low-Q Peak Research Articles

Are NaTFSI and NaFSI Salt-Based Water-in-Salt Electrolytes Structurally Similar or Different?

Water-in-salt electrolytes (WiSEs) are a promising class of electrolytes due to their wide electrochemical stability window and nonflammability. In this study, we explore the structural organization of sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) and sodium bis(fluorosulfonyl)imide (NaFSI) salt-based aqueous electrolytes, covering dilute to highly concentrated regions, by employing an all-atom molecular dynamics simulation. For the NaTFSI-based electrolyte, we observe that Na+ ions are mostly surrounded by water molecules at all the salt concentrations due to the very strong interaction between them. While TFSI anions weakly coordinate with Na+ ions and other TFSI anions, they also mostly prefer to be surrounded by water molecules. These interactions were found to have moderate dependence on the concentration of the NaTFSI salt. For the NaFSI-based electrolyte, while the Na+-water interaction is stronger at lower salt concentrations, the number of nearest neighbor FSI anions is found to be more than that of water at higher concentrations (≥20 m). This is because the increase in the salt concentration leads to expulsion of water molecules from the solvation shell of Na+ ions and enhances the interaction between Na+ ions and oxygen atoms of FSI. At the highest salt concentration (solubility limit), the bulk-like water structure is completely disrupted and dominated by an anionic network in the FSI-based electrolyte. In contrast, water-water hydrogen bonding network is still present even in the highly concentrated TFSI-based electrolyte. The simulated X-ray scattering pattern displays a low-q peak, revealing the presence of an intermediate range ordering due to alternating anion-rich and water/Na+-rich regions in both the electrolytes. However, the characteristic length scale corresponding to the low-q peak decreases with increasing the salt content in both the electrolytes.

Backbone and Side Group Interchain Correlations Govern Wide-Angle X-ray Scattering of Poly(3-hexylthiophene).

Identifying the origin of scattering from polymer materials is crucial to infer structural features that can relate to functional properties. Here, we use our recently developed virtual-site coarse graining to accelerate atomistic simulations and show how various molecular features govern wide-angle X-ray scattering from a conjugated polymer, poly(3-hexylthiophene) (P3HT). The efficient molecular dynamics simulations can represent the structure and capture the emergence of crystalline order from amorphous melts upon cooling while retaining atomistic details of chain configurations. The scattering extracted from simulations shows good agreement with wide-angle X-ray scattering experiments. Amorphous P3HT exhibits broad scattering peaks: a high-q peak from interchain side-group correlations and a low-q peak from interchain backbone-backbone correlations. During amorphous to crystalline phase transitions, the distance between backbones along the side-group direction increases because of lack of interdigitation in the crystalline phase. Scattering from π-π stacking emerges only after crystallization takes place. Intrachain correlations contribute negligibly to the scattering from the amorphous and crystalline phases.

Structure of sodium silicate water glass—X-ray scattering experiments and force-field molecular dynamics simulations

Sodium silicate water glass is used in various applications as adhesives and lubricants. Herein, we used X-ray scattering experiments and force-field molecular dynamics (MD) simulations to investigate the structure of sodium silicate water glass. The concentrated sodium water glass had a low-Q pre-peak; in contrast, the diluted glass did not, as observed from X-ray scattering experiments. The intensity of the low-Q peak decreased with an increase in pressure. MD simulations provided a structure of sodium silicate water glass with sodium clusters surrounded by hydrated silicate anions at low pressures. These sodium clusters were the origin of the low-Q peaks in the simulated patterns. The sodium clusters dissolved in water with an increase in pressure, and consequently, the intensity of the low-Q peak drastically decreased.

Do Deep Eutectic Solvents Form Uniform Mixtures Beyond Molecular Microheterogeneities?

