Thermal and structural study of the crystal phases and mesophases in the lithium and thallium(I) propanoates and pentanoates binary systems: Formation of mixed salts and stabilization of the ionic liquid crystal phase

Article abstract of DOI:10.1021/jp1031702

The temperature and enthalpy vs composition phase diagrams of the binary systems [xC₂H₅CO₂Li + (1 - x)C ₂H₅CO₂Tl], and [x(n-C₄H ₉CO₂Li) + (1 - x)n-C₄H₉CO ₂Tl], where x is the mole fraction, were determined by DSC. Both binary systems display the formation of one 2:1 mixed salt each (at x = 0.667) that appear as a peritectic (incongruent melting) at T_fus = 512.0 K, and T_fus = 461.1 K, with ??_fusH_m = 13.76 and 8.08 kJ?·mol^-1 for Li-Tl (I) propanoates, and n-pentanoate mixed salts, respectively. The thermotropic liquid crystal of the thallium(I) n-pentanoate transforms into a more stable liquid-crystal phase, which appears in the phase diagram between 380 and 488 K and for x = 0 up to x = 0.56. The crystal structure of thallium(I) propanoate and of the two mixed salts were obtained via X-ray synchrotron radiation diffraction measurements. These compounds present a bilayered structure similar to the two pure lithium salts previously found by our group. ? 2010 American Chemical Society.

Full text of DOI:10.1021/jp1031702

J. Phys. Chem. B 2010, 114, 10075–10085
10075
Thermal and Structural Study of the Crystal Phases and Mesophases in the Lithium and
Thallium(I) Propanoates and Pentanoates Binary Systems: Formation of Mixed Salts and
Stabilization of the Ionic Liquid Crystal Phase
F. J. Mart´ınez Casado,† M. Ramos Riesco,§ I. da Silva,‡ A. Labrador,† M. I. Redondo,§
M. V. Garc´ıa Pe´rez,§ S. Lo´pez-Andre´s,| and J. A. Rodr´ıguez Cheda*,§
BM16-Laboratori de Llum Sincroto´ (LLS), c/o European Synchrotron Radiation Facility, 38043 Grenoble,
France; BM25-Spline, c/o European Synchrotron Radiation Facility, 38043 Grenoble, France; Departamento
de Qu´ımica F´ısica I, Facultad de Ciencias Qu´ımicas, UniVersidad Complutense, 28040 Madrid, Spain; and
Departamento de Crystalograf´ıa y Mineralog´ıa, Facultad de Ciencias Geolo´gicas, UniVersidad Complutense,
28040 Madrid, Spain
ReceiVed: April 8, 2010; ReVised Manuscript ReceiVed: June 18, 2010
The temperature and enthalpy vs composition phase diagrams of the binary systems [xC2H5CO2Li + (1 -
x)C2H5CO2Tl], and [x(n-C4H9CO2Li) + (1 - x)n-C4H9CO2Tl], where x is the mole fraction, were determined
by DSC. Both binary systems display the formation of one 2:1 mixed salt each (at x ) 0.667) that appear as
a peritectic (incongruent melting) at Tfus ) 512.0 K, and Tfus ) 461.1 K, with ∆fusHm ) 13.76 and 8.08
kJ · mol-1 for Li-Tl (I) propanoates, and n-pentanoate mixed salts, respectively. The thermotropic liquid
crystal of the thallium(I) n-pentanoate transforms into a more stable liquid-crystal phase, which appears in
the phase diagram between 380 and 488 K and for x ) 0 up to x ) 0.56. The crystal structure of thallium(I)
propanoate and of the two mixed salts were obtained via X-ray synchrotron radiation diffraction measurements.
These compounds present a bilayered structure similar to the two pure lithium salts previously found by our
group.
1. Introduction
mesomorphism, even when none of both components are
mesogens, but “potentially mesogenic compounds”. According
to these authors, the reason for the appearance of an ionic liquid
crystal mesophase would be the different size of the cations,
which increases the electrical anisotropy of the carboxylates,
favoring the formation of the liquid-crystal mesophases.
A variety of binary systems with common cations or common
anions formed by alkali alkanoates have been analyzed, and
some of them were reviewed and critically evaluated.16 In
particular, some of the already studied binary systems are the
ones between lithium and cesium alkanoates, from formiates
to butyrates.17 In these four cases, a mixed salt with the
composition 2:1 between LiCn:CsCn (where n is the total
number of carbons in the alkanoate anion, hereafter) is formed,
and a liquid-crystal phase appears from the acetates mixture,
even when the shortest members of the CsCn and LiCn series
presenting mesogenicity are the ones with 6 and 12 C atoms,
respectively. In addition, the formation of two mixed salts (with
a composition of 1:1 and 2:1) and a stable liquid crystal phase
was detected in the LiC4-RbC4 binary system, studied by us.18
The size of thallium(I) is comparable to the cesium or rubidium
ones: thus, this cation could be appropriate in order to stabilize
the probable liquid-crystal phase in the mixtures with LiCn
compounds. Moreover, thallium(I) is a low polarizing cation
that forms weaker bonds than alkali cations, and the melting
points of TlCn compounds are much lower than those of the
corresponding alkali carboxylates, and so the mesogenic phase
in their binary systems are expected to appear at even lower
temperatures than for pure TlCn members.
Polymorphism and/or polymesomorphism are one of the most
interesting aspects of the organic salts in general, and of metal
alkanoates in particular. The stepwise melting process from the
totally ordered crystalline phase at very low temperature until
the isotropic liquid can be rather complicated, involving all of
the transitions taking place as solid-to-solid, solid-mesophase,
mesophase-mesophase, and melting. Thereby, different solid
(plastic crystal, rotator,1,2 or condis3) or fluid mesophases (liquid
crystal), as well as polymorphs in the crystal phase can be found
in these salts.
