Mixed-dimer formation in binary systems of 4-substituted benzoic acids and structure considerations

Article abstract of DOI:10.1139/V08-008

The phase behavior of binary systems of 4-substituted benzoic acids is governed by the formation of mixed, dimers. This study was conducted to determine the effect of the components' structural difference on mixed-dimer formation in crystalline and liquid-crystalline phases. The phase diagrams of two systems, with 4-[(S)-(-)-2-methylbutoxy]benzoic acid (MBOBA) as one component and 4-(hex-5-enoxy)benzoic acid (HOBA) and 4-(dec-9-enoxy)benzoic acid (DOBA), respectively, as the second component, were determined by differential scanning calorimetry, polarized-light microscopy, and X-ray diffraction. The MBOBA-HOBA system exhibited a cholesteric phase, two solid solutions, and above 58°C for compositions between 40 and 80 mol% HOBA a crystalline phase of mixed dimers. The MBOBA-DOBA system showed a crystalline phase of mixed dimers at all compositions, a cholesteric phase, and a twisted smectic C phase, which was dominated by mixed dimers at 60 and 70 mol.% DOBA. We conclude that liquid-crystalline phases are generally dominated by mixed dimers, but in crystalline phases the formation of mixed dimers is promoted by a greater difference in molecular structure. The crystal structure of two of the pure compounds MBOBA and DOBA and comparable compounds have been determined for an evaluation of the arrangements of the molecules in the crystal and liquid-crystalline state.

Full text of DOI:10.1139/V08-008

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Mixed-dimer formation in binary systems of
4
-substituted benzoic acids and structure
1
considerations
Maren Roman, Annett Kaeding-Koppers, and Peter Zugenmaier
Abstract: The phase behavior of binary systems of 4-substituted benzoic acids is governed by the formation of mixed
dimers. This study was conducted to determine the effect of the components’ structural difference on mixed-dimer for-
mation in crystalline and liquid-crystalline phases. The phase diagrams of two systems, with 4-[(S)-(–)-2-
methylbutoxy]benzoic acid (MBOBA) as one component and 4-(hex-5-enoxy)benzoic acid (HOBA) and 4-(dec-9-
enoxy)benzoic acid (DOBA), respectively, as the second component, were determined by differential scanning calorime-
try, polarized-light microscopy, and X-ray diffraction. The MBOBA-HOBA system exhibited a cholesteric phase, two
solid solutions, and above 58 °C for compositions between 40 and 80 mol% HOBA a crystalline phase of mixed
dimers. The MBOBA-DOBA system showed a crystalline phase of mixed dimers at all compositions, a cholesteric
phase, and a twisted smectic C phase, which was dominated by mixed dimers at 60 and 70 mol% DOBA. We conclude
that liquid-crystalline phases are generally dominated by mixed dimers, but in crystalline phases the formation of
mixed dimers is promoted by a greater difference in molecular structure. The crystal structure of two of the pure com-
pounds MBOBA and DOBA and comparable compounds have been determined for an evaluation of the arrangements
of the molecules in the crystal and liquid-crystalline state.
Key words: benzoic acid, crystal arrangement, phase diagrams, liquid crystal.
Résumé : Le comportement de phase des systèmes binaires d’acides benzoïques substitués en position 4 est régi par la
formation de dimères mixtes. On a réalisé cette étude dans le but de déterminer les effets des différences de structure
des composants sur la formation de dimères mixtes dans les phases cristallines et à cristaux liquides. Les diagrammes
de phase de deux systèmes dans lesquels ont retrouve d’une part, dans tous les deux, de l’acide 4[(S)-(–)-2-
méthylbutoxy]benzoïque (AMBOB) et, d’autre part, respectivement de l’acide 4-(hex-5-énoxy)benzoïque (AHOB) et de
l’acide 4-(dec-9-énoxy)benzoïque (ADOB) en faisant appel à la calorimétrie à balayage différentiel, à la microscopie à
lumière polarisée et la diffraction des rayons X. Le système AMBOB/AHOB, on observe une phase cholestérique, deux
solutions solides et, au-dessus de 58 °C, pour les compositions entre 40 et 80 mol% d’AHOB, on observe aussi une
phase cristalline des dimères mixtes. Le système AMBOB/ADOB, présente une phase cristalline de dimères mixtes
pour toutes les compositions, une phase cholestérique et une phase smectique C décalée qui était dominée par les di-
mères mixtes pour les compositions allant de 60 à 70 mol% d’ADOB. On en conclut que les phases à cristaux liquides
sont généralement par les dimères mixtes, but que dans les phases cristallines la formation de dimères mixtes est faci-
litée par une plus grande différence dans les structures moléculaires. On a déterminé les structures cristallines des com-
posés AMBOB et ADOB et de composés apparentés afin d’évaluer les arrangements des molécules dans le cristal et
dans l’état à cristal liquide.
