Physico-chemical properties and nanoscale morphology in N-alkyl-N-methylmorpholinium dicyanamide room temperature ionic liquids

Article abstract of DOI:10.1016/j.molliq.2013.08.002

A series of new room temperature ionic liquids based on N-alkyl-N-methylmorpholinium cations (the alkyl chain ranging from ethyl to nonyl) and a hydrophilic anion (dicyanamide), a mostly unexplored class of room temperature ionic liquids (RTILs), has been synthesized and characterized. Selected physico-chemical properties (density, viscosity and ionic conductivity) have been measured and the salts' propensity to organize into ionic clusters has been explored. The inspection of their Walden plots and electrospray ionization spectra strongly prompts for the occurrence of clustering phenomena, especially for salts bearing long alkyl chains. This structural organization has been confirmed using X-ray scattering techniques and the existence of a high degree of intermediate range order has been detected, as fingerprinted by distinct diffraction features at low Q. These evidences are rationalized in terms of a structural model where, similarly to other RTILs, the alkyl chains tend to segregate from the charged moieties.

Full text of DOI:10.1016/j.molliq.2013.08.002

Journal of Molecular Liquids 187 (2013) 252–259
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Physico-chemical properties and nanoscale morphology in
N-alkyl-N-methylmorpholinium dicyanamide room temperature
ionic liquids
c
c
Olga Russina a, Ruggero Caminiti a, Alessandro Triolo b, , Sunita Rajamani , Bernardo Melai ,
⁎
d,⁎⁎
Alessandra Bertoli d, Cinzia Chiappe
a
Università di Roma “La Sapienza”, Dipartimento di Chimica, Piazzale Aldo Moro, 5 00185 Rome, Italy
Istituto di Struttura della Materia, Consiglio Nazionale delle Ricerche, Via del Fosso del Cavaliere 100, I-00133 Rome, Italy
Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Via del Risorgimento, 35-56126 Pisa, Italy
Dipartimento di Farmacia, Università di Pisa, Via Bonanno, 33-56126 Pisa, Italy
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 May 2013
Received in revised form 4 August 2013
Accepted 8 August 2013
Available online 22 August 2013
A series of new room temperature ionic liquids based on N-alkyl-N-methylmorpholinium cations (the alkyl chain
ranging from ethyl to nonyl) and a hydrophilic anion (dicyanamide), a mostly unexplored class of room temperature
ionic liquids (RTILs), has been synthesized and characterized. Selected physico-chemical properties
(density, viscosity and ionic conductivity) have been measured and the salts' propensity to organize into
ionic clusters has been explored. The inspection of their Walden plots and electrospray ionization spectra
strongly prompts for the occurrence of clustering phenomena, especially for salts bearing long alkyl chains.
This structural organization has been confirmed using X-ray scattering techniques and the existence of a
high degree of intermediate range order has been detected, as fingerprinted by distinct diffraction features
at low Q. These evidences are rationalized in terms of a structural model where, similarly to other RTILs, the
alkyl chains tend to segregate from the charged moieties.
Keywords:
Room temperature ionic liquids
Dicianamide
Morpholinium
Mesoscopic organization
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
[13–15]. Earlier this year, N-alkyl-N-methylmorpholinium bistriflamide
(alkyl = even chains from ethyl to octyl) has been thoroughly
Over the past ten years, as a consequence of their physico-chemical
properties [1], ionic liquids (ILs) have been the object of an increasing
interest, being proposed as environmental friendly solvents for chemical
synthesis, [2–4] (bio)catalysis, [5–7] separation technologies [8,9] and
as solvents/templates for nanomaterial preparations [10,11]. Moreover,
owing to their unique electrochemical properties [12] a large number of
ILs have been investigated to promote the safety and performance of
solar cells, fuel cells, capacitors and lithium batteries. The most commonly
studied ILs are based on imidazolium cations, combined with different
anions, such as hexafluorophosphate [PF6]−, tetrafluoroborate [BF4]− or
bistriflimide [Tf2N]−. Imidazolium-based ILs have been extensively inves-
tigated in the last decade; on the other hand very few studies exist on
other classes of ILs, such as morpholinium-based ones: these are peculiar
salts as they bear a cyclic cation with a significant dipolar moment and
can also bear a side alkyl chain that can affect several physico-chemical
properties: recently, some long chain salts of this class have attracted
attention because of their structural properties for the design of IL crystals
investigated probing the fluorescence response of Coumarin-153
and rotational dynamics of dipolar probes when dissolved in these
liquids [16,17]. The authors highlighted that morpholinium-based
RTILs are characterized by a high level of structural heterogeneity.
