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Nanostructure of Trialkylmethylammonium Bistriflamide Ionic Liquids Studied by Molecular Dynamics Karina Shimizu,*,†,‡ Agı´lio A. H. Pa´dua,§,| and Jose´ N. Canongia Lopes*,†,‡ Centro de Quı´mica Estrutural, Instituto Superior Te´cnico, 1049 001 Lisboa, Portugal, Instituto de Tecnologia Quı´mica e Biolo´gica, UNL, AV. Repu´blica Ap. 127, 2780 901 Oeiras, Portugal, Clermont UniVersite´, UniVersite´ Blaise Pascal, Laboratoire Thermodynamique et Interactions Mole´culaires, BP 10448, F-63000 Clermont-Ferrand, France, and CNRS, FRE3099, LTSP, F-63177 Aubiere, France ReceiVed: September 3, 2010; ReVised Manuscript ReceiVed: October 7, 2010
Our previous simulation studies on the nanostructuration of ionic liquids are extended to the homologous series of trialkylmethylammonium bis(trifluoromethanesulfonyl)amide ionic liquidss[N1 n n n][NTf2] (n ) 4, 6, and 8)srecently studied by Pott and Me´le´ard using small-angle X-ray scattering. Comparisons between experimental and simulation results allowed us to conclude that ionic liquids of this and other homologous series with sufficiently large alkyl side chains are nanostructured media composed by polar and nonpolar domains and that their complex structure can be further subdivided according to different classes of morphology (globular, filamentous, stratified). These different topologies are a result of the specific ionic frames and interactions, characteristics of each type of cation and anion present in the homologous series. Introduction The enormous success of ionic liquids as a new class of chemical compounds stems from their unique properties: their extremely low volatility1-3 stands out against the volatility of traditional organic solvents and the corresponding pollutionrelated issues; their complex nature as solvents4 allows their use as task-specific reaction or separation media. In addition, the success is also due to the possibility of combining different series of cations and anions to yield different subclasses of ionic liquids whose properties can be fine-tuned for specific applications.5,6 It was recently reported in different molecular simulation studies7-9 that ionic liquids exhibit medium-range ordering; that is, there are persistent microscopic domains in the liquid phase. Other simulation studies that focused on the microscopic dynamics of ionic liquids have also pointed out their slow dynamics and the persistence of local environments, typical of the glassy state.10 Several pieces of experimental evidence can also be related to these heterogeneities in the liquid phase.11 These include direct evidence such as X-ray diffraction data12 but also thermo-physical properties such as viscosity, diffusion, and electrical conductivity13 and also spectroscopic data from fluorescence experiments.14,15 On the other hand, the structure of some ILs that are solid at room temperature was determined by X-ray diffraction methods.16 For ionic liquids based on the 1-alkyl-3-methylimidazolium cation, [Cnmim]+, the structural studies showed that the solid consists of an extended network of cations and anions connected together by hydrogen bonds (mainly through the aromatic hydrogens connected to the C2, C4, and C5 carbon atoms of the imidazolium ring), with each cation surrounded by at least three anions and each anion surrounded by at least three cations. Although the number of * Corresponding authors. E-mail:
[email protected] (K.S.);
[email protected] (J.N.C.L.). † Instituto Superior Te´cnico. ‡ UNL. § Universite´ Blaise Pascal. | CNRS.