Although liquid-crystal formation in alkali soaps was dis-
covered by Vorlander4 in 1910, it is only after five decades that
the organic salts are being widely studied, providing an
important advance in the characterization of different meso-
phases. Many workers have studied in detail the mesomorphism
of alkali, alkaline earth, and transition metal soaps, often finding
very complex phase behavior. Numerous reviews5-14 on organic
salts and their formation of ionic liquid crystals are available.
The long-chain members of the organic salt series have
received more attention than the short-chain ones because they
form thermotropic mesomorphs more easily. The first member
exhibiting thermotropic mesophase depends on the metal; e.g.,
for the sodium or thallium series, they are the propanoate and
pentanoate members, respectively.5 Mirnaya and co-workers15
have investigated many binary phase diagrams of short-chain
metal alkanoates and reported the surprising formation of
* Corresponding author. Tel.: +91-3944306. Fax: +91-3944135. E-mail:
cheda@quim.ucm.es.
Thallium(I) and lithium alkanoates (TlCn and LiCn) have
been widely studied in the past decades by several techniques,
such as adiabatic calorimetry,19-30 DSC,31-44 DTA,45,46 XRD,47,48
and others.49 However, many aspects about the thermal behavior,
† BM16-Laboratori de Llum Sincroto´ (LLS).
‡ BM25-Spline.
§ Departamento de Qu´ımica F´ısica I, Universidad Complutense.
| Departamento de Crystalograf´ıa y Mineralog´ıa, Universidad Complutense.
10.1021/jp1031702  2010 American Chemical Society
Published on Web 07/19/2010

10076 J. Phys. Chem. B, Vol. 114, No. 31, 2010
Mart´ınez Casado et al.
TABLE 1: Experimental Parameters and Main Crystallographic Data for the Studied Compounds
data
TlC3
Li2Tl(C3)3
empirical formula
TlC3H5O2
277.44
orthorhombic
Pca21 (29)
Li2TlC9H15O6
437.46
orthorhombic
P212121 (19)
Mr (g·mol-1
)
crystal system
space group (no.)
crystal size (mm)
temperature (K)
a (Å)
0.08 × 0.04 × 0.01
0.05 × 0.03 × 0.01
298(2)
298(2)
7.3020(15)
10.636(2)
21.028(4)
1633.1(6)
4.9090(10)
11.892(2)
23.014(5)
1343.5(5)
4
b (Å)
c (Å)
V (Å3)
Z
12
λ (Å)
0.7513
3.390
32.946
2535
1584
165
0.9786
2.163
17.265
1135
1035
165
Dc (g·cm-3
)
µ (mm-1
)
reflection collected
reflections with I > 2σ(I)
parameters refined
hydrogen treatment
R-factor
H-atom parameters not refined (geom.)
H-atom parameters not refined (geom.)
0.0700
0.2100
1.020
0.0520
0.1316
1.114
wR-factor
goodness of fit
CCDC deposition numbers
770314
770315
TABLE 2: Temperatures, Enthalpies, and Entropies of the Transitions of the Pure Compounds
compound
LiC3
transition
T/K
∆H/kJ·mol-1
∆S/J·mol-1 ·K-1
SII-SIa
SI-IL
549.7 ( 0.7
606.4 ( 0.5
205.5 ( 0.5
325.2 ( 0.7
576.5 ( 0.3
278.3 ( 0.5
365.8 ( 0.4
466.6 ( 0.5
205.2 ( 0.4
219.2 ( 0.5
234.3 ( 0.3
283,8 ( 0.3
351.9 ( 0.3
453.1 ( 0.5
487.6 ( 0.4
367.7 ( 0.3
512.0 ( 0.7
406.6 ( 0.5
461.2 ( 0.3
3.1 ( 0.1
16.3 ( 0.2
1.31 ( 0.05
3.0 ( 0.1
5.6 ( 0.2
26.9 ( 0.4
6.3 ( 0.3
9.3 ( 0.3
37.71 ( 0.07
14.2 ( 0.2
1.0 ( 0.1
22.1 ( 0.2
2.0 ( 0.3
0.4 ( 0.2
1.7 ( 0.3
8.5 ( 0.1
5.6 ( 0.2
13.1 ( 0.2
6.9 ( 0.1
13.3 ( 0.2
81.8 ( 0.4
16.7 ( 0.2
52.7 ( 0.5
LiC5
TlC3
TlC5
SIII-SII
SII-SI
SI-IL
21.74 ( 0.03
3.94 ( 0.05
0.39 ( 0.04
10.3 ( 0.1
0.40 ( 0.05
0.08 ( 0.04
0.40 ( 0.05
2.41 ( 0.03
1.96 ( 0.05
5.92 ( 0.05
3.36 ( 0.04
4.9 ( 0.05
41.9 ( 0.2
8.0 ( 0.1
SIII-SII
SII-SI
SI-IL
SVI-SV
SV-SIV
SVI-SIII
SIII-SII
SII-SI
SI-LC
LC-IL
b
Li2Tl(C3)3
Li2Tl(C5)3
SII-SI
incongruent fusion
SII-SI
b
incongruent fusion
24.3 ( 0.2
a Referred to the S-S transition in the first heating of the LiC3. b Values referred to the mixed salts as pure compounds (the molecular
weight of these salts is 3 times the average molecular mass weight of the mixtures with that composition).
n-pentanoates diagram. Special effort was paid to solve the
crystal structures of the pure and mixed salts in order to infer
information of their thermal behavior and properties.
the crystal phase structures, and their mesophases are still not
explained. Other reasons to have chosen TlCn and LiCn for
this study were also the presence of the condis phase in the
TlCn series,37 and a possible rotator phase in the LiCn series.44
2. Experimental Section
Moreover, a mixed salt with different-sized cations can allow
the movement of the smallest one (Li), and an order-disorder
transition could appear, accompanied by an increase of its
electrical conductivity.