Mots-clés : acide benzoïque, arrangement cristallin, diagrammes de phase, cristal liquide.
[
Traduit par la Rédaction] Roman et al. 532
Introduction
dimers. The bonding is so strong that, on heating, the dimers
persist from the crystalline state through to the isotropic
state (1). A dimer has a larger aspect ratio and twice the
number of polarizable aromatic rings and carbonyl groups
Molecules of benzoic acid and its derivatives generally
pair through intermolecular hydrogen bonds and form
Received 1 November 2007. Accepted 1 December 2007. Published on the NRC Research Press Web site at canjchem.nrc.ca on
4
April 2008.
2
3
4
M. Roman, A. Kaeding-Koppers, and P. Zugenmaier. Institute of Physical Chemistry, Clausthal University of Technology,
Arnold-Sommerfeld-Str. 4, D-38678 Clausthal-Zellerfeld, Germany.
1
This article is part of a Special Issue dedicated to Professor R. Marchessault.
Present address: Department of Wood Science and Forest Products, 230 Cheatham Hall (0323), Virginia Tech, Blacksburg, VA
2
2
4061, USA.
3
4
Present address: Wipak-Walsrode GmbH & Co KG, P.O. Box 1661, 29656 Walsrode, Germany.
Corresponding author (e-mail: zugenmaier@pc.tu-clausthal.de).
Can. J. Chem. 86: 525–532 (2008)
doi:10.1139/V08-008
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Can. J. Chem. Vol. 86, 2008
Table 1. Properties of the pure components.
a
Determined by the lengths of the molecules in the crystal structure; for HOBA by adding the lengths of appropriate segments.
Change of d values upon increase of temperature.
b
with respect to a single molecule. However, due to the for-
mation of dimers by hydrogen bonding by the carbonyl
groups, the dipoles are compensated and an antiparallel ar-
rangement of the molecules is favored. For this reason, most
of the 4-substituted benzoic acids with linear substituent
groups are liquid-crystalline (2). When 4-substituted benzoic
acids are mixed, dimers containing two different molecules
can be formed. In studies of binary systems of 4-substituted
benzoic acids (3, 4), a number of systems showed deviation
from eutectic phase behavior. Mixed-dimer formation is
thought to be the reason for the high melting points in these
systems.
In this study, crystalline and liquid-crystalline phases of
these systems have been investigated with a focus on the for-
mation of mixed dimers. To determine the effect of the dif-
ference in molecular structure of the components on mixed-
dimer formation, we studied two systems that have one com-
mon component and whose second components differ only
in the length of the substituent groups. In the first system,
the molecular structures of the components are fairly similar
so that mutual solubility is expected. In the second system,
the molecular structures of the components are distinctly dif-
ferent, which can result in eutectization. The component that
both systems have in common is the chiral 4-[(S)-(–)-2-
methylbutoxy]benzoic acid (MBOBA). This compound is
not liquid-crystalline but a tendency to form a mesophase is
anticipated because 4-butoxybenzoic acid is mesomorphic.