anions that surround a cation (and vice versa) can change depending on the anion size and the imidazolium alkyl substituents, this hydrogen-bonded network of ions is a common feature of crystals of imidazolium salts that is retained (at least partially) in the liquid phase. Neutron diffraction analysis can yield the complementary liquid structure information. In dialkylimidazolium salts, a close relationship between the crystal structure and the liquid structure was found, emphasizing once again the importance of the hydrogen-bonding interactions between ions.17 The existence of hydrogen-bonded clusters both in the solid and liquid phases was also confirmed by IR and Raman spectroscopy,18-20 NMR,21 and mass spectrometry.22,23 Ionic liquids based on the 1-alkyl-3-methylimidazolium cation, [Cnmim]+, are among the most popular and commonly studied. Structural studies, including the discovery of the abovementioned nanosegregated domains, were mostly performed using a homologous series based on [Cnmim]+ cations with different lengths of one alkyl side chain.7-9,12 In this paper, we have decided to extend our previous simulation studies on the nanostructuration in ionic liquids to a different series of homologous ionic liquids, namely, those based on cations with more than one alkyl side chain. In this context, Pott and Me´le´ard24 have recently investigated the structure of trialkylmethylammonium bis(trifluoromethanesulfonyl)amide, [CH3N(CnH2n+1)3][N(SO2CF3)2] or [N1n n n][NTf2] (n ) 4, 6, and 8), ionic liquids using small-angle X-ray scattering. One important difference between trialkylmethylammonium and 1-alkyl-3-methylimidazolium cations is that the former can have three long (and symmetrical) alkyl side chains. Those authors have found that the [N1 n n n][NTf2] salts have a structure similar to a disordered smectic-A phase. In the present contribution, we have extended their study on the liquid state organization of [N1 n n n][NTf2] salts using molecular dynamics simulations. Other differences in terms of molecular structure and interactions that are expected to have an impact on the properties of the trialkylmethylammonium ionic liquids, when compared to those of alkylmethylimidazolium compounds, are (i) the absence
10.1021/jp108420x 2010 American Chemical Society Published on Web 11/08/2010
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of strong H-bonds and (ii) the absence of polarizable electrons of an aromatic π-system. In imidazolium cations, the H bonds and the aromatic system induce stronger directional effects, that need to be taken into account together with the symmetry and flexibility of the cation due to the alkyl side chains. In tetralkylammonium, the anisotropy of the interactions will be mainly due to the length of the side chains, and by focusing on this family of cations, we intend to better isolate this effect. The cation-anion interactions should normally be simpler to describe in the tetraalkylammonium salts, since they can be reduced to electrostatic forces plus repulsion and dispersion. As for the anions, the traditional halide, hexafluorophosphate, and tetrafluoroborate anions are being replaced in many studies and applications by the hydrolytically and thermally more stable and less viscous bis(trifluoromethanesulfonyl)amide anion, [NTf2]-,18,19 which we chose for the present work.
TABLE 1: Experimental and Calculated Densities at 293 K of the Three [N1 n n n][NTf2] Ionic Liquids Studied in This Work
Experimental Section
