edng

g

llenidinelleridints. Usl pro

the alkyl chain and the pyridinium ring of the cation, have been modelled in terms of the so-called Gaussianlength of the alkyl chain, attached to the cation, on the dynamical processes is

velengtompareore, drogen,

ing) ranges from a fraction of picoseconds to nanoseconds and, hence, sation of our sample, followed by a short introduction into the method

-based ILs (See Fig. 1).s and ILs.

Journal of Molecular Liquids 192 (2014) 199207

Contents lists available at ScienceDirect

Journal of Mole

.e ls[811], however, the number of the works on this technique remainslimited.

In this paper we summarise our neutron scattering results obtained1. C4H9\C5H5N, protonated [BuPy] cation

2. C4D9\C5D5N, completely deuterated [BuDPyD] cationlong-lived cages [5], and correlated motions [6,7] are some of the cur-rently discussed topics concerning heterogeneous dynamics in ILs. Theadvantages of QENS has also been used to study these questions

In this paperwe present results on pyridiniumThe list below contains all relevant cations, anionthis method is a valuable tool for studying such processes as diffusion,rotation, libration etc. of molecular and ionic species [1,2]. On theother hand, information about macroscopic dynamics in ionic liquids(ILs) is highly required for their manifold applications, especially, forelectrochemical devices [3]. Ion pair formation [4], and existence of

of quasielastic neutron scattering (QENS). In Section 3 of this paperwe present and discuss our results and we nish this paper with ashort outlook.

2. Materials and methodson pyridinium-based ionic liquids with [Tf2N]has been selected because of the negligible in

Corresponding author.E-mail address: r.hempelmann@mx.uni-saarland.de (R

http://dx.doi.org/10.1016/j.molliq.2014.03.0070167-7322/ 2014 Elsevier B.V. All rights reserved.sing neutron scatteringsielastic neutron scatter-

of an IL in a thermally stable inorganic skeleton [13].We start with a short description of the preparation and characteri-are very well suited for an investigation umethods. The time scale covered by QENS (quaenergies in condensed matter. Furthermcoherent scattering cross section of hyd1. Introduction

Cold and thermal neutrons havewaic distances as well as energies that cthat clearly demonstrate the melting of the alkyl chain and the activation of methyl end-group rotations at lowtemperatures. Finally, the power of deuterium labelling is evidenced for inelastic neutron scattering data.

2014 Elsevier B.V. All rights reserved.

hs thatmatch interatom-very well to excitation

ue to the outstanding in-hydrogen-rich materials

section of the anion and the fact that the cation can be easily modiedin terms of the alkyl chain length and the possibility to get partially deu-terated cations. This latter possibility enables us to switch on/off scatter-ing contributions from the different parts of the cations. Moreover, thisgroup of ILs shows some benecial properties for such applications asdye-sensitized solar cells [12] and ionogels, based on the connementdiscussed in detail. Furthermorewe showneutron backscattering data, obtained on partially deuterated samples,

model. The inuence of theThe dynamics of cations in pyridinium-basquasielastic- and inelastic neutron scatteri

Tatsiana Burankova a,b, Elena Reichert b, Verlaine Fossoa Laboratory for Neutron Scattering, Paul Scherrer Institut, 5232 Villigen, Switzerlandb Physical Chemistry, Saarland University, 66123 Saarbrcken, Germany

a b s t r a c ta r t i c l e i n f o

Available online 20 March 2014

Keywords:Neutron scatteringIonic liquidsQuasielastic neutron scatteringInelastic neutron scattering

Neutron scattering is an excesubsume our results on pyrNeutron scattering is an excwe subsume our work on pyneutron scattering experimetemperatures in ionic liquidssition of localized dynamica

j ourna l homepage: wwwanions. This family of ILscoherent scattering cross

. Hempelmann).ionic liquids by means of

b, Rolf Hempelmann b,, Jan Peter Embs a

t tool to study the dynamics of hydrogen-rich materials. In the present workweium-based ionic liquids. We have performed inelastic, quasielastic and elastic.nt tool to study the dynamics of hydrogen-rich materials. In the present worknium-based ionic liquids. We have performed inelastic, quasielastic and elasticaiming to understand the different dynamical processes that occur at differenting quasielastic scattering we obtained data that can be described as a superpo-cesses and long range diffusion. The localized processes, which originate from

cular Liquids

ev ie r .com/ locate /mol l iq3. N(SO2CF3)2, [Tf2N] anion4. IL [C4H9\C5H5N]+[N(SO2CF3)2] ([BuPy][Tf2N])5. IL [C4D9\C5D5N]+[N(SO2CF3)2] ([BuDPyD][Tf2N])6. C12H25\C5H5N, protonated [C12Py] cation7. IL [C12H25\C5H5N]+[N(SO2CF3)2] ([C12Py][Tf2N])

