Effect of exchange of terminal substituents on the mesophase behavior of laterally methyl substituted phenyl azo benzoates in pure and mixed systems

Article abstract of DOI:10.1016/j.tca.2011.07.024

Two groups of the formulae, 4-(4′-hexyloxy)phenylazo-2-(and 3)methyl phenyl-4″-substituted benzoates (I_a-e and II_a-e) were prepared and investigated for their mesophase behavior. Each group consists of five derivatives that differ fr

Full text of DOI:10.1016/j.tca.2011.07.024

Thermochimica Acta 525 (2011) 78–86
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Effect of exchange of terminal substituents on the mesophase behavior of
laterally methyl substituted phenyl azo benzoates in pure and mixed systems
M.M. Naoum∗, A.A. Fahmi, M.A. Alaasar, N.H.S. Ahmed
Department of Chemistry, Faculty of Science, Cairo University, Egypt
a r t i c l e i n f o
a b s t r a c t
Two groups of the formulae, 4-(4ꢀ-hexyloxy)phenylazo-2-(and 3)methyl phenyl-4ꢀꢀ-substituted ben-
zoates (Ia–e and IIa–e) were prepared and investigated for their mesophase behavior. Each group consists
of five derivatives that differ from each other by the terminal polar substituent X which varies between
CH3O, CH3, Br, NO2, and CN. Two other isomeric groups, IIIa–e and IVa–e, that exchange the terminal sub-
stituents, X and C6H13O, were also prepared and similarly characterized. Four different types of binary
phase diagrams were constructed in which both components are corresponding isomers one from each of
the four groups (I–IV). In the two binary mixtures, I/II and III/IV, the two components are positional iso-
mers that differ in the position of the lateral methyl group, while in the other two systems, I/III and II/IV,
the two components are again positional isomers that exchange the two wing substituents.The study
is undertaken to investigate the effect of exchange of the terminal substituents on the mesomorphic
properties of the produced derivatives in their pure or mixed states.
Article history:
Received 10 June 2011
Received in revised form 7 July 2011
Accepted 25 July 2011
Available online 30 July 2011
Keywords:
Lateral substitution
Binary mixtures
4-(4ꢀ-Hexyloxy)phenylazo-2-(or
3-)methylphenyl-4ꢀꢀ-substituted benzoates
4-4ꢀ-Substituted phenylazo-2-(or
3-)methylphenyl-4ꢀꢀ-hexyloxy benzoates
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
molecular attractions, electron-withdrawing substituents, which
are situated in a para position with respect to the ester-oxygen
Generally polar substituents, through mesomeric interactions
with the mesogenic portion of the molecule, affect the polariz-
ability of the aromatic ring to which they are attached. Compact,
polar, and/or polarizable groups appear to be very effective in
creating mesophases with relatively high stability [1]; that is,
extending upward the temperature region for the persistence of
the mesophase. This has been attributed [1–4] to the increase
of intermolecular attraction with the increase of polarity and/or
polarizability due to substitution. In the phenyl benzoate ester
system, the mesomorphic phase becomes more persistent when
the mesophase stability effect of the mutual conjugation between
the terminal substituent and the ester carbonyl is increased. In
previous works [5–7], the effect of exchange of the terminal sub-
stituents on the mesophase behavior of two groups of the phenyl
benzoate derivatives, namely, 4-alkoxyphenyl-4ꢀ-substituted ben-
zoates and 4-substituted phenyl-4ꢀ-alkoxybenzoates, as well as of
their binary mixtures, was investigated. Molecules of these types
represent extremes in the conjugative interactions between the
terminal substituent and the remainder of the molecule. Thus,
while electron-donating substituents promote the polarity of the
ester carbonyl group, which in turn results in stronger inter-
atom, reduce the polarity of the carbonyl group, and hence, lower
the clearing point of the mesophase. In another work [8], the
exchange of the two terminal carboxy and hexadecyloxy groups
attached to the two opposite sides of the phenyl benzoate molecule,
were found to affect the stability of the mesophase to variable
extents due to differences in the mesomeric interaction in either
case.
In view of the high mesophase stability of a three-ring molecule
compared with a two-ring one, it seems interesting to investigate
the effect of exchange of the two wing substituents attached to the
three-ring phenylazo phenyl benzoate molecule on the mesophase
behavior of the resulting isomers. The azo-ester molecule was
chosen on the basis of its relatively higher mesophase stability.
