Phase diagrams and optical properties of binary liquid-crystal systems of cobalt(II) caprylate with a divalent metal caprylate

Article abstract of DOI:10.1134/S0036023612080141

Phase equilibria in the binary systems of cobalt(II) caprylate with a divalent metal (lead, cad- mium, or zinc) caprylate were studied by differential thermal analysis and polarization microscopy, and the concentration and temperature ranges of the existence of liquid crystals and glasses were determined. Con- tinuous or boundary smectic liquid-crystal solutions were found to form in all the systems. The temperature and concentration dependences of the electronic absorption spectra of liquid crystals and glasses in binary systems based on cobalt caprylate were investigated. Pleiades Publishing, Ltd., 2012.

Full text of DOI:10.1134/S0036023612080141

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2012, Vol. 57, No. 8, pp. 1141–1145. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © I.I. Tokmenko, T.A. Mirnaya, G.G. Yaremchuk, 2012, published in Zhurnal Neorganicheskoi Khimii, 2012, Vol. 57, No. 8, pp. 1220–1224.
PHYSICAL METHODS
OF INVESTIGATION
Phase Diagrams and Optical Properties of Binary LiquidꢀCrystal
Systems of Cobalt(II) Caprylate with a Divalent Metal Caprylate
I. I. Tokmenko, T. A. Mirnaya, and G. G. Yaremchuk
Vernadskii Institute of General and Inorganic Chemistry, National Academy of Sciences,
pr. akademika Palladina 32/34, Kiev, 03142 Ukraine
Received March 29, 2010
Abstract—Phase equilibria in the binary systems of cobalt(II) caprylate with a divalent metal (lead, cadꢀ
mium, or zinc) caprylate were studied by differential thermal analysis and polarization microscopy, and the
concentration and temperature ranges of the existence of liquid crystals and glasses were determined. Conꢀ
tinuous or boundary smectic liquidꢀcrystal solutions were found to form in all the systems. The temperature
and concentration dependences of the electronic absorption spectra of liquid crystals and glasses in binary
systems based on cobalt caprylate were investigated.
DOI: 10.1134/S0036023612080141
Metal alkanoates are ionic metallomesogens with ponents to obtain information on the structure of
valuable optical, nonlinear optical, and electrophysical caprylate mesomorphic melts and glasses and the
coordination of Co(II) ions in the field of caprylate
ligands.
properties [1]. Optically anisotropic glasses of metal
alkanoates in the form of a frozen liquidꢀcrystal phase
are of particular interest for creating modern optical
materials, media for generation and modulation of laser
radiation, and holographic information recording [2].
A lowꢀmelting glassꢀforming mesomorphic metal
alkanoate composite can be obtained by producing
multicomponent systems or synthesizing lowꢀmelting
glassꢀforming mesogenic metal alkanoates. Among
metal alkanoates, alkanoates of some divalent metals
EXPERIMENTAL
Metal caprylates (С7Н15СОО)2М were produced
by metathesis by adding saturated aqueous solutions of
divalent metal nitrate to a methanol solution of potasꢀ
sium caprylate (Fluka puriss grade). The obtained
cobalt caprylate was washed repeatedly with hot disꢀ
tilled water. Lead, cadmium, and zinc caprylates were
crystallized several times from hot toluene. All the salts
(lead and cadmium) and
3d transition metals are known
to have the lowest melting points and the abilities to be
supercooled and to form glass [3, 4]. To study specific
features of the structure and properties of these comꢀ
pounds in various aggregation states (melt, mesophase,
or glass) are of undoubted interest for developing glass
and liquidꢀcrystal science and materials science. Howꢀ
ever, to date, data on systematic investigation of the
thermophysical and optical properties of systems based
were dried in a vacuum drying cabinet at 50°С for a
day. The IR spectra of the synthesized salts indicated
that these salts contained neither water, nor carbonic
acid. Components weighed in a necessary molar ratio
were thoroughly mechanically mixed and alloyed in an
argon atmosphere for preventing the possible decomꢀ
position of samples. The samples were stored in a dry
argon atmosphere.
