, M.; Aparicio, S. Theoretical Study on the Solvation of C60 Fullerene by Ionic Liquids. J. Phys. Chem. B 2014, 118, 11330-11340. (43) Fileti, E. E.; Chaban, V. V. Imidazolium Ionic Liquids Helps to Disperse Fullerenes in Water. J. Phys. Chem. Lett. 2014, 5, 1795-1800. (44) Aparicio, S., Atilhan, M. Molecular Dynamics Study of Carbon-nanostructures in N-Methylpieprazinium Lactate Ionic Liquid. J. Phys. Chem. C 2013, 117, 22046-22059. (45) Li, S.; Feng, G.; Cummings, P. T. Interfaces of Dicationic Ionic Liquids and Graphene: a Molecelar Dynamics Simulation Study. J. Phys. Cond. Matt. 2014, 26, 284106. (46) Rouha, M.; Cummings, P. T. Thickness-Dependent Structural Arrangement in Nano-Confined Imidazolium - Based Ionic Liquid Films. Phys. Chem. Chem. Phys. 2015, 17, 152-4159. (47) Chaban, V. V., Prezhdo, O. V. Nanoscale Carbon Greatly Enhances Mobility of a Highly Viscous Ionic Liquid. ACS Nano 2014, 8, 8190-8197. (48) Akbarzadeh, H.; Abbasspour, M.; Salemi, S.; Abdollahzadeh, S. Investigation of the Melting Point of Ionic Liquid [emim][PF6] Confined Inside CarbonNanotubes Using Molecular Dynamics Simulations. RSC Adv. 2015, 5, 3868-3874. (49) Perkin, S. Ionic Liquids in Confined Geometries. Phys. Chem. Chem. Phys. 2012, 14, 5052-5062. (50) Chen, S.; Wu, G.; Sha, M.; Huang, S. Transition of Ionic Liquid [bmim][PF6] room Liquid to High-Melting Point Crystal When Confined in Multiwalled Carbon Nanotubes. J. Am. Chem. Soc. 2007, 129, 2416-2417. (51) Sha, M.; Wu, G.; Liu, Y.; Tang, Z.; Fang, H. Drastic Phase Transition in Ionic Liquid [Dmim][Cl] Confined Between Graphite Walls: New Phase Formation. J. Phys. Chem. C 2009, 113, 4618-4622. (52) Greaves, T. L.; Drummond, C. J. Protic Ionic Liquids: Properties and Applications. Chem. Rev. 2008, 108, 206-237.

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(53) Greaves, T. L.; Ha, K.; Muir, B. W.; Howard, S. C.; Weerawardena, A.; Kirby, N.; Drummond, C. J. Protic Ionic Liquids (PILs) Nanostructure and Physicochemical Properties: Development of High-Throughput Methodology for PIL Creation and Property Screens. Phys. Chem. Chem. Phys. 2015, 5, 2357-2365. (54) Plechkova, N. V.; Seddon, K. R. Applications of Ionic Liquids in the Chemical Industry. Chem. Soc. Rev. 2008, 37, 123-150. (55) Lyubartsev, A. P.; Laaksonen, A. MDynaMix - A Scalable Portable Parallel MD Simulation Package for Arbitrary Molecular Mixtures. Comput. Phys. Commun. 2000, 128, 565-589. (56) Martínez, L.; Andrade, R.; Birgin, E. G.; Martínez, J. M. Packmol: A Package for Building Initial Configurations for Molecular Dynamics Simulations. J. Comput. Chem. 2009, 30, 2157-2164. (57) Breneman, C. M., Wiberg, K. B. Determining Atom-Centered Monopoles from Molecular Electrostatic Potentials. The Need for High Sampling Density In Formamide Conformational Analysis. J. Comput. Chem., 1990, 11, 361-373. (58) Essmann, U. L.; Perera, M. L.; Berkowitz, T.; Darden, H.; Lee,H.; Pedersen, L. G. A Smooth Particle Mesh Ewald Method. J. Chem. Phys. 1995, 103, 8577-8593. (59) Tuckerman, M.; Berne, B. J.; Martyna, G. J. Reversible Multiple Time Scale Molecular Dynamics. J. Chem. Phys. 1992, 97, 1990-2001. (60) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09, revision D.01; Gaussian, Inc.: Wallingford, CT, 2010. (61) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785-789. (62) Becke, A. D., Density‐functional thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648-5652. (63) Becke, A. D., Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior. Phys. Rev. A 1988, 38, 3098-3100. (64) Dapprich, S.; Komaromi, I.; Byun, K. S.; Morokuma, K.; Frisch, M. J., A New ONIOM Implementation in Gaussian 98. 1. The Calculation of Energies, Gradients and Vibrational Frequencies and Electric Field Derivatives. J. Mol. Struct. (Theochem), 1999, 462, 1-21. (65) Perdew, J.P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple. Phys. Rev. Lett., 1996, 77, 3865-3868. (66) Grimme, S. Semiempirical GGA-type Density Functional Constructed with a Long-Range Dispersion Correction. J. Comp. Chem., 2006, 27, 1787-1799. (67) Xu, X.; Goddard III, W. A. The Extended Perdew-Burke-Ernzerhof Functional with Improved Accuracy for Thermodynamic and Electronic Properties of Molecular Systems. J. Chem. Phys., 2004, 121, 4068-4082. (68) Le Bahers, T.; Rérat, M.; Saulet, P. Semiconductors Used in Photovoltaic and Photocatalytic Devices, Assessing Fundamental Properties from DFT. J. Phys. Chem. C, 2014, 118, 5997. (69) Cohen, A. J.; Mori-Sánchez, P.; Yang, W., Challenges for Density Functional Theory. Chem. Rev. 2012, 112, 289-320. (70) Grimme, S.; Ehrlich S.; Goerigk, L. Effect of the Damping Function in Dispersion Corrected Density Functional Theory. J. Comp. Chem. 2011, 32, 1456-1465. (71) Grimme, S.; Hujo, W.; Kirchner, B. Performance of Dispersion-Corrected Density Functional Theory for the Interactions in Ionic Liquids. Phys. Chem. Chem. Phys. 2012, 14, 4875-4883. (72) Johnson, E. R.; Keinan, S.; Mori-Sánchez, P.; Contreras-García, J.; Cohen, A. J.; Yang, W., Revealing Noncovalent Interactions. J. Am. Chem. Soc. 2010, 132, 6498-6506. (73) Lu, T.; Chen, F., Multiwfn: A Multifunctional Wavefunction Analyzer. J. Comput. Chem. 2012, 33, 580592. (74) O'Boyle, N. M.; Tenderholt, A. L.; Langner, K. M., CCLIB: A Library for Package Independent Computational Chemistry Algorithms. J. Comput. Chem. 2008, 29, 839-845. (75) Hayes, R.; Imberti, S.; Warr, G. G.; Atkin, R. Amphiphilicity Determines Nanostructure in Protic Ionic Liquids. Phys. Chem. Chem. Phys. 2011, 13, 3237-3247. (76) Hettige, J.; Araque, J. C.; Margulis, C. J. Bicontinuity and Multiple Length Scale Ordering in Triphilic Hydrogen - Bonding Ionic Liquids. J. Phys. Chem. B. 2014, 118, 12706-12716. (77) Skarmoutsos, I.; Weltin, T.; Hunt, P. A. The Importance of Timescale for Hydrogen Bonding in Imidazolium Chloride Ionic Liquids. Phys. Chem. Chem. Phys. 2014, 16, 3675-3685. (78) Eckert, F.; Klamt. A. COSMOthermX, Version C21_0111; COSMOlogic GmbH & Co. KG: Leverkusen, Germany, 2010. (79) Palomar, J.; Ferro, V. R.; Torrecilla, J. S.; Rodríguez F. Density and Molar Volume Predictions Uisng COSMO-RS for Ionic Liquids. An Approach to Solvent Design. Ind. Eng. Chem. Res. 2007, 46, 6041-6048.

