Insights on the isotropic-to-smectic A transition in ionic liquid crystals from coarse-grained molecular dynamics simulations: the role of microphase segregation.

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Insights on the Isotropic-to-Smectic A Transition in Ionic Liquid Crystals from Coarse-Grained Molecular Dynamics Simulations: the Role of Microphase Segregation Giacomo Saielli, Alessandro Bagno, and Yanting Wang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp5104565 • Publication Date (Web): 10 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Insights on the Isotropic-to-Smectic A Transition in Ionic Liquid Crystals from Coarse-Grained Molecular Dynamics Simulations: the Role of Microphase Segregation Giacomo Saielli,a* Alessandro Bagno,b Yanting Wangc a

CNR Institute on Membrane Techonlogy, Unit of Padova, Via Marzolo, 1 – 35131, Italy

b

c

Department of Chemical Sciences, University of Padova, Via Marzolo, 1 – 35131 Padova, Italy

State Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy

of Sciences, 55 East Zhonggunacun Rd., P.O. Box 2735, Beijing, 100190, China KEYWORDS Ionic Liquids, Liquid Crystals, Micro-segregation, Heterogeneity Order Parameter.

ABSTRACT We have investigated the role of microphase segregation as the driving force in the stabilization of thermotropic ionic liquid crystals of smectic type. To this end we have applied the heterogeneity order parameter, initially proposed for ionic liquids, to the coarse-grained molecular dynamics simulation results for a model system of an imidazolium nitrate ionic liquid

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crystal, [C16mim][NO3], whose phase diagram was recently studied. We have found that the heterogeneity order parameters become larger when the system goes through the transition from the isotropic phase to the smectic A phase as the temperature decreases. This can be understood by considering that, in the smectic A phase, the layered structure allows the tail groups to have a larger degree of aggregation than in the isotropic phase. Our results highlight the role of microsegregation in the stabilization of ionic liquid crystals, which cannot be revealed by the commonly used translational and orientational order parameters used to describe liquid crystal phases.

1. Introduction Ionic liquid crystals (ILCs) are attracting the interest of the chemistry and materials science community since they combine the anisotropy of liquid crystalline phases (LCs) with the peculiar solvation and conductive properties of ionic liquids (ILs).1-3 Imparting a long-range order to ionic liquids could be beneficial in many cases and proof of principle applications based on ILCs have already been reported concerning batteries,4, 5 solar cells,6 electrochemical sensors7, 8 membranes for water desalination,9 and electrofluorescence switches.10 ILCs are usually obtained from the same kind of quaternary ammonium salts that form ILs with the additional requirement that the alkyl chain(s) has to be sufficiently long to induce microphase segregation. This occurs, for example for imidazolium salts, for alkyl chains of at least 12 carbon atoms.11, 12 Similarly, silver alkanoates exhibit several smectic phases for chains longer than ten carbon atoms.13 Therefore, ILCs are almost invariably of smectic type since microphase


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segregation induces a layered structure, similarly to what have been observed in lamellar phases of water-surfactant mixtures.14 Interestingly, microphase segregation has also been reported for the isotropic phase of short chain ILs, first from computer simulations15-17 then confirmed experimentally,18 and the degree of segregation in the isotropic phase increases as the length of the alkyl chain is increased. Clearly, the difference between the isotropic phase and the LC phase of a long-chain imidazolium salt resides in the long-range order and symmetry of the phase: no long-range order and spherical symmetry for the isotropic phase; long-range orientational and translational order and cylindrical symmetry for the smectic LC phase. The question arises, however, about what is the difference, if any, between the microscopic structure and microphase segregation that is present in the isotropic and in the smectic phase. In other words, if microphase segregation is already present in the high temperature isotropic phase of a long-chain imidazolium salt, what is exactly the driving force that is responsible of the formation of a smectic A phase rather than a crystal phase, on cooling down the temperature from the isotropic melt? Moreover, microphase segregation is usually invoked to explain the instability of ionic nematic phases. As of now, very few reports concerning ionic nematic phases have appeared19-22 and a classical DFT description of the phase diagram of model ILCs have highlighted the narrow range of existence of the ionic nematic phase compared to the ionic smectic phase.23 However, as mentioned above, microphase segregation is already present in the isotropic phase, and thus it is not incompatible with a translationally disordered phase. Therefore, is microphase segregation really the reason for the instability of the ionic nematic phase?