Morpholinium-based ILs have been considered also of interest as
catalysts for organic synthesis, [18] heat stabilizers, or antioxidants
for lubricating oils [19] and for electrochemical applications
[20–24]. Protic morpholinium-based ILs have been proposed as elec-
trolytes for fuel cell devices and thermal transfer fluids [25].
Melting points in these ILs are often above or close to room temper-
ature; for example, N-ethyl-N-methylmorpholinium bromide is a solid
with a high melting (mp = 188 °C) and the association with common
anions, such as [BF4]−, [Tf2N]−, [PF6]−, gives ILs that are solid at room
temperature, characterized by higher viscosities and lower conductivities
than the analogous pyrrolidinium- or piperidinium-based ILs due to the
presence of an oxygen atom in the cyclic ring [26].
Significantly, sporadic data of toxicity reported for some N-
dialkylmorpholinium based ILs have evidenced [27–29] lower environ-
mental impact of these salts with respect to other commonly used ILs. A
recent study on bromides salts has shown that low toxicity to Vibrio
fischeri is a peculiarity of morpholinium salts bearing short alkyl chains
[30]. Moreover, in the case of the ethyl derivative this low toxicity is
⁎
Corresponding author. Tel.: +39 06 45488136.
⁎⁎ Corresponding author. Tel.: +39 050 2219669.
E-mail addresses: triolo@ism.cnr.it (A. Triolo), cinziac@farm.unipi.it (C. Chiappe).
0167-7322/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molliq.2013.08.002

O. Russina et al. / Journal of Molecular Liquids 187 (2013) 252–259
253
associated to a significant biodegradability (around 40%), comparable
to that characterizing the more biodegradable functionalized
imidazolium-based ILs. Recent studies have however shown that
dicyanamide ([N(CN)2]−) anion is able to give highly fluid and
conductive ILs, [31–33] which are generally characterized by toxicity
levels at least to aquatic species (algae and crostaceous) lower than
analogous salts bearing other common anions, such as tetrafluoroborate
and trifluoromethanesulfonate [34].
In this framework of sporadic studies on this class of ILs, we
conducted a systematic characterization of the physico-chemical and
solvent properties of N-alkyl-N-methylmorpholinium dicyanamide
(alkyl = ethyl-nonyl), exploring the effect of the alkyl chain length on
several properties (including density, viscosity and conductivity) and
on the degree of association between the ions. The mesoscopic order
in these ionic liquids has also been explored using X-ray scattering
techniques, in order to extract information on their propensity to
structurally organize via alkyl tail segregation, a phenomenon that
is ubiquitous in ionic liquids (see e.g. [35–41]), but was not so far
observed in ILs bearing cyclic ether functionalized cations (we mention
that the Samanta's study [16,17] provides preliminary indication of
such a behavior also in morpholinium-based salts).
ionization spectra were recorded on a Finnigan LCQ Advantage (Thermo
Finnigan, San Jose, CA, USA) ion trap instrument equipped with an Ex-
calibur software. Solid samples were dissolved in proper volume of ace-
tonitrile (HPLC grade) and injected directly (20 ppm) by a pump
syringe (5 μl/min) in the ESI-MS system. The MS spectra were
performed in the 50–2000 m/z range by the following ESI-MS
parameters: positive mode–sheath gas flow 20 arb., spray voltage
4.5 kV, capillary temperature 280 °C, capillary voltage 24 V, tube
Lens Offset 45 V; negative mode–sheath gas flow 20 arb., spray voltage
4.5 kV, capillary temperature 280 °C, capillary voltage −16 V, and tube
Lens Offset −25 V.