Because of its asymmetric carbon atom, MBOBA is further
expected to twist the molecular arrangements in the
mesophases of the second components, which are 4-(Hex-5-
enoxy)benzoic acid (HOBA) and 4-(Dec-9-enoxy)benzoic
acid (DOBA). Further clues for the formation of chiral
phases can be provided by consideration of the unusual ar-
rangement of the pure compounds.
consisting of longer molecules, exhibited a nematic
mesophase. DOBA showed a nematic and a smectic C
mesophase. A crystalline phase transition was observed be-
low 80 °C in X-ray measurements. The transition was very
slow, which we attributed to impurities (10). As a conse-
quence, the crystalline phase transition of the DOBA was
only indirectly revealed by differential scanning calorimetry
(DSC): the enthalpy of melting upon cooling was lesser by
12.4 kJ/mol than upon first heating. This decrease implied
that in the heating run the crystalline phase transition oc-
curred at the melting temperature and was supercooled be-
low 0 °C in the cooling run. The transition did not occur
during the second heating, as the enthalpy of melting was in
good agreement with the one for cooling.
Methods
Eleven mixtures were prepared for the MBOBA–HOBA
system and nine for the MBOBA–DOBA system by co-
dissolving various amounts of the components in warm etha-
nol. The solvent was allowed to evaporate over a period of
one to two weeks after the solutions had cooled to room
temperature.
Transition temperatures were determined with
PerkinElmer DSC7. Samples were prepared by weighing 4–
mg of a mixture into an aluminum pan and then sealing it.
Every sample was heated, cooled, and heated again at a rate
a
6
–
1
of 5° min .
Mesophases were identified using an Olympus BH-2 po-
larized-light microscope with a Linkam THM600 hot stage.
The crystalline regions were investigated with an Anton Paar
Kratky Compact Small Angle System with a KH-R sample
heater. Samples were prepared by melting a small amount
(
about 2 mg) of a mixture on a microscope slide and driving
it by capillary action into a quartz capillary, where it
recrystallized on cooling.
Experimental
Crystals of the mixture 65 mol% HOBA in MBOBA, as
obtained from solution, were investigated with an in-house
flat-film camera equipped with an FP82-Mettler hot stage.
Single crystals suitable for X-ray investigations were ob-
tained by slow crystallization. The data collections was per-
formed on a CAD4 single-crystal diffractometer with Mo K
α radiation and the MolEN package (Enraf Nonius, Delft)
(11), and (or) the Shelxl-93 program (12) was used for data
processing and refinement. The starting models were ob-
tained by SIR97 (13). Here, we concentrate on the confor-
mation and packing arrangement of the molecules only. A
full description and discussion of the data will be published
Materials
MBOBA was synthesized by brominating (S)-(–)-2-
methylbutan-1-ol (5, 6) with phosphorus tribromide (7, 8)
and subsequently reacting the obtained (S)-(–)-2-methyl-1-
bomobutane with 4-hydroxybenzoic acid. The different
alkenoxy benzoic acids used in this study were prepared as
described in ref. (9). The properties of the pure components
(
Table 1) were determined as described in the methods sec-
tion below. For MBOBA, no liquid-crystallinity was ob-
α
β
served; it showed two crystalline phases, C and C . HOBA,
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Roman et al.
527
Fig. 1. Representation of 4-[(S)-(–)-2-methylbutoxy]benzoic acid
(MBOBA), which crystallizes in the triclinic two-molecule
acentric space group P1 in three projections. (A) Projection in
elsewhere. The drawings were obtained with SCHAKAL-92
(
14).
[
010] direction on the a–c plane showing the formation of
Results and discussion
dimers through hydrogen bonding. (B) Projection in [100] direc-
tion on the b–c plane showing the continuous rod formation.
Crystal structures
(
C) Projection along the continuous rods showing the relative
Almost all benzoic acids, composed of single non-chiral
compounds and forming dimers by hydrogen bonds, crystal-
lize in centric space groups with an inversion center between
the carbonyl groups (15, 16). However, a 1:1 mixture of two
different benzoic acids is devoid of the inversion center and
an acentric space group can describe the crystal structure
packing arrangement of the molecules.
(
16) with consequences for the chirality of the liquid-
crystalline phases (17).