The temperatures of phase equilibria in binary sysꢀ
tems were studied by polythermal polarization microsꢀ
copy and differential thermal analysis with a Paulikꢀ
PaulikꢀErdey Qꢀ1500 D derivatograph (Hungary)
with a Pt/Pt–Rh thermocouple and Al2O3 as a stanꢀ
dard substance. The heating rate in all the experiments
was 2.5 deg/min.
on
able. There is only information on phase equilibria in
the system of a transition metal (cobalt, manganese,
3d transition metal alkanoates have been unavailꢀ
3d
nickel, or copper) decanoate with lead decanoate [5]
and the cobalt laurate–lead laurate system [6], and also
on the optical amd mesomorphic properties of the
cobalt caprylate–lithium caprylate binary system [7].
In this work, we studied the phase diagrams of the
binary systems of cobalt caprylate with lead, cadꢀ
mium, or zinc caprylate and determined the concenꢀ
tration and temperature ranges of the existence of liqꢀ
uid crystals and glasses. We also explored the elecꢀ
tronic absorption spectra of the Co(II) cation in
mesophases and glasses in these binary systems as
An Amplival polarizing microscope with a hot stage
was used for identifying the possible mesophase and
also for estimating the temperatures of the phase equiꢀ
libria crystal–mesophase (
tropic liquid ( clear).
The electronic absorption spectra of melts and
Tmelt) and mesophase–isoꢀ
T
functions of temperature and concentrations of comꢀ glasses were recorded within the range 400–800 nm
1141

1142
TOKMENKO et al.
(С7Н15СОО)2Pb undergoes solidꢀphase transition at
79 and melts to give a smectic mesophase at 81
this liquid clears at 107 , which agrees with the preꢀ
vious data [3]. The thermal behavior of cadmium
caprylate (С7Н15СОО)2Cd has not been described in
T, °C
°
С
°С;
°
С
160
140
120
100
80
IM
the literature; according to our data, it melts at 98
°С
to yield a smectic mesophase, which clears at 168°С.
On cooling, the mesophase vitrifies. Zinc caprylate
(С7Н15СОО)2Zn does not form a liquidꢀcrystal phase.
It melts at 140°С to give a viscous isotropic liquid,
LC
SS
SS
which agrees with the literature data [9].
60
0
20
40
60
80
100
х, mol %
RESULTS AND DISCUSSION
Figure 1 presents the phase diagram of the
Fig. 1. Phase state diagram of the {
х(C Н СОО) Со–
7 15 2
{x
(C7H15COO)2Со–(100 –
system.
In the system, continuous smectic liquidꢀcrystal
solutions and continuous solid solutions form at 70
and = 36 mol %. The clearing points Tclear of the
x)(C7H15COO)2Pb} binary
(100 – )(C H COO) Pb} binary system. IM, LC, and
х
7
15
2
SS are the singleꢀphase regions of the existence of isotropic
melt, liquidꢀcrystal solution, and solid solution, respecꢀ
tively.
°С
х
mesophase are negatively deviated from the additive
straight line connecting the Tclear values of the initial
components. The twoꢀphase region of the coexistence
of the mesophase and the isotropic liquid in our experꢀ
iments was less than <2°С wide and is not indicated in
the diagram. The supercooling and the formation of
optically isotropic (mesomorphic) glasses occur
T, °C
IM
LC
160
140
120
100
80
within the concentration range 10
Figure 2 shows the phase diagram of the
(C7H15COO)2Со–(100 – )(C7H15COO)2Cd} binary
system.
< х ≤ 100 mol %.
2
1
{
x
x
SSCd
SSCo
In the system, continuous smectic liquidꢀcrystal
solutions form by a eutectic reaction between the iniꢀ
3
60
tial components at 79
of the clearing points
°
С
and
х = 85 mol %. The curve
0
20
40
60
80
100
Tclear of the mesophase has proꢀ
х
, mol %
nounced negative deviations from the additive straight
line. The twoꢀphase region of the coexistence of the
mesophase and the isotropic liquid in our experiments
Fig. 2. Phase state diagram of the {х(C Н СОО) Со–
7 15 2
(100 –
SSCd, (
х
)(C H COO) Cd} binary system. (
1) LC +
7
15
2
less than <2°С wide and is not indicated in the diaꢀ
2) LC + SSCo, and (3) SSCd + SSCo are twoꢀ
gram. In the cobalt caprylate–cadmium caprylate
binary system, mesomorphic glasses form throughout
the concentration range because both cobalt capryꢀ
late, and cadmium caprylate tend to glass formation.