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Recently we have investigated the structure and dynamics of a model ILC ([C16mim][NO3]) by means of molecular dynamics simulation using a coarse-grained Force Field.24, 25 The CGFF was initially developed by Voth and Wang for the isotropic phase of short-chain imidazolium salts.26, 27

However, after a proper extension to longer alkyl chains, the model potential has demonstrated

the capability to model the transition from the crystal to the smectic A (SmA) phase and from the SmA phase into the isotropic (Iso) phase.24 The corresponding Iso-to-SmA and SmA-to-crystal (Cr) transition have been observed on cooling the system. The structural and dynamic properties have been extensively investigated and the effect of varying the chain length has also been analysed.28 The CGFF has been capable of reproducing the qualitative trends observed experimentally, including the appearance of a smectic phase for chains of at least twelve carbons.28 Although atomistic computer simulations have been successfully used recently to investigate an ionic liquid crystal,29 CGFF models still represent the only choice if one wishes to study the full phase diagram or if he/she needs to get qualitative information as several structural parameters are varied. In this paper, we extend our previous computational studies in order to provide some insights concerning the role of microphase segregation in driving the Iso-to-SmA transition and to identify the mechanism for the microscopic structural changes occurring when the isotropic melt is cooled down. To this end we will compare the behaviour of the commonly used order parameters describing the order and structure of LC phases, the orientational order parameter and the translational order parameter ̅ , with the behaviour at the transition of the heterogeneity order parameter (HOP), or , introduced by Voth and Wang in Refs. 30, 31 to describe the degree of micro-segregation, at the nanoscale level, in the isotropic phase of ionic liquids. 2. Results and Discussion


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The technical details of the simulations have already been discussed in Refs. 24, 25 We briefly summarise them here for the sake of clarity. The model system of 1-hexadecyl-3methylimidazolium nitrate, [C16mim][NO3], is composed of 19 different sites after coarsegraining of the imidazolium ring, the methyl and methylene groups and the anion into single sites, see Figure 1. Details of the coarse-graining procedure can be found in the original papers.26, 27

Partial charges are present on the imidazolium ring site, the 3-methyl group, the first four

methylene sites bound to nitrogen 1 and the anion, while the remaining carbon groups of the hexadecyl chain bear no charge. Simulations for 512 ion pairs were run in the NPT ensemble using the DL_POLY software.32 For each temperature a trajectory file containing 1000 configurations separated by 27 ps was generated after several nanoseconds of equilibration (between 50 and 300 depending on the temperature). The trajectory file was used for the calculation of the average properties. Structures and configurations are visualized with VMD software.33

Figure 1. (Top) Structural formula of [C16mim][NO3] and (bottom) schematic representation of the Coarse-Grained Force Field (CGFF) model with some of the sites labelled. Sites with nonzero charge are also indicated.


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In addition to the equilibrated trajectories at various temperatures, we have two sets of data corresponding to phase transitions: the first is the trajectory for the transition from the isotropic phase into the SmA phase obtained by starting from the final configuration at 575 K (isotropic) and changing the temperature to 505 K, where a stable SmA phase is expected (and in fact observed during the heating run).24 The second is the trajectory corresponding to the transition from the SmA phase into the crystal phase which has been obtained by starting from an equilibrium configuration at 505 K (SmA phase) and changing the temperature to 480 K, where a stable crystal phase is observed during the heating run. Although the phase structure has been discussed in Ref. 24, it is useful to recall here the main results and to define the order parameters used to characterise the phases. For the sake of clarity a snapshot of each of the three phases is shown in Figure 2 with the intent to visually highlight the order, or absence thereof, along the director (z axis) as well as in the xy plane perpendicular to the director.