MBOBA and four benzoic acids with a linear group at-
tached in 4-position of the benzyl ring, namely 4-(but-3-
enoxy)benzoic acid (BOBA), 4-(pent-4-enoxy)benzoic acid
(
(
POBA), 4-(dec-9-enoxy)benzoic acid (DOBA), and 4-
undec-10-enoxy)benzoic acid (UOBA), have been investi-
gated by single crystal analysis to get insight into the con-
formation and packing arrangements of these molecules,
which are important for a discussion of the liquid crystal
phases observed. All five compounds are arranged in
antiparallel fashion to compensate the strong dipoles of the
molecules and form dimers. They have in common a similar
distance of the two C–O bonds of the carbonyl group of
which both oxygens are involved in hydrogen bonds and a
gauche position of the alkoxy segment of the linear
substituent.
The chiral MBOBA crystallizes in the enantiomorphous
space group P1 and projections representing the conforma-
tion and packing arrangement are shown in Fig. 1. The
dimer formations are revealed. Both molecules forming the
asymmetric unit have similar conformations. The carbon
atom of the methyl branch of the substituent lies in plane
with the preceding carbon atoms and oxygen, while the
alkoxy segment of the linear group is placed in gauche posi-
tion involving the oxygen attached to the benzene ring. The
molecules are placed in a tilted direction in the unit cell,
which means that the length of the molecule, which amounts
to 12.2 Å, does not agree with the longest unit-cell axis of c
=
9.4 Å. The molecules pack in planes with the benzene
rings lying flat in these planes and they are staggered
(
Figs. 1A, 1B, 1C).
BOBA crystallizes in the acentric space group Pa with
two antiparallel molecules in the asymmetric unit, unique
axis b (Fig. 2). Although the torsion angles of the alkoxy
segment have reversed signs, as expected for
centrosymmetric arrangements, the torsion angle of the ter-
minal atom does not follow this reversal, and two distinct
different molecules form the dimer and cannot crystallize in
a centric space group with an inversion center between the
carboxyl groups of the dimers. Here, the dimer is tilted in
the a–c plane, and the length of the dimer of ~20 Å is also
longer compared with the largest unit-cell parameter c =
linear properties. The packing arrangement of the dimers
formed is represented in Fig. 3 in two projections. Two
antiparallel molecules form the asymmetric unit, which by
application of a 2 screw axis lead to the four molecules
1
within the unit cell. The molecular axes are placed almost
perpendicular to the screw axis b in contrast to DOBA, to be
discussed. The overall arrangement of POBA resembles the
one of BOBA. The length of the dimers amounts to 26.8 Å
and is much longer than the c parameter of the unit cell with
15.4 Å. Here, a the tilt is observed in the a–c plane. If
mesophases are formed, they are expected to be chiral.
1
4.9 Å. According to our experience, the centric or acentric
packing of molecules in the crystalline state may also lead to
different kinds of arrangement in the liquid-crystalline state.
Surprisingly, the space group of crystalline POBA is P21,
unique axis b, which belongs to the optically active space
groups and the ones, which may exhibit second-order non-
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Can. J. Chem. Vol. 86, 2008
Fig. 2. Representation of 4-(but-3-enoxy)benzoic acid (BOBA),
which crystallizes in the monoclinic four-molecule unit cell of
the acentric space group Pa in three projections. (A) Projection
in [010] direction on the a–c plane showing the formation of
dimers through hydrogen bonding. (B) Projection in [100] direc-
tion on the b–c plane showing the overlapping molecules of the
plane represented in A. (C) Projection in rod direction showing
the lateral packing arrangement of the molecules. The hydrogen
atoms at the terminal carbon could not be localized.
Fig. 3. Representation of 4-(pent-4-enoxy)benzoic acid (POBA),
which crystallizes in the monoclinic four molecule unit cell of
the acentric space group P2 (b unique axis) in two projections.
1
(A) Projection showing the formation of dimers through hydro-
gen bonding. (B) Projection in rod direction showing the lateral
packing arrangement of the molecules.
dimer through hydrogen bonds as in almost all benzoic acids
(
Fig. 4). The gauche conformation of the alkoxy segment
forces a change of direction between the central part of the
dimer and the linear chain, which exhibits an almost trans
conformation. A chain length is difficult to define and
amounts to 39.3 Å between the two terminal groups of the
dimer and differs considerably, if this length is calculated
with a trans conformation of the alkoxy segment.