Figure 3 demonstrates the phase diagram of the
phase regions, where SSCd and SSCo are the solid phases
of cadmium and cobalt caprylates, respectively. IM and
LC are the singleꢀphase regions of the existence of isotroꢀ
pic melt and liquidꢀcrystal solution, respectively.
{
x
(C7H15COO)2Со–(100 –
x)(C7H15COO)Zn} binary
with a Perkin Elmer Lambda 35 UV/Vis spectrophoꢀ
system.
tometer with a heating unit to 200°С. Quartz cells
As Fig. 3 shows, in the system, a boundary smectic
liquidꢀcrystal solution forms by a eutectic reaction
between the solid phases of the initial components and
exists within the concentration range 50
A eutectic point occurs at 90 and = 90 mol %. The
curve of the clearing points of the mesophase interꢀ
sects the melting curve at 118 and = 53 mol %. At
this invariant (metatectic) point, the solid phase coexꢀ
ists with two liquid phases, one of which is isotropic
and the other is mesomorphic. The twoꢀphase region
of the coexistence of the mesophase and the isotropic
10 m thick were used.
µ
The phase transition temperatures of the synthesized
individual salts agree well with the published data [3, 8,
9]. For example, cobalt caprylate (С7Н15СОО)2Со melts
at 95°С to form a smectic mesophase, which is a bireꢀ
fringent liquid with illꢀdefined microscopic texture,
probably, because of the strong tendency to the
homeotropic orientation of liquidꢀcrystal domains.
< x ≤100 mol %.
°С
х
°
С
x
This liquid clears at 164
erature data [8]. The isotropic melt bears the marks of
decomposition near 200 . On cooling, the cobalt
°С, which agrees with the litꢀ
°С
liquid in our experiments was less than <2°С wide and
caprylate mesophase vitrifies. Lead caprylate is not indicated in the diagram. In the system, solid
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 8 2012

PHASE DIAGRAMS AND OPTICAL PROPERTIES
1143
solutions based on zinc caprylate form over a wide
concentration range. However, in our experiments,
the boundaries of their existence were not accurately
determined and are shown in the diagram schematiꢀ
cally by a dashed line. in the system, on cooling isotroꢀ
pic melts, optically isotropic glasses form within the
T, °C
160
140
120
100
80
IM
4
LC
range 20
melts, optically anisotropic (mesomorphic) glasses
form within the range 53 100 mol %.
< х < 53 mol %, and on cooling mesomorphic
SSZn
2
≤ х ≤
1
To identify the coordination state of Co(II) ions
surrounded by caprylate ligands in liquidꢀcrystal melts
and glasses in the binary systems of cobalt(II) capryꢀ
late with a divalent metal (lead, cadmium, or zinc)
caprylate, we studied the electronic absorption spectra
as functions of temperature and concentrations of
components.
3
SSCo
60
0
20
40
60
80
100
х, mol %
The glasses forming in the binary systems
Fig. 3. Phase state diagram of the {
х(C Н СОО) Со–
7 15 2
{x
(C7H15COO)2Со–(100
–
x)(C7H15COO)2М},
(100 – )(C H COO) Zn} binary system. (1) LC +
х
7
15
2
SSZn, (2) LC + SSCo, (3) SSZn + SSCo, and (4) IM +
SSZn are twoꢀphase regions, where SSZn and SSCo are
the solidꢀphases of zinc and cobalt caprylates, respectively.
IM and LC are the singleꢀphase regions of the existence of
isotropic melt and liquidꢀcrystal solution, respectively.
where M = Cd, Pb, or Zn, are pink, and the color
intensity increases with increasing cobalt caprylate
concentration. For all the samples, there was thermoꢀ
chromism, i.e., a change in their color with changing
temperature. At
T > 120°С, the obtained mesophase
or isotropic liquid (depending on the composition)
was always blue, and with cooling to room temperature
the samples became pink. In all the cases, the temperꢀ
atureꢀdependent color changes are reversible.
cooled to produce stable mesomorphic or isotropic
glasses, depending on composition. For all the samꢀ
ples, decreasing temperature decreases the absorption
intensity (ΔD/ΔT > 0), which is characteristic of cenꢀ
trosymmetric octahedral complexes [10].