We have performed small-angle neutron scattering in a momentum transfer range (0.05 < Q < 0.5 Å-1) to study long-range order and concentration fluctuations in deep eutectic solvents (DESs) and their aqueous solutions. Ethaline (choline chloride/ethylene glycol), glycerol/lactic acid, and menthol/decanoic acid mixtures were selected to illustrate individually the case of ionic, nonionic, and hydrophobic mixtures. Carefully designed isotopic labeling was used to emphasize selectively the spatial correlations between the different solvent components. For ethaline DESs and their aqueous solutions, a weak low-Q peak observed only for certain compositions and some partial structure factors revealed the mesoscopic segregation of ethylene glycol molecules that do not participate in the solvation of ionic units, either because they are in excess with respect to the eutectic stoichiometry (1:4 neat ethaline) or substituted by water (4w-ethaline and higher aqueous dilutions). For the nonionic hydrophilic solutions, such a mesoscopic segregation was not observed. This indicates that the better balanced interactions between the three nonionic H-bonded components (water, lactic acid, and glycerol) favor homogeneous mixing. For the hydrophobic DESs, we observed an excess of coherent scattering intensity centered at Q = 0, which could be reproduced by a model of noninteracting spherical domains. Local concentration fluctuations are not excluded either. However, unlike liquid mixtures with a tendency to demix, we have found no evidence of expansion of domains with different compositions to a large scale.

Microscopic insights into the phase transition of poly(N-isopropylacrylamide) in aqueous media: Effects of molecular weight and polymer concentration

We study a temperature (T)-induced phase transition of semi-dilute poly(N-isopropylacrylamide) (pNIPAm) in H2O and D2O, focusing on effects of molecular weight and polymer concentration. We use simultaneous small- and wide-angle x-ray scattering (SWAXS) and dielectric relaxation spectroscopy (DRS). The hydration number Zhyd of pNIPAm is determined by DRS. We observe a gradual decrease of Zhyd with increasing T below TS − 6 K, where TS is the spinodal temperature, and its subsequent steep decrease on approaching TS. This eccentric two-mode dehydration behavior implies strong cooperative nature of hydration/dehydration of the pNIPAm chains. Furthermore, the observed dehydration is coupled to the critical fluctuations manifested by a power-law divergence of the correlation length ξ. The emergence of a low-q interference peak provides evidence that high-density microglobules are formed even far below Ts, where a swollen coil state of polymer chains is generally considered to be predominant. The local number density of the microglobules is almost unchanged below Ts, but then exhibits a step-like increase just above Ts, where most pNIPAm chains are already collapsed. This observation suggests that in addition to the ordinary (macroscopic) density order parameter, the local number density of the microglobules should serve as an additional order parameter monitoring the phase transition of aqueous pNIPAm. The correlation length significantly decreases at higher polymer concentration because of the stronger interpenetration effect between different chains. At higher concentration, ξ converges to similar values independent of molecular weight, which indicates an already congested state of the pNIPAm chains. Lowering averaged molecular weight leads to a stronger tendency of inter-chain aggregation and interface formation especially at high T.

Medium range interactions evidences in compounds with aliphatic lateral chain: 1-pentanoic acid, 1-pentanol and pentylammonium nitrate as test cases

In this study we have explored by means of polarizable molecular dynamics simulations and small-angle X-ray diffraction, a subset of liquids belonging to the following three different classes of compounds: pentyl ammonium nitrate, 1-pentanol and 1-pentanoic acid. The presence of a low-Q peak in the X-ray spectra of the liquids provides evidence that a long alkyl chain is connected to the polar moiety and it is sufficient to establish nanodomain segregation in the liquid. From the calculations, we have obtained the theoretical scattering patterns that reproduce very nicely the experimental spectra in the low-Q ranges. From detailed analyses of the radial distribution functions, we infer that the medium-range ordering in these liquids stems from the joined effects of both strong and weak interactions.

Coupled hydroxyl and ether functionalisation in EAN derivatives: the effect of hydrogen bond donor/acceptor groups on the structural heterogeneity studied with X-ray diffractions and fixed charge/polarizable simulations.

We present a study by energy-dispersive X-ray diffraction of liquid 2-(2-hydroxyethoxy)ethan-1-ammonium nitrate, NH3CH2CH2(OCH2CH2OH)+NO3- (22HHEAN). This ionic liquid is derived from the parent ethylammonium nitrate (EAN) with an ether link in the chain and a hydroxyl group in the terminal position. The absence of peaks at low-q values in the experimental diffraction curve indicates that the added polar groups and the high conformational isomerism of the cations alter strongly the nanosegregation of the parent EAN liquid. Aggregation between ionic species may involve hydrogen bonding between cations and anions and a variety of intermolecular hydrogen bonds between cations. Diffraction patterns are compared with the results of molecular dynamics simulations with two different force fields: the fixed point charge force field (GAFF) with different charge scaling protocols and the polarizable AMOEBA force field. Most point charge models lead to the appearance of a quite evident low q-peak which decreases gradually, when the percentage and type of the scaling (uniform vs. non-uniform) are increased. In the polarisable model and in the model where only anion charges are scaled to 20%, instead, the pre-peak is absent in agreement with our experiments.