A similar conformation for the single molecule is found for
DOBA as compared to UOBA, but the packing arrangement
in the crystalline state differs considerably as shown in Fig. 5.
The long dimension c is almost doubled due to the centric
space group P2 /n, c = 41.51 Å for DOBA and P-1 with
1
2
2.34 Å for UOBA. The unit cell of DOBA has to accommo-
date two succeeding molecules related by a 2 screw axis,
1
which changes the direction of the central part of the dimer in
space. Nevertheless, quite similar dimensions of the unit cell
are found for the lateral packing of the molecules for DOBA
and UOBA. A single molecule represents the asymmetric unit
in both structures. A distance between the terminal hydrogen
of 32.8 Å was determined for the dimer of DOBA as com-
pared with 39.3 Å for UOBA, and both lengths deviate from a
trans conformation of the alkenoxy substituent.
In contrast, centric space groups are found for longer lin-
ear chains attached in the 4-position on the benzene ring of
benzoic acids as DOBA and UOBA. But again the packing
arrangement differs for these two compounds.
UOBA crystallizes in the triclinic space group P-1. The
unit cell contains two molecules, which are connected by an
inversion symmetry element, thus forming the expected
The crystal and molecular structures of the four homolo-
gous achiral benzoic acids show a broad variation of possible
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Roman et al.
529
Fig. 5. Representation of 4-(dec-9-enoxy)benzoic acid (DOBA),
Fig. 4. Representation of 4-(undec-10-enoxy)benzoic acid
(
UOBA), which crystallizes in the triclinic two-molecule unit cell
which crystallizes in the monoclinic four-molecule unit cell of
of the centric space group P-1 in two projections. (A) Projection
in [100] direction on the b–c plane showing the formation of
dimers through hydrogen bonding. (B) Projection in [010] direc-
tion on the a–c plane showing the relative packing arrangement
of the molecules.
the centric space group P2 /n (b unique axis) in two projections.
1
(A) Projection in [100] direction on the b–c plane showing the
formation of dimers through hydrogen bonding. (B) Projection in
[001] direction on the a–b plane representing the lateral packing
arrangement of the molecules.
Fig. 6. Crystalline-cholesteric and cholesteric–isotropic transition
line for the MBOBA–HOBA system (peak temperatures of the
first DSC heating run); TVN*I: virtual cholesteric–isotropic transi-
tion temperature (– – –): theoretical melting curves for a eutectic
system.
low energy states, which may lead to a variety of arrange-
ments of molecules in the liquid-crystalline state. Moreover,
chiral molecules are created when the chiral MBOBA forms
dimers with achiral benzoic acid homologues regardless of
the conformation of the achiral partner.
MBOBA–HOBA system
All MBOBA–HOBA mixtures were liquid-crystalline
Fig. 6). The only mesophase upon heating revealed an “oily
(
streak” texture in the polarized-light microscope, which is
characteristic of the cholesteric phase. As anticipated, the
nematic molecular arrangement was twisted by the chiral
MBOBA molecules and became cholesteric. The fact that 10
mol% of HOBA was enough to induce a mesophase indi-
cated the expected tendency of MBOBA to show liquid-
crystallinity. In accordance, extrapolation of the cholesteric–
isotropic transition line to 100 mol% MBOBA gave a value
of 101.8 °C for the acid’s virtual cholesteric–isotropic transi-
tion temperature (TVN*I). This value defines how disruptive
the non-mesomorphic compound is towards the mesophase
thermal stability of the crystalline region, we investigated
the mixtures at different temperatures below their melting
temperatures by X-ray diffraction. Below 58 °C two crystal-
line phases (C Aα and C ) were found, represented by differ-
B
α
ent reflections in the diffraction patterns (Fig. 7). The CA
(
1
18). Since the value was close to the melting point of
10.5 °C, the disruptive effect of MBOBA, due to the
phase was formed by mixtures containing less than 80 mol%
HOBA (Fig. 8). It had a characteristic d value equal to the d
value of MBOBA and was therefore most likely a solid solu-
methyl branch of the substituent group, on the nematic mo-
lecular arrangement was small.