The efciency of interaction between an incident neutron and a sam-ple nucleus is represented bymeans of the quantity called neutron crosssection. As the neutron can be either scattered or absorbed by the target,the values of the scattering (scatt) and absorption (abs) cross sections,respectively, are of relevance for planning a neutron scattering experi-ment. The scattering cross-section can be further subdivided into coher-ent (coh) and incoherent (inc) scattering cross sections, which showwhether collective effects or uncorrelated motions will inuence thespectrum of the scattered neutrons. Table 1 summarizes thementionedneutron cross sections for the studied species. We see that the incoher-ent scattering cross section of the [Tf2N] anion can be completelyneglected and that the scattering cross section can highly be inuenced

200 T. Burankova et al. / Journal of Molecular Liquids 192 (2014) 199207by exchanging H by D.

2.1. Sample preparation

Weused a two-step process to synthesize protonated as well as par-tially or completely deuterated pyridinium based ionic liquids.

2.1.1. Synthesis of N-butylpyridinium bis(triuoromethylsulfonyl)imideIn the rst step we prepared N-butylpyridinium bromide. To do so,

pyridine was reuxed, in a standard reux apparatus, tted with adrying tube, which contained P2O5 as drying medium. An equimolarquantity of 1-butylbromide was slowly added and the reaction mix-ture was reuxed in a dark environment for four days. After coolingto room temperature, the resulting white and shining crystals werepuried by recrystallization with ethyl acetate and quickly trans-ferred into a bottle. In the second step, N-butylpyridinium bromidewas dissolved in distilled water, and an equimolar amount of lithiumbis(triuoromethylsulfonyl)imide was added. The aqueous phasewith dissolved LiCl was removed; the remaining ionic liquid waswashed 5 times with distilled water and dried 3 days in vacuumat 60 C, resulting in a colourless viscous liquid with a yield greaterthan 95%.

2.1.2. Synthesis of [BuDPy][Tf2N], [BuPyD][Tf2N] and [C12Py][Tf2N]The partially and fully deuterated ILs were synthesized by the same

process detailed above, but with deuterated educts (Butylbromide(d9)),(Pyridine(d5)), depending on the cases. The same process was also ap-plied for the [C12Py][Tf2N], by replacing the N-butylpyridinium bromidewith N-dodecylpyridinium bromide. Thewater content (b80 ppm) of allthe synthesized ionic liquids was determined by means of Karl-Fischertitration. Furthermore, we checked the purity and quality of the partialdeuteration (98.16 b %D b 99.70) using NMR-techniques.

2.2. Differential scanning calorimetry, DSC

For the characterization of the studied ionic liquids, determination ofthe temperatures of phase transitions, DSC measurements have beencarried out with a Netzsch DSC 204 F1 System. Measurements wereperformed on heating and cooling with a rate of 510 K/min using2030 mg samples encapsulated in standard Al crucibles. An argonstream was used during the whole experiment as a protective gas. The

Table 1Summary of the neutron scattering and absorption cross sections of the different speciesmentioned in this paper. abs is given for neutrons with 5.75 incident wavelength.1b = 1028 m2.

System scatt [b] abs [b] inc [b] coh [b] inc/scatt [%]

1 1209.75 21.07 1124.15 85.60 92.92 168.43 6.20 29.21 139.22 17.33 65.70 9.67 0.52 65.18 0.84 1275.45 30.74 1124.67 150.78 88.25 234.13 15.87 29.73 204.40 12.76 2566.48 38.18 2408.32 158.16 93.87 2632.18 47.85 2408.84 223.34 91.5DSC curves of both samples exhibit a distinct dependence on the tem-perature history (Fig. 2). Both crystallization and cold crystallizationpoints are affected by the rate of cooling or heating. The onset pointsfor the melting peaks seem to be independent of this parameter(Table 2).