Reversible Lamellar Periodic Structures Induced by Sequential Crystallization/Melting in PBS-co-PCL Multiblock Copolymer

Reversible periodic structures in a symmetric poly(butylene succinate)-co-poly(e-caprolactone) (PBS-co-PCL) multiblock copolymer have been detected for the first time. A phase-segregated structure can be observed under the phase contrast optical microscope at 150 °C, but it has no significant effect on the subsequent crystallization behavior of the PBS component, which can break out at lower temperatures (i.e., 82 °C) forming spherulites that contain the molten PCL component within them. During PBS chains crystallization at 82 °C, two peaks are detected by SAXS experiments. The high-q peak corresponds to a periodic structure formed within PBS-rich domains consisting of PBS lamellae and amorphous regions containing PBS and molten PCL chains. The low-q peak arises from a periodic structure formed within PCL-rich domains consisting of PBS lamellae and thick amorphous layers needed to accommodate the large fraction of molten PCL chains at 82 °C. When the temperature is decreased to 36 °C, the PCL component cr...

Morphology of Polymerized Ionic Liquids from X-Ray and Neutron Scattering: An Atomistic Simulation Perspective

The design of solid-state electrolytes for electrochemical applications that utilize polymerized ionic liquids (polyILs) would greatly benefit from a molecular-level understanding of structure-property relationships. We herein use atomistic molecular dynamics simulations to investigate the structural properties of a homologous series of poly(n-alkyl-vinylimidzolium bistrifluoromethylsulfonylimide) poly(CnVim Tf2N) and present the first direct comparison of the structure factors obtained from scattering and simulations. Excellent agreement is found in terms of peak position and shape. Specifically, three characteristic peaks can be identified: low-q backbone-to-backbone peak or polarity peak, immediate ionic peak or charge peak and high-q pendant-to-pendant peak or adjacency peak. As the alkyl chain length increases, the backbone-to-backbone peak becomes stronger, moving to larger distance, the anion-to-anion separation slightly increases with weaker intensity, and the pendant-to-pendant or close contact peak remains essentially intact. The longer alkyl chains lead to the longer backbone-to-backbone separation and the larger nonpolar nanodomains. The quantitative cluster analysis along with color-coded snapshots vividly demonstrates that discrete nonpolar islands first form within the continuous polar network, then nonpolar domains grow beyond the percolation threshold, finally interconnect with the polar network into a bicontinuous ‘sponge-like’ nanostructure. Moreover, we exploit the selective labeling technique of neutron scattering using hydrogen/deuterium substitution to afford further insight into the morphology of poly(CnVim Tf2N). The neutron scattering profiles markedly depend on the isotopic substitution pattern. The total neutron structure factors of the backbone deuterated samples reveal the most noticeable low-q peak and are much more intense than those observed in single isotope neutron scattering. Our results strongly suggest that the X-ray scattering and distinct isotopic labeled neutron scattering data do not necessarily give rise to the same positioned scattering peaks. We hope these insights will lead to a fundamental understanding in structure and morphology of polyILs and pave a path forward towards the rational design of future polyILs for electrochemical devices.

Alkyl Chain Length Dependence of Backbone-to-Backbone Distance in Polymerized Ionic Liquids: An Atomistic Simulation Perspective on Scattering

The backbone-to-backbone correlation length in polymerized ionic liquids (polyILs) manifested as the low-q peak in scattering profiles increases with increasing alkyl chain length, concomitant with ever-growing nonpolar domain size. Understanding the dependence of the correlation length on the pendant alkyl chain length is crucial to effectively designing polyILs for electrochemical applications. Herein, extensive atomistic MD simulations for a complete homologous series of poly(n-alkylvinylimidazolium bis(trifluoromethylsulfonyl)imide), poly(CnVim Tf2N) (n = 2–8), have been carried out to investigate the liquid structure with emphasis on scattering. Neutron scattering with isotopic labeling affords more versatile ways to vet the intrinsic structure, but different isotopically labeled neutron and X-ray scattering data do not necessarily lead to the same position of the low-q peaks. Such an unintended consequence has a nontrivial implication on exploitation of scattering experiments.