tion of HOBA in MBOBA. The C phase was formed by
B
The melting curve indicated that the MBOBA–HOBA sys-
tem was not eutectic. To elucidate the deviation from
eutectic behavior, we calculated eutectic melting curves
mixtures with more than 65 mol% HOBA and had a d value
equal to that of HOBA. In this phase, MBOBA molecules or
dimers are probably incorporated into the HOBA crystal lat-
tice. In the mixtures with 65 to 80 mol% HOBA, the two
(
Fig. 6) on a thermodynamic basis using the Schroeder–
vanLaar equations (19, 20). The observed melting tempera-
tures were notably higher than the theoretical temperatures
for the mixtures with 20 to 70 mol% HOBA, which sug-
gested solid-solubility of the two components or the forma-
tion of phases of mixed dimers. To identify the cause for the
phases, C Aα and C , coexisted.
B
The high-temperature crystalline phase of the MBOBA,
β
C , was found only in the mixtures with a HOBA content
A
less than 20 mol%. For the mixtures containing 40 to 80
mol% HOBA an additional phase (CAB) was found above
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Can. J. Chem. Vol. 86, 2008
Fig. 7. X-ray diffraction patterns of 70 mol% HOBA in
Fig. 8. Crystalline phase diagram of the MBOBA–HOBA system
MBOBA.
(temperatures from X-ray diffraction measurements).
5
8 °C. The characteristic d value (d = 12.5 Å) lay between
Fig. 9. DSC traces of 50 mol% HOBA in MBOBA; the arrows
indicate the CA –CAB phase transition.
α
α
the d values of CA and C and thus indicated the formation
B
of mixed dimers. The d value of MBOBA amounts to ~9 Å
that of HOBA to ~16 Å. In the crystalline state, the molecu-
lar length of the monomer does not correlate with the d
spacing obtained by X-ray diffraction due to possible tilted
positions within the unit cell. The d value of MBOBA is
confirmed by the longest d spacing derived from the unit
cell (d = 9.12 Å). The length of the mixed dimer was calcu-
lated to 27.4 Å as opposed to 24.2 and 30.2 Å for the pure
MBOBA and HOBA dimers (Table 1). The d value in-
creased slightly with increasing temperature to d = 12.6 Å
before melting. The phase transition from the two-phase re-
gion into the high-temperature phase was clearly evident at
5
8 °C in X-ray experiments (Fig. 7).
To assure that this high-temperature phase was not a more
highly ordered liquid-crystalline phase, we investigated the
mixture with 65 mol% HOBA above 70 °C by polarized-
light microscopy. We did not observe any fluidity. A flat-
film X-ray diffraction pattern of the same mixture, taken at
mol%. In the latter case, the molecules had to arrange from
one crystal lattice into two different crystal lattices.
8
0 °C, showed discrete Bragg maxima indicating the pres-
ence of small crystalline domains. We concluded that this
was not a smectic phase of higher order but a fourth crystal-
line phase dominated by mixed dimers.
MBOBA–DOBA system
As in the MBOBA–HOBA system, all mixtures exhibited
a cholesteric mesophase (Fig. 10). The cholesteric–isotropic
transition temperature of the mixture 10 mol% DOBA in
MBOBA was 105.1 °C as opposed to 105.9 °C for 10 mol%
HOBA in MBOBA, reflecting that the molecular arrange-
ment of the cholesteric phase was only weakly affected by
the greater difference in molecular lengths of the compo-
nents. The cholesteric–isotropic transition line, however, was
not as smooth as in the case of the MBOBA–HOBA system.
It was straight at the extremes of the composition range and
had a local minimum at the composition with a one-to-one
ratio of the components. The shape of the cholesteric–isotropic
transition line indicated that the cholesteric phase at DOBA
contents less than 20 and more than 80 mol% was domi-
nated by pure MBOBA or DOBA dimers, respectively, but
that mixed dimers dominated at DOBA contents in between.
This conclusion was supported by microscopy results: the
textures observed for the cholesteric phase on heating were
“oily streak” textures for all mixtures; on cooling, the mix-
tures with a DOBA content less than 70 mol% showed fan
textures; whereas the mixtures with 80 and 90 mol% DOBA
showed textures of a cholesteric phase with greater pitch.