Volumetric properties are the most reliable and extensive set of experimental data that can be used to validate simulated results in ionic liquid systems. To evaluate the quality of the present simulations, the molar densities of [N1 4 4 4][NTf2], [N1 6 6 6][NTf2], and [N1 8 8 8][NTf2] were calculated and compared with experimental values,24 cf. Table 1. In all three cases, the liquid densities were predicted within 3% uncertainty, which is the usual level of accuracy in this type of purely predictive calculation (no parameters in the molecular model were fit to volumetric or other liquid state thermodynamic data). In a previous work,9 some of the authors have shown that, in 1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids (alkyl ) ethyl, butyl, hexyl, and octyl), the clustering of the nonpolar alkyl side chains into nonpolar domains can be analyzed using an intermolecular end-carbon to end-carbon site-site radial distribution function (RDF). In fact, this was one of the first clues that subsequently led to the postulation of the existence of nanosegregated domains in ionic liquids.32 The same type of analysis was performed for the [N1 n n n][NTf2] ionic liquids studied in this work. Figure 1 shows the site-site intermolecular radial distribution function of the end-carbons (CT) of the alkyl side chain for those ionic liquids. The indication that clustering of the nonpolar side chains of the trialkyl-methyl-ammonium cations occurs can be ascertained from the analysis of Figure 1 and also from the comparison between Figures 1 and 2 from ref 9. In both cases, the density
Simulation. The molecular force field used in the present simulations is based on the OPLS-AA model25 with parameters specifically tailored for the ions that compose the ionic liquids.26-29 Following the spirit of OPLS-AA, intramolecular terms related to the covalent bonds and angles are taken from the AMBER force field,30 and efforts are concentrated on carefully describing conformational and intermolecular terms. The potential energy function has the general form given in eq 1 with the traditional decomposition of the pairwise-additive potential energy, uRβ, into covalent bonds, valence angles, torsion (dihedral) angles, and atom-atom pairwise repulsive, dispersive, and electrostatic contributions. The Coulomb interactions are defined in terms of atomic point charges, while the repulsive and dispersive terms are described by the Lennard-Jones 12-6 potential. Details concerning the development and implementation of the force-field can be found elsewhere.26 bonds
uR,β )
∑
kij (r - r0,ij)2 + 2 ij
ij dihedrals
4
∑ ∑
ijkl m)1 nonbonded
∑ ij
angles
∑ ijk
θij (θ - θ0,ijk)2 × 2 ijk
Vm,ijkl [1 - (-1)m cos(mφijkl)] + 2
{ [( ) ( ) ] 4εij
σij rij
12
-
σij rij
6
+
1 qiqj 4πε0 rij
}
(1)
All simulations were performed using molecular dynamics, implemented using the DL_POLY code.31 In the case of the ionic liquids [N1 n n n][NTf2] with n ) 4, 6, or 8, we have started from low-density initial configurations in systems composed of 150 ion pairs. These were equilibrated under constant N-p-T ensemble conditions for 500 ps at 293 K using a Nose´-Hoover thermostat and an isotropic Hoover barostat with time constants of 0.5 and 2 ps, respectively.31 The equilibrium density was attained after about 50 ps. Further simulation runs of 100 ps were used to produce equilibrated systems at the studied temperatures. Electrostatic interactions were treated using the Ewald summation method considering six reciprocal-space vectors and a convergence parameter (R) value of 0.3 Å-1, and repulsive-dispersive interactions were explicitly calculated below a cutoff distance of 16 Å (long-range corrections were applied assuming the system has a uniform density beyond that cutoff radius). Finally, 1000 configurations were stored from production runs of 300 ps for each one of the ionic liquids. Four successive 300 ps runs (totaling 1.2 ns) showed no drift
-3
Fexp /mol dm Fsim/mol dm-3 δF/% 24
[N1 4 4 4][NTf2]
[N1 6 6 6][NTf2]
[N1 8 8 8][NTf2]
2.5961 2.5123 -3.33
2.0613 2.0086 -2.63
1.7104 1.6871 -1.38
in the corresponding equilibrium properties and selected pair radial distribution functions. Possible ergodicity problems and finite-size effects were checked by the comparison of results from simulations starting from different conditions (distinct initial configurations, temperature annealing, charge scrambling) and boxes with different sizes and cutoff distances. Results and Discussion
Figure 1. Site-site intermolecular radial distribution functions of the terminal carbon (CT-CT).