Pressure-dependent morphology of trihexyl(tetradecyl)phosphonium ionic liquids: A molecular dynamics study.

In the present molecular dynamics study, we investigate the effects of increasing pressure on the structural morphology of trihexyl(tetradecyl)phosphonium bromide (P666,14+/Br-) and trihexyl(tetradecyl)phosphonium dicyanamide (P666,14+/DCA-) ionic liquids (ILs). Special attention was paid to how charge and polarity orderings, which are present in the microscopic structure of these ILs at ambient conditions, respond to very high external pressure. The simulated X-ray scattering structure functions, S(q)s, of the two systems reveal that both the characteristic orderings show appreciable responsiveness towards the applied pressure change. At a given pressure, a slight difference between the polarity ordering (PO), charge ordering (CO), and adjacency correlations (AC) for both the systems points towards different microscopic structure of the two ILs due to change in anion. Beyond a certain pressure, we observe emergence of a new low-q peak in the S(q)s of both the systems. The new peak is associated with formation of crystalline order in these systems at higher pressures and the real space length-scale corresponding to the crystalline order lies in between those of polarity- and charge-ordering. Beyond the transition pressure, the crystallinity of both the systems increases with increasing pressure and the corresponding length-scale shifts towards smaller values upon increasing pressure. We also observe that the extent of the usual polarity ordering decreases upon increasing pressure for both the P666,14+/Br- and P666,14+/DCA- systems. We demonstrate that the disappearance of the usual polarity peak is due to decreased polar-polar and apolar-apolar correlations and enhanced correlations between the charged and uncharged groups of the ions. This scenario is completely reversed for the components corresponding to the crystalline order, the polar-polar and apolar-apolar correlations are enhanced and polar-apolar correlations are diminished at higher pressure. In addition, the charge ordering peak, which is not so obvious from the total S(q) but from ionic and sub-ionic partial components of it, shifts towards lower q values for P666,14+/Br-. Instead, for the P666,14+/DCA-, at the highest pressure studied the CO peak occurs at a q-value higher than that at the ambient pressure.

Low-Q peak in X-ray patterns of choline-phenylalanine and -homophenylalanine: A combined effect of chain and stacking

In this contribution we report for the first time the X-ray patterns of choline-phenylalanine and choline-homophenylalanine ionic liquids. The presence of a low Q peak in both systems is another evidence that a long alkyl chain is not always needed to establish a nanodomain segregation in the liquid sufficient to be revealed by the diffraction experiment. These new data are compared with the diffraction patterns and the theoretical calculations of other choline-aminoacid ionic liquids recently reported. A significant role might be played by the stacking interactions between aromatic rings.

Small-Angle X-ray Scattering Study on Internal Microscopic Structures of Poly(N-isopropylacrylamide-co-tris(2,2'-bipyridyl))ruthenium(II) Complex Microgels.

Internal microscopic structures of poly(N-isopropylacrylamide-co-tris(2,2'-bipyridyl))ruthenium(II) complex microgels were investigated using small-angle X-ray scattering (SAXS) in the extended q-range of 0.07 ≤ q/nm(-1) ≤ 20. The microgels were prepared by aqueous free-radical precipitation polymerization, resulting in formation of monodispersed, submicrometer-sized microgels, which was confirmed by transmission electron microscopy and dynamic light scattering. To reveal the changes in the microscopic structures of the microgels during swelling/deswelling or dispersing/flocculating oscillation, the redox state of Ru(bpy)3 complexes was fixed in the microgels using Ce(IV) or Ce(III) ions under high ionic strength (1.5 M) during the SAXS measurements. The scattering intensity of the microgels manifested five different structural features. In particular, the correlation length (ξ), which was obtained from the fitting analysis using the Ornstein-Zernike equation, of the microgels both in the reduced and oxidized Ru(bpy)3 states exhibited divergent-like behavior. In addition, a low-q peak centered at q ≈ 5 nm(-1) did not appear clearly in both the reduced [Ru(bpy)3](2+) and oxidized [Ru(bpy)3](3+) states, indicating that the formation of a polymer-rich domain was suppressed; thus, Ru(bpy)3 complexes can be active even though the microgels are deswollen or flocculated during the oscillation reaction.

Thermal and structural studies of imidazolium-based ionic liquids with and without liquid-crystalline phases: the origin of nanostructure.