The table presents the frequencies of the absorption
band maxima of the spectra of mixtures of cobalt
caprylate with lead, cadmium, and zinc caprylates.
For all the samples, a decrease in the cobalt caprylate
concentration by diluting with divalent metal capryꢀ
The electronic absorption spectrum of Co(II) ions
in the mesophase and glass of individual cobalt capryꢀ
late is a broad band in the range 480–630 nm with a
maximum at 562.7 nm and a shoulder at 535.7 nm [7].
Varying temperature hardly changes the shape and
position of the absorption band. This wavelength range
is known [10] to contain an absorption band of Co(II)
ions with multiplet structure, which corresponds to
the 4Т1 )–4Т1 transition in an octahedral or pseuꢀ
(P
g g
Maxima of the main absorption band of Co(II) ion in opꢀ
tically anisotropic mesomorphous glasses in the
dooctahedral Co(II) complex, and also bands for d–d
transitions in both tetrahedrally and dodecahedrally
coordinated Co(II) ion. In the liquidꢀcrystal melt or
glass with high cobalt ion concentration, several coorꢀ
dination states of Co(II) ions can be assumed to coexꢀ
ist to cause the broad absorption band with unresolved
structure. However, because the cobalt caprylate melt
has low ionicity and, correspondingly, low activity of
alkanoate ligands, the dominant coordination state of
Co(II) ions should be octahedral [11], although the presꢀ
ence of other coordination states is not ruled out [7].
In the binary caprilate systems in the presence of
zinc, lead, or cadmium ions, the electronic absorption
spectrum of Co(II) ions noticeably depends on temꢀ
perature, namely, increasing temperature leads to
longꢀwavelength shifts of the positions of the absorpꢀ
tion maxima of Co(II) ions and to an increase in their
intensity.
{x(C7H15COO)2Co–(100 –
x)(C7H15COO)2M} binary sysꢀ
tems
Absorption band maximum, nm
x
, mol %
M = Pb
M = Cd
M = Zn
100
90
80
70
60
50
40
30
20
10
562.7
562.4
561.7
561.2
558.8
554.4
554.5
555.6
556.7
–
562.7
559.1
557.4
557.2
553.7
543.8
541.5
540.7
539.2
538.7
562.7
557.3
540.9
532.9
531.5
538.9
539.2
540.3
–
Figure 4 presents the electronic absorption spectra
of Co(II) ions in samples of the systems
{x(C7H15COO)2Со–(100 – x)(C7H15COO)2М}, where
M = Pb, Cd, or Zn, of equimolar composition that
were recorded on cooling the melts that are superꢀ
–
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 8 2012

1144
TOKMENKO et al.
(а)
D
0.4
D
2
170°C
150°C
0.3
0.3
0.2
0.1
130
°
C
110°C
3
90
°
C
0.2
0.1
70
°C
1
0
20
40
60
80
, mol %
(C7H15COO)2Co
х
400 450 500 550 600 650 700 750
(b)
170°C
Fig. 5. Concentrationꢀdependent optical density of mesoꢀ
morphic caprylate glasses obtained by supercooling melts
0.4
0.3
0.2
0.1
in the binary systems
{
x
(C H COO) Со–(100
–
7
15
2
x
)(C H COO) М}, where M = (
1
) Zn, (
2
) Cd, or (
3
) Pb,
7
15
2
150°C
at room temperature. The quartz cell thickness is 10
µm.
130°C
110°C
absorption by tetrahedrally coordinated Co(II) ions,
the existence of which in individual cobalt caprylate
cannot be ruled out.
90°C
70°C
Figure 5 illustrates the concentration dependences
of the optical density of glasses obtained by supercoolꢀ
{x(C7H15COO)2Со–(100 –
700 750
500
400 450
550
600 650
(c)
shows, a decrease in the Co(II) ion concentration in
the systems causes a natural decrease in the optical
density of glass because of the dilution of the chroꢀ
mophore concentration.
0.4
0.3
0.2
Figure 6 presents the dependences of the intensity
of the absorption band of Co(II) ions on the average
ionic radius for equimolar compositions. It is seen that
the smaller the ionic radius of the metal cation in the
second coordination sphere of the Co(II) ion and the
stronger its counterpolarizing action on caprylate
ligands, the less the intensity of the optical absorption
of caprylate glass. This is related to the decrease in the
170
°C
110°
C
90°C
70°C
d–d electronic transition oscillator strength because of
the increase of the ionic contribution to the bond of
the Co(II) ion with caprylate ligands in its octahedral
complex [12].