Trialkylmethylammonium Bistriflamide Ionic Liquids of CT terminal carbon atoms in the vicinity (ca. 0.4 nm) of a given CT carbon atom is much larger than the average (bulk) density of CT atoms. In imidazolium-based ionic liquids, the intensity of the main peak (Figure 2 of ref 9) also increased markedly when lengthening the alkyl side chain, indicating that the tendency to form segregated nonpolar domains increased as the size of the nonpolar moieties of the molecule (alkyl side chain) became larger. It is important to stress that the size of the anion (that will be segregated to the polar domain of the ionic liquid) also plays a role in the morphology of the resulting domains of the ionic liquids. In the study discussed in ref 9, the main focus was on the [PF6]-anion. In a subsequent study,33 also concerning the nanosegregation on 1-alkyl-3-methylimidazolium-based ionic liquids, the focus shifted to the (larger) [NTf2]- anion and only minor deviations in rdf’s under discussion were observed. The general conclusion is that when the alkyl side chains are small (ethyl to butyl in the case of [Cnmim][PF6],9 ethyl to hexyl in the case of [Cnmim][NTf2]33), the nonpolar domains consist of hydrocarbon-like “islands” in the midst of a continuous polar network, which is highly cohesive due to cation-anion interactions, whereas for longer alkyl side chains those nonpolar islands start to connect forming a second continuous microphase, establishing in this way a bicontinuous segregated phase (the morphology is analogous to that of a sponge). The most important feature to notice here is that, since the [NTf2]- anion is bulkier than the [PF6]- anion, the “percolation” limit of the nonpolar domains seems to be slightly shifted to longer alkyl side chains (around hexyl instead of butyl). This makes sense just from a topological perspective: if the polar network is larger due to bulkier ions (in this case, the term “ions” refers just to the high charge density parts of the cation or the anion), one also needs larger islands before the percolation between them starts to happen. In the present case, no clear shifts in the intensity of the first peak of the CT-CT rdf’s of [N1 n n n][NTf2] have been observed when n increases from 4 to 8. This probably means that with three long alkyl side chains (starting at n ) 4) the percolation limit of the nonpolar domains has already been attained, even taking into account the effect of the bulkier [NTf2]- anion. However, the slightly less intense peak and lack of a shoulder around 0.5 nm in the RDF of [N1 4 4 4][NTf2] (when compared to the rdf’s of [N1 6 6 6][NTf2] and [N1 8 8 8][NTf2]) seems to indicate that the structure of the former ionic liquid is not as bicontinuous as in the case of the other two. A simple and easy (and quite well-known) way to visualize the polar and nonpolar domains is to use color-coded snapshots of the simulation boxes. In Figure 2, we have used the color scheme introduced in a previous work:33 the negative and positive parts of the polar network have been colored in red and blue, respectively, whereas the nonpolar domain was painted in (neutral) gray. The negative part includes all atoms of the anion; the positive part contains the nitrogen atom of the cation, the four carbon atoms (C1) bonded to it, and the nine hydrogen atoms (H1) attached to C1; the nonpolar domain includes all atoms of the alkyl side chains except C1 and H1. A qualitative topological analysis of the snapshots presented in Figure 2 shows that the nonpolar islands/domains in [N1 4 4 4][NTf2] are comparable to those found in [C6mim][NTf2] or in [C4mim][PF6]:33 the nonpolar domains are close to or just passed their percolation limit from discontinuous islands in the midst of the polar network to a second continuous phase. On the other hand, the snapshots of [N1 6 6 6][NTf2] and [N1 8 8 8][NTf2] show much bulkier and continuous nonpolar domains.
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Figure 2. Snapshots of simulation boxes containing 150 ions of [CH3N(CnH2n+1)3][NTf2]. The application of a coloring code enables clear identification of the cations and anions of the polar network (blue and red, respectively) and atoms belonging to the nonpolar region (in gray) of the nanosegregated ionic liquid. (a) [CH3N(C4H9)3][NTf2]; (b) [CH3N(C6H13)3][NTf2]; (c) [CH3N(C8H17)3][NTf2].