Density was determined using the vibrating tube method with a
Mettler Toledo DM45 instrument at 25 °C. Conductance measurements
were performed using a CON 510 bench meter supplied with
conductivity/TDS electrode. This electrode has a stainless steel ring
and a cell constant of K = 1.0 cm−1. It also has an inbuilt tempera-
ture sensor for automatic temperature compensation. Viscosity and
rheological measurements were conducted using a Brookfield DV-
II + Pro concentric cylinder viscometer and a rotational rheometer
(Rheostress RS150, Haake). Data were determined at shear rate
ranging from 0 to 200 s−1
.
X-ray scattering experiments were conducted at the high brilliance
beam line ID02 and at the high energy diffraction beamline ID15b of
the European Synchrotron Radiation Facility (ESRF) in Grenoble,
France. The first beamline makes use of an instrumental setup that
covers the momentum range Q = 0.1–2 Å−1. Measurements were
performed at 25 °C using a thermostated bath. The samples were kept
inside a temperature controlled flow-through cell with an internal
diameter of 1.9 mm. The corresponding empty cell contribution
was subtracted. Calibration in absolute units (mm−1) was performed
2. Experimental part
2.1. Synthesis and characterization
The investigated N-alkyl-N-methylmorpholinium dicyanamides
have been synthesized as reported in Scheme 1. In the first step, the
corresponding alkyl bromide and N-methylmorpholine were reacted
in acetonitrile at 70 °C to produce N-alkyl-N-methylmorpholinium
bromides, [Mor1,n]Br, 1. In the case of short chain bromides, an excess
of the alkyl halide was used to prevent problems arising from evaporation
due to the reaction temperature. Subsequent metathesis of 1 with silver
dicyanamide (Ag[N(CN)2]), freshly prepared by reacting AgNO3 and Na
[N(CN)2] in equimolar quantities, resulted in the corresponding
dicyanamide salts 2, i.e. [Mor1,n][N(CN)2].
with a neat water sample in
conducted at ID15b allowed covering a much larger Q range (up to
20 Å−1), given the higher incoming X-ray energy (Eincoming
a 2-mm capillary. Measurements
=
55.60 keV). The instrumental setup was based on a LINKAM TMS-600
thermostat that allows measurements to be conducted as a function
of temperature, using a 1 mm outer diameter capillary. The correspond-
ing empty cell contribution was subtracted.
All synthesized N-alkyl-N-methylmorpholinium bromides are
colorless solids at room temperatures whereas the corresponding
dicyanamides are colorless liquids. The structures and compositions
of all ILs were confirmed by electrospray ionization mass spectrometry
(ESI-MS), IR, 1H and 13C NMR spectroscopy. 1H NMR and head space
GC–MS analyses were used to check for residues of unreacted reagents
or solvents. On the other hand, ESI-MS spectra showed that species such
as Ag+ or Br−, arising from not completely conducted metathesis
reactions, were not detectable in the samples used for physico-
chemical property determination. Finally, to ensure that the amount
of water and other volatile compounds in ILs was reduced as much as
possible, each IL was kept under vacuum (pressure around 10−3 mbar)
with a stirring at 80–90 °C for 24 h before each test. On the other hand,
measurements conducted in not completely closed systems (for
example, IR spectra) were performed within 1–2 min to reduce the
measurement errors arising from the slight increase in water content.
1H NMR and 13C NMR measurements were conducted on a Bruker
Advance II 250 spectrometer operating at 250 MHz. Electrospray
3. Results and discussion
For all synthesized N-alkyl-N-methylmorpholinium dicyanamides,
fundamental properties including density (ρ), viscosity (η) and conduc-
tivity (σ) have been measured after accurate drying (see above) and the
corresponding data are shown in Table 1. Data for density and viscosity
are plotted as a function of the alkyl chain length in Fig. 1.
The density values of the eight investigated morpholinium salts lie in
the range 1.17 to 1.03 g/cm3. Thus, a moderate effect of cation structure
on density can be evidenced: density decreases as the alkyl chain on the
cation gets longer; moreover indications of an odd–even effect can be
observed, the odd members showing a significantly higher density
trend than the one followed by the even members of the homologous
series.
Dicyanamide-based ILs are known to be characterized by small
viscosity [31,32]; however, the inclusion of the ether oxygen in the
Scheme 1. Synthetic route to prepare N-alkyl-N-methylmorpholinium dicyanamide ILs.