400 450 500 550 600 650 700 750
λ
, nm
Thus, our studies of the optical properties of liquidꢀ
crystal melts and glasses in the binary systems of
cobalt(II) caprylate with a divalent metal (lead, cadꢀ
mium, or zinc) caprylate showed that the Co(II) ions
in these systems occur primarily in one coordination
state—octahedral. It was established that the formaꢀ
tion of a multicomponent liquidꢀcrystal system based
on metal caprylkates makes it possible to create stable
functional mesomorphic glasses and to optimize their
spectral characteristics, such as the positions of
absorption bands and the optical density, which is
Fig. 4. Electronic absorption spectra of Co(II) ions in
melts of the systems (C H COO) Со–(100
)(C H COO) М}, where M = (a) Zn, (b) Cd, or (c) Pb,
{x
–
7
15
2
x
7
15
2
of equimolar composition at various temperatures.
lates causes a shortꢀwavelength shift of the absorption
maximum. This indicates of an increase in the field of
caprylate ligands in octahedral complexes of Co(II)
ions in the binary systems in comparison with that in
individual cobalt caprylate. Probably, this also suggests
a decrease in the contribution to the total optical important for designing new photorefractive liquidꢀ
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 8 2012

PHASE DIAGRAMS AND OPTICAL PROPERTIES
1145
matics, Physics and Chemistry), Ed. by R. D. Rogers,
et al. (Kluwer Academic Publishers, Dordrecht, 2002),
p. 439.
D
0.10
2. G. V. Klimusheva, S. A. Bugaychuk, Yu. A. Garbovskiy,
0.08
0.06
0.04
0.02
0
et al., Opt. Lett. 31, 235 (2006).
Pb
Cd
3. A. Sanchez Arenas, M. V. Garcia, M. I. Redondo, and
J. A. R. Cheda, Liq. Cryst. 18, 431 (1995).
Zn
4. T. A. Mirnaya, L. S. Sudavtsova, G. G. Yaremchuk,
A. S. Tolochko, and L. N. Lisetskii, Russ. J. Inorg.
Chem. 49, 1440 (2004).
5. T. A. Mirnaya, G. G. Yaremchuk, L. S. Sudavtsova, and
S. V. Volkov, Ukr. Khim. Zh. 73 (7), 3 (2007).
6. D. V. Bylina, T. A. Mirnaya, and S. V. Volkov, Russ.
0.7
0.8 0.9
1.0 1.1
1.2
1.3 1.4
r, Å
J. Inorg. Chem. 51, 639 (2006).
7. T. A. Mirnaya, I. I. Tokmenko, G. G. Yaremchuk, and
A. A. Ponomarenko, Ukr. Khim. Zh. 75 (7), 40 (2009).
Fig. 6. Dependence of the intensity of the main optical
absorption band of Co(II) ions in mesomorphic caprylate
8. R. W. Corkey, PhD Thesis (Australian National Univ.,
1998).
glasses in the binary systems {
x(C H COO) Со–(100 –
7 15 2
x)(C H COO) М}, where M = Zn, Cd, or Pb, of
7
15
2
equimolar composition on the average ionic radius of the
metal of the second component.
9. R. A. Taylor and H. A. Ellis, Spectrochim. Acta A 68
,
99 (2007).
10. A. B. P. Lever, Inorganic Electronic Spectroscopy
(Elsevier, Amsterdam, 1984; Mir, Moscow, 1987),
Vol. 2.
crystal materials and matrices for holographic inforꢀ
mation recording.
11. V. A. Maroni and M. L. Maciejewski, In Proceedings of
the Fourth Int. Symp. on Molten Salts, US Electrochem.
Soc., 1983, 84/2, 359.
REFERENCES
1. T. A. Mirnaya and S. V. Volkov, Green Industrial Appliꢀ 12. S. V. Volkov and K. B. Yatsimirskii, Spectroscopy of Molꢀ
cations of Ionic Liquids. NATO Science Series II (Matheꢀ ten Salts (Nauk. dumka, Kiev, 1977) [in Russian].
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 8 2012