Dye-containing materials, on the other hand, are applied to a wide
variety of fields such as high technology and nano-technology [9].
On the other hand, the introduction of the lateral-methyl group into
either position (2 or 3) of the central benzene ring is an attempt to
reduce the melting point of the laterally-neat analogues. In this
respect, two groups of laterally methyl substituted arylazophenyl-
4-alkoxy benzoates were investigated [10] for their mesophase
behavior. All members of the first group, in which the methyl group
is introduced into position-2 of the central benzene ring, were
found to be purely nematogenic, but of little stability compared
with their laterally-neat analogues. In the second group, where the
methyl group is introduced this time into position-3, the orienta-
tion of the methyl group is not in favor of mesphase stability so
∗
Corresponding author. Tel.: +20 127132834; fax: +20 235673673.
E-mail address: magdinaoum@yahoo.co.uk (M.M. Naoum).
0040-6031/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2011.07.024

M.M. Naoum et al. / Thermochimica Acta 525 (2011) 78–86
79
that it destabilizes the nematic phase. In both cases, extension of
the alkoxy chain gives rise to a decrease in the nematic stability.
The present study aims to investigate, first, the effect of the
methyl-lateral substitution into the central benzene ring of the pre-
viously investigated azo-ester [10], but with shorter alkoxy-chain
length (n = 6) which is expected to be of higher nematic stability
(series IIIa–e and IVa–e). Secondly, it is to investigate the effect
of exchange of the two wing substituents (C6H13O and X) on the
mesophase behavior of the resulting isomers (Groups Ia–e and IIa–e).
While their terminally substituted isomers, IIIa–e and IVa–e, were
prepared according to Scheme 2:
2.1.1. Preparation of 4-(4ꢀ-hexyloxy phenylazo)-2-(or 3-)methyl
phenol (A)
One molar equivalent of the 4-hexyloxy aniline in ice-cold dilute
hydrochloric acid was diazotized with cold sodium nitrite solution
and then added slowly to a cold 2-(or 3-)cresol/sodium hydroxide
solution (1:1). The solid azo product was filtered and crystallized
twice from ethanol. The melting points of the prepared azo dyes
are 93.1 and 88.2 ◦C, respectively.
O
C6H13O
N N
O C
X
2.1.2. Preparation of 4-(4ꢀ-substituted phenylazo)-2-(or
3)-methyl phenol (B)
These were prepared in a similar manner from 4-substituted
aniline and 2-(or 3-) cresol. The melting points of the prepared azo
dyes were in good agreement with those reported in literature [11].
CH3
(Ia-e)
Ia, X=CH3O, Ib, X=CH3, Ic, X=Br, Id, X=NO2, and Ie, X=CN.
O
C6H13O
N N
O C
X
H3C
(IIa-e)
IIa, X=CH3O, IIb, X=CH3, IIc, X=Br, IId, X=NO2, and IIe, X=CN.
O
X
N N
O C
OC6H13
CH3
(IIIa-e)
IIIa, X=CH3O, IIIb, X=CH3, IIIc, X=Br, IIId, X=NO2, and IIIe, X=CN.
O
X
N N
O C
OC6H13
H3C
(IVa-e)
IVa, X=CH3O, IVb, X=CH3, IVc, X=Br, IVd, X=NO2, and IVe, X=CN.
2.1.3. Preparation of 4-(4ꢀ-hexyloxy phenylazo)-2-(or
3)-methylphenyl-4ꢀꢀ-substituted benzoates, Ia–e and IIa–e
Molar equivalents of the 4-(4ꢀ-hexyloxy phenylazo)-2-(or 3-)
methyl phenol (A) and 4-substituted benzoic acid were dissolved
in dry methylene chloride. To the resulting solution, 2-molar equiv-
alents of dicyclohexyl carbodiimide (DCC) and few crystals of
4-(dimethylamino) pyridine (DMAP), as catalyst, were added and
the solution left to stand for 72 h at room temperature with stir-
ring. The solid separated was then filtered off and the solution
evaporated. The obtained solid residue was crystallized twice from
ethanol to give TLC pure products, as indicated by one spot in their
TLC thermograms.