Figure 2. Snapshots of the equilibrated boxes obtained in cascade during the heating run. (a) The crystal phase at 450 K, side view; (b) the same box as in (a), top view; (c) the SmA phase at 505 K; (d) the isotropic phase at 600 K.


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Despite the orthorhombic symmetry of the simulation box and periodic boundary conditions, the alkyl chains in the crystal phase are slightly tilted with respect to the director, in agreement with the fact that the true structure of long-chain imidazolium salts is usually triclinic.34 However, since our interest is mostly focussed on the smectic-isotropic transition, we did not consider the possibility of triclinic periodic boundary conditions. As we can see in Figure 2 a) chains are very well ordered and packed in a hexagonal fashion, which can be better appreciated by the inspection of Figure 2 b). The top layer in Figure 2 a) does not clearly show an ordered arrangement of chains only because the two hexagonal structures are rotated, one with respect to the other. Tilting the view by 60° does show a crystal arrangement of alkyl chains in the top layer as well. In the SmA phase shown in Figure 2 c) all order is lost except for a density modulation along the z axis (the director). Hydrophobic layers of alkyl chains are alternating with ionic layers composed of the cation heads and anions. However, no translational order is present within the layers as expected for a fluid SmA phase. Finally, the isotropic phase at 600 K (Figure 2 d)) shows the complete lack of long-range orders, both orientational and translational, although microphase segregation can be observed at a nanoscale level, as well documented for ionic liquids. To describe more rigorously the structure of the three phases we first introduce the following two macroscopic order parameters widely used for characterising LC structures. The orientational order parameter, P2 = P2 (cos β ) , is defined as the ensemble average of the second Legendre polynomial of cosβ, where β is the angle between the long axis of the molecule and the director. The long axis of the molecule is defined as the vector joining the cation head, site A, with the cation tail, site E. The translational order parameter, τ1 = cos(2πz / d ) , is the ensemble average of a cosine function having a full cycle within the distance of a layer, d. At the isotropic-


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to-smectic A phase transition both order parameters jump from zero to a finite value, as expected for a first-order transition involving a change in orientational and translational order; however, because of the finite size effect, the values in the isotropic phase are not exactly zero. The low value of the orientational order parameter in the SmA phase is due to the high flexibility of the molecule and the lack of a rigid anisotropic core. An additional jump in the order parameters is observed at the SmA-to-Cr transition: there, being the alkyl chains mostly in the all-trans conformation, the parameter reaches values typical of highly ordered phases. This analysis has been reported already in Ref.

24

In Table 1 we show the values of the macroscopic order

parameters in the three phases.

Table 1. Macroscopic order parameters for the three phases of the CGFF model [C16mim][NO3]. From Ref. 24 Cr 450 K

SmA 505 K Iso 600 K

0.88 ± 0.004 0.24 ± 0.04

0.06 ± 0.02

0.72 ± 0.004 0.44 ± 0.04

0.04 ± 0.02

In addition to the numerical values of the order parameters, various radial pair distribution functions (RDF), decomposed into their components parallel and perpendicular to the director, complete the description of the order of each phase. These functions have been fully described in Ref.

24

and they will not be repeated here. They confirm the absence of long-range order in the

isotropic phase, the presence of just a density modulation parallel to the director in the smectic phase (that is a layered structure with alternating hydrophobic and ionic layers with liquid-like structure within each layer) and a 3-dimensionally ordered phase for the crystal.


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6 A-A

E-E

g(r)

4

2

0 6 A-D

C10-C10

g(r)

4

2

0 6 E-D

D-D

4 g(r)

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2

0

5

10

15

5 r/Å

10

15

20 r/Å

Figure 3. Radial distribution functions, g(r), of some sites (see Figure 1 for labels) in the three phases: (black) Cr at 450 K; (red) SmA at 505 K; (blue) Iso at 600 K.