2.3. Quasielastic neutron scattering, QENS

In a QENS experiment the intensities are measured as a function ofboth, scattering angle 2 and energy transfer between neutrons andsample, E= = i f= Ei Ef, where Ei and Ef denote the initialandnal neutron energy, respectively; this allows for an investigation ofthe dynamics of a system. QENS [14,1,2,15] investigates small energyexchanges peaked at E = 0. Neutrons are electrically neutral andtherefore do not interact with the electrons of the atoms/moleculesin condensed matter, but are scattered at their nuclei. Since thewavelength and the kinetic energy of cold and thermal neutrons iscomparable to interatomic distances and excitation energies in con-densed matter, respectively, the diffusion process can be studied onatomic scales of space and time simultaneously, and this feature isunique. For X-rays with wavelengths of a few the energies are inthe keV range, and for Raman or Brillouin light scattering the energyts to excitation energies in condensedmatter, but the wavelength isfar too large.

2.3.1. Neutron scattering basicsThe intensity of the scattered neutrons is proportional to the double

differential cross-section

d2d d

kfki

coh4

Scoh Q ; inc4

Sinc Q ; n o

1

where ki is the wave number of the incident neutrons and kf is the wavenumber of the scattered neutrons.

Strictly speaking, the scattering process is a quantum mechanicaltransition and has to be treated in terms of wave functions, transitionprobabilities, time-dependent perturbation theory etc. and, eventually,ensemble averages. On the other hand, classical particle diffusion incondensedmatter is treated in terms of statistical physics. The interfacebetween these quite different types of descriptions is given by thermalaverages which can be expressed in terms of so-called van Hove corre-lation functions, G(r, t) and Gs(r, t). The correlation function G(r, t) dr isthe conditional probability that, given a particle was at time t= 0 at theorigin r=0, any particle is found at time t at the position r in a volumeelement dr. The self-correlation function Gs(r, t) dr is the conditionalprobability that, given a particle was at time t = 0 at the origin r= 0,the same particle is found at time t at the position r in a volume elementdr.

The Fourier transformation of G(r,t) with respect to space yieldsthe so-called intermediate scattering function I(Q,t), and a subse-quent temporal Fourier transformation yields the dynamical scatter-ing function

S Q ; 2 1e

i Qrt G r; t drdt 2

The analogous treatment of the self-correlation function yields, viathe self-part of the intermediate scattering function Is(Q,t), the incoher-ent scattering function Sinc(Q,).With respect to diffusion, S(Q,) givesaccess to collective diffusion and Sinc(Q,) gives access to self-diffusion.

2.3.2. Applications of the QENS methodConcerning the different types of stochastic motions which can be

studied by the QENSmethodwe have to differentiate between localizedand long-range diffusion. In the case of long-range self-diffusion, Is(Q,t)

decays to zero for t , while for localized motions (like rotations or

The solution of Eq. (8) in the energy domain then reads (taking intoaccount appropriate boundary conditions)

SRs Q ; j20 QR Xl1

2l 1 j2l QR 1

DRl l 1 DRl l 1 2 2

9

where jl(x) denotes spherical Bessel functions of the order l.

2.3.6. ReorientationsThe simplest form of a reorientational type of motion consists of

jumps between equivalent sites; for a complete derivation of the scat-

201T. Burankova et al. / Journal of Molecular Liquids 192 (2014) 199207diffusion inside a connement) this function approaches a nite valuefor t .

2.3.3. Self-diffusionAs the most simple application of the formalism presented in

Section 2.3.1, we consider long-range self-diffusion. The self-correlation function obeys Fick's second law of diffusion:

t Gs r; t Ds

2Gs r; t 3

with the initial condition Gs(r,t = 0) = (r). Thus we treat the dif-fusion process as if all diffusing particles start at the origin at timezero; this leads to the correct result because of temporal and spatialinvariance. A Fourier transform in space and time nally results in:

Sinc Q ; 1

DsQ2

DsQ2 2 2 4

which is a Lorentzian function with the half-width at half maxi-mum (HWHM)

Q DsQ2 5

where Ds represents the self-diffusion coefcient. At sufcientlysmall Q this so-called Q2-law is generally valid irrespective of thedetails of the diffusion process, which can be elucidated from mea-surements at nite or larger Q, but any long-range diffusion modelconverges to the Q2 law at small Q.