A more quantitative analysis can be performed using rdf’s that correlate the relative positions of selected atoms or atomic centers of mass belonging to the polar network (cation-anion, cation-cation, and anion-anion combinations) or atoms belonging to the nonpolar domains. All of these rdf’s, for the three ionic liquids under discussion, are collected in Figure 3. The anion-anion rdf’s (Figure 3a) show first-neighbor distances (first peak maxima) at around 10 Å that slightly increase with the length of the alkyl chains of the cationsfrom 9.75 Å for [N1 4 4 4][NTf2] to 10.35 Å for [N1 8 8 8][NTf2]. This increase represents a small departure from the behavior observed in the corresponding rdf’s of [Cnmim][PF6], where the first-peak distances (at around 7 Å) are independent of the alkyl side chain length. The larger anion-anion distance is easy to understand: not only is [NTf2]- larger than [PF6]- (the centers of mass of larger ions will necessarily lie further apart), but the cation (or the charged part of it that will lie between two anions) is also bulkier. In the literature, it was reported that the first-peak anion-anion distances rise to 8.5 Å in N-alkyl-3-methylpyri-
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Figure 3. Selected site-site intermolecular radial distribution functions (rdf’s) in trialkylmethylammonium bistriflamide systems. In all cases, black, gray, and light gray lines represent systems containing cations with butyl, hexyl, and octyl side chains, respectively: (a) anion center-of-mass to anion center-of-mass (aa) rdf’s; (b) cation center-of-mass to cation center-of-mass (cc) rdf’s; (c) nitrogen atom of cation to oxygen atoms of anion (cao) rdf’s; and (d) alkyl side chain center-of-mass of cation to alkyl side chain center-of-mass of cation (cca2) rdf’s. The inset in part c depicts the rdf’s between the nine hydrogen atoms closest to the nitrogen atom of the cation and the oxygen atom of the anion (specific anion-cation interaction).
dinium bistriflamide, or to almost 9 Å in N-alkyl-3,5-dimethylpyridinium bistriflamide ionic liquids.34 Also, Pott and Me´le´ard24 estimated for the [N1 n n n][NTf2] systems an average anion-anion separation distance of 9.3 Å from their X-ray data (the d2 value presented in ref 24 corresponds to the interlayer distance (d2 ) 8 Å) of a hexagonal grid containing the anions; the anion-anion distance is obtained by dividing that value by 31/2/2). Such as in the case of our MD results, these distances also show a slight increase with n. There are two important issues to retain here: (i) on one hand, the anion-anion distance will be a function of the size, shape, and flexibility of both the anion and the cations; (ii) on the other hand, the interionic distances (see also below) will not be strongly affected by the size of the alkyl side chains attached to the cation. This means that longer alkyl chains (and the corresponding nonpolar domains) will have to be accommodated within the ionic liquid without disruption of the cation-anion network. Figure 3b shows the cation-cation center-of-mass rdf’s. These results are similar to those obtained for the anion-anion pairs. The major differences between the two types of rdf’s are the much more complex aspect (multiple peaks, valleys, and shoulders) of the curves and their decrease in intensity with increasing size of the alkyl side chains. Obviously, the positions of the centers of mass of the cations become more “blurred” as the three flexible and articulated alkyl side chains become
longer. As a result, the corresponding rdf’s will span a larger range of distances and the corresponding intensities will be more subdued. The rdf’s between the anion and the cation were also considered and are shown in Figure 3c. In this case, we have selected the nitrogen atoms of the cations, N1, and the oxygen atoms of bistriflamide, OBT, as proxies for the anion-cation pair correlation functions (the atoms in the anion that carry most of the negative charge and are responsible for the strongest anion-cation interactions). The cation-anion radial distribution function shows first peaks at closer distances than the cation-cation or anion-anion pair functions. This just means that the ions of a given type prefer to be surrounded by ions of opposite charge in order to fulfill local electroneutrality conditions. Furthermore, when these rdf’s (Figure 3c) are analyzed with their anion-anion and cation-cation counterparts (Figure 3a and b), it can be seen that the former function is in phaseopposition with the last two (peaks coinciding with troughs and vice versa), showing the typical local charge-ordering arrangement of ionic condensed phases. As stated above, such behavior is logical if we take into account the electrostatic nature of the interactions between the high-charge density parts of the anions and cations that compose the ionic liquid: the first neighbor shell of a given ion is always populated by ions with opposite charge, the second shell, by ions with charges of the same sign, and so
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Figure 4. Spatial distribution functions of systems containing [CH3N(CnH2n+1)3][NTf2]. The images show the distribution of the oxygen (OBT) atoms of the anion (in red), the N1 nitrogen of the trialkyl-methyl-ammonium (in blue), and the CT carbon of the latter around a central cation molecule. The isosurfaces depicted correspond to densities of 2.8 for OBT, 2.2 for N1, and 2.0, 2.2, 2.4, 2.6, and 2.8 for the CT in (a) [CH3N(C4H9)3][NTf2], (b) [CH3N(C6H13)3][NTf2], and (c) [CH3N(C8H17)3][NTf2].
