Bicontinuity and Multiple Length Scale Ordering in Triphilic Hydrogen

Article pubs.acs.org/JPCB

Bicontinuity and Multiple Length Scale Ordering in Triphilic Hydrogen-Bonding Ionic Liquids Jeevapani J. Hettige,† Juan Carlos Araque,† and Claudio J. Margulis* Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States S Supporting Information *

ABSTRACT: Triphilic ionic liquids (containing polar, apolar, and fluorinated components) that can hydrogen bond present a new paradigm in ionic liquid structural morphology. In this study we show that butylammonium pentadecafluorooctanoate and its nonfluorinated analogue butylammonium octanoate form disordered bicontinuous phases where a network of charge alternating hydrogen bonds continuously percolate through the whole liquid. These systems show order on multiple length scales, the largest length scale given by the percolating network. Separation between filaments in the network gives rise to a prepeak or first sharp diffraction peak. In the case of the fluorinated system, shorter range order occurs due to apolar-fluorinated alternation that decorates the surface of each individual filament. The backbone of the filaments is the product of the shortest organized length scale, namely, charge alternating hydrogen bonds. Liquid structure obtained via molecular dynamics simulations is used to compute coherent X-ray scattering intensities, and a full picture of the liquid landscape is developed. A careful mathematical analysis of the simulation data proposed here reveals individual molecular correlations that importantly contribute to each feature of the experimental structure function. from the group of Drummond69 lead us to believe that there possibly is a very interesting interplay between components with different polarity in these liquids and that this combined with hydrogen bonding can lead to new and exciting topological arrangements of ions as recently proposed for protic ionic liquids.13,26,70−74 However, a full mathematical and computational description of the subionic components of S(q) is still missing, and hypotheses about the origin of the different peaks in S(q) for these types of systems are yet to be fully explored. The current work focuses on the computational description of the triphilic butylammonium pentadecafluorooctanoate (BAOF) IL as compared to its nonfluorinated analogue butylammonium octanoate (BAO) shown in Figure 1, which the Drummond group has synthesized and studied via smalland wide-angle X-ray scattering (SWAXS).69 In this study we go beyond what so far our group has been able to accomplish in terms of analyzing S(q). The mathematical and computational analysis technique that we will present in section 2.3 allows us to accurately identify and explain at the molecular level the origin of all structural features in S(q). We will show that both of these systems are continuously percolated by charge alternation strings or filaments that bifurcate creating disordered bicontinuous phases and that peaks in S(q) can either be associated with interfilament characteristic distances

1. INTRODUCTION The ionic liquids (ILs) community has made tremendous progress toward a complete understanding of the structural landscape of conventional ionic liquids. Because of many neutron and X-ray structural studies1−36 combined with revealing molecular dynamics (MD) simulations,6,12,30−32,37−59 we now have intuition and predictive capabilities about ionic liquid morphology that were not available 10 years ago. Our own work6,30,52,60−65 has led us to the conclusion that three main topological characteristics, namely, polarity alternation (as in apolar tails separated by charged groups), charge alternation (as in positive−negative alternation of cationic heads and anions/anion heads), and adjacency correlations of all sorts are common features to almost all biphilic ionic liquids in cases where alkyl tails are moderately long. These three features correspond to the typical three peaks or shoulders in the intermolecular region of S(q) below 1.5 Å−1. However, recently novel and exciting design ideas have emerged that challenge our current understanding of IL morphology. In some cases these involve ionic liquids with polar tails; we call these systems monophilic as opposed to the proverbial case of polar heads and apolar tails which are normally called biphilic. An example of these systems are ionic liquids containing tail ether functionalities.42,47,63 Other very recent developments are the so-called triphilic ionic liquids in which polar groups, apolar alkyl tails, and significantly large fluorinated tails are present. Pioneering experimental studies by the groups of Yoshida,66 Triolo,67 and Rebelo68 as well as work © 2014 American Chemical Society