2.3.4. Jump diffusionThe diffusion on the microscopic scale can be successfully described

by so-called jump diffusionmodels. Thesemodels assume that diffusiontakes place as successive jumps. Parameters are Dtr, the diffusion coef-cient, 0, the so-called residence time and the mean-jump length, fromwhich only two are independent. Differentmodels have been developedfor different jump length distributions, for example, the model pro-posed by Chudley and Elliott [16] with a xed jump-length or Singwiand Sjlander's [17] model for an exponential distribution of jumplengths. In the latter case the dynamic structure factor reads:

Sinc Q ; 1

tr2tr 2

6

tr Q DtrQ

2

1 DtrQ20: 7

2.3.5. Rotational dynamicsNowwe consider the isotropic rotational diffusion (continuous rota-

tional diffusion on the surface of a sphere). The idea behind thismodel isthat the reorientation of the molecule is due to small-angle random ro-tations. The orientation of a particle is describedby an orientation vector! ; . For the orientation probabilityP !; t

of an orientation

!at

time t it can be written, in analogy to Fick's law:

t P

!; t

DR2P

!; t

: 8tering function see [2]. In the case of three energetically equivalentsites the scattering function is given by

S Q ; A0 Q 1A0 Q 1

3=3= 2 2 10

A0 Q 13

1 2 j0 Qr3

p 11

where denotes the time between two successive jumps.

2.3.7. Gaussian model for localized translational motionThe Gaussian model for localized motion has been recently intro-

duced by Volino et al. [18]. It considers the diffusion of particles in a con-tinuous and innitely derivable potential, that corresponds to restricteddynamics in a spherewith a soft boundary. For the onedimensional casethe displacement of a particle moving along the x-axis is assumed to bea centred Gaussian random variable:

ux t ux 0 h i 2 t 12

where is the variance of this variable, and (t) is the correlation func-tion. If (t) decreases exponentially with time, the following dynamicstructure factor can be derived in the three-dimensional case (ux2 =uy

2 = uz2):

SG Q ;; eQ22

Xn1

Q22 n

n!1

nDloc=2

nDloc=2

2 224

35

13

where Dloc is the diffusion coefcient for the localized translational mo-tion and is the characteristic size of the domain with soft boundaries,in which the particles are moving.

2.4. Neutron backscattering [19]

So-called backscattering spectrometers provide a very good energyresolution, typically E 1 eV. Elastic or inelastic xed windowscans (FWS) allow for an overview of the onset of dynamical processes

Fig. 1. Sketch of the 1-alkylpyridinium [CnPy]+ cation (left) and thebis(triuoromethylsulfonyl)imide [Tf2N] anion (right). R denotes the alkyl chain

CnH2n + 1. Here we present results for n = 4 and n = 12.

202 T. Burankova et al. / Journal of Molecular Liquids 192 (2014) 199207faster than the time scale that corresponds to the energy resolutionof the instrument used (typically in the range of nanoseconds). Toperform this type of experiment the monochromator and theanalysers are chosen to reect neutrons with xed wave vectors kiand kf. For ki=kfwe will perform elastic scans and ki kf denes in-elastic scans.

In Fig. 3 we illustrate themethod schematically. The left hand part of

Fig. 2. Heat ux vs temperature for N-butylpyridinium bis(triuoromethylsulfonyl)imide(upperpanel) andN-dodecylpyridiniumbis(triuoromethylsulfonyl)imide (lower panel).For further information, see text.Fig. 3 shows the elastic intensity as a function of temperature. At a cer-tain temperature,marked by the vertical bar on the temperature scale, adynamic process gets activated and correspondingly the elastic intensi-ty will drop (i.e. the energy will be redistributed now into somequasielastic channels). At the same time the inelastic intensity will in-crease; this is presented in the right hand part of Fig. 3.