Nevertheless, in view of the analysis based on the HOP parameter and its relationship with the RDF, it is useful to discuss the g(r) of some CG sites in the three phases. These are shown in Figure 3. The structure of the isotropic phase is the typical structure of an ionic liquid with an alternation between cations and anions, as indicated by the peaks in the A-D g(r) which approximately correspond to minima in the A-A g(r) and D-D g(r). The tail-tail g(r), E-E, is qualitatively similar in the isotropic and smectic phase; however the peak intensity is clearly higher in the SmA phase indicating an increased aggregation. On the other hand, in the isotropic and smectic phases the correlation between C10 sites (roughly in the middle of the alkyl chain) is


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very similar and very low. In contrast, the C10-C10 g(r) in the crystal phase indicates a high order and a high correlation which is enhanced by the close proximity of C10 sites even if belonging to chains with opposite orientation. This is confirmed by the fact that the intensity of the first peak drops from 6.2 for the C10-C10 g(r) to 5.0 for the C9-C9 and C11-C11 g(r) and down to 2.8 for C8-C8 and C12-C12 g(r), still in the crystal phase (data not shown). As we have already mentioned, the long-range macroscopic order parameters do not contain any information about the microscopic structure of the nanosegregated domains usually found in ionic fluid systems. Moreover, the RDFs give many insights on the structure but it is not easy to use them to quantify the degree of microphase segregation because they are, in fact, functions of the distance, so, in practice, they consist in an array of numbers. To better quantify this microscopic structure and how it changes during the transition, we conveniently adopt the HOP defined in Refs. 30, 31 as: 1 h = N

 rij2  ∑i ∑j exp − 2σ 2   

(1)

which replaces a function with a single value. Here, rij is the distance, corrected for periodic boundary conditions, between two identical sites on different molecules, while σ is a scale length parameter given by (1/ρ)1/3, with ρ being the density of sites under consideration. A detailed derivation of the relation between the HOP parameter and the radial distribution function can be found in Ref. 31. In short, measures the deviation of a given site distribution from a perfectly uniform distribution, for which the theoretical value is predicted (and verified by simulations) to be 15.75. Values of higher than this indicate the emergence of microphase segregation.


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SmA

Cr

Iso

24

E A D C10

22 20

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


18 16 14 400

450

500

550

600

T/K

Figure 4. Heterogeneity Order Parameter (HOP) for some sites of the CGFF model as a function of the temperature. Where error bars are not visible, they are smaller than the size of the markers. In contrast to the orientational and translational order parameters, which are long-ranged and measure a single-molecule property averaged over the whole phase, the HOP parameter gives a description of the local structure on a scale length given by the σ parameter. This, for low molecular weight systems in condensed phases, is in the scale of nanometers. In Figure 4 we show the HOP values for some of the sites of the CGFF model as a function of the temperature obtained from the production runs of the equilibrated boxes. As shown in Ref. 24 the system undergoes a first-order transition from the isotropic to the SmA phase around 560 K and a transition from the SmA to a crystal phase around 500 K. The parameter for cation head and anion follow a very close trend, both increasing as the temperature is lowered. Although we note a discontinuity at the transition temperatures, the changes may simply reflect an increased order of the ionic part as the temperature is lowered. A more interesting behaviour is shown by the parameter of the tail, site E. In this case the parameter value first


11


increases on going from the isotropic (where it is around 18.5) to the SmA phase, with a clear jump at 550 K, reaching a maximum of about 21.5 in the smectic phase at 505 K, just before the transition to the crystal phase. However, in the crystal phase the HOP drops down significantly to values around 16.5, not too far from to the value expected for a uniform distribution. Finally, we also show the value for the carbon in the middle of the alkyl chain (C10): this is rather large in the crystal phase, though it hardly changes from the isotropic to the SmA phase remaining below 18. It is noteworthy that the trend shown in Figure 3 shares some qualitative features with the trend observed in Ref. 28 as the chain length was varied at a fixed temperature. There the emergence of a smectic phase for imidazolium based ILs was correctly predicted to occur for alkyl chains of 12 carbon atoms as found experimentally for imidazolium salts.11, 12 Therefore, the parameter , particularly that one of the tail, site E, appears very useful to describe the phase transitions of the model system and, complementary to the macroscopic orientational and translational order parameters, it contains useful information on the microscopic structure and microphase segregation of the system at the nanoscale level. E A D C10