forth. This feature of the rdf’s has already been observed for imidazolium-based ionic liquids.9,35 In imidazolium bistriflamide-based ionic liquids, the firstneighbor shell comprises in fact two peaks (at 3.5 and 5.5 Å (C2-OBT distance) for [Cnmim]+ and 4.8 and 7.0 Å (N1-OBT distance) for [N1 n n n]+) corresponding to the two pairs of OBT oxygen atoms present in the anion. The difference between the two peakss1.5 Å for imidazolium-based bistriflamide ionic liquids, 2.2 Å for the ionic liquids studied in this workscan be attributed to (i) the way the oxygen atoms of bistriflamide interact with the most positively charged parts of the cations (the simultaneous interaction of two oxygen atoms with the hydrogen atom linked to the C2 carbon of the imidazolium ring is a common situation in the former case that probably does not occur in the second one). However, it must be stressed that the oxygen atoms of the anion also interact strongly with the H1 hydrogen atom of the cations studied in this work (inset of Figure 3c). These are not hydrogen bonds (the H1 hydrogen atoms are not acidic enough) but are specific enough to yield rdf’s with a clearly defined first peak. An important issue that can be inferred from the existence of these specific interactions is that, given the number of H1 atoms (nine) and their more or less even distribution around the central nitrogen atom (except for the places where the three long carbon backbones protrude), one should expect on average a more or less isotropic distruibution of anions around the nitrogen atom of the cations (cf. the discussion based on spatial distribution functions below); (ii) different proportions of the anti and gauche conformers of [NTf2]- in the two types of ionic liquid (the distances between oxygen atoms belonging to the two sulfonate groups of bistriflamide are different in the two conformational isomers of bistriflamide).36 Actually, the second fact is not independent from the first one: two interacting oxygen atoms in bisstriflamide instead of one will shift the balance between the amount of each bistriflamide conformer present in the ionic liquid. Finally, the rdf’s between the centers of mass of each single alkyl side chain of the cations were considered, and are presented
in Figure 3d. The most conspicuous information we can take from these rdf’s is that the first peaks appear around 5 Å. Pott and Me´le´ard have found a peak around 4.6 Å, and they have attributed it to short-range order of the cation aliphatic chains. These results emphasize the agreement between molecular dynamics simulations and X-ray data. Further structural information can be acquired from the molecular dynamic simulations using spatial distribution functions (SDFs). SDFs can provide very useful visual insights into the spatial relations between different structural features present in the ionic liquids. Figure 4 shows selected SDFs for each of the three ionic liquids studied in this work. Each figure depicts four types of SDFs. The first type illustrates the probability of finding both the anion (red) and the cation (blue) around the cation, the second and third ones represent the probability of finding the anion and cation separately, and the last one is the probability of finding the end-carbon (gray) around the cation. The analysis of the first series of SDFs clearly shows the spherical character of the first and second ionic coordination shells around the central nitrogen atom of the [N1 n n n]+ cations. These shells correspond to the spatial correlation functions between the oxygen atoms of the anion and the nitrogen atoms of the cations. The spherical SDFs are only interrupted by the protruding long alkyl chains of the cations. These results are in fact a three-dimensional representation of the rdf’s presented before, with the sequence of opposite-charge and same-charge shells clearly visible in the first three rows of Figure 4. For technical reasons, the end-carbon (CT) SDFs presented in the last row of Figure 4 were produced by assembling together different isosurfaces, calculated with reference to different central points. The SDFs that were created by just considering the N atoms as a central point did not provide enough information about the spatial probability of finding a CT atom near a given atom of the alkyl side chain. To overcome this difficulty, diverse SDFs, with different central points along the alkyl side chain, were calculated and joined together in order to obtain a complete isosurface (in fact, a mosaic of isosurfaces)