2.5. Inelastic neutron scattering

The measured Sexp(Q, ) contains, in addition to the elastic andquasielastic signal, also inelastic contributions (phonons, inter- andintramolecular vibrations etc.). The most convenient way to presentand discuss inelastic neutron scattering data is to calculate the so-called (pseudo) density-of-states (PDOS or DOS) or the generalized

T T

elas

tic in

tens

ity

inel

astic

inte

nsity

Fig. 3. Scheme to illustrate how the xed window scans either for elastic (left hand gure)or for inelastic (right hand gure) experiments work.susceptibility. The relation between Sexp(Q, ) and the DOS gexp()reads:

gexp Q2

Sexp Q ; coth2kBT

1

114

and for the generalized susceptibility we nd

E Sexp Q ; E nB T ; E

15

with the so-called Bose factor nB T; E e=kBT1h i1

. Note, that

(E) can be compared to () as measured with dielectric relaxation

spectroscopy.

3. Results

In our previous studies [20,21] we considered the dynamics of onlyone IL ([BuPy][Tf2N]) in the liquid state using both totally protonatedand partially deuterated samples. In the present work, rst, we continueanalysis of the pyridinium-based ILs, comparing two samples, which dif-fer by the length of the alkyl substituent attached to the pyridinium ringbut which have the same anion. The size and the shape of the cation de-termine themagnitude of interaction between the ions, so that this effectcan be seen for both global diffusion and localizedmotions. Understand-ing of the inuence of the alkyl chain on microscopic dynamics is essen-tial for designingmolten salts. Many groups of ILs have alkyl side groups,which allow one to change their physicochemical properties (viscosity,diffusivity, density, conductivity, phase transition temperatures) [22].

Second, applying the backscattering technique we investigated therestricted dynamics of the [BuDPy] and [BuPyD] cations at low tempera-tures, before global diffusion and rotation set in. The advantage of thedeuterium labelling was also employed for studying inelastic spectraof the [BuPy] cation.

As this work is tightly connected with the previously published re-sults, a short summary of the latter is justied. The most descriptivemodel-independent analysis allowed us to make out two distinct pro-cesses occurring in [BuPy][Tf2N] on picosecond time scale: spatially re-stricted dynamics and unrestricted diffusion, described by Sloc(Q, E)and Sglob(Q, E), respectively. The general form of the scattering law,used to t the spectra, was given by

S Q ; E I0 Q Sglob Q ; E Sloc Q ; E R E a bE 16

where I0(Q) includes an intensity factor and the DebyeWaller factor,the term a + bE represents a linear background, which accounts forprocesses faster than the instrument observation time. The long-rangediffusion was satisfactorily described by the jump-diffusion model(Eqs. (6)(7)), the diffusion coefcients changing with temperatureaccording to Arrhenius' equation (D= D0exp(-EA/RT)).

We also performed the quantitative analysis of the experimental dataonpartially deuterated samples ([BuDPy] and [BuPyD]) in the liquid phasein order to separate contributions from the different parts of the butyl-pyridiniumcation and to simplify the observedpicture of diversemotionshidden in Sloc(Q, E) [21].While the global diffusionalmotion of both sam-ples was characterized by the same values of the diffusion coefcient Dtrwithin the limits of the experimental accuracy, the QENS-spectradisplayed differences in the localized dynamics. The diffusion coefcientof the conned motion of the ring turned out to be about four timeshigher than that of the chain, being less sensitive to temperature at thesame time. Both contributions lead to the quasielastic broadening of thespectra of the totally protonated sample. As the characteristic times liequite close on the time scale of the experiment, it is very difcult to dis-tinguish them in this case. The localized process occurs in the conne-ment of ~1 , as it followed from the t results. From these values we

draw the conclusion that the considered localized dynamics do not

correspond to the movement of the cation as a whole in the cage formedby its neighbours, but to chain conformations or ring libration.

In [20] we compared the spectra of the totally protonated [BuPy][Tf2N] anddeuterated [BuDPyD][Tf2N] samples. The aimwas to eliminatethe incoherent scattering and to observemore pronounced interferenceeffects reected in the coherent contribution. The deuteration of thecation led to themore noticeablemodulation of the linewidths account-ing for the diffusional process at those Q-values, where the maxima ofthe diffraction spectrum were detected. At the same time thelinewidths of [BuDPyD][Tf2N] were slightly narrower, in otherwords, the correlation times are longer. This is an indication of thelocal arrangements among particles and correlation in their diffu-sional motion.