24

22

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20

18

16 0

20

40

60

80

100

t (ns)


12

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Figure 5. Heterogeneity Order Parameter (HOP, ) for some sites of the CGFF model as a function of the time during the transition from the isotropic phase into the smectic A phase. In order to get more insights on the Iso-to-SmA transition itself we will now analyze in detail the changes in the HOP as the system undergoes such a transition. In Figure 5 we show the trend of for the same sites as in Figure 4 but now as a function of time: the starting configuration is the equilibrium configuration at 575 K, which is in the isotropic phase. At time t = 0 the temperature is changed at 505 K, at which the smectic phase is the stable phase. After about 30 ns the layers are almost fully developed and the system has undergone a phase transition. The transition can be nicely traced by following the evolution of the parameter, in particular that of the tails: starting with a low value around 18.5, corresponding to an isotropic phase, it rises up to around 22.5 in the smectic phase.

Figure 6. Snapshots of the simulation box during the transition from the isotropic to the SmA phase taken after 0, 48 and 72 ns with respect to the time when the temperature was set at 505 K. In Figure 6 three snapshots are shown at different times during the simulation of the Iso-toSmA transition. The development of a layered structure with time is clearly visible. We note that the starting isotropic box was obtained, by increasing the temperature, from a layered SmA structure with the layers parallel to the box axis, see Ref. 24, while here the layers have reformed


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with a different orientation because the isotropic phase has lost memory of its previous state. Small differences in the layer thickness, due to the periodic boundary conditions, might be responsible for the slightly different value of the parameter for the two SmA boxes at the same temperature, that is ca. 22.5 in Fig. 5 vs ca. 21.5 in Figure 4. Although our main interest is in the Iso-to-SmA transition, also because of the limitations due to the symmetry of the box that does not allow a proper description of the triclinic structure of the crystal phase, it is useful to discuss also the behavior of the parameter at the SmA-to-Cr transition of the CGFF model. In Figure 7 we show the values for the same sites discussed above during a 27-ns run obtained starting from the box in the SmA phase obtained at 505 K after setting the temperature to 480 K, where we expect a crystal phase. The system indeed undergoes a transition to the crystal phase during the first 6 ns of simulation after which it remains in a steady state. The alkyl chain values show the most significant changes: a sharp decrease for the tails and a sharp increase for the middle site C10. E A D C10

24

22

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20

18

16 0

5

10

15

20

25

30

t (ns)

Figure 7. Heterogeneity Order Parameter (HOP) for some sites of the CGFF model as a function of the time during the transition from the smectic A phase into the crystal phase.


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In Figure 8 we show three snapshots of the simulation box highlighting the packing of the alkyl chains as the crystal phase is formed. In the SmA phase, although the long-range order exists along the director, in the other two disordered dimensions, the cationic alkyl tails still aggregate to form nanoscale tail domains. Since now the tail groups combine both heterogeneities coming from layering and tail aggregation, the HOP value for site E is very high (around 21). The charged groups (sites A and D) also take a relatively high HOP value around 19.5 since they are now layered but still retain a continuous polar network by having a small amount of charged groups connecting adjacent layers.28 The C10 groups take a relatively low HOP value of 17 as a compromise between the intramolecular constraints and the intermolecular interactions from both charged and tail groups.

Figure 8. Snapshots of the simulation box during the transition from the SmA to the crystal phase taken after 0, 5, and 20 ns with respect to the time when the temperature was set at 480 K.

In contrast, in the crystal phase the tail groups no longer aggregate, so they only have the heterogeneity coming from layering, resulting in a drop down of their HOP to a moderate value of 17. The charged groups can no longer distribute between layers, so their HOP values increase to between 20 and 21. The HOP values for charged groups are higher than the value for tail


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groups, indicating that the charged groups are more ordered due to the relatively strong electrostatic interactions between them, while the tail groups in the crystal phase are not arranged so tightly as they are in the SmA and isotropic liquid phases when they aggregate to form tail domains. Because of the long-range order of cationic side chains in the crystal phase, the C10 groups are now perfectly aligned in layers to have a very high HOP value of 22. These microscopic mechanisms are schematically illustrated below.