2.1. Pure Components. The thallium(I) salts were prepared
by chemical reaction between the corresponding acids (propionic
acid, Fluka, g 99.5%, and valeric acid, Fluka, g 99%), and
thallium(I) carbonate (Riedel-de Hae¨n, g 99%), in methanol
(Merck, g99.8%), being later recrystallized several times in an
1:1 ethanol/ether mixture (Merck, g99.8%, and Fluka, > 99.8%,
respectively), and vacuum-dried.37,46 LiC3 and LiC5 were
prepared as described elsewhere.44 Special attention was paid
to the purity of all the samples to avoid the formation of acid
soap or the presence of water.50-56
The aim of the present paper is to study the possible liquid-
crystalline mesomorphism15,50 in mixtures of nonthermotropic
salts containing a common organic anion and two metallic
cations, very different in size. In particular, we present a
thorough study of the binary phase diagrams of lithium and
thallium(I) propanoates and pentanoates over the whole com-
position range by means of DSC, XRD, FTIR, and polarizing
light microscopy. A further target is to provide a good
description of the mixed salts for both systems and of the liquid-
crystal region in the case of the lithium and thallium(I)
The purity was assessed by DSC, obtaining values of 99.72,
99.91, 99.97, and 99.98% for TlC3, TlC5, LiC3, and LiC5,
respectively.

Li and Tl(I) Propanoates and Pentanoates Binary Systems
J. Phys. Chem. B, Vol. 114, No. 31, 2010 10077
Figure 1. (A) 3D plot (heat flow vs composition vs temperature) for LiC3 + TlC3 system. (B) Contour plot (with z corresponding to the heat flow)
of the diagram composition vs temperature.
Figure 3. ∆H (per mole of mixture) vs composition plot for the LiC3
+ TlC3 system, for the transitions: (0) solubilization of the TlC3; (4)
SIII-to-SII transition of TlC3; (O) SII-to-SI transition of TlC3; (+)
eutectic reaction; (b) peritectic reaction; (1) solubilization of the mixed
salt (MS); (f) SII-to-SI transition of the mixed salt; ([) SII-to-SI
transition of the LiC3; (9) first solubilization of LiC3; (×) second
solubilization of LiC3.
Figure 2. T vs composition plot for the LiC3 + TlC3 system, showing
the eutectic (E) and the peritectic (P) points, and points T (solid-to-
solid transition of the mixed salt), and M (joint of the two solubilizations
of LiC3). IL, isotropic liquid; ILC, ionic liquid crystal; SN(TlC3), where
N ) I-III, N solid phase of TlC3; SN(LiC3), where N ) I-II, N solid
phase of LiC3.
dissolved the other one, in inert atmosphere, followed by a
solidification and grinding of the mixture (repeating at least 3
times this process),17,57 and (b) dissolving the weighed samples
in methanol (Merck, 99.8%), and then removing the solvent
with a rotavapor; mixtures were finally high vacuum-dried, in
order to get homogeneous and dried powdered samples.18 The
second method of homogenization is preferred as it avoids
heating the samples repeatedly before running the DSC mea-
surements, therefore getting a real first heating on the sample
when measuring by DSC. In both cases, the estimated error in
the molar fraction x was at most ((0.002). The two routes were
followed to prepare the [LiC3 + TlC3] system, showing the
same result, and only the second one was used for the [LiC5 +
TlC5] system.
2.3. Techniques. Differential Scanning Calorimetry. A TA
Instruments DSC, Model Q10, was used to register all the
thermograms. Tightly sealed aluminum pans were used, in dry
nitrogen, flowing at 50.0 mL ·min-1. A MT5 Mettler microbal-
ance was used to weigh the samples, ranging between 3 and 10
mg (with an error of (0.001 mg). The calorimeter was calibrated
in temperature using standard samples of In and Sn, supplied
by TA (purity >99.999% and >99.9%, respectively), and of
Once the binary phase diagrams were solved, the mixed salts,
Li2Tl(C3)3 and Li2Tl(C5)3, were prepared in a different way,
apart from the routes described above. Stoichiometric amounts
(2:1) of LiC3 and TlC3, and of LiC5 and TlC5, respectively,
were dissolved in hot ethanol. When the solution was colled, a
salt precipitated. In the case of Li2Tl(C5)3, it was necessary to
evaporate some ethanol from the solution. In both cases, the
two salts were washed with acetone (Merck, > 99.5%) and
vacuum-dried. The thermal behavior of the mixed salts thus
obtained is similar to that of the mixtures with the same
composition.
All the compounds here synthesized grow into flake-shaped
crystals. Thus, crystallization produced very tiny crystals, whose
sizes are given in Table 1. Crystals used for X-ray studies were
grown by slow evaporation of the solvents from the solutions
of compounds in ethanol and in ethanol/ether (1:1) for
Li2Tl(C3)3 and TlC3, respectively.
2.2. Mixed Salts and Mixtures Preparation. In order to
cover the whole composition range, 20 mixtures (of about 0.3 g
each) were prepared by weighing the proper amounts of both
components in Pyrex vials. Two methods for homogenization
were used: (a) heating the mixtures until one of the components

10078 J. Phys. Chem. B, Vol. 114, No. 31, 2010
Mart´ınez Casado et al.
benzoic acid (purity >99.97%), supplied by the former NBS
(lot 39i), and in enthalpy with the In and Sn standards already
described.
All the measurements were done at a heating rate of 5
K·min-1. All the scans were usually explored from 200 K to
the temperature corresponding to the isotropic liquid in each
case (a maximum of 650 K).