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that describes those probabilities along the alkyl side chains of the whole molecule. For the [N1 4 4 4]+ cation, four isosurfaces were needed: one centered around the central nitrogen atom plus three centered on each CT atom of each butyl chain. For the [N1 6 6 6]+ and [N1 8 8 8]+ cations, only the SDFs along one of the long chains were representedsthe isosurfaces obtained for the other chains provided similar information and were not depicted for the sake of simplicity. Three SDFs were required for [N1 6 6 6]+: the first centered at N1, the second at the C4 atom in the middle of the chain, and the third one at the CT atom at the end of the chain. Finally, four SDFs were needed for the [N1 8 8 8]+ cation: a central one plus three along the tail (N1, C4, C6, and CT). The visual analysis of those SDFs shows that the CT of the cation can be found very close to the carbons of the alkyl side chain, especially toward the tail of the chain. The probability of finding the CT atoms near the N1 atoms is smaller than the probability of finding them at positions near the middle or end of the alkyl side chains, with the isosurfaces appearing more fragmented near the center of the cation (its high-chargedensity moiety) and a more compact appearance at the outer portion of the cation (the low-charge-density moieties). From a morphological point of view, the SDFs collected in Figure 4 also demonstrate a quite striking difference between the spatial distribution of the polar and nonpolar regions contained in this type of ionic liquid: whereas the polar network around a given cation develops as a series of almost spherical concentric isoprobability shells only punctured by the long alkyl side chains, the nonpolar domain is characterized by alkyl chains that come very close together from any direction except the nitrogen “core”, giving rise to sleeve-like isosurfaces. The interplay between the different length scales of these two types of interaction and morphology (polar versus nonpolar and spherical versus sleeve-like) causes not only the structural nanosegregation that is typical of many classes of ionic liquids, but in the particular case of [N1 n n n][NTf2] it provides the molecular nexus to understand the proto-smectic nature of their isotropic liquid phase, as pointed out previously by the study of Pott and Me´le´ard.24 The overall segregation into (proto)layers is consistent with the fact that the long alkyl chains tend to lie side by side whereas the charged moieties of the cations and anions tend to form a charge-interspersed network. Unlike the trialkylmethylammonium cations, their 1,3-dialkylimidazolium counterparts do not exibit such “isotropic” polar SDFs (because of the possibility of establishing strong H-bonds, some regions surronding the aromatic hydrogen atoms of the imidazolium ring are privileged in terms of cation-anion interactions, cf. for instance, the SDFs presented in Figure 4 of ref 37). This means that the polar network in imidazolium-based ionic liquids will be more branched (with each cation being surrounded by an average of three anions that act as closestneighbors), less homogeneous (with some other anions relegated to secondary positions in the first shell), and with a globularshape morphology.31 On the other hand, the fact that there are no special directional interactions in [N1 n n n][NTf2] ionic liquids (the anions will just try to get as close as possible to the central nitrogen atom of the cation or its four surrounding methyl/ methylene groups while avoiding the rest of the alkyl side chains) will lead to a stratified (proto-smectic) arrangement, where the cations and anions will arrange themselves in more or less two-dimensional grids (hexagonal according to the data from ref 24) that will “sandwich” the nonpolar domains. It must be stressed that the presence of three symmetrical long alkyl chains and one methyl group in the present ammonium cations is compatible with the development of this morphology (the