Figure 9. Cluster of chains (only C5-C16 carbons for clarity) extracted from the last configuration of the crystal phase at 450 K. Top panels: view along the director; bottom panels: in-plane view. From left to right: alkyl chains; E sites highlighted; C10 sites highlighted.

In Figure 9 we show the arrangement of a small cluster of first neighbor molecules belonging to one layer of the crystal phase at 450 K (final configuration). The Figure highlights the hexagonal arrangement of the alkyl chains in the crystal phase, particularly clear in the top view on the left. The other panels show the same cluster with the tail (E site) in the central panels and the C10 sites on the right panels highlighted in light blue. Inspection of Figure 9 reveals why the HOP parameter of the tails is going down in the crystal phase while that one of the central carbon atom C10, is going up: imidazolium cations in the crystal phase are randomly oriented,


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concerning the orientation (pointing “up” or “down”) of the head-to-tail vector, similarly to the structure of the SmA phase, which is non polar; therefore, sites E belonging to two adjacent and aligned cations are not necessarily close to each other. In fact they are separated by a large distance if the two cations have opposite orientations (one pointing up, the other pointing down). In contrast, C10 sites, being in the middle of the alkyl chain, are always in close contact, irrespective of the orientation of the cation. If pairs of same sites are not sufficiently close then, based on Eq. (1) they will not give a contribution to the HOP. It is therefore noteworthy that the value of for crystal phases, where tridimensional order is present, is also influenced by the symmetry and periodicity of the crystal lattice itself, rather than simply describing the degree of homogeneity of the system.

Figure 10. Pictorial representation of the structure of the nano-domains of the alkyl chains in the (from left to right) isotropic, smetic A, and crystal phases. See text for a discussion. A pictorial representation of the change in the microscopic structure, which can explain the trend observed in the parameter, is shown in Figure 10. The relatively low values of for the tail groups in the crystal phase, on the one hand, can be easily understood as a result of a strong interdigitation of the alkyl chains. X-ray scattering experiments clearly demonstrate that the layer thickness for imidazolium-based ILCs is shorter than the full length of the molecule,


17


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thus suggesting an interdigitated structure.35

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The picture in Figure 10 does not intend to

represent the real crystal structure of imidazolium salts, rather it is aimed at highlighting the relatively large separation, poor aggregation and uniform distribution (being a periodic lattice) of the tails in the crystal structure. On the other hand, the value of for the tails in the isotropic phase can be explained as a result of roughly spherical aggregates of chains due to the wellknown nanosegregation effect.15-18 However, tails cannot be all segregated in the center of the roughly spherical aggregate, since this would amount to large voids in the structure and unphysical overlap of particles. Therefore, together with tails (sites E) in close contact, in the isotropic phase there must be several “non-segregated” tails as well. In contrast, the layered structure of the SmA phase, which is at the same time highly disordered within each hydrophobic layer, is the only one that allows the largest degree of close contacts, on average, between tails in the intermediate region of the hydrophobic layer. Since such close contacts are exactly what is measured by the heterogeneity order parameter it is not surprising that the values increases in the smectic phase. We would also like to try to give an answer to the other question we posed in the Introduction: is microphase segregation responsible for the instability of ionic nematics? Based on the discussion above the answer is, at least in part, yes. As we have seen, a significant change in the HOP parameter at the isotropic-to-smectic transition suggests that microphase segregation is increased in the layered structure of the smectic phase. Therefore, for a system with the potential of forming a fluid LC phase, but where microphase segregation plays an important role, we expect that on cooling the temperature from the isotropic liquid the system undergoes a transition to a phase where microphase segregation is increased, as in the SmA phase. Such a contribution


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of increased microsegregation coming from layering, would be absent in the nematic phase where there are no layers. This single observation would not exclude, in principle, the possibility of a nematic phase (in fact, as mentioned in the Introduction, very few examples exist) but we need also to consider that, in contrast to normal non-ionic LCs, which usually have a rigid anisotropic core responsible for the appearance of a nematic phase in a suitable temperature range, common quaternary ammonium salts with long alkyl chains lack such nematogenic rigid part, being composed of a highly flexible alkyl chain and a relatively small and not very anisotropic rigid core. Inspection of the schematic arrangements pictured in Figure 10 seems to suggest that a nematic phase would not be able to have a relatively high value of the HOP parameter for the alkyl tails, since the phase lacks both the possibility to have spherical-like aggregates as in the isotropic phase (because molecules in the nematic phase should be on average oriented) as well as the possibility to be arranged in layers, because then it would be a smectic phase.