Single-Crystal X-ray Diffraction (SCXRD). Two crystal
structures, for Li2Tl(C3)3 and TlC3, were solved and refined
by means of single-crystal difraction. X-ray measurements were
conducted using synchrotron radiation (with λ ) 0.9786 Å for
Li2Tl(C3)3, and λ ) 0.7513 Å for TlC3) at the BM16 Spanish
beamline of ESRF. Data sets of 360 images were collected in
a ADSC-q210r CCD detector. The oscillation range (∆ꢀ) used
for each image was 1°. Both salts were measured at room
temperature.
Optical Microscopy. To identify the nature of the existing
phases (solid, isotropic liquid, or liquid crystal phases), a Carl
Zeiss-Jena polarizing optical microscope (model Jenalab Pol-
30-G0527), equipped with a LINKAM hot stage (model
THMS600) connected to a LINKAM programmable tempera-
ture-controller (model TMS94), was used.
3. Results
3.1. Thermal Analysis. Pure Components. The solid-to-solid
transitions of LiC3, LiC5, TlC3, and TlC5 were studied by
adiabatic calorimetry28,30,37 and by DSC44,31,32,46 in the super-
ambient region (including here the melting and clearing
processes). However, DSC thermograms were carried out again
on the pure samples used in the present work to prepare all of
the mixtures, which data are listed in Table 2, and are in fair
agreement with the previously reported values.
Binary Systems. (a) LiC3 + TlC3. To study this phase
diagram, a total of 29 mixtures of different molar fractions were
prepared (by the two methods described in the Experimental
Section). The output data obtained by DSC were normalized to
“heat flow per mole of mixture” in order to make thermograms
of different mixtures comparable with each other. Thus, a three-
dimensional (3D) representation heat flow/temperature/molecular
fraction (Figure 1A) could be built. The 2D projection of the
former representation is also given in Figure 1B, showing the
construction of the T vs x phase diagram. In order to make easier
the construction of the phase diagram, only the pure TlC3 and
two more mixtures were measured from 250 K just to confirm
that the solid III-to-solid II transition of the pure salts appears
as an invariant in the range 0 e x(LiC3) < 0.667.
In Figure 2 the complete T vs x phase diagram is illustrated.
In all the defined regions, except in the isotropic liquid (IL),
two separated phases are observed. ∆H vs x phase diagram
(shown in Figure 3 for all of the reactions and characteristic
points) has been developed following Tammann’s method. This
method is used commonly in the construction of the phase
diagram to help in finding the characteristic points or invariants
(isotherms), by plotting enthalpies (per mole of mixture) vs
composition for each process (e.g., eutectic reaction).18,60
The characteristic reactions, eutectic and peritectic, in this
binary system are given in Scheme 1.
The structures of these compounds are bilayered (ionic and
lipidic alternating layers), as can be observed from the solved
crystals. Thus, the joint between these latter layers is very weak,
and it is of great difficulty to obtain good crystals due to the
ease with which they exfoliate. The biggest single crystals
obtained of these salts presented a size of 0.05 × 0.03 × 0.01
and 0.08 × 0.04 × 0.01 mm for Li2Tl(C3)3 and TlC3,
respectively. Furthermore, crystals for both compounds do not
diffract beyond 0.9 Å, and also suffer severe radiation damage,
specially TlC3, so the data presented a great intricacy to be
treated, but they were still reliable enough to solve the structures.
The structures were solved by direct methods and subsequent
Fourier syntheses using the SHELXS-97 program58 and were
refined by the full-matrix least-squares technique against F2 with
the SHELXL-97 program,59 in anisotropic approximation for
all non-hydrogen atoms being all the parameters for the
hydrogen atoms calculated geometrically in both compounds.
An absorption correction was done for both crystals, due to the
presence of heavy atoms. The main crystallographic data and
some experimental details are shown in Table 1.
Further crystallographic details for the structures reported in
this paper may be obtained from the Cambridge Crystallographic
Data Center (see Supporting Information).
Powder X-ray Diffraction. High-resolution powder diffraction
(HRPD) measurement was performed at SpLine beamline
(BM25A) of the Spanish CRG at the European Synchrotron
Radiation Facility (ESRF, Grenoble) with a fixed wavelength
of 0.8266 ( 0.0001 Å, at room temperature. The powdered
sample was placed inside a 0.5 mm diameter glass capillary,
which was rotated during exposure. Data collection was done
in a 2θ-step scan mode with 0.015° step and 5 s acquisition
time per point.
As can be appreciated (dotted line in Figure 2), a “premelting”
effect appears just before the eutectic transition (as a small
shoulder before the eutectic peak in these thermograms).
SCHEME 1: Description of the Eutectic and Peritectic
Reactions for the LiC3 + TlC3 Systema
Routine powder XRD measurements were carried out with a
Philips X’Pert PRO MPD X-ray diffractometer with vertical
goniometer θ/θ (Cu KR1 radiation, 1.540 56 Å, Ni filter),
X’Celerator detector, equipped with a high-temperature chamber
Anton Paar HTK1200. In situ XRD area was scanned from 2θ
) 2° to 70° at several temperatures in the range from 273 to
600 K during the heating and the cooling, at a rate of 3 °C/
min.
FTIR Spectroscopy. Mid-infrared spectra of the samples as
KBr pellets and powders between two pure KBr pellets were
recorded at a resolution of 4 cm-1 in a Nicolet Magna 750 FTIR
spectrometer. No significant differences between the spectra
obtained by both procedures were observed. A commercial
variable-temperature cell, SPECAC VTL-2, adapted for solid
samples was employed to obtain IR spectra at different
temperatures.
a All of the phases are solid, except the ones indicated with liq.
(liquid) with the corresponding composition of LiCn.

Li and Tl(I) Propanoates and Pentanoates Binary Systems
J. Phys. Chem. B, Vol. 114, No. 31, 2010 10079
Figure 4. (A) 3D plot (heat flow vs composition vs temperature) for LiC5 + TlC5 system. (B) Contour plot (with z corresponding to the heat flow)
of the diagram composition vs temperature.