The most interesting nding was that the linewidths characterizingthe faster localized process remained unchanged. For both of the sam-

0) = 0, whereas for the Gaussian function f (r = 0) N 0. If the radiusof connement is equal to zero, the corresponding particle is immobile.This feature of the Gaussian distribution could be benecial, because acertain fraction of hydrogen atoms in the alkyl chain close to thepyridinium ring could be immobile. However, when this fraction hasquite a large value, the tting routine produces divergent results. Zeroor small radii generate a at contribution to the spectrum (Dloc/a2 in Eq. (13)) and can be indeed mixed up with faster componentstaken into account by the at background (a + bE item) in Eq. (16).To describe the inuence of partial immobility, one can also introducean additional parameter pmob (the fraction of mobile particles) in thescattering law. But as pmob and the variance of the distribution functionsare not independent variables, thetting routinedoes not provide stableresults aswell. To avoid this ambiguity the log-normal distribution func-tion was applied:

(ins

203T. Burankova et al. / Journal of Molecular Liquids 192 (2014) 1992073.1. FOCUS results, inuence of the alkyl-chain length

The rst step in the comparison of the totally protonated [BuPy][Tf2N] and [C12Py][Tf2N] was done in the terms of the model-indepen-dent approach [21]. For further analysis explicit analytical models arerequired. While the translational diffusion of both ILs can be satisfacto-rily modelled by the jump-diffusion model (Eqs. (6)(7)), the descrip-tion of the spatially restricted dynamics is not straightforward. Themain difculty of the studied systems is that it possesses several degreesof freedom. It is necessary to model the exibility of the alkyl substitu-ents and to take into account librational motion of the pyridinium ringat the same time. For this purpose either the Gaussian distribution orthe log-normal distribution can be used to describe the diversity oflocalized processes occurring in volumes of different sizes. Themain dif-ference between the two (among many possible) functions is the be-haviour at r = 0. The log-normal distribution function ensures f (r =

Dtr

Dloc

Fig. 4. Sketch of the different possible dynamic processes in ourmodel. The localizedmotionples we did not see the inuence of the static structure factor on theconned motion. Therefore we concluded that this process is purelyincoherent in nature in contrast to the long-range diffusion. Thetwo structural maxima in the Q-range covered in our experiment(Q b 2.0 1) are at Q1 = 0.8 1 (R1 = 2 / Q1 = 7.8 ) and Q2 =1.4 1 (R2 = 2 / Q2 = 4.5 ). The corresponding lengths R1 and R2may be assigned to the characteristic distances between the closestions of the same and opposite charge [23]. However, deuterationhelps to judge about the input of the both contributions only approxi-mately. The only experimental way to separate coherent and incoher-ent scattering is to apply polarization analysis of incident andscattered neutrons. So far we performed this kind of an experiment onthe triple-axis spectrometer TASP at SINQ atQ=1.31, where the co-herent contribution is expected to be considerable. Though the accessi-ble Q-range was limited, we were able to corroborate our assumptionthat the long-range diffusion is at least partially of a collective nature;the conned motion is a true single-particle process.whereas the global process is characterized by the diffusion coefcient Dtr.f ; a; exp ln =a

2 222

h i

2

p 17

where a and are the mode and the shape parameter of the lognormaldistribution. The nal form of the scattering law is given by:

SI Q ; E I0 Q 1

tr2tr E2

Z

0

f ; a; SG Q ; E; d8>>:

9>>=>>;

2664

3775

19

lnSel T;Q ACTQ2 ln1pmob pmob

13

1 2 j0 QR3

p 23

1 j0 QR3

p resres 1:5=

i

20

where res is the halfwidth of the resolution function, pmob is the fractionof hydrogen atoms involved in the methyl group rotation. Our data iscompatible with both models, yielding the parameter valuespresented in Table 4. The fraction of mobile hydrogen atoms is around1/3. This number corresponds to the three protons at the end of thealkyl chain, whereas the total number of protons in the chain is nine.The radius of the sphere is 1.1 , which is in agreement with thecharacteristic radius of the methyl group.