Received: July 9, 2014 Revised: August 26, 2014 Published: August 26, 2014 12706

dx.doi.org/10.1021/jp5068457 | J. Phys. Chem. B 2014, 118, 12706−12716

The Journal of Physical Chemistry B

Article

S(q) =

ρo ∑i ∑j xi xj fi (q) f j (q) ∫

L /2

0

4πr 2(gi j(r ) − 1)

sin qr W (r ) qr

dr

2

[∑i xi fi (q)]

(1)

where, as is customary, gij(r) stands for the full system radial distribution function comprising both an intra- and intermolecular pair of atomic species of types i and j, xi and xj are corresponding atomic mole fractions of atoms of types i and j, and f i(q) and f j(q) are tabulated X-ray atomic form factors that can be found in ref 89. In eq 1 the total number density for the system is denoted as ρo and the box length for our periodically replicated system is referred to as L. As is common practice, a Lorch function W(r) = sin(2πr/L)/(2πr/L)90,91 is used to reduce the effect of finite truncation of gij(r) at large values of r. In this work we split S(q) into different additive subcomponents based on convenient chemical intuition. The full mathematical description of these partitionings have been thoroughly provided in recent publications and need not be repeated here.6,60,61 To provide a comparison with experiments, the coherent intensities, Icoh(q) for BAOF and BAO were derived from the calculated total S(q) using

Figure 1. Scheme showing the definition of group partitions of the structure function S(q) that are used throughout this work in the case of BA (left), OF (upper right), and O (lower right).

(the prepeak) or intrafilament repeating patterns (peaks at larger q values).

2. COMPUTATIONAL AND THEORETICAL METHODS In order to ensure proper size sampling 1000 ion pairs were simulated in the case of butylammonium (BA) octanoate (O) as well as in the case of BA coupled with pentadecafluorooctanoate (OF). All MD simulations were carried out using the GROMACS75,76 package. As we have done in previous studies,6,30,52,60−63 during the process of equilibration we start with constant pressure and temperature simulations on systems in which ions have 0% of their charge and slowly ramp the charges up to 100%. This protocol was then followed by a 3 ns temperature annealing scheme in which the temperature of the system was ramped up by 150 K and then brought back down to the target value of 323 K. As a final equilibration/production step all systems were further run in the NPT ensemble for more than 7.0 ns at 323 K and pressure of 1 bar consistent with the conditions at which SWAXS experiments were collected.69 During this final step, the temperature was controlled using the Nosé−Hoover77,78 thermostat and the pressure using the Parrinello−Rahman79 barostat. The coordinates of atoms during the final 1 ns of this constant temperature and pressure simulation were saved every 1 ps and used for structural analysis. The Leap-Frog algorithm with a time step of 1 fs was used to integrate equations of motion. Coulomb and LennardJones cutoffs were set to 1.5 nm and the particle mesh Ewald (PME)80,81 method with an interpolation order of 6 and Fourier grid spacing of 0.8 Å was used to calculate electrostatic interactions. 2.1. Force Field Parameters. In the case of BAO, a combination of parameters available in the Canongia Lopes− Pádua,46 OPLS-AA,82−84 and AMBER85−87 force fields was used. The full set of parameters is provided in the Supporting Information. In the case of BAOF, the Fourier coefficients for the F−C−C−O dihedrals were not available and were instead approximated by those of H−C−C−O given in ref 82. To the best of our knowledge, atomic partial charges for the OF anion were not available in the literature and were fitted using the ChelpG protocol implemented in the Gaussian 03 code88 from a B3LYP geometry optimized structure at the 6-311++G** basis set level. 2.2. Structure Function and Coherent Intensity. The structure function S(q) for the two systems studied was computed using

S(q) =

Icoh(q) − ∑i xi fi 2 (q) [∑i xi fi (q)]2

(2)