The more significant process in this binary system is the
formation of a mixed salt, Li2Tl(C3)3 (with a stoichiometry 2:1).
Li2Tl(C3)3 has a solid-to-solid transition, SII-to-SI (point T and
dash-dotted line in Figure 2), at 367.7 K (with an enthalpy of
4.9 kJ ·mol-1 per mole of mixed salt, or 1.6 kJ ·mol-1 per mole
of mixture). Since the mixed salt is actually a pure compound,
it is important to note that its molecular mass has to be 3 times
the average molecular mass of the mixture with the composition
x(LiC3) ) 0.667, that is, 437.46 and 145.82 g ·mol-1, respec-
tively. The salt melts incongruently at 512.0 K as it is described
in the peritectic reaction. The full thermal data for Li2Tl(C3)3
are shown in Table 2.
(b) LiC5 + TlC5. This phase diagram was studied by
preparing 20 different molar fractions, including the pure
components, as explained in section 2.2. A 3D plot representing
heat flow vs composition vs temperature of several mixtures
and the corresponding contour plot constructing the phase
diagram are shown in Figure 4, A and B, respectively.
The complete T vs x and ∆H vs x phase diagrams are
represented in Figures 5 and 6, respectively. All the defined
regions, except in the isotropic liquid and the ionic liquid crystal
(IL and ILC, respectively), correspond to two phase-separation
regions, as explained for the LiC3 + TlC3 diagram. It is
important to note that the transition SIII-to-SII of the LiC5 could
not be measured in the range of 0.667 < x (LiC5) < 1 due to
the great undercooling of this phase change,44 and so this
transition is represented as a dotted line in Figure 5.
The characteristic reactions of this binary system, and also
the eutectic and peritectic reactions, are described in Scheme
2.
A mixed salt with the composition (2:1) appears, as in the
propanoate phase diagram. This compound was later isolated
by crystallization in ethanol and studied by DSC. Li2Tl(C5)3
has a solid-to-solid transition, SII-to-SI (point T and dash-dotted
line in Figure 5), and an incongruent fusion, as described in
the peritectic reaction. The same considerations are taken for
the molecular mass of this mixed salt, as in the case of the
Li2Tl(C3)3. The thermal data for the Li2Tl(C5)3 are also given
in Table 2.
Figure 5. T vs composition plot for the LiC5 + TlC5 system, showing
the eutectic (E) and the peritectic (P) points, and points T (solid-to-
solid transition of the mixed salt), A (joint of the two solubilizations
of the mixed salt), and M (joint of the clearing reaction with the
solubilization of LiC5). IL, isotropic liquid; ILC, ionic liquid crystal;
SN(TlC5), where N ) I-VI, N solid phase of TlC5; SN(LiC5), where
N ) I-II, N solid phase of LiC5.
pure TlC5 to 110 K in the eutectic mixture (x(LiC5) ) 0.19).
It is remarkable that the fact of finding the clearing reaction of
this ILC phase as an invariant that was not detected in similar
phase diagrams reported.17,18 This indicates the high stability
of the liquid-crystal phase in the binary diagram.
Polarizing light microscopy was used to identify the liquid-
crystal phase as “smectic A-like” or neat phase. In Figure 7,
the typical focal conic domain of a neat phase is shown.
3.2. Structural Analysis of Pure and Mixed Salts. The
structures of TlC3 and Li2Tl(C3)3 (by SCXRD) and of
Li2Tl(C5)3 (by HRPD) were solved in this work. The expected
bilayered arrangement is the main common feature in every
compound, but a great difference in the packing of the ionic
layer (and subsequently in the alkyl chains) was found.
TlC3 was measured at 298 K (SII phase) and presents an
orthorhombic cell in the Pca21 space group, shown in Figure
8A. Three slightly different thallium(I) cations are found, but
sharing the same characteristics: they are coordinated by 6
oxygen atoms with a trigonal prism geometry, which results to
be hemidirected, the thallium being in the same plane as four
of these oxygen atoms (see Figure 8B).
Besides the mixed salt, the most important feature in this
binary system is the large interval of existence of an ionic liquid-
crystal phase (ILC): from x(LiC5) ) 0 to 0.56, point M in Figure
5. The existence of this phase extends as well from 35 K in the

10080 J. Phys. Chem. B, Vol. 114, No. 31, 2010
Mart´ınez Casado et al.
Figure 6. ∆H (per mole of mixture) vs composition plot for the LiC5
+ TlC5 system, for the following main transitions: (×) solubilization
of TlC5; (f) clearing of the ILC phase; (]) SIII-to-SII transition for
TlC5; (4) SII-to-SI transition for TlC5; (O) eutectic reaction; (solid
left-pointed triangle) peritectic reaction; (b) first solubilization of the
mixed salt; (3) second solubilization of the mixed salt; (9) SII-to-SI
transition of the mixed salt; (open right-pointed triangle) SII-to-SI
transition of LiC5; ([) solubilization of LiC5. In the inset B, solid
right-pointed triangle, 9, and O represent SVI-SV, SV-SIV, and
SIV-SIII transitions for TlC5.
SCHEME 2: Description of the Eutectic and Peritectic
Reactions for the LiC5 + TlC5 Systema
Figure 7. Typical fan shape domains of a “smectic A-like” (or neat
phase). White crosses in the corners shows the orientation of the crossed
polars. Pictures taken on a sample of x(LiC5) ) 0.19 (eutectic mixture)
at 460 K.
group; after an automatic intensity extraction procedure, the
statistical algorithm for finding the correct space group66 found,
as the most probable, the P21/c extinction group.
Structure solution, by means of Direct Methods, was at-
tempted with the Expo2004 program, within the P21/c space
group, without success. Thus, a new and successful attempt was
carried out, this time using the Monte Carlo technique, with
the FOX67 program.
a All of the phases are solid, except the ones indicated with “liq.”
(liquid) with the corresponding composition of LiCn.