A third aim is to investigate the effect of steric interaction,
on the binary phase behavior, that may be encountered between
any two corresponding positional isomers, once based on different
methyl-protrusion (I/II and III/IV), and another between alterna-
tively wing-substituted isomers (I/III and II/IV).
2. Experimental
Chemicals have been purchased from the following companies:
Aldrich, Wisconsin, USA; E. Merk, Darmstadt, Germany; and Fluka,
Buchs, Switzerland.
2.1.4. Preparation of 4-(4ꢀ-substituted phenylazo)-2-(or
3)-methylphenyl-4ꢀꢀ-n-hexyl oxy benzoates, IIIa–e and IVa–e
These were similarly prepared using the phenols (B) and 4-n-
hexyloxy benzoic acid.
2.1. Preparation of materials
Compounds Ia–e and IIa–e were prepared according to Scheme 1:

80
M.M. Naoum et al. / Thermochimica Acta 525 (2011) 78–86
i) HCl/NaNO2
C6H13O
N N
OH
C6H13
O
NH2
O
ii) o- or m-cresol/NaOH
CH3
(A)
DCC/DMAP
+
(A)
X
C OH
CH2Cl2/ stirring
O
O C
X
C6H13O
N N
CH3
Ia-e and IIa-e
Scheme 1. Steps for the preparation of compounds Ia–e and IIa–e
.
2.2. Physical characterization
3.2. Mesophase behavior
3.2.1. Simple molecules
Calorimetric measurements were carried out on a PL-DSC of
Polymer Laboratories, Epsom Surrey, England. The instrument was
calibrated for temperature, heat and heat flow according to the
method recommended by Cammenga, et al. [12]. Measurements
were carried out for small samples (2–3 mg) placed in sealed
aluminum pans. All of the thermograms have been achieved at
a heating rate of 10 ◦C/min in inert atmosphere of nitrogen gas
(10 mL/min).
Transition temperatures were checked and types of mesophases
identified for compounds prepared and their binary mixtures with a
standard polarized-light microscope (Wild, Heerbrugg, Germany),
attached to a home made hot-stage. The temperature is measured
by thermocouple connected to a Brookfield temperature controller,
Derbyshire England.
Since the mesophase behavior of a liquid crystalline compound
is dependent, in addition to other factors, upon the intermolecu-
lar attractions, in which molecular polarity plays a significant role,
it has been shown [5] that, in a series of compounds, of nearly
the same molecular shape, the dipole moment of any compound
is dependent upon the nature of both the substituent and the
mesogenic portion of the molecule to which this substituent is
attached. A change in the extent of electronic interactions between
the substituent, X, and the remainder of the molecule, alters the
polarizability and resultant dipole moment of the molecule. In
our case, electronic interactions between the substituents, X, and
molecules of type I and II are expected to be different from that
occurring with molecules of types III and IV. The phase tran-
sition temperatures and enthalpies of the prepared compounds
(Ia–e–IVa–e), as well as the entropies of the nematic–isotropic tran-
sitions, are collected in Table 2. The data in Table 2 revealed that,
irrespective of either the position of the lateral methyl group, the
polarity of the terminal substituent, X, or the relative positions
of wing substituents, all compounds investigated were found to
be monomorphic exhibiting the enantiotropic nematic phase as
the only mesophase observed. To make the comparison of data
in Table 1 clear, the substituent X attached to the mesomorphic
compounds can be placed in the order of enhancing nematic phase
stability of group I–IV compounds as:
Thin layer chromatography was performed with TLC sheets
coated with silica gel (E. Merck); spots were detected by UV irradi-
ation.
Infrared spectra (4000–400 cm−1) were measured (Table 1) with
a Perkin-Elmer B25 spectrophotometer, and 1H NMR-spectra with
Varian EM 350L.
3. Results and discussion
3.1. Confirmation of molecular structure
Infrared spectra, mass spectra, and elemental analyses for com-
pounds investigated were consistent, within permissible limits,
with the structures assigned. 1H-NMR data showed the expected
integrated aliphatic to aromatic proton ratios in all compounds
investigated.