2.3. Inverting S(q) To Discover Important Real Space Liquid Structural Motifs. The work presented in several previous articles30,60,61 has enabled us to achieve an understanding of ionic liquids in terms of three main features, namely, charge alternation, polarity alternation, and diverse adjacency correlations. Whereas this understanding has tremendously helped in properly analyzing S(q), many ILs show the same three peaks. Going beyond this analysis to fully discern the three-dimensional (3D) real space context in which such adjacencies and alternations manifest has not been easy. We therefore seek to mathematically identify those characteristic atomic correlations that at distances around 2π/q contribute significantly to S(q). The problem with what has been our proverbial approach to computing partial contributions to S(q)30,60 is that identifying particular atomic contributions is difficult in real space by using eq 1. The difficulty arises because the information about individual atomic pairs is not directly available from g(r) since this is a collective function. This information becomes readily available if instead of writing S(q) as a 3D Fourier transform of g(r) we instead cast it in terms of a sum of Fourier space static correlation functions as in S(q) =

1 ∑i ∑j fi (q) f j (q)⎡⎣ N ⟨ρqi ρ−j q ⟩ − xiδi , j ⎤⎦

[∑i xi fi (q)]2

(3)

In eq 3 the angle brackets denote the ensemble average of Fourier space densities defined in ρqi =

Ni

∑ exp(−iq·rα) α=1 Nj

ρ−j q =

∑ exp(iq·rβ) β=1

12707

(4) dx.doi.org/10.1021/jp5068457 | J. Phys. Chem. B 2014, 118, 12706−12716


Article

Figure 2. Total structure function S(q) and coherent intensities Icoh(q) (inset) for (a) BAOF and (b) BAO.

Figure 3. Cation head−anion head partial subcomponent of S(q) for (a) BAOF and (b) BAO.

where Ni and Nj are the total number of atoms of species i and j, respectively. Equation 3 can be easily derived by disregarding forward scattering and noticing that92 1 i j ⟨ρ ρ ⟩ = xi δi , j + ρo xi xj N q −q

∫0

L /2

4πr 2(gi j(r ) − 1)

Sαi , βj (q)

sin qr dr qr

During simulation the ensemble average ⟨ρiq ρj−q⟩ is obtained as β a sum over exponentials ρiqα ρj−q = exp(−iq·(rα − rβ)) that is real, where iα and jβ correspond to atoms α and β from species i and j, respectively. It is clear now that one can identify for a q value of interest atomic pairs α and β that for each simulation frame significantly contribute to ⟨ρiq ρj−q⟩. Many such important contributions exist at wave numbers corresponding to peaks and antipeaks in the partial subcomponents of S(q). However, only a few structural motifs satisfy both that their contribution at the q value of interest is large and atomic distance is close to dαβ = 2π/q. To easily identify those pairs of atoms that satisfy both requirements, we propose to use the following function: 1

w

2π q

(7)

3. RESULTS AND DISCUSSION Drummond and co-workers performed SWAXS experiments on a set of amonnium based and pyrrolidinium based ionic liquids coupled with fluorinated carboxylate anions.69 Of particular interest are studies that couple the BA cation with the OF anion because these ions incorporate a cationic alkyl tail and a significantly long anionic fluorinated tail. Furthermore, this fluoroprotic ionic liquid can be compared with its nonfluorinated analogue BAO. Interestingly, Drummond and coworkers found that SWAXS intensity profiles show three peaks at about 0.33, 0.63, and 1.22 Å−1 in the case of BAOF but only two peaks at around 0.36 and 1.45 Å−1 in the case of BAO.

i,j Re(Sαβ (q))

− dαβ

[∑i xi fi (q)]2

The overbar in eq 7 implies an angular average over q vectors of the same magnitude. Equation 6 identifies pairs of atomic contributions that significantly contribute to S(q) and that at the same time are resonant with the Bragg condition at wavenumber q. Thus, eq 6 provides a tool to identify individual structural motifs in real space (relative distance and orientation) that at distances 2π/q give rise to the different peaks and antipeaks in partial subcomponents of S(q).