3. Conclusions By means of coarse-grained MD simulations of a model system exhibiting an ionic liquid crystal phase we have presented a detailed microscopic view of the isotropic-to-smectic A transition. The role of microphase segregation in driving the formation of ionic liquid crystal phases has been highlighted and discussed after monitoring the changes in several heterogeneity order parameters. Despite giving no account of the free energy changes at the transitions, such structural information provides very useful insights on the differences in the microscopic organization of the hydrophobic and ionic parts in the isotropic liquid and LC phases.


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We can, therefore, say that the liquid-like structures of the hydrocarbon chains of the imidazolium ionic liquids, by arranging themselves into a bidimensional layers are indeed allowed to microsegregate more efficiently, at the nanoscale level, from the ionic parts. This can only be done at the expense of reducing the spherical symmetry of the isotropic phase to form a long-ranged translationally ordered smectic phase. The reason why this happens for chains of at least 10-12 carbon atoms (it slightly depends on the anion type) has been investigated in Ref. 28: there we observed a clear increase in the van der Waals (VDW) interaction energy as a function of the chain length for simulations. With a cationic side chain of 12 carbons or shorter, the electrostatic interactions between charged groups dominate, so the nonpolar tail groups are passively “pushed” together to form aggregated tail domains. When the chain is longer, the effective VDW interactions between chains overcome the electrostatic interactions, so the chains are aligned to form ILC structures. Therefore, the effect of side-chain length on IL structures reported there seems to share the same mechanisms as the effect of temperature analyzed in this paper. A two-dimensional phase diagram with respect to both temperature and chain length would provide a unified and complete microscopic physical picture for the role of microphase segregation as a driving force in the IL structural changes. We leave the possibility of conducting this interesting but time-consuming study open for future investigations. Finally, following the analysis of the HOP values, we have qualitatively discussed the stability of ionic nematic phases: we have concluded that, for long chain imidazolium salts and structurally related systems, there is neither a reason in the structure and shape of the molecule nor in the structure and aggregation of the phase which push the system to undergo a transition, on cooling, to an ionic nematic phase if an ionic smectic phase is allowed instead.


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This is not surprising since it is also well known that micro-segregation between the alkyl chains and the aromatic cores (a much weaker micro-phase segregation effect) in the typical example of cyanobiphenyls (non-ionic LC systems) is responsible for the appearance of the smectic phase for chains longer than 7 carbon atoms and suppression of the nematic phase for alkyl chains longer than 9 carbon atoms.36 Therefore, stabilization of ionic nematic phases will require a very subtle balance between different and competing effects.

AUTHOR INFORMATION

Corresponding Author *Email: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT Computer time was granted by CINECA (ISCRA projects CGILC_HP10BB7HYH and SMACG_HP10BZQZGE), and the Laboratorio Interdipartimentale di Chimica Computazionale (LICC) of the University of Padova. This work was supported by the Italian National Research Council (CNR) and Chinese Academy of Sciences (CAS) through a bilateral agreement 20142016. We also thank MIUR (PRIN 2010N3T9M4, FIRB RBAP11C58T), Fondazione