Atomic coordinates found by FOX were introduced in the
FullProf68 program in order to perform a Rietveld refinement;
the atomic coordinates of the 24 independent non-H atoms were
fitted, but solid rigid constraints on two parts of the alkyl chains
were applied to limit the number of free parameters. Two
different isotropic temperature factors were introduced: one for
the Tl and one for the rest of atoms.
Crystal data for this compound can be found in Table 3, while
in Figure 10 the result of the Rietveld refinement is shown.
Despite belonging to different crystal systems, the molecular
arrangement is practically the same as in the case of Li2Tl(C3)3,
principally in the ionic layer (see Figure 11). Thus, the only
significant and logical difference is the increase in the d-spacing
from Li2Tl(C3)3 to Li2Tl(C5)3.
The crystal for Li2Tl(C3)3 corresponds to the low-temperature
crystalline phase (SII) and presents an orthorhombic unit cell,
with symmetry P212121. The structure presents two bilayers
per unit cell, as can be seen in Figure 8A. The coordination of
the lithium cations is tetrahedral, as in the pure lithium
alkanoates, while, in the case of the thallium atom, four O atoms
coordinate it in a pyramidal geometry, the thallium being in
apical position (see Figure 9B). There are other oxygens
surrounding the Tl atom, but at distances longer than 3.25 Å,
which is considered the limiting distance for a Tl-O bond in
the literature.61 Crystal data for these two salts are given in Table
1.
The structure for Li2Tl(C5)3 was solved by HRPD and by
Monte Carlo methods, and refined by means of the Rietveld
method.62 Reflection positions of the first 20 peaks were
determined using the WinPLOTR63 program and peak indexing
was carried out with the DicVOL0664 program, finding a
monoclinic cell. Expo200465 program was used to find the space
The d-spacings of the bilayers for the initial compounds and
the mixed salts were also analyzed as a function of the
temperature and are shown in Figure 12, indicating the different
ordered phases and reactions. A decrease in the d-spacing in
the SII-SI transitions, which will be discussed later, and the

Li and Tl(I) Propanoates and Pentanoates Binary Systems
J. Phys. Chem. B, Vol. 114, No. 31, 2010 10081
Figure 8. (A) Crystal structure for the TlC3 in the bc projection. (B) Coordination of the thallium atoms in a trigonal prism geometry.
intermolecular end chain interactions that are different in the
two solid phases. This is consistent with the shortening observed
by powder XRD in the d-spacing from SII to SI. No bands
corresponding to the acid in any of the samples are observed,
which shows the high purity of both samples.
4. Discussion
The main feature of the LiC3-TlC3 and LiC5-TlC5 phase
diagrams is the formation of a mixed salt, which occurs at the
same composition (2:1) in both systems. The molecular packing
is the same in both mixed salts: a bilayered arrangement of ionic
(lithium, thallium, and carboxylates ions) and lipidic (alkyl
chains) planes. Consequently, the existence of a mixed dilithium
Figure 9. (A) Crystal structure for the Li2Tl(C3)3 in the bc projection.
and thallium(I) alkanoates series, Li2Tl(Cn)3, can be predicted,
since the growth of the alkyl chain does not affect to the ionic
packing.
(B) Coordination of the lithium (tetrahedral) and thallium atoms
(pyramidal geometry, with thallium in apical position).
decomposition in the peritectic reaction are clearly detected for
both mixed salts. In the case of the Li2Tl(C5)3, the ILC and the
SI(LiC5) phases, both ordered phases, coexist after this peritectic
reaction.
Apparently, no significant differences are found when com-
paring the room temperature crystalline structure of the pure
and mixed salts. However, despite all of them presenting a
bilayered packing, their ionic arrangement differs enough to
enable or disable the movements in the alkyl chains. This fact
can be quantified with the value of the cross-sectional area of
the alkyl chains (S′), that is, the space that the alkyl chain has
to be able to form gauche defects. This parameter depends on
the area per polar head (S),69 and on the torsion (tilt) of the
plane of the chains with respect to the ionic plane (R), with
this relation: S′ ) S cos(R). The torsion is almost negligible for
TlC3 (4-6°), and for Li2Tl(C3)3 and Li2Tl(C5)3, since they have
two of the three chains with 2 and 6° of tilt. In the case of
LiC3 and LiC5 R is 33°. The values of S (and S′, if necessary)
are 25.6 Å2 for TlC3; 19.5 and 19.7 Å2 for Li2Tl(C3)3 and
Li2Tl(C5)3, respectively; and 21.4 (18.6) and 21.8 (18.7 Å2),
for LiC3 and LiC5, respectively. From these results, it is clear
to infer the more packed arrangement in the cases of the lithium
and mixed alkanoates than for the thallium salts. The structure
of the thallium(I) alkanoates was studied previously by Dorfler
et al.,47,48 giving values of around 24 Å2 for S, for longer
members of the series, comparable with the one found in this
work for the TlC3. This great value of S (and S′) explains the
presence of disorder in the chains of the thallium(I) alkanoates
at room temperature, observed by FTIR, forming the so-called
“condis” phase (solid phase where the alkyl chains are confor-
mationally disordered), already studied.37,49 It also fits perfectly
with the difficulty in obtaining good results in the X-ray study
(in single crystal and in powder) of the ionic layer structure in
the compounds of the TlCn series when n g 4: the single-
crystalline flakes were found to be “plastic”, and easily bendable.