(5)

Sαβi ,j(q) =

=

1 fi (q) f j (q)⎡⎣ N ρqiα ρ−jβq − xiδi , j ⎤⎦

(6)

where, in analogy with eq 3, Si,α jβ(q) is defined as 12708



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Figure 4. Hydrogen-bonding unit observed in BAOF. Red corresponds to anion O, and dark blue, to cation N. Figure 6. Typical structure of hydrogen-bonded filaments in the case of BAOF. In this depiction we see all three structural features that define this liquid, namely, hydrogen-bonded charge alternation (highlighted with a dashed line is the typical nitrogen−nitrogen separation distance), fluorinated tail−alkyl tail alternation (highlighted with a dashed line on the left filament), and the typical separation between filaments in the network (highlighted with a dashed line connecting two different filaments). The blue and red spheres represent the van der Waals volume of the nitrogen and oxygen atoms in the cation and anion head, respectively. The red and green sticks represent the configuration of cationic and anionic tails, respectively. The yellow isosurface is as described in Figure 5

Their interpretation of the three peaks in the case of BAOF was based on the assumption of fluorinated and hydrocarbon domains that each are segregated accounting for the two peaks with lowest q value and cation tail−cation tail adjacency correlations accounting for the peak at higher q value. In the case of BAO the absence of one of the three peaks was interpreted as an indication that only one type of hydrophobic domain (and not two as in the case of BAOF) was associated with the long anionic alkyl tails.69 Parts a and b of Figure 2 show our computationally derived structure functions for BAOF and BAO, respectively. Insets include the Icoh(q) for comparison with the results of Drummond and co-workers (see Figure 11 in ref 69). In close agreement with experimental results our Icoh(q) also show two peaks in the case of BAO and three peaks in the case of BAOF approximately at the correct wavenumber. Having established that our computational model is accurate to reproduce the overall S(q) and Icoh(q), we proceed to analyze in detail the physical origin of the different scattering features. To better dissect and understand liquid morphology, in Figure 1 we define and label groups of atoms for which partial contributions to the overall S(q) will be emphasized throughout the work. 3.1. Defining Property of Both Ionic Liquids: Hydrogen-Bonding Charge Alternation Network That Percolates Them Resulting in Bicontinuous Phases. We seek first to understand charge and polarity alternation. We recall from prior work62,64 that the polar cationic−polar anionic partial subcomponent of S(q) defined as Scathead−anhead(q) + Sanhead−cathead(q) is the most natural mathematical function to investigate the distinct q ranges where polarity and charge alternation in ionic liquids manifest.64 The reader is encouraged

Figure 5. Isosurfaces representing the subvolume occupied by the hydrogen bond network as defined in ref 93 for the case of (a) BAOF and (b) BAO.

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Figure 7. Polar−polar, polar−anion tail, and anion tail−anion tail subcomponents of S(q) for the case of (a) BAOF and (b) BAO. A vertical line in each panel indicates the q value at which polarity alternation occurs which should be interpreted in the context of Figure 8 as well as Figure 6