α
The C –C or C –C phase transitions, respectively, in
A
AB
B
AB
the mixtures with 40 to 90 mol% HOBA were also revealed
in DSC measurements as shown for 50 mol% HOBA in
MBOBA in Fig. 9. In most mixtures, the transition occurred
just below or at the melting temperature in the first heating
run and was strongly supercooled in the cooling run. The
shift of the crystalline transition to lower temperatures upon
cooling differed for the different mixtures. At 40, 50, and 90
mol% HOBA in MBOBA, the transition was supercooled by
about 15 °C, 40 °C (Fig. 9), and 50 °C, respectively. In the
mixtures with 60, 70, and 80 mol% HOBA, the transition
was supercooled below 0 °C and thus not seen in the cooling
trace. It occurred in both directions (exotherm and
endotherm) during the second heating run for the mixture
with 60 mol% HOBA, but the mixtures with 70 and 80
mol% HOBA did not show the transition upon second heat-
ing. The different thermal behavior upon cooling reflected
that the transition in the mixtures with 40, 50, 60, and 90
mol% HOBA was into a one-phase region; whereas it was
into a two-phase region in the mixtures with 70 and 80
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Roman et al.
531
Fig. 11. d Values of the smectic C phases at different MBOBA–
DOBA compositions: (a) as a function of temperature (numbers
indicate DOBA content in mol%), (b) at the smectic–nematic
transition temperatures.
Fig. 10. Phase diagram of the MBOBA–DOBA system (peak
temperatures of the first DSC heating run); DSC transition peak
has a shoulder (o).
The low thermal stability at 50 mol% DOBA in MBOBA
was attributed to the structural difference of the substituent
groups of the components. The linear decenoxy group of
DOBA allowed a narrower molecular arrangement than the
branched 2-methylbutoxy group of MBOBA. The DSC
thermogram of the mixture 50 mol% DOBA in MBOBA
showed a shoulder peak for the cholesteric–isotropic transi-
tion, which implied that the cholesteric phase of this mixture
is not homogeneous. It might separate into domains with dif-
ferently tight molecular arrangements.
The mixtures that contained more than 50 mol% DOBA
exhibited a twisted smectic C phase. In the polarized-light
microscope the phase appeared in form of a fine Schlieren
texture.
Table 2. d Values of the crystalline phases of the MBOBA–
DOBA mixtures (at 30 °C for the α-phases and at the tempera-
ture of the phase transition for the β-phases).
d Value (Å)
DOBA
content mol%
Cα
Cβ
Cα
Cβ
Cα
Cβ
A
A
AC
AC
C
C
0
8.7
8.8
8.8
14.6
1
2
3
4
5
6
7
8
9
0
0
0
0
0
0
0
0
0
14.6
15.1
19.2
16.2
a
b
17.2
19.8
20.8
21.7
22.1
25
25.6
25.9
To determine whether this phase contained mixed dimers,
we analyzed the mixtures by X-ray diffraction at tempera-
23.9
24.1
tures between their melting points and S *–N* transition
C
a
temperatures. The temperature dependence of the d value
was similar in all mixtures (Fig. 11a) and the absolute value
increased with increasing DOBA content (Fig. 11b). The in-
crease in d value was linear with a discontinuity and change
in slope at 80 mol% DOBA. In comparison the longest d
value calculated from the size of the unit cell depicted in
Fig. 5 with interdigitated molecules of DOBA amounts to
d = 20.82 Å. We concluded that similar to the cholesteric
phase, the twisted smectic C phase at a DOBA content less
than 80 mol% was dominated by mixed dimers, but pure
DOBA dimers dominated at a DOBA content higher than 80
mol%. This conclusion was supported by DSC results,
1
00
25.2
a
Appeared at ~57 °C upon heating.
Values not available.
b
at the extremes of the composition range (CA and CC,
Fig. 10) were most likely the crystalline phases of the pure
components with a low- and a high-temperature modifica-
α
tion (α and β). The C phase presumably contained differ-
AC
ent amounts of mixed dimers in the different mixtures, as
the d value varied with composition. Most of the mixtures
(30 to 90 mol% DOBA in MBOBA) melted at about 58 °C.