254
O. Russina et al. / Journal of Molecular Liquids 187 (2013) 252–259
Table 1
Selected physical properties of [Mor1,n][N(CN)2] ILs at 25 °C.
n
ρ [g/cm3]
σ [mS cm−1
]
η [cP]
2
3
4
5
6
7
8
9
1.1682
1.1637
1.1183
1.1224
1.0697
1.0631
1.0347
1.0325
n.a.
n.a.
1.340
1.410
0.724
0.701
0.260
0.209
0.156
410.93
398.82
509.05
518.24
598.20
666.55
721.09
morpholinium ring [42] leads to high values for the viscosity of the
studied compounds. The alkyl chain length exerts a significant effect
on the viscosity (see Fig. 1), as well. Consistently with the proposal
from Bonhote et al. [43] and Tokuda et al. [44], the upraise in viscosity
upon chain length increasing can be explained as due to the increasing
inductive forces between alkyl chains with increasing number of
methylene units, which enhance the frictional forces between ions,
aggregates, and clusters, and the corresponding decreasing of electrostatic
interaction between the ions with increasing number of methylene units.
Similar trend has recently been observed on a large series of
bis(trifluoromethanesulfonyl)imide-based salts (namely 1-alkyl-
3-methylimidazolium [Tf2N], alkyl = ethyl-dodecyl) [45].
In Fig. 1, an odd–even alkyl chain length effect on viscosity can be
observed: namely the salts having odd alkyl chain lengths exhibit
viscosity values significantly higher than those of adjacent salts having
even alkyl chain lengths. This behavior was also recently observed in
the series of imidazolium based [Cnmim][Tf2N] RTILs, where data for
1 ≤ n ≤ 9 were reported [46]. Dynamic viscosities of all the investigated
salts follow a rather similar trend that deviates from the Arrhenius one
(Fig. 2) and data have been modeled with the Vogel–Tamman–Fulcher
(VTF) equation:
Fig. 2. Temperature dependence of the viscosity (and corresponding fit according to
Eq. (1)) for the series of N-alkyl-N-methylmorpholinium dicyanamides, [Mor1,n][N(CN)2]
(n = 3–9), as a function of the alkyl chain length.
viscosity and a corresponding decrease in conductivity, in agreement
with findings for other RTILs [44,47].
The conductivity for the different salts [Mor1,n][N(CN)2] salts (with
varying n) follows a non-Arrhenius behavior (Fig. 3), similarly to the
viscosity results; conductivity data were modeled using the Vogel–
Tamman–Fulcher (VTF) law (Eq. (2)):
σ ¼ σ0exp½−B=ðT−T0Þꢀ
ð2Þ
where the σ0, B and T0 parameters are reported in Table 3.
The pre-factor σ0 follows the same decreasing trend on increasing
the alkyl chain length observed with electrical conductivity at room
temperature, whereas the Vogel temperatures (To) are characterized
by a moderate increase upon increasing the alkyl chain length, B tends
to progressively decrease with n. As previously observed for other ILs,
the good data description by means of the VFT equation, compared to
the Arrhenius one may be a consequence of the conductivity mechanism
being affected by association phenomena, ion pairing or formation of
ionic clusters [48]. The ability of these ILs to give ionic clusters has
been shown also by ESI-MS measurements (vide infra). Nevertheless,
another way of assessing ionicity of ILs is to use the classification
diagram based on the classical Walden rule. This rule relates ionic mobil-
ities, represented by the equivalent conductivity (Λ), to the fluidity (or
reciprocal viscosity (1/η)) of the medium through which the ions
move. Fig. 4 shows the equivalent conductivity versus the reciprocal
viscosity in logarithmic form for the present morpholinium salts at
25 °C.
η ¼ η0exp½B=ðT−T0Þꢀ
ð1Þ
where η0 (cP), B (K) and T0 (K) are fitting parameters. The best-fit
parameters are given in Table 2.
Upon increasing the alkyl chain length, one observes a decrease of ηο
(similarly to what has been recently reported for [Tf2N]-based salts
[45]) and To (which seems to reach a plateau level for long enough
alkyl chains); on the other hand the values found for B tend to progres-
sively increase.