The geometry of the localized dynamics at low temperatures wasalso considered by analysing QENS-spectra. Unfortunately the availableenergy-window (~25 eV) did not allow one to retrieve reliable infor-mation about the timescale of the localized motion of the chain andthe ring, as the observed QENS-broadening was either comparablewith the energy-window or much wider and had a visible contribution

only for high Q-values, resulting in high EISF-values (0.71.0). It can be

Fig. 10. Elastic scan data for the partially deuterated samples at Q= 1.85 1. A dashedline represents the result of the t using Eq. (20).

explained by the assumption that only a certain part of hydrogen atomsis mobile in the given temperature range. We applied the rotational dif-fusion model to analyse QENS-data for the two partially deuteratedsamples. The results of the ts are listed in Table 5. At T=290 K the lo-calized motion of the ring can be envisaged as a diffusion on a spherewith a radius R=1.5 and a relaxation time of =12 ps. The fractionof mobile atoms amounts to about 20%. We suppose that the QENS-

the samplewith the protonated chain, the obtained values for the radius

Neutron scattering experiments were performed to study the dy-namic processes in pyridinium-based ionic liquids with Tf2N anions.This selection allows for a detailed investigation of the cation dynamicsin this IL family.

We used different neutron scattering techniques (QENS, inelasticand elastic scattering) to cover the very rich dynamic landscape of theselected pyridinium-based ionic liquids. Furthermorewe demonstratedhow themethod of selective deuteration of the cation enables us to dis-entangle the different cationic dynamical processes, i.e. chain- and ringdynamics, and thus to get a deeper understanding of the system underinvestigation.

Table 4Fit results of the elastic intensity scans according to Eqs. (19) and (20).

3-Site jump Rot. diffusion

0, ps 9.0 2.0Ea, kJ/mol 6.5 7.0R, 1.1 1.1pmob 0.37 0.33

206 T. Burankova et al. / Journal of Molecular Liquids 192 (2014) 199207of connement and the fraction of mobile atoms are in agreement withthe values evaluated from the elastic intensity scans (R = 1.0 , pmob 0.3). So we may conclude that we observe the dynamics of the endmethyl-group of the alkyl chain. The melting of the butyl chain, ob-served in the elastic scans measured on MARS at T N 150 K, is also ahint of the thermally activated methyl group rotations. The drop of theelastic intensity does not have a step-like feature in that case, becauseit can be observed when the linewidth of the QENS-broadening is com-parable to the linewidth of the resolution function. For MARS it wouldoccur around T=285K, very close to the temperature of thephase tran-sition, and hence it is not visible. On the other hand, the onset of thefaster ring librations is more pronounced for the spectrometer with ashorter observation time.

3.3. Inelastic neutron scattering

In Fig. 11 we present the dynamic susceptibility as a function ofthe energy transfer (neutron energy gain); the energy is given in GHzunits (1 meV = 242 GHz), measured at 200 K.

The results in Fig. 11 have beenmeasured on four different ionic liq-uids: completely protonated IL ([BuPy][Tf2N], red curve), ring-deuterat-ed IL ([BuPyD][Tf2N], blue curve), chain-deuterated IL ([BuDPy][Tf2N],

Table 5Fit results of the QENS-spectra according to the rotational diffusion model.

[BuDPy][Tf2N]

T [K] [ps] R [] pmob

290 12 1.5 0.20broadening of [BuDPy][Tf2N] is connected with out-of-plane librationsof the ring and the dynamics are essentially frozen at the lower tem-peratures, as pmob does not reach high values.We also claim that the ob-served broadening is not a trace of a weak incoherent contribution fromthe deuterated chain. Otherwise the elastic scans of the two sampleswould look very similar; the difference would become apparent onlyin the size of a step connected with the thermally activated methyl-group rotation. With the temperature decrease pmob drops almost tozero, while the relaxation time of the ring librations increases. As for250 16 1.5 0.15175 ~20 1.5 0.07130 ~0

[BuPyD][Tf2N]

T [K] [ps] R [] pmob

290 20 1.0 0.35250 ~20 1.0 0.32175 20 1.0 0.30130 20 1.0 0.26green curve) and completely deuterated IL ([BuDPyD][Tf2N], yellowcurve).