Group I: CN > CH3O > NO2 > CH3 > Br
Group II: CN > CH3O > NO2 > CH3 > Br
Group III: CN > CH3O > NO2 > CH3 > Br
Group IV: CN > CH3 > NO2 > Br > CH3O
i) HCl/NaNO2
X
N N
OH
NH2
X
ii) o- or m-cresol/NaOH
CH3
(B)
O
C OH
DCC/DMAP
+
(B)
OC6H13
CH2Cl2/ stirring
O
X
N N
OC
OC6H13
CH3
IIIa-e and IVa-e
Scheme 2. Steps for the preparation of compounds IIIa–e and IVa–e
.

M.M. Naoum et al. / Thermochimica Acta 525 (2011) 78–86
81
Table 1
Infrared absorption spectra of represented compounds of Groups Ia–e–IVa–e
.
Comp. No.
Subst, X
ꢀCH
ꢀCH
ꢀC
ꢀC
N
ꢀC–O
ꢀC–O
ꢀNO (or ꢀCN)
2
Asym.
Sym.
O
3
3
Ia
Ib
Ic
Id
CH3O
CH3
Br
NO2
CN
CH3O
CH3
Br
NO2
CN
CH3O
CH3
Br
NO2
CN
CH3O
CH3
Br
NO2
CN
2930
2931
2930
2930
2933
2928
3929
2929
2931
2936
2922
2921
2933
2916
2936
2920
2919
2932
2928
2940
2858
2857
2858
2861
2863
2856
2855
2860
2862
2862
2851
2851
2862
2851
2865
2849
2847
2857
2850
2863
1730
1727
1734
1732
1731
1734
1729
1735
1731
1736
1727
1728
1725
1735
1724
1732
1730
1736
1734
1731
1600
1594
1589
1596
1594
1603
1594
1586
1594
1590
1603
1605
1602
1608
1601
1606
1604
1599
1626
1597
1465
1473
1471
1476
1475
1470
1474
1478
1484
1484
1465
1467
1475
1472
1490
1470
1467
1468
1474
1474
1246
1244
1250
1251
1254
1253
1254
1252
1254
1255
1242
1243
1258
1241
1256
1260
1253
1256
1248
1256
–
–
–
1535/1341
2227
Ie
IIa
IIb
IIc
–
–
–
IId
IIe
IIIa
IIIb
IIIc
IIId
IIIe
IVa
IVb
IVc
IVd
IVe
1524/1341
2226
–
–
–
1525/1345
2224
–
–
–
1532/1344
2223
That is, although wing substituents being exchanged between ana-
logues of groups I and III, both groups obey the same order of
nematic stability. Furthermore, while the lateral methyl group
is oriented differently in groups I and II their nematic stabil-
ity decreases in a parallel order. It can also be observed that,
while the linear strong electron-withdrawing, CN group, is the
best to enhance the nematic stability, the non-linear, also electron-
withdrawing, NO2 group occupies an intermediate order. The CH3
and CH3O groups nearly exchange their order in group IV com-
pounds.
3.2.1.1. Lateral substitution and entropy change of transition, ꢁSN–I.
The entropy of the nematic–isotropic transition was calculated for
all members of the four series I–IV, and the results were appended
to Table 2. It has been shown [13], for a related groups of com-
pounds, that lateral substitution with the bulky methyl group is
associated with lower values of ꢁSN–I than those of the corre-
sponding laterally chloro-substituted analogues, independent of
the length of the alkoxy chain. The decrease observed in ꢁSN–I
was presumably in part a reflection of the increase in the biaxiality
of the mesogenic group, which resulted in that the flexible termi-
nal alkoxy chain being less anchored at its end, giving a resulting
decrease in the conformational entropy [14]. Upon terminal substi-
tution with different polar groups at either wing of the molecule,
the behavior becomes more complex reflecting the variable extent
of the strength of electronic interaction with the mesogenic core as
a result of the change of the degree of conjugation. Thus, for the ter-
minally 4-CH3O substituted analogues in the four series, Ia–IVa, the
order of decrease in ꢁSN–I was mainly dependent on the nature of
the mesogenic core, which definitely affects the lateral intermolec-
ular interaction to a variable degree. That is, the order of decrease
in ꢁSN–I was IIIa > Ia > IIa > IVa. Terminal substitution by the 4-CH3
group is accompanied by an increase in ꢁSN–I in series I and IV,
Alternatively, with respect to the effect of the terminal polar
substituent, X, attached to any of the four isomeric groups, I–IV,
the nematic stability was found to decrease in the order:
For
X = CH3O
X = CH3
X = Br
IIIa
212.6
IVb
>
>
Ia
>
>
>
>
>
IIa
187.6
IIb
>
>
>
>
>
IVa
◦
204.1
Ib
151.2
C
C
C
C
C
IIIb
◦
206.7
180.8
168.4
138.9
Ic > IVc
180.3
IIc
IIIc
◦
172.3
Id
159.5
IId
133.0
X = NO2
X = CN
IIId
199.2
>
IVd
◦
184.1
186.2
179.2
IIIe > IVe
222.5
IIe
213.2
Ie
209.0
◦
216.8
Table 2
Phase transition temperatures (◦C), enthalpy of transition (ꢁH, kJ/mole), and entropy of clearing (ꢁSN–I, J/mole/K) for Group Ia–e–IVa–e
.