The order gets lost in the ionic plane, being only clearly visible
3.3. FTIR Spectroscopy. A FTIR study as a function of
temperature was carried out for Li2Tl(C3)3, Li2Tl(C5)3, TlC3,
and TlC5 in order to study structural changes at the different
phase transitions. The infrared spectra recorded for Li2Tl(C5)3
at room temperature (SII) and after the solid-to-solid transition
(SI) are displayed in Figure 13A. Three bands corresponding
to the asymmetric vibrations of coordinate carboxylate groups
(νas(COO)) appear at 1582, 1560, and 1545 cm-1 in the crystal
at room temperature, according to the three different types of
coordination detected by XRD, but only two of them at 1578
and 1556 cm-1 are observed in the high-temperature solid phase,
SI. The corresponding components for the νs(COO) symmetric
stretching vibration are measured at 1449, 1436, and 1410 cm-1
in the SII, and at 1436 and 1410 cm-1 in the SI. The difference
in wavenumbers between these vibrations is related to the type
of carboxylate-to-metal coordination present in the salt. In this
compound, such difference is between 146 and 138 cm-1 which
is consistent with the bridging and chelating coordination
determined by X-ray data. The frequency of bands correspond-
ing to methyl group deformation and methylene wagging and
rocking vibrations is constant with temperature; only broadening
of these bands is observed as a consequence of the temperature
increase. This would imply that the all-trans order is present in
both phases, SII and SI.
The methyl and methylene stretching vibrations region is
displayed in Figure 13B. The symmetric stretching band
corresponding to the CH3 group shifts from 2955 cm-1 in SII
to 2960 cm-1 in SI. This frequency shift must be related to

10082 J. Phys. Chem. B, Vol. 114, No. 31, 2010
Mart´ınez Casado et al.
TABLE 3: Crystallographic Data, Experimental Parameters and Structure Refinement Data for Li2Tl(C5)3
Crystal Data
formula
Li2TlC15H27O6
521.67
monoclinic, P21/c (14)
298(2)
Mr (g·mol-1
)
cell setting, space group (no.)
temperature (K)
a, b, c (Å), ꢁ (deg)
4.92113(7), 12.0299(2), 33.8176(5), 91.3463(12)
volume (Å3)
2001.46(5)
4, 1.6402
0.8266(1)
3.75
Z, Dc (g·cm-3
)
wavelength (Å)
µ (mm-1
)
Data Collection
Refinement
diffractometer
Spline (BM25A) at the ESRF, Grenoble
borosilicate glass capillary
transmission
2θ-step scan
1-50, 0.015
specimen mounting
data collection mode
scan mode
2θ range (deg), step size (2θ deg)
refinement method
Rp, Rwp, Rexp
full-matrix least-squares on Inet
0.00964, 0.0138, 0.0136
RF, RBRAGG
0.0267, 0.0353
goodness-of-fit
1.901
profile function
pseudo-Voigt with axial divergence asymmetry
no. of contributing reflections
no. of parameters
no. of rigid bodies
CCDC deposition no.
2217
96
6
770316
the d-spacing between bilayers. This value of S can also explain
the great thermal agitation found in the O and C atoms in the
TlC3, where the carboxylate anions could rotate perpendicularly
to the Tl planes.
arrangement was found to be the same for both salts, the
difference in the chain length being the only possible reason
for the difference in the transition enthalpy found by DSC, which
increases with the alkyl chain length. Moreover, the FTIR study
shows that the order in the all-trans conformation is maintained
also in the high-temperature phase (SI). These results point that
the SI could be a rotator phase, as it happens in the lithium and
in the lead(II) alkanoates.44,69,73,74
Besides the formation of the mixed salts, these binary systems
give information about the existence and/or the stabilization of
an ionic liquid-crystal phase (ILC) as a homogeneous region in
the binary phase diagram. On the one hand, when mixing
organic salts with different-sized metal cations, mesogenicity
is favored, and, at the same time, its thermal stability,15 which
is the case for the LiC5-TlC5 binary system. On the other hand,
there is a critical length for the alkyl chain to form ILC phases,
similarly to what happens for pure metal alkanoates series.5
While no liquid-crystalline phase is observed in the LiC3-TlC3
phase diagram, a broad and greatly stabilized ILC phase is found
Thallium(I) is comparable in size and in charge with some
alkaline cations, such as potassium and cesium. A similar
trigonal prism coordination (6 coordinating oxygen atoms), is
found for some potassium alkanoates,70,71 with the only differ-
ence that the potassium cation is inside the prism in these cases,
instead of in its base, as it occurs with the thallium(I) in the
TlC3 (hemidirected coordination). This makes the value of S
for TlC3 (25.6 Å2) larger than the one for KC10 (22.9 Å2). The
packing changes significantly for cesium carboxylates, which
present a cubic coordination (by 8 oxygen atoms) and a S of
22.2 Å2.72
One solid-to-solid transition was found for both dilithium and
thallium(I) mixed salts. The nature of this phase transition may
be explained by the different techniques used here. The ionic
Figure 10. Rietveld refinement for the Li2Tl(C5)3, comparing the
observed and calculated diffraction patterns, their difference, and Bragg
positions.
Figure 11. (A) Crystal structure for the Li2Tl(C5)3 in the bc projection.
(B) Coordination of the lithium (tetrahedral) and thallium atoms
(pyramidal geometry, with thallium in apical position).

Li and Tl(I) Propanoates and Pentanoates Binary Systems
J. Phys. Chem. B, Vol. 114, No. 31, 2010 10083
metal soaps.75 Comparing both binary systems, the effect of the
chain length becomes clearly crucial in the formation of the
ILC phases.