32


Page 32 of 33

Page 33 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(80) Aparicio, S.; Atilhan, M. A Computational Study on Choline Benzoate and Choline Salicylate Ionic Liquids in the Pure State and After CO2 Adsorption. J. Phys. Chem. B. 2012, 116, 9171-9185. (81) AlTuwain, M. S., Alkhaldi, K. H. A. E.; Al-Jimaz, A. S.; Mohammad, A. A. Temperature Dependence of Physicochemical Properties of Imidazolium-, Pyroldinium-, and Phosphonium-Based Ionic Liquids. J.Chem. Eng. Data 2014, 59, 1955-1963. (82) Chhoaray, P. K.; Gardas, R. L. Thermophysical Properties of Ammonium and Hydroxylammonium Protic Ionic Liquids. J. Chem. Thermodyn. 2014, 72, 117-124. (83) Chhotaray, P. K.; Jella, S.; Gardas, R. L. Physicochemical Properties of Low Viscous Lactam Based Ionic Liquids. J. Chem. Thermodyn. 2014, 74, 255-262 (84) Margulis, C. J. Computational Study of Imidazolium-based Ionic Solvents with Alkyl Substituents of Different Lengths. Mol. Phys. 2004, 102, 829-838. (85) Huang, X.; Margulis, C. J.; Li, Y.; Berne, B. J. Why is the Partial Molar Volume of CO2 So Small When Disolved in a Room Temperature Ionic Liquid? Structure and Dynamics of CO2 Dissolved in [Bmim+][PF6]. J. Am. Chem. Soc. 2005, 127, 17842-17851. (86) Andreussi, O.; Marzari, N. Transport Properties of Room Temperature Ionic Liquids from Classical Molecular Dynamics. J. Chem. Phys. 2012, 137, 044508. (87) Wang, S.; Li, S.; Cao, Z.; Yan, T. Molecular Dynamic Simulations of Ionic Liquids at Graphite Surface. J. Phys. Chem. C 2010, 114, 990−995. (88) Page, A. J.; Elbourne, A.; Stefanovic, R.; Addicoat, M. A.; Warr, G. G.; Voïtchovsky, K.; Atkin, R. 3Dimensional Atomic Scale Structure of the Ionic Liquid - Graphite Interface Elucidated by AM-AFM and Quantum Chemical Simulations. Nanoscale 2014, 6, 8100-8106. (89) Giorgino, T. Computing Diffusion Coefficients in Macromolecular Simulations: The Diffusion Coefficient Tool for VMD, Submitted (2014). Available from http://multiscalelab.org/utilities/DiffusionCoefficientTool (accessed January, 20 2015).

33


Theoretical Study of Renewable Ionic Liquids in ... - ACS Publications

Recommend Documents