The alkyl chain length also plays a noticeable effect on the conduc-
tivity of the studied salts; longer alkyl chains lead to an increase in
Salts bearing a chain from propyl to hexyl lie on or around the so-
called “ideal” Walden product line. The other morpholinium salts
(heptyl–nonyl) are characterized by conductivities significantly lower
than the ideal behavior and a trend can be observed on increasing
alkyl chain from [Mor1,3] to [Mor1,9]. Although, the interpretation of
IL's Walden plots with respect to their deviation from the so-called
Table 2
VTF fitting parameters (Eq. (1)) for the viscosity data of [Mor1,n][N(CN)2] ILs.
n
η
o [cP]
B [K]
505.2
676.7
911.9
To [K]
3
4
5
6
7
8
9
0.6914
0.3157
0.1287
0.1112
0.0502
0.0458
0.0376
218.6
203.4
188.0
183.9
169.2
167.6
167.1
964.3
1209.3
1250.2
1291.3
Fig. 1. Density (blue squares) and viscosity (red circles) of N-alkyl-N-methylmorpholinium
dicyanamides, [Mor1,n][N(CN)2], as a function of the alkyl chain length at 25 °C.

O. Russina et al. / Journal of Molecular Liquids 187 (2013) 252–259
255
Fig. 3. Conductivity of [Mor1,n][N(CN)2] (n = 3–9) and the corresponding fit in terms of
the VFT law (Eq. (2)).
Fig. 4. Walden plot (log Λ vs log (1/η), where Λ is the equivalent conductivity and 1/η is
the fluidity) for [Mor1,n][N(CN)2] (n = 3–9).
ideal KCl line (determined with 1 mol L−1 aqueous KCl solution [49])
has been recently questioned [50], the present Walden plot prompts
for the existence in the neat ILs of aggregates whose dimensions, or
association degree, are influenced by the alkyl chain length. In view of
these data (together with the ESI-MS experiments that will be discussed
in the following), one can envisage that for medium-short chains, upon
increasing the alkyl chain length (e.g. [Mor1,n][N(CN)2] with n = 3–6),
the steric effects and dispersive interactions lead to a decrease of the
Coulombic interaction extent, probably giving rise to labile charge-
driven aggregates; accordingly the corresponding ILs appear to have
high ionicity [44]. In the case of longer alkyl tails ([Mor1,n][N(CN)2]
with n N 6), dispersive forces among alkyl chains become dominating,
leading to the formation of large dispersion-driven aggregates that can
strongly diminish the conductivity performances: accordingly long
chained morpholinium dicyanamides can be considered as “poor ILs”.
In Fig. 5, conductivity data for the whole set of samples at 25 °C are
shown. Once again the existence of a distinct even–odd effect can be
appreciated in this series. Similar trend was found by Leys and co-
workers in the series of [Cnmim][BF4] salts, although, in that case
the effect was not as clear as it appears in our present data set [47].
The enhanced viscosity (and the reduced conductivity) of the odd
members as compared to the even ones suggests that the alkyl tails of
the studied compounds are densely packed into alkyl domains. Such a
structural feature has been proposed in the last few years on the
basis of several computational [51–55] as well as experimental
[37,38,46,56–71] evidences. Furthermore, on the basis of fluores-
cence data, Samanta and coworkers recently proposed a high degree
of structural organization in morpholonium-based RTILs [16,17], that
can be tentatively related to the formation of mesoscopic domains of
alkyl chains segregating from the charged environment. As a matter of
fact, a nano-scale segregation model for the side alkyl chains in these
salts is confirmed by our synchrotron Small-Wide Angle X-ray Scattering
data. In Fig. 6 we show the diffraction patterns from a series of [Mor1,n
[N(CN)2] salts (namely n = 2, 4–6, 8, 9).