A careful comparison of the four curves in Fig. 11 reveals some inter-esting features. First, we see that below 1 THz (4 meV, region I) all 4curves show a (more or less pronounced) peak at about 500 GHz(2 meV); this is the well known Bose-peak, specic for glass-formingsystems. The second observation concerns the energy region above 1THz. In this energy region we observe a feature that originates fromthe contribution of the ring only (region III), while the contributionsin the regions II, IV, V and VI are related to the butyl-chain. From this ob-servation it is evident that all features measured on the completely pro-tonated sample at energies above 2.9 THz represent inter-chain modes.The energy-range between 1 and 2.9 THz contains vibrational modesfrom both, the ring and the alkyl-chain. Our ndings can be describedonly on a qualitative level, however they demonstrate the powerof deuteration labelling to disentangle contributions from differentparts of a given ion. In order to get more insight and a full interpre-tation of these data, also with respect to the role of the anion, a com-parison to MD results is highly demanding and matter of our currentintentions.

4. Conclusion and outlook

120

100

80

60

40

20

0102

2 3 4 5 6 7 8 9103

2 3 4 5 6 7 8 9104

energy transfer [ GHz ]

T = 200 K

completely protonated ring deuterated butyl-chain deuterated completely deuterated

I

II

III

IV V

VI

dyn.

sus

cept

ibilit

y '

' [ a

rb. un

its ]

Fig. 11.Dynamic susceptibility vs energy transfer (neutron energy gain). For better visibil-ity the curves have been gradually shifted in vertical direction.Several further steps in studying ILs by means of neutron scatteringcould be proposed. First, a broader dynamic range could be covered byusing spectrometers with different resolution functions. Whilechanging the experimental observation time, we could focus eitheron the long-range translational motion, which was seen as a singleLorentzian for the usually applied FOCUS setting, or on very fast pro-cesses, presented by the at background in our t models. For a morethorough investigation these experiments should be performed for aset of temperatures (close to the melting point, below and above, forboth the liquid and solid/amorphous state). This information would

be even more precious, if it were acquired for partially deuteratedsamples, because it makes the independent analysis of differentgroups of atoms feasible. However, deuteration of the samplesleads to the more intense coherent contribution. In this case itwould be an important step to separate coherent and incoherentcontributions in a QENS experiment using longitudinal polarisationanalysis. Moreover, the coherent scattering itself contains valuableinformation about collective, strongly coupled motions in ILs [29,7].In order to receive its more detailed interpretation, we intend to per-form MD simulations, which give a direct access to the dynamicstructure factor in the same (Q,E)-range covered by the QENS meth-od. The computed quantities could then be compared with the ex-perimentally obtained ones.

Acknowledgement

Financial support by the Deutsche Forschungsgemeinschaft(DFG) within the Scientic Priority Program SPP 1191 Ionic Liquids(project no. HE2403/8-3) is gratefully acknowledged. This work isbased on experiments performed at the Swiss spallation source SINQ,Paul Scherrer Institut, Villigen, Switzerland. The FOCUS spectrometerat the Swiss spallation neutron source SINQ, Paul Scherrer Institute,Villigen, Switzerland, has nancially been supported by the BMBF (pro-ject no. 05KN7TSA) in the framework of Forschung mit Grogerten.

The authors acknowledge the Institute LaueLangevin (ILL) forbeam time on IN10 backscattering spectrometer.We thankDr. T. Seydelfor his support during our experiments at IN10.

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The dynamics of cations in pyridinium-based ionic liquids by means of quasielastic- and inelastic neutron scattering1. Introduction2. Materials and methods2.1. Sample preparation2.1.1. Synthesis of N-butylpyridinium bis(trifluoromethylsulfonyl)imide2.1.2. Synthesis of [BuDPy][Tf2N], [BuPyD][Tf2N] and [C12Py][Tf2N]

2.2. Differential scanning calorimetry, DSC2.3. Quasielastic neutron scattering, QENS2.3.1. Neutron scattering basics2.3.2. Applications of the QENS method2.3.3. Self-diffusion2.3.4. Jump diffusion2.3.5. Rotational dynamics2.3.6. Reorientations2.3.7. Gaussian model for localized translational motion

2.4. Neutron backscattering [19]2.5. Inelastic neutron scattering

3. Results3.1. FOCUS results, influence of the alkyl-chain length3.2. Backscattering experiments3.2.1. MARS results [24]3.2.2. IN10 results [25]

3.3. Inelastic neutron scattering

4. Conclusion and outlookAcknowledgementReferences