Comp. No.
X
TCr–N
ꢁHCr–N
TN–I
ꢁHN–I
ꢁSN–I
Ia
Ib
Ic
Id
CH3O
CH3
Br
NO2
CN
CH3O
CH3
Br
NO2
CN
CH3O
CH3
Br
NO2
CN
CH3O
CH3
Br
NO2
CN
118.5
94.0
45.2
45.0
36.9
58.0
37.2
46.0
58.5
44.7
47.3
48.6
47.8
27.5
28.3
50.9
56.2
27.1
59.4
56.9
47.9
50.1
204.1
180.8
180.3
184.3
209.0
187.6
168.4
159.5
186.2
213.2
212.6
138.9
133.0
199.2
222.5
151.2
206.8
172.3
179.2
216.8
1.66
1.78
1.35
0.83
1.37
1.49
1.06
1.39
1.18
2.45
2.50
2.08
0.57
0.95
0.80
0.86
1.39
0.89
1.18
0.90
3.48
3.92
2.98
1.82
2.84
3.23
2.40
3.21
2.57
5.04
5.15
5.05
1.40
2.01
1.61
2.03
2.90
2.00
2.61
1.84
105.9
119.2
136.9
111.6
110.5
117.3
133.2
150.7
111.3
74.0
Ie
IIa
IIb
IIc
IId
IIe
IIIa
IIIb
IIIc
IIId
IIIe
IVa
IVb
IVc
IVd
IVe
90.5
115.5
121.8
84.9
106.4
105.7
107.6
112.4
Cr, solid crystal; N, nematic; and I, isotropic.

82
M.M. Naoum et al. / Thermochimica Acta 525 (2011) 78–86
√
The TC values are plotted as a function of ꢁ˛X, for series I–IV
in Fig. 1. As shown from Fig. 1, except for the data of Group IV,
acceptable linear correlations were obtained for the other groups
of compounds. Comparison of the slopes of the linear correlations
in Fig. 1 revealed that, in a molecular core of type I or II, termi-
nal substituent effect is more pronounced when the lateral methyl
group is introduced in position 3 (IIa–e). This is evidenced from the
higher slope of their linear correlations for group II compared with
group I. The reverse holds good for molecular cores of groups III and
IV. That is the slope for group III correlation, with lateral methyl
group introduced into position 2, is higher than that or group IV
(position 3).
3.2.2. Binary mixtures
It is already known that materials which exhibit its mesophase
at room temperature and retain liquid crystalline character over
a wide range of temperatures are preferred for practical applica-
tions. One way to achieve this property is to use eutectic mixtures
of materials exhibiting liquid crystallinity in their pure state, or
at least when the molecules in question resemble one another
structurally. This is because under such condition, the mesophase-
isotropic line is usually straight or slightly enhanced, and the
temperature range of the mesophase is consequently greater for
the low-melting eutectic mixture than either components of the
mixture.
Based on the type of the two components of the binary mixture,
we can formulate, using the compounds Ia–e–IVa–e, two types of
binary mixtures. The first is based on the difference in the posi-
tion of the lateral CH3-substituent, i.e., mixtures I/II and III/IV. The
second type of binary mixtures to be investigated is based on the
difference in the relative positions of the wing substituents (X and
C6H13O), i.e., mixtures I/III and II/IV. Such mixtures aim to achieve
a reasonable balance between improving mesomorphic properties
and retaining advantageous physical properties of the laterally sub-
stituted components.