To clarify the origin of the nanostructure of ionic liquids (ILs), we have investigated two series of ILs 1-alkyl-3-methylimidazolium hexafluorophosphate (CnmimPF6, n = 4-16, n is an alkyl-carbon number) and 1-alkyl-3-methylimidazolium chloride (CnmimCl, n = 4-14) using differential scanning calorimetry and X-ray diffraction techniques. The PF6 samples with n > 13 and the Cl samples with n > 10 exhibited the liquid-crystalline (LC) to liquid (L) phase transitions, as reported before. We found that both samples with smaller n also exhibited the LC to L transitions under supercooled states as far as the ionic motions were not frozen-in at the glass transition temperatures Tg. The Tg of the LC phase was close to that of the L phase, indicating that the characteristic length of the glass transition is shorter than that of the nanostructure. A low-Q peak due to the nanostructure in the L phase and a diffraction peak due to the layer structure in the LC phase appeared at almost the same Q positions in both samples. On the basis of the above results and some thermodynamic analysis, we argue that the nanostructures of ILs are essentially the same as the layer structures in the LC phases.

Relationship between low-Q peak and long-range ordering of ionic liquids revealed by high-energy X-ray total scattering.

The structures of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([CnmIm(+)][TFSA(-)]) ionic liquids (alkyl-chain length n = 4, 8, 10, and 12) have been studied by high-energy X-ray total scattering at T = 298-453 K. The low-Q peaks observed in the X-ray structure factors S(Q)s at 0.2 < Q < 0.4 A(-1) for n≤ 8 were almost unchanged up to T = 453 K, whereas they shifted to lower Q values and their peak intensity decreased for n = 10 and 12. The radial distribution functions G(r)s for n≤ 8 showed no temperature dependence, but the G(r)s in the long r region showed a significant temperature dependence, particularly for r = 15-25 Å for n = 10 and 12. To discuss the relationship between the low-Q peak intensity in S(Q) and the profile of G(r) at long range, we propose a new function S(Qpeak)(r). It is elucidated that the low-Q peak observed for n = 12 is a signature of long-range ordering in the r range 15-25 Å, which corresponds to both of the anion-anion correlations at the first neighbor and the alkyl group aggregates among the C12mIm(+) cations, whereas the anion-anion correlations at the first neighbor are predominant for reproducing the low-Q peaks in the n = 8 and 10 systems.

Structure and Aggregation in the 1-Alkyl-3-Methylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquid Homologous Series

A new comprehensive Molecular Dynamics study using large simulation boxes has been performed in order to complete and extend the structural analysis on the mesoscopic segregation observed in the ionic liquids of the 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide homologous series, [CnC1im][Ntf2] (2 ≤ n ≤ 10). The analysis includes the discussion along the whole family of the corresponding structure factors, S(q), in the low-q range (1.6 ≤ q/nm(-1) ≤ 20); the confirmation of the periodicity of the polar network of the ionic liquid and its intermediate low-q peak equivalence; and the introduction of five statistical functions that probe the existence and characterize the polar network and the nonpolar aggregates that are formed along the [CnC1im][Ntf2] series. The later functions comprise aggregate size distributions, average number of contact neighbors within an aggregate, neighbor distributions, distributions of aggregate maximum length, and distributions of aggregate volume.

Intermediate range order and structure in colloidal dispersions with competing interactions

Colloidal dispersions with a short-range attraction and long-range repulsion can exhibit an intriguing intermediate range order, manifested in scattering experiments as a low-q peak in the structure factor. Monte Carlo simulations are performed on fluids that exhibit intermediate range order to explicitly determine its connection to a possible state of microphase separation, equilibrium clustering. This is accomplished by decomposing the structure factor into cluster-cluster, monomer-monomer, and cross-correlations that cannot be extracted from experimental scattering patterns. Our simulation results indicate that the intermediate range order arises from either monomeric or cluster species, depending on solution conditions, and reflects the presence of a preferred length scale that is not trivially related to the interparticle potential. Further, criteria are established to define monomer, cluster, and percolated states in these systems that facilitate further studies. Combining scattering techniques with simulations provides an effective method for identifying clustered states in complex fluids.