]
These data are characterized by two amorphous halos for
Q b 20 nm−1. At high Q (Q N 7 nm−1) a complex peak superposition
is found. A more detailed study on a high energy X-ray diffraction
beamline allowed a better characterization of this complex diffrac-
tion pattern. In Fig. 7, we show that the amorphous halo between 7
and 30 nm−1 is built up by the superposition of at least three different
peaks. For the cases of [Mor1,n][N(CN)2] with n = 2 and 8 (for which
high Q data are available) it appears that the peaks' positions (that are
the fingerprint of structural correlations both at intra- and inter-
molecular level) are rather unaffected by the alkyl chain length and
only their amplitude changes with n, presumably as a consequence of
their being built up by a combination of both negative and positive
contributions to the experimental I(Q). (see e.g. references [54,55])
By fitting these data sets with three Gaussian functions, we estimate
that the peaks' positions are 10.2, 14.2 and 21.4 nm−1 (the centers of
these peaks are indicated by arrows in Fig. 7).
These features correspond to structural correlation of sizes: 6.2, 4.4
and 3 Å, respectively. It is very likely that the combination of
these peaks corresponds to the structural correlations that were identi-
fied by Binnemans and coworkers in their study on longer chain
morpholinium salts which showed smectic order: as a matter of fact
they refer to diffuse signals at 4.7 and 5.0 Å [14]. We stress however,
Table 3
VTF fitting parameters (Eq. (2)) for the conductivity data of [Mor1,n][N(CN)2] ILs.
n
σ
0 (mS cm−1
)
B (K)
T0 (K)
3
4
5
6
7
8
9
0.134
0.061
0.072
0.062
0.016
0.014
0.011
484
305
395
363
306
283
267
193
217
212
216
223
230
235
Fig. 5. Alkyl chain length dependence of the conductivity of for [Mor1,n][N(CN)2] (n = 3–9)
at 25 °C.

256
O. Russina et al. / Journal of Molecular Liquids 187 (2013) 252–259
Fig. 8. Alkyl chain length dependence of the characteristic size (D) of the structural hetero-
geneities fingerprinted by the low Q peak in the diffraction patterns from [Mor1,n][N(CN)2]
(n = 2–6, 8, 9) at ambient temperature. The red line represents the linear fit to the data
for n ≥ 4.
Fig. 6. SWAXS data for the series of [Mor1,n][N(CN)2] (n = 2, 4–6, 8, 9) at ambient
temperature.
that the present set of salts has a relatively short chain length and so
none of them shows hints of smectic structural organization. A more
careful rationalization of these results will be presented in a following
contribution reporting on joined computational and experimental
data, aiming to carefully characterize the structural organization in
these salts.
the one previously reported for other long alkyl chain RTILs and it
is noticeable that it corresponds to approximately twice the van der
Walls' radius of a CH2 group. Such a finding led some of us to propose
that the segregated clusters are composed of non interdigitating alkyl
chains [56,72].
The diffraction patterns are furthermore characterized by a distinct
low-Q peak (Q b 7 nm−1), whose position and amplitude is, on the
other hand, strongly dependent on the alkyl chain length. This is a
structural feature that turned out to be ubiquitous in RTILs bearing
long alkyl chains and reflects the occurrence of alkyl chains self-
assembling as a consequence of their segregation from the charged
environment (composed by the anions and the charged cations'
heads) that is built up by the strong, long ranged coulombic interactions.
These mesoscopic heterogeneities are characterized by a spatial scale, D,
that can be estimated on the basis of the Bragg's law (D = 2π/Q, where
Q is the peak position in the reciprocal space). In Fig. 8 we report the
alkyl chain dependence for this characteristic spatial scale. Similarly to
other RTILs, the observed trend is appreciably linear in the probed
chain length range (when n N 4); in particular the slope of this linear
trend is: ∂DMorpholinium/∂n = 2.3 Å/CH2 unit. This value is similar to
We stress here that at odd with other cases of ILs, where the low Q
peak had been reported as a function of the alkyl chain length, in the
present case, a clear indication of the existence of such a diffraction
feature is already observed for as short alkyl chain as the ethyl. In
the case of aprotic ILs this is a totally new result, as in general the
first hint for a low Q peak or shoulder is observed only for n N 4–6,
depending on the cation head. In the present case, similarly to the
case of protic ILs (e.g. alkylammonium nitrates) [39,62,73–78], a
chain as short as the ethyl is already long enough to determine the
formation of alkyl tails clusters. Molecular dynamics simulations
are currently under way in order to try to rationalize this peculiar
observation that might be a specific effect due to the high dipole of
the cation polar head.