bonded filaments that percolate the whole liquid. These filaments are separated by alkyl and fluorocarbon spacer chains in the case of BAOF and by short and long alkyl tails in the case of BAO. Instead in this subsection we focus only on the antipeak that stems from charge alternation within single filaments and is reflected at q values around 1.15−1.3 Å−1. Figure 4 shows a typical example in which the analysis described in section 2.3 is used to identify important contributions to the charge alternation region qch‑alt that are at a distance about 2π/qch‑alt. We see that two cations and two anions form a hydrogen-bonded ring that allows for second hydrogen-bonded connections to the second carboxylate oxygens of each of the partner anions. Further connections are also possible at other proton locations of ammonium nitrogen. On average in the case of BAOF there are close to three hydrogen bonds per cation head and about 3.5 oxygen atoms in the first solvation shell. This is shown in Figures S.2 and S.3 of the Supporting Information where the number of hydrogen bonds as a function of time and the integral of the N−O g(r) are plotted, respectively. What is shown in Figure 4 is just an example of a “hydrogen-bonding unit”, and many variations of this structure are indeed found by inspection and careful mathematical analysis of the polar subcomponent of S(q). Whereas Figure 4 shows a particular hydrogen-bonding unit, such units are part of a complex network made of onedimensional filaments shown in Figure 5a,b. A closer look of such a hydrogen bond network is shown in Figure 6. We will refer to this figure repeatedly because it clearly conveys all of the main features present in BAOF, namely, hydrogen-bonding charge alternation, the typical separation between filaments giving rise to the prepeak and the apolar-fluorinated alternation within single filaments that contributes significantly to the intermediate peak in S(q). In terms of hydrogen bonding and charge alternation, the case of BAO is not too different from what we see for BAOF. Figure 3b is certainly very similar to Figure 3a, and analysis of S(q) to look for important contributors to charge alternation results in patterns very similar to those derived for BAOF. Perhaps an important difference between the two systems, as can be seen from Figure S.2 of the Supporting Information, is

Figure 8. Realistic molecular level view of polar-F tail alternation in BAOF. Green corresponds to fluorine atoms and white to hydrogen atoms, light blue to carbon atoms, red to oxygen atoms, and dark blue to nitrogen atoms.

to read ref 64 for a thorough explanation of why this is the case, but briefly it can be shown that in the range of q values where charge alternation occurs this function is expected to have a negative going peak (commonly called antipeak) and in the range of q values where polarity alternation occurs this same function is expected to have a positive going peak (the prepeak or first sharp diffraction peak). Parts a and b of Figure 3 show the polar cationic−polar anionic partial subcomponent of S(q) in the cases of BAOF and BAO, respectively. Vertical arrows indicate the q values at which polarity alternation (smaller q peak) and charge alternation (larger q peak) occur. We will show in subsequent subsections that the polarity alternation peak in Figure 3a,b corresponds mainly to the separation between hydrogen12710



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Figure 9. (a) Anionic fluorocarbon tails (green) and cationic alkyl tails (red) continuously percolated by a charge alternating hydrogen bond network (yellow) in the case of BAOF. (b) Anionic alkyl tails (green) and cationic alkyl tails (red) continuously percolated by a charge alternating hydrogen-bonded network (dark blue) in the case of BAO. In both panels the van der Waals volumes of oxygen and nitrogen atoms in the network are depicted in red and blue, respectively.

somewhat below 1 Å−1. In the case of BAOF and BAO charge alternation occurs at around 1.15 and 1.3 Å−1, respectively, overlapping to some extent with the q range in which adjacency correlations of different nature appear in S(q) as properly interpreted by Drummond and co-workers.69 Because in these protic systems charge alternation occurs at shorter distance, it is expected that the antipeak in Figure 3 will be broader. This is in part because a given range of distances corresponds to a much larger range of q values when q is large. 3.2. Molecular Origin of the First Sharp Diffraction Peak. Both for BAO and BAOF experimental and computational I(q) show prominent prepeaks at around 0.36−0.4 Å−1. If this peak is the result of some type of polarity alternation (in the case of BAOF polar-fluorocarbon alternation and in the case of BAO polar-alkyl tail alternation), there should be clear signature of this in the subcomponents of S(q).60,61 At qprepeak, in the case of BAOF, one can expect a peak both in the polar− polar subcomponent of S(q) (Scathead−cathead(q) + Sanhead−anhead(q) + Scathead−anhead(q) + Sanhead−cathead(q)) as well as in the fluorocarbon− fluorocarbon subcomponent of S(q) (SFtail−Ftail(q)). At qprepeak the polar-fluorocarbon subcomponent (S cat head −F tail (q) + SFtail−cathead(q) + Sanhead−Ftail(q) + SFtail−anhead(q)) should instead show an antipeak.60,61 Figure 7a clearly confirms this prediction. A similar situation can be observed in the case of BAO (see Figure 7b). One should simply conclude that in the case of BAOF (Figure 7a) the first sharp diffraction peak is a result of polar-fluorocarbon alternation and in the case of BAO (Figure 7b) it is a result of alternation between polar groups and anion alkyl tails. A typical ionic arrangement giving rise to the prepeak in the case of BAOF is displayed in Figure 8. This structure was not extracted from visual inspection of the simulation box but instead via the protocol described in section 2.3. Such structure corresponds to two distinct polar groups at distance 2π/qprepeak separated by fluorocarbon tails that strongly contribute to the prepeak intensity. To better understand the role of this common liquid motif in the context of the whole liquid, the reader should envision each polar end in Figure 8 as forming