Fig. 1. Dependence of the mesophase stability (TC
anisotropy (ꢁ˛X) of the substituent (X) attached to compounds of: (a) Group I, (b)
Group II, (c) Group III, and (d) Group IV.
) on the polarizability
1/2
a significant decrease in series II, and slight decrease in series III
where it decreases from 5.15 J mol−1 K−1 for IIIb to 5.05 J mol−1 K−1
for IIIa. Alternatively, when a comparison is made between ꢁSN–I
values within members of any group of derivatives of the same core
structure, ꢁSN–I was observed to decrease in an order which differ
according to the nature of the terminal substituent as well as its
position.
3.2.2.1. Binary mixtures of isomers bearing differently pro-
truded methyl group I/II and III/IV. The binary phase diagrams
for
the
4-hexadecyloxyphenylazo
(2-methylphenyl)-4-
substituted benzoates (Ia–e
)
with their isomeric derivatives,
An explanation of the fact that the entropies do not correlate
well with either the polarity of the substituent or with the clear-
ing temperature, TC, may be found in the conjugative interaction
of the substituents (lateral or terminal) with the remainder of the
molecule [1,15].
4-hexadecyloxyphenylazo(3-methylphenyl)-4-substituted
benzoates (IIa–e), are represented graphically in Fig. 2. The corre-
sponding phase diagrams for the other two groups of isomers that
bear the central methyl group with different protrusion, namely,
4-substituted phenylazo (2-methylphenyl)-4-hexadecyloxy ben-
zoates (IIIa–e
) with their isomeric derivatives, 4-substituted
phenylazo-(3-methylphenyl)-4-hexadecyloxybenzoates (IVa–e),
are represented graphically in Fig. 3. As can be seen from
Figs. 2 and 3, irrespective of the terminal substituent attached to
either side of the molecule or the position of the central methyl
group, linear nematic behaviors were observed in all systems
investigated, indicating that the addition of one component to
the other does not disturb the nematic arrangements exhibited
by both molecules. Such behavior necessitates the back-to-face
arrangements of molecules that do not allow steric methyl–methyl
interaction that would have lead to a negative deviation from the
linear behavior. Figs. 2 and 3 also indicate that all systems exhibit
eutectic composition, with melting points as low as 60 ◦C for the
binary system IIIb/IVb, Fig. 3b.
The composition, phase transition temperatures, and the
nematic ranges (ꢁT = TN–I − TCr–N) of the eutectic mixtures, as
deduced from Figs. 2 and 3, are collected in Table 3, from which
it can be seen that, according to the substituent X, the mesophase
ranges (ꢁT = Tm − TN–I) for the two systems investigated at their
eutectic compositions decreases in the order
3.2.1.2. Clearing temperature and polarizability anisotropy of the
Car− X bonds. The relationship between the stability of the
mesophase, expressed as the clearing temperature, TC, and the
anisotropy of polarizability (ꢁ˛X) of bonds to small compact ter-
minal substituent (Car− X) was studied by van der Veen [16]. The
relation has the form:
TC ∝ (ꢁ˛M + ꢁ˛X )2,
(1)
where TC is measured in Kelvin. The term ꢁ˛M is the anisotropy
of the polarizability for the whole molecular structure except the
terminal substituent, X. Eq. (1) can be put in the form [7]:
ꢀ
TC ∝ (ꢁ˛M + ꢁ˛X ) = aꢁ˛M + aꢁ˛X
(2)
where “a” is the proportionality constant. The values of ꢁ˛X are
√
given else where [17]. Thus, if TC is plotted against ꢁ˛X for any
series of liquid crystalline compounds, a straight line is expected,
the slope of which equals “a” and intercept equals “aꢁ˛M”. Conse-
quently, ꢁ˛M will be given by: ꢁ˛M = intercept/slope.

M.M. Naoum et al. / Thermochimica Acta 525 (2011) 78–86
83
Fig. 2. Binary phase diagrams of the binary systems (I/II) with various terminal substituents. (a) X = CH3O, (b) X = CH3, (c) X = Br, (d) X = NO2, and (e) X = CN.
Fig. 3. Binary phase diagrams of the binary systems (III/IV) with various terminal substituents. (a) X = CH3O, (b) X = CH3, (c) X = Br, (d) X = NO2, and (e) X = CN.