The mixtures with 60 to 90 mol% DOBA separated above
which revealed a shoulder peak for the S *–N* transition at
C
8
0 mol% DOBA in MBOBA, indicating that the S * phase
C
in this mixture was not homogenous. The mixed dimers with
a length of 28.9 Å did not form a smectic phase on their
own. The smectic phase in the mixtures with 60 and
60 °C into a S * phase and the high-temperature crystalline
C
phase of DOBA. On melting, the d value of the smectic
phase showed a distinct shift to higher values, which re-
flected increasing incorporation of DOBA molecules into
the smectic molecular arrangement.
7
0 mol% DOBA was induced by the dimers of excess
DOBA molecules.
At 58 °C, the mixtures with 10 and 20 mol% DOBA
β
showed a phase transition into another mixed phase (C
,
AC
Conclusions
Fig. 10). This phase must have a slightly different composi-
tion (higher DOBA content), since the crystalline phase of
the MBOBA is formed simultaneously with it.
With respect to the crystalline region, X-ray diffraction
gave a variety of d values (Table 2). We were able to distin-
guish at least six different crystalline phases. The two phases
A variety of shapes and packing arrangements was found
for crystalline 4-substituted benzoic acids. Achiral pure
compounds with longer substituents attached crystallize in
acentric and with shorter substituents in centric crystal struc-
tures with dimers formed by hydrogen bonding. The symme-
©
2008 NRC Canada

5
32
Can. J. Chem. Vol. 86, 2008
try elements present in the crystalline state also influence the
kind of arrangements in liquid crystals.
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4
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Regarding the crystalline phases in the mixed systems, the
formation of mixed dimers was found to be promoted by a
greater difference in molecular structure. The system with
the more structurally similar components, the MBOBA–
HOBA system, exhibited a crystalline phase of mixed
dimers at compositions between 40 and 80 mol% HOBA and
temperatures above 58 °C. The system with the greater
structural difference in components (the MBOBA–DOBA
system) showed crystalline phases containing mixed dimers
at room temperature and for all compositions. A greater dif-
ference in molecular structure led to a decrease in solubility.
Due to the tendency of these systems to form mixed dimers,
the region in which mixed dimers were found was enlarged,
as opposed to enlarging the region where two crystalline
phases coexisted.
(
9
(
6
7
8
3
128 (1948).
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1
1
0. B. Sternberg, W. Pilz, P. Reich, and H.D. Koswig. Z. Phys.
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Nonius, Delft, The Netherlands. 1990.
The liquid-crystalline phases were found to be dominated
by mixed dimers at compositions in the midrange. At the ex-
tremes of the composition range, pure dimers dominated the
liquid-crystalline phases. Concerning the cholesteric phase,
mixed-dimer formation in the composition midrange was in-
dicated by the shape of the cholesteric–isotropic transition
line and was more obvious for the system with the more
structurally different components. Regarding the twisted
smectic C phase, the change from mixed-dimer prevalence
to pure-dimer prevalence with composition was evident by a
discontinuity in the increase of the d value with increasing
DOBA content.
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for the refinement of single-crystal diffraction data. University
of Göttingen, Göttingen, Germany. 1993.
13. A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi,
A.G.G. Moliterni, M.C. Burla, G. Polidori, M. Camalli and
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5. G.M. Frankenbach and M.C. Etter. Chem. Mat. 4, 272 (1992).
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Acknowledgement
17. F. Heinemann and P. Zugenmaier. In Chiral liquid crystals.
Edited by W. Kuczynski. Institute of Molecular Physics, Polish
Academy of Sciences, Poznan. 2003. pp. 99.
We thank the European Commission’s Directorate-General
Education and Culture for an ERASMUS scholarship for
M.R. and Prof. Corrie T. Imrie at the University of Aberdeen
for hosting M.R. during her study abroad. We also thank Mrs.
Astrid Peschel for assistance with the DSC measurements.
1
1
2
8. R.J. Diefendorf. Ext. Abstr. Program– Bienn. Conf. Carbon.
6, 26 (1983).
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(
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©
2008 NRC Canada

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