Heterogeneous Slow Dynamics of Imidazolium-Based Ionic Liquids Studied by Neutron Spin Echo

We have investigated structure and relaxation phenomena for ionic liquids 1-octyl-3-methylimidazolium hexafluorophosphate (C8mimPF6) and bis(trifluoromethylsulfonyl)imide (C8mimTFSI) by means of neutron diffraction and neutron spin echo (NSE) techniques. The diffraction patterns show two distinct peaks appeared at scattering vectors Q of 0.3 and 1.0 Å(-1). The former originates from the nanoscale structure characteristic to ionic liquids and the latter due to the interionic correlations. Interestingly, the intensity of the low-Q peak drastically grows upon cooling and keeps growing even below the glass transition temperature. The NSE measurements have been performed at these two Q positions, to explore the time evolution of each correlation. The relaxation related to the ionic correlation (ionic diffusion) is of Arrhenius-type and exhibits nonexponential behavior. The activation energy (Ea) of the ionic diffusion, which is linked to viscosity, depends on the type of anion; the larger is the anion size, the smaller Ea becomes for most of anions. On the other hand, two kinds of relaxation processes, slower and faster ones, are found at the low-Q peak position. The most significant finding is that the fraction of the slower relaxation increases and that of the faster one decreases upon cooling. Combining the NSE data with the diffraction data, we conclude that there exist two parts in ILs: one with the ordered nanostructure exhibiting the slow relaxation, and the other with disordered structure showing faster relaxation. The structure and dynamics of ILs are heterogeneous in nature, and the fraction of each part changes with temperature.

The Interpretation of Diffraction Patterns of Two Prototypical Protic Ionic Liquids: a Challenging Task for Classical Molecular Dynamics Simulations

In this study, we discuss the performance of classical molecular dynamics in predicting the experimental X-ray diffraction patterns of liquid ethylammonium nitrate (one of the simplest protic room-temperature ionic liquid showing amphiphilic behavior) and of its hydroxy derivative (2-ethanolammonium nitrate, 2-HOEAN). Newly recorded energy-dispersive X-ray diffraction structure factors are compared with the corresponding quantities extracted from molecular dynamics simulations. Other useful theoretical and experimental indicators are used as a probe of the local structure of the title ionic liquids. We shall show that the use of a general purpose, two-body terms only, force field, such as OPLS/AA is able to describe most of the structural experimental data. However, we shall also point out that an improved description of some key structural features observed in the X-ray radial distribution function, can be obtained very easily by adding a general three-body potential energy term instead of changing the two-body potential parameters, in order to optimize the agreement with experimental data. This three-body term turns out to be naturally able to describe the complex polarization effects due to hydrogen bonding without requiring a quanto-mechanical treatment or a polarizable force field. In addition the present model turns out to be able to account for the presence of a low-Q peak in the scattering patterns of EAN, which has been commonly interpreted as a manifestation of the amphiphilic nature of this compound.

Experimental evidences for molecular origin of low-Q peak in neutron/x-ray scattering of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ionic liquids

Short- and long-range liquid structures of [C(n)mIm(+)][TFSA(-)] with n = 2, 4, 6, 8, 10, and 12 have been studied by high-energy x-ray diffraction (HEXRD) and small-angle neutron scattering (SANS) experiments with the aid of MD simulations. Observed x-ray structure factor, S(Q), for the ionic liquids with the alkyl-chain length n > 6 exhibited a characteristic peak in the low-Q range of 0.2-0.4 Å(-1), indicating the heterogeneity of their ionic liquids. SANS profiles I(H)(Q) and I(D)(Q) for the normal and the alkyl group deuterated ionic liquids, respectively, showed significant peaks for n = 10 and 12 without no form factor component for large spherical or spheroidal aggregates like micelles in solution. The peaks for n = 10 and 12 evidently disappeared in the difference SANS profiles ΔI(Q) [=I(D)(Q) - I(H)(Q)], although that for n = 12 slightly remained. This suggests that the long-range correlations originated from the alkyl groups hardly contribute to the low-Q peak intensity in SANS. To reveal molecular origin of the low-Q peak, we introduce here a new function; x-ray structure factor intensity at a given Q as a function of r, S(Q) (peak)(r). The S(Q) (peak)(r) function suggests that the observed low-Q peak intensity depending on n is originated from liquid structures at two r-region of 5-8 and 8-15 Å for all ionic liquids examined except for n = 12. Atomistic MD simulations are consistent with the HEXRD and SANS experiments, and then we discussed the relationship between both variations of low-Q peak and real-space structure with lengthening the alkyl group of the C(n)mIm.