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M.M. Naoum et al. / Thermochimica Acta 525 (2011) 78–86
Fig. 4. Binary phase diagrams of the binary systems (I/III) with various terminal substituents. (a) X = CH3O, (b) X = CH3, (c) X = Br, (d) X = NO2, and (e) X = CN.
Fig. 5. Binary phase diagrams of the binary systems (II/IV) with various terminal substituents. (a) X = CH3O, (b) X = CH3, (c) X = Br, (d) X = NO2, and (e) X = CN.

M.M. Naoum et al. / Thermochimica Acta 525 (2011) 78–86
85
Table 3
Composition, melting temperatures (Tm
,
◦C), clearing temperatures (TN–I
,
◦C), nematic ranges (ꢁT = TN–I–Tm
Tm (◦C)
,
◦C), and the calculated TE values of the eutectic binary mixtures,
I
a–e/IIa–e, IIIa–e/IVa–e, Ia–e/IIIa–e, and IIa-e/IVa–e.
Mixture
Subst. X
Composition (%)
TE (◦C)
TN–I (◦C)
ꢁT (◦C)
Ia/IIa
Ib/IIb
Ic/IIc
Id/IId
CH3O
CH3
Br
NO2
CN
CH3O
CH3
Br
NO2
CN
CH3O
CH3
Br
NO2
CN
CH3O
CH3
Br
NO2
CN
65.3% IIa
41.3% IIb
50.5% IIc
69.3% IId
50.0% IIe
76.7% IVa
24.0% IVb
40.0% IVc
65.3% IVd
75.3% IVe
73.3% IIIa
76.0% IIIb
71.3% IIIc
43.0% IIId
57.7% IIIe
72.7% IVa
58.7% IVb
41.3% IVc
75.3% IVd
76.1% IVe
90.0
84.0
90.1
114.0
108.0
70.0
60.0
68.0
88.0
84.0
90.0
61.0
65.5
88.0
106.8
68.0
86.0
78.0
66.0
94.2
95.7
81.3
84.4
101.6
112.4
74.7
64.3
71.7
91.0
92.0
84.5
61.6
69.4
95.7
106.9
72.8
92.9
88.0
75.4
105.8
215.5
182.0
172.0
179.5
209.0
164.5
152.0
143.0
184.0
219.2
209.0
158.0
139.0
182.0
216.8
160.0
190.0
170.0
182.0
215.0
125.5
98.0
82.0
65.5
101.0
96.5
92.0
75.0
96.0
135.2
119.0
97.0
73.5
94.0
110.0
92.0
104.0
92.0
116.0
120.8
Ie/IIe
IIIa/IVa
IIIb/IVb
IIIc/IVc
IIId/IVd
IIIe/IVe
Ia/IIIa
Ib/IIIb
Ic/IIIc
Id/IIId
Ie/IIIe
IIa/IVa
IIb/IVb
IIc/IVc
IId/IVd
IIe/IVe
Mixture I/II: CH3O > CN > CH3 > Br > NO2.
Mixture III/IV: CN > NO2 > CH3O > CH3 > Br.
ideal system, follows the vant’t Hoff relation, which is conveniently
recast into the Schroeder-van Laar equation [18–20]:
ꢁ
ꢂꢁ
ꢂ
ꢁ
ꢂ
ꢁHi
R
1
TE
1
Ti
Whereas, the nematic stability, according to the substituent X,
for the eutectic compositions for these two groups of binary mix-
tures decreases in the order:
lnꢂi =
−
(3)
in which the melting point (TE) of the eutectic mixture is related
to the mole fraction (ꢂi), crystal-mesophase transition tempera-
ture (Ti), and the molar enthalpy of fusion (ꢁHi) of component “i”
in a mixture. R is the gas constant. Knowing the values of ꢂi, Ti,
and ꢁHi, TE could be calculated, using Eq. (3), and the calculated
TE values were included to Table 3. The difference between the
measured, Tm, and calculated, TE, eutectic melting temperatures
can be taken as a measure of deviation from ideal solid behavior.
Coincidence or deviations are dependent on differences in the core
structures of both components of the mixture, orientation of the
lateral methyl group in both components, and/or the polarity of the
terminal group, X. Deviations from ideal behavior are sufficiently
common and large to make this approach unreliable [21].