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CARIPARO (Progetti di Eccellenza NanoMode 2010), and National Natural Science Foundation of China (Nos. 11274319 and 11121403) for financial support. Snapshots are created with VMD. VMD is developed with NIH support by the Theoretical and Computational Biophysics group at the Beckman Institute, University of Illinois at Urbana-Champaign. ABBREVIATIONS CGFF: Coarse-Grained Force Field; DFT: Density Functional Theory; HOP: Heterogeneity Order Parameter; IL: Ionic Liquid; ILC: Ionic Liquid Crystal; LC: Liquid Crystal; VDW: van der Waals REFERENCES (1) Binnemans, K. Ionic Liquid Crystals. Chem. Rev. 2005, 105, 4148-4204. (2) Axenov, K. V.; Laschat, S. Thermotropic Ionic Liquid Crystals. Materials 2011, 4, 206-259. (3) Causin, V.; Saielli, G. In Ionic Liquid Crystals; Mohammad, A., Inamuddin, D., Eds.; Green Solvents II. Properties and Applications of Ionic Liquids; Springer: UK, 2012; pp 79-118. (4) Yoshio, M.; Kagata, T.; Hoshino, K.; Mukai, T.; Ohno, H.; Kato, T. One-Dimensional IonConductive Polymer Films: Alignment and Fixation of Ionic Channels Formed by SelfOrganization of Polymerizable Columnar Liquid Crystals. J. Am. Chem. Soc. 2006, 128, 5570-5577. (5) Judeinstein, P.; Huet, S.; Lesot, P. Multiscale NMR Investigation of Mesogenic Ionic-Liquid Electrolytes with Strong Anisotropic Orientational and Diffusional Behaviour. RSC Adv.

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Insert Table of Contents Graphic and Synopsis Here

Although microphase segregation is present both in the isotropic as well as in the smectic phase of long-chain imidazolium salts, its increased magnitude is an important contribution to the stabilization of the low-temperature ordered phase.


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Dynamics of water confined in lyotropic liquid crystals: Molecular dynamics simulations of the dynamic structure factor.

alkali salt mixtures interface.

Insights into the role of Asp79(2.50) in β2 adrenergic receptor activation from molecular dynamics simulations.

Molecular dynamics simulations of the protonated G4 PAMAM dendrimer in an ionic liquid system.

Self-assembly and mesophase formation in a non-ionic chromonic liquid crystal system: insights from dissipative particle dynamics simulations.

The liquid surface of chiral ionic liquids as seen from molecular dynamics simulations combined with intrinsic analysis.

Rigidity and soft percolation in the glass transition of an atomistic model of ionic liquid, 1-ethyl-3-methyl imidazolium nitrate, from molecular dynamics simulations--Existence of infinite overlapping networks in a fragile ionic liquid.

Charge templates in aromatic plus ionic liquid systems revisited: NMR experiments and molecular dynamics simulations.

The role of Cys179-Cys214 disulfide bond in the stability and folding of prion protein: insights from molecular dynamics simulations.

The role of non-native interactions in the folding of knotted proteins: insights from molecular dynamics simulations.

Thermotropic Ionic Liquid Crystals.

Replica-exchange molecular dynamics simulations of cellulose solvated in water and in the ionic liquid 1-butyl-3-methylimidazolium chloride.

A Comparative Study for Molecular Dynamics Simulations of Liquid Benzene.

lithium salt solutions: insights from implicit and explicit solvent models and molecular dynamics simulations.

The Integrin Receptor in Biologically Relevant Bilayers: Insights from Molecular Dynamics Simulations.

Understanding the thermostability and activity of Bacillus subtilis lipase mutants: insights from molecular dynamics simulations.

DNA Polymerase Conformational Dynamics and the Role of Fidelity-Conferring Residues: Insights from Computational Simulations.

Solubility of gases in a common ionic liquid from molecular dynamics based free energy calculations.

Key Developments in Ionic Liquid Crystals.

Mean ionic activity coefficients in aqueous NaCl solutions from molecular dynamics simulations.

Impact of ionic liquids in aqueous solution on bacterial plasma membranes studied with molecular dynamics simulations.

Comparison of three ionic liquid-tolerant cellulases by molecular dynamics.

Insights into molecular interactions between CaM and its inhibitors from molecular dynamics simulations and experimental data.

Temperature effects on the capacitance of an imidazolium-based ionic liquid on a graphite electrode: a molecular dynamics simulation.