A comparison between the LiC3-TlC3 and the LiC3-CsC3
systems17 will be useful in order to find what the influence of
the different cations is, maintaining the same chain length. The
former does not exhibit a mesogenic phase, but the latter
surprisingly does, even when none of the pure compounds is
mesogenic, showing that the different nature of the cation plays
a basic role in mesogenicity. The chain length and cation size
dependence have been deeply studied in several metal alkanoate
series for the appearance of liquid-crystal phases.5,6,13,14 Taking
into account only the monovalent metal alkanoate series, the
appearance of the liquid-crystal phase starts from n g 4 for
NaCn and KCn, from n g 5 for RbCn and TlCn, from n g 6
for CsCn, and from n g 12 for LiCn,5,6 (being n the total number
of carbon atoms of the shortest member forming this mesophase,
or critical length). In this sense, we may consider these lithium
and thallium alkanoate systems, LiCn-TlCn, as a series itself,
and compare it with those series. The formation of an ionic
liquid-crystal phase can be interpreted on the basis of the size
and polarizability of the cations. Thus, the series with intermedi-
ate-sized cations presents a ILC phase with lower values of n,
0.59, 1.02, 1.38, 1.52, 1.67, and 1.50 Å being the ionic radii
for monovalent Li, Na, K, Rb, Cs, and Tl, respectively.76 This
agrees with the LiC4-RbC4 phase diagram, which presents an
ILC phase, and where n decreases from 5 and 12 (for RbCn
and LiCn, respectively) to 4 (in this binary system).18 This is
the same case for the LiC3-CsC3 system. According to that,
the formation of the ILC phase may be also more favored in
the LiCn-TlCn systems than in pure LiCn and TlCn, but this
phase seems to start with n ) 5, as in the TlCn series. However,
the stability of this mesophase is greatly improved in the phase
diagrams. It is important to remark the low polarizability of
thallium ion, which forms less ionic and weaker bonds than
the alkaline ions. This could be the reason why n does not
decrease from the TlCn series (n ) 5) to the LiCn-TlCn
systems and it does in the case of binary systems with alkaline
atoms. That also explains the lower melting points of thallium(I)
alkanoates in comparison with those of the alkaline ones.
Figure 12. d-spacing vs temperature indicating the different phases,
for (b) LiC3, (9) TlC3, (4) Li2Tl(C3)3, ([) LiC5, (2) TlC5, and (O)
Li2Tl(C5)3. In the case of the mixed salts, the reactions are displays
with dash-dotted lines, where S-S T. ) solid-to-solid transition, P.R.
) peritectic reaction, C.R. ) clearing reaction, and S. ) solubilization.
The resulting compounds after the incongruent melting of the mixed
salts (peritectic reactions) are also indicated.
5. Conclusions
The phase diagrams of the lithium and thallium(I) propanoate
and pentanoates have been thoroughly studied by DSC, XRD,
FTIR, and polarizing light microscopy, in the temperature range
from 200 K up to the temperatures of existence of the isotropic
liquid phase.
Similarities were found in the two phase diagrams studied.
The existence of a mixed salt with a composition of 2:1 (in the
lithium:thallium(I) ratio) has been demonstrated. The two mixed
salts, Li2Tl(C3)3 and Li2Tl(C5)3, have a similar arrangement of
the ions in the ionic layers, the chain length being the only
difference. They have been synthesized and characterized here
for the first time. Furthermore, it has been proved that it is
possible to obtain them by crystallization. Their thermal behavior
is quite similar: both salts present a solid-to-solid transition,
from crystal to a rotator phase, previous to their incongruent
melting point. Similarly to the pure metal alkanoates series, the
mixed salts constitute a series as well: dilithium and thallium(I)
alkanoates.
Figure 13. FTIR spectra for Li2Tl(C5)3 (A) in the ν(COO) region at
T ) 298 and 408 K, and (B) in the ν(CH) region at T ) 298, 398, and
408 K.
in the LiC5-TlC5 system. Thus, the temperature range of
existence of the ILC phase extends from 35 to 108 K, from the
pure TlC5 to the eutectic composition, since the clearing points
remain as an invariant (see section 3.1). Moreover, the tem-
perature of appearance of the ILC decreases to 419.9 K in the
eutectic composition, which is a very low temperature for molten
The stabilization of the ILC phase, which was the initial
desired feature, was found for the pentanoates phase diagram
but not for the propanoates. Once again, the dependence on the
alkyl chain becomes crucial for the existence of an ionic liquid-

10084 J. Phys. Chem. B, Vol. 114, No. 31, 2010
Mart´ınez Casado et al.
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crystalline phase. In the pentanoates binary system, the range
of existence of the ILC phase increases from 35 K, in the pure
TlC5, to 108 K in the eutectic mixture, showing the stabilization
when mixing salts with different sized metal cations, as predicted
by Mirnaya et al.15 In this sense, ILC phases with low melting
points can be used to prepare “ordered solvents” for selective
reactions (“tailor-made” solvents). The liquid-crystal phase
studied here cannot be considered as a “green solvent” because
of the presence of thallium cation, but there is no doubt that it
will help to establish correlation rules to find such kind of
materials.
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Besides lithium propanoate and pentanoate, previously studied
by us,44 three new crystalline structures were also analyzed,
allowing us to study the complex polymorphism in these organic
salts. As can be inferred from this work, there is a direct
dependence between the cross-sectional area of the alkyl chains
and their ability to show chain defects in the different poly-
morphic or mesomorphic states, which form the stepwise
melting process, typical of all the organic salts. The cross-
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mesomorphic phases, such as rotator or condis, which currently
are not well characterized.
Acknowledgment. Partial support of this research by the
Beca de Especializacio´n en Organismos Internacionales (ES-
2006-0024) and by the DGICYT of the Spanish Ministerio de
Ciencia e Innovacio´n (Project CTQ2008-06328/BQU) are
gratefully acknowledged. The authors thank BM16-LLS and
BM25 (both Spanish beamlines at the ESRF, in Grenoble,
France) and the CAIs (Centro de Asistencia a la Investigacio´n)
of XRD and Spectroscopy of the UCM for the use of their
technical facilities.
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Supporting Information Available: The structures reported
in this paper may be obtained from the Cambridge Crystal-
lographic Data Center, on quoting the depository numbers
770314, 770315, and 770316, for TlC3, Li2Tl(C3)3, and
Li2Tl(C5)3, respectively. This material is available free of charge
via the Internet at http://pubs.acs.org.
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