Mixture I/II: CH3O > CN > CH3 > NO2 > Br.
Mixture III/IV: NO2 > CH3O > CN > CH3 > Br.
3.2.2.2. Binary mixtures of isomers with exchanged terminal sub-
stituents, I/III and II/IV. The binary phase diagrams for the
4-hexadecyloxyphenylazo (2-methylphenyl)-4-substituted ben-
zoates (Ia–e
) with their isomeric derivatives, 4-substituted
phenylazo-(2-methylphenyl)-4-hexadecyloxy benzoates (IIIa–e),
are represented graphically in Fig. 4. The binary phase diagrams
for the corresponding two groups of isomers that bear the central
methyl group in position-3, namely, 4-hexadecyloxyphenylazo-
(3-methylphenyl)-4-substituted benzoates (IIa–e) with their iso-
meric derivatives, 4-substituted phenylazo-(3-methylphenyl)-4-
hexadecyloxy benzoates (IVa–e), are represented graphically in
Fig. 5. As can be seen from Figs. 4 and 5, irrespective of the nature
of the lateral substituent, X, the introduction of such substituent on
either wings of the molecule, namely, I compared to III or II com-
pared to IV, do not alter the type of molecular arrangement within
the nematic mesophase to such an extent that all binary mixtures
behaved ideally. With respect to their effect on the nematic range
and stability at the eutectic compositions, ꢁTN–I decreases in the
order:
4. Conclusions
Four homologous series (Ia–e–IVa–e) of the laterally methyl
substituted derivatives were prepared, the first two groups (I
and II) have the molecular formula 4-(4ꢀ-hexyloxyphenylazo)-2-
(or 3-)methylphenyl-4ꢀꢀ-substituted benzoates, while the other
two groups, III and IV, are their respective isomers in which
the last two groups (II and IV) have the molecular formula 4-
(4ꢀ-substituted phenylazo)-2-(or 3-) methylphenyl-4ꢀꢀ-hexyloxy
benzoates. All compounds prepared were investigated for their
mesophase behavior in mixed states.
Combining the added properties of the lateral CH3 and the ter-
minal polar, X, substituents, significant changes were observed
in the mesomorphic behavior of the resulted derivatives and
their binary mixtures. Varying extents of mesomeric interactions
between the central CH3 group (in position 2 or 3) as well as the
terminal polar substituent, X, with the ester C O group, which def-
initely depend on the polarity and position of X, is reflected on the
resultant dipole moments, and consequently on their mesomor-
phic properties. The study revealed that all compounds prepared,
irrespective of the position of the three substituents (CH3, C6H13O,
or X), are enantiotropically nematogenic.
Mixture I/III: CH3O > CH3 > NO2 > CN > Br.
Mixture II/IV: NO2 > CH3 > CH3O > Br > CN.
And the nematic range decreases in the order:
Mixture I/III: CH3O > NO2 > CH3 > CN > Br.
Mixture II/IV: CH3 > NO2 > Br > CH3O > CN.
3.2.3. Eutectic behavior of mixtures
Predicting the crystal-mesophase transition temperatures, Tm
of a mixture is quite a bit less straight forward. The solubility of
compounds in a mixture, which behaves as a thermodynamically
,

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M.M. Naoum et al. / Thermochimica Acta 525 (2011) 78–86
Two types of binary phase diagrams, namely, I/II and III/IV, were
mixtures. III. Binary mixtures of alkoxyphenyl arylates and aryl alkoxyarylates,
Liq. Cryst. 25 (1998) 165–173.
constructed for all possible binary mixtures made from any two
isomers of the same skeletal structure except the orientation of the
central lateral CH3 substituent. The other two groups of diagrams
investigated were made from isomers of similar molecular core but
of the terminal (C6H13O and X) are being exchanged, i.e., the two
binary systems I/III and II/IV.
The binary mixtures investigated were found to exhibit, in all
cases, eutectic compositions with relatively low melting points that
also differ according to the polarity and relative position of the
terminal polar group, X, as well as the orientation of the central
methyl group attached to the two components of the mixture. Lin-
ear nematic behavior was observed in all mixtures investigated as
indicated from the linear TN–I – composition dependencies.
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