Figure 10. Important component contributing to the intermediate peak at about 0.75 Å−1 in the case of BAOF where we can observe cation alkyl tail−anion fluorinated tail alternation. This pattern was chosen because it was deemed important at this q value from the analysis described in section 2.3. This alternation should be interpreted in the context of Figures 12 and 13.

that the number of hydrogen bonds per cation is larger in the case of BAO. This is similarly reflected in the integral of g(r) between N and O atoms shown in Figure S.3 of the Supporting Information. This is likely a consequence of the excluded volume effect due to the bulkiness of fluorinated tails. Interestingly, because of the geometries in which these hydrogen bond interactions occur, the charge alternation antipeak appears at significantly larger q values (shorter real space distance) than commonly observed in other ILs. Normally adjacency correlations occur as a broad peak at around 1.5 Å−1, whereas charge alternation occurs at or 12711



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Figure 11. Cation tail−cation tail, anion tail−anion tail, and cation tail−anion tail subcomponents of S(q) in the case of (a) BAOF and (b) BAO. A vertical dashed line indicates the q value at which long anionic and short cationic tail alternation occurs. This alternation should be understood in the context of Figure 12 and Figure 13.

Figure 13. Context within single filaments in which the long anionic alkyl tails (green) and short cationic alkyl tails (red) alternate in the case of BAO.

Figure 12. Context within single filaments in which the anionic fluorinated tails (green) and cationic alkyl tails (red) alternate and give rise to the peak at 0.75 Å−1 in S(q) in the case of BAOF.

part of a hydrogen bond filament and the fluorinated tails as filling the space in between. The filaments are shown in Figure 5, and the pattern of filaments separated by fluorocarbon are as shown in Figure 6. In Figure 5, the empty space is filled with fluorocarbon and alkyl chains. However, it is the separation between hydrogen bond filaments defined mostly by the length between opposing fluorocarbon tails that gives rise to the prepeak. The case of BAO is not too different from that of BAOF. The prepeak in this case corresponds to the distance between hydrogenbonded filaments separated by anionic hydrocarbon tails. Figure 9a shows an example of such a common liquid pattern in the case of BAOF, and Figure 9b shows the same pattern in the case of BAO.

In brief, both in the case of BAOF and that of BAO the prepeak is due to polar-anion tail alternations. The liquid landscape is dominated by a network of hydrogen bonds, and the typical separation between hydrogen bond filaments is associated with the prepeak length scale. It is important to notice however that there are many other flourinated tail correlations in the case of BAOF as well as alkyl anion tail correlations in the case of BAO that are also consistent with the prepeak wave vector. Such correlations certainly contribute to the tail−tail subcomponents of S(q) in both liquids making assignment of each peak to a single liquid feature inappropriate. 3.3. Intermediate S(q) Peak in BAOF. Figure 2 shows a peak at around 0.75 Å−1 in the case of BAOF that is absent in the case of BAO. The presence of a peak in one case and the 12712