Fig. 9. SWAXS data for [Mor1,8][N(CN)2] at different temperatures between −80° and
60 °C. In the inset the temperature dependence of the characteristic size of the structural
heterogeneities is reported.
Fig. 7. SWAXS data for [Mor1,n][N(CN)2] (n = 2, 8) at ambient temperature, highlighting
the complex nature of the amorphous halo centered at ca. 15 nm−1
.

O. Russina et al. / Journal of Molecular Liquids 187 (2013) 252–259
257
A more careful inspection of the low Q portion of the diffraction data
indicates that the mentioned peak has a complex nature and appears as
built up by two separate contributions. This is a new effect that was not
previously observed in other ILs. Such a feature can be better evidenced,
when measuring the diffraction patterns as a function of temperature.
In Fig. 9 we show the temperature dependence of the diffraction pattern
for [Mor1,8][N(CN)2], between −80° and 60 °C. It can be appreciated
that at very low temperature (−80 °C, black line), the low Q feature is
composed by a strong contribution centered at ca. 0.25 Å−1 and a
weak shoulder centered at ca. 0.35 Å−1. Upon increasing temperature,
the low Q component's amplitude decreases and, for example, at 60 °C
(red line) the two components have almost comparable amplitude.
We tentatively relate the observed complex behavior found in the
present case to the recent report from Margulis and coworkers that
found strong correlations between the liquid morphology and the
corresponding structure observed in the crystal phase of a given IL
[54]. In particular the authors propose that many of the crystal struc-
tural features persist in an average way in the liquid phase and that
the alternating nature of the polar components, which tend to stay
away from the nonpolar alkyl tails, corresponds to what is found in
the solid state.
be reliably determined and its temperature dependence is plotted
in the inset of Fig. 9.
It can be noticed that the spatial extent of the structural heterogene-
ities fingerprinted by the low Q peak increases from ca. 21 Å (at 60 °C)
up to 24 Å when approaching the IL's glass transition. We already
reported that the low Q peak does not shift further when the sample
is cooled below the glass transition, as a consequence of the structural
arrest in that regime [79]. When heating the sample, the characteristic
size progressively decreases: this behavior that is common with other
observations for RTILs, is at odd with the trend that would be expected
on the basis of the density's temperature dependence [79].
Finally, in order to provide a clearer picture of the clustering behavior
in this class of ILs, ESI-MS experiments on diluted acetonitrile solutions of
[Mor1,n][N(CN)2] (n = 2–9) salts were performed under conditions able
to give information on IL propensity to structurally organize into aggre-
gates or clusters of the constituent ions [80–85]. The positive ion ESI
mass spectra exhibited parent peaks (C+) at m/z values corresponding
to the expected cations, while, on the other hand, the anion parent
peak (A−) has never been observed. Nevertheless, aggregates based on
cation–anion clusters characterized the negative and positive spectra of
all examined ILs (in Fig. 10 we report selected spectra). In particular, in
agreement with previous reports for the imidazolium salts, ILs bearing
short alkyl chains (n = 2–5) presented both C+ and CA2− (CA2− indicates
an ionic cluster composed of a cation and two anions) as the major peaks
in positive and negative mode, respectively.
A quantitative fitting of the experimental data did not provide
reliable results, also due to the presence of a third component (located
at higher Q, at ca. 0.5 Å−1) that becomes apparent at high enough
temperatures. However, the position of the low Q component can
Fig. 10. Relative percentages of the more relevant peaks observed in the positive and negative ESI-MS spectra of selected [Mor1,n][N(CN)2] salts (n = 2,4,6,8).

258
O. Russina et al. / Journal of Molecular Liquids 187 (2013) 252–259
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[N(CN)2] (the alkyl chain ranging from ethyl to nonyl) RTILs, a largely
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Acknowledgements
We acknowledge the European Synchrotron Radiation Facility for
the provision of synchrotron radiation facilities and would like to
thank Dr. T. Narayanan and M. Di Michiel for their kind and competent
assistance in exploiting beamline ID02 and ID15b, respectively. A.T.
acknowledges support from FIRB-Futuro in Ricerca (RBFR086BOQ)
and PRIN (2009WHPHRH). C.C. acknowledges support from Fondazione
CARIPI.
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