Shimizu et al. anions will prefer to approach the nitrogen atoms of the cations by the less hindered methyl side, causing it to be incorporated into the polar strata). In the case of tetra-alkylphosphonium ionic liquids, where the cations exhibit four long alkyl side chains, the resulting morphology of the polar network is filamentous instead of layered.31 Conclusions The nanostrucuture of trialkylmethylammonium bistriflamide ionic liquids was analyzed using a molecular dynamics technique. Computer simulations using an all-atom force field have revealed that these ionic liquids show microphase separation between polar and nonpolar domains, similar in nature to those observed for other classes of ionic liquids. The polar domains have the structure of a tridimensional network of ionic channels and clusters, while the nonpolar part is arranged as a continuous microphase permeating the polar network. These results agree with the experimental evidence reported by Pott and Me´le´ard24 using an X-ray diffraction technique and also with previous simulation studies presented by Canongia Lopes and Pa´dua9 for [Cnmim][PF6] and by Shimizu et al.35 for [Cnmim][NTf2]. Moreover, a more detailed analysis of our MD data using spatial distribution functions allowed us to distinguish between different types of morphologies characteristic of different classes of ionic liquids. In the particular case of [N1 n n n][NTf2] ionic liquids, our SDF results strongly support the evidence previously reported in the diffraction studies that the structure of the liquid is similar to a disordered smectic (layered) phase. This study provides another confirmation of the idea that not only most ionic liquids are nanostructured media composed by polar an nonpolar domains but that their complex structure can be further subdivided according to different classes of morphology (globular, filamentous, stratified) that are a result of the specific ionic frames and interactions, characteristic of each ionic liquid homologous series. Namely, the presence of directional interactions such as hydrogen bonds, the degree of electrostatic charge delocalization, and the symmetry and the flexibility of the ions are factors that affect the structure of the liquid phases. Acknowledgment. Work supported by projects PTDC/QUIQUI/101794/2008 (FCT/Portugal) and PICS 3090 (CNRS/ France and GRICES/Portugal). K.S. acknowledges the support of grant SFRH/BPD/38339/2007 (FCT/Portugal). References and Notes (1) Rebelo, L. P. N.; Canongia Lopes, J. N.; Esperanc¸a, J. M. S. S.; Filipe, E. On the critical temperature, normal boiling point, and vapor pressure of ionic liquids. J. Phys. Chem. B 2005, 109, 6040–6043. (2) Paulechka, Y. U.; Zaitsau, Dz. H.; Kabo, G. J.; Strechan, A. A. Vapor pressure and thermal stability of ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide. Thermochim. Acta 2005, 439, 158–160. (3) Earle, M. J.; Esperanc¸a, J. M. S. S.; Gilea, M. A.; Canongia Lopes, J. N.; Rebelo, L. P. N.; Magee, J. W.; Seddon, K. R.; Widegren, J. A. The distillation and volatility of ionic liquids. Nature 2006, 439, 831–834. (4) (a) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; Wiley VCH: Weinheim, Germany, 2007. (b) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772–3789. (5) (a) Santos, C. S.; Baldelli, S. Alkyl Chain Interaction at the Surface of Room Temperature Ionic Liquids: Systematic Variation of Alkyl Chain Length (R ) C1-C4, C8) in both Cation and Anion of [RMIM][R-OSO3] by Sum Frequency Generation and Surface Tension. J. Phys. Chem. B 2009, 113, 923–933. (b) Torrecilla, J. S.; Palomar, J.; Garcia, J.; Rodrı´guez, F. Effect of Cationic and Anionic Chain Lengths on Volumetric, Transport, and Surface Properties of 1-Alkyl-3-methylimidazolium Alkylsulfate Ionic Liquids at (298.15 and 313.15) K. J. Chem. Eng. Data 2009, 54, 1297– 1301. (6) Blesic, M.; Swadzba-Kwasny, M.; Holbrey, J. D.; Canongia Lopes, J. N.; Seddon, K. R.; Rebelo, L. P. N. New catanionic surfactants based on
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