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more electron rich than anionic alkyl tails in the case of BAO, certain differences exist in the liquid morphology that are reflected in S(q). One example is the complete cancellation of the peak at qintermediate in the case of BAO that is present in the case of BAOF. BAOF is one in a class of ionic liquids that has been called triphilic. The name is well deserved in that the charge alternation imposes two phases bicontinuously intertwined, one purely made of filaments within which charges alternate and the other made of fluorinated and alkyl components. Within the noncharged subcomponent, fluorinated and alkyl tails are also segregated by an arrangement imposed by charge alternation along filaments. The organization is in the form of strips that decorate the charged filaments, giving the liquid the desired three segregated component identity.

absence in the other was originally assigned by Drummond and co-workers69 to some type of aggregation of cationic alkyl tails in the case of BAOF that is absent in the case of BAO. Our analysis of S(q) described in section 2.3 provides a clear explanation for the origin of the qintermediate peak. Because of the charge alternating nature of the hydrogen bond filaments, cationic tails and anionic tails must naturally alternate. Another way of expressing this is that both cationic and anionic tails often point in directions perpendicular to the hydrogen bond filaments and therefore alkyl and fluorinated tails alternate in sync with the charges on the network. One can get an idea of what such an alternation looks like from Figure 10. The fact that there is an alternation in BAOF at qintermediate can be gleaned from Figure 11a where the typical pattern of two peaks and one antipeak is highlighted. A peak at qintermediate appears in the F tail−F tail subcomponent of S(q), another peak in the cation tail−cation tail subcomponents of S(q). These are naturally matched by an antipeak in the F tail−cation tail subcomponent of S(q) at the same q value. That this is the signature of F tail−cation tail alternation can be understood from derivations and arguments in refs 61 and 60. Whereas cationic alkyl tail−anionic F tail alternation is not the only contribution responsible for the large F tail−F tail subcomponent of S(qintermediate), it certainly is an important one. How does the alternation shown in Figure 10 fit within the context of the liquid? Are fluorinated tails and alkyl tails segregated? We can begin understanding this by considering Figure 12. We see that charge alternating hydrogen bond filaments are decorated by anionic fluorinated tails and cationic alkyl tails that each form strips (green in the case of the F tails and red in the case of the alkyl tails). Within a single hydrogen bond filament, red and green decorating strips alternate and tend not to mix. The alternation between these strips gives rise to individual ionic correlations such as that shown in Figure 10. Interestingly the case of BAO is strikingly similar (see Figure 13), and the reason why a peak is not seen in the overall S(q) at qintermediate is because of perfect cancellations. This can be verified from the pattern of peaks and antipeaks in Figure 11b. In the case of BAO the two peaks and the antipeak contributing to cationic tail−anionic tail alternation simply add up to an overall flat S(qintermediate). This phenomenon of perfect cancellation of alternations is certainly not uncommon.52

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing atom labeling, the number of hydrogen bonds per N atom for BAOF and BAO, and the radial distribution function and its integral for N and all O atoms and tables listing partial atomic charges and force field parameters used in the simulations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions †

J.J.H. and J.C.A. contributed equally to this work

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was funded by Grant No. CHE-1112033 from the National Science Foundation awarded to C.J.M. All opinions are those of the authors and not of the funding agency.

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REFERENCES

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4. CONCLUSION: EMERGING PICTURE OF THE LIQUID LANDSCAPE Both in the case of BAOF and that of BAO, the emerging picture arising from this study is that of multiscale ordering that encompasses from the atomic to the fully macroscopic. The main liquid feature is a fully connected percolating network of charge alternating hydrogen bonds that renders these systems bicontinuous. The distance between hydrogen bond filaments gives rise to the prepeak. Because the long anionic tails define the separation between filaments, they also define the q value at which the prepeak occurs. Filaments in the hydrogen bond network are decorated by strips of long anionic tails and short cationic tails that alternate. In the case of BAOF this alternation gives rise to intermediate range order via apolar-fluorinated segregation within the surface of each individual filament. In both liquids, long anionic and short cationic tails are anchored to the filaments by charged atoms. These charged atoms are capable of hydrogen bonding, and their charge alternation defines the backbone of the filaments forming the network. Because in the case of BAOF fluorinated tails are bulkier and 12713



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