Carbohydrate liquid crystals: Synthesis and characterisation of the methyl-6-O-(n-acyl)-α-D-glucopyranosides

Article abstract of DOI:10.1016/j.chemphyslip.2010.11.007

Seven members of the methyl-6-O-(n-acyl)-α-d-glucopyranosides have been synthesised and their transitional properties determined. The undecanoyl and octadecanoyl members do not exhibit liquid crystallinity while the members having chain lengths between dodecanoyl and hexadecanoyl exhibit a monotropic smectic A phase. Variable temperature infrared spectroscopy reveals that the hydrogen bonding within the system shows a marked change at the melting point but apparently no change at the smectic A-isotropic transition. This observation is interpreted in terms of Goodby's model for the smectic A phase in which the carbohydrate moieties are located at the centre of the smectic bilayer and assuming that hydrogen bonded aggregates persist into the isotropic phase. Within this framework, the unusually low values of the entropy change associated with the smectic A-isotropic transition may also be accounted for.

Full text of DOI:10.1016/j.chemphyslip.2010.11.007

Chemistry and Physics of Lipids 164 (2011) 118–124
Contents lists available at ScienceDirect
Chemistry and Physics of Lipids
journal homepage: www.elsevier.com/locate/chemphyslip
Carbohydrate liquid crystals: Synthesis and characterisation of the
methyl-6-O-(n-acyl)-␣-d-glucopyranosides
Andrew G. Cook, James L. Wardell, Corrie T. Imrie∗
Chemistry, School of Natural and Computing Sciences, University of Aberdeen, Meston Walk, Old Aberdeen AB24 3UE, Scotland, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Seven members of the methyl-6-O-(n-acyl)-␣-d-glucopyranosides have been synthesised and their
transitional properties determined. The undecanoyl and octadecanoyl members do not exhibit liquid
crystallinity while the members having chain lengths between dodecanoyl and hexadecanoyl exhibit
a monotropic smectic A phase. Variable temperature infrared spectroscopy reveals that the hydrogen
bonding within the system shows a marked change at the melting point but apparently no change at
the smectic A-isotropic transition. This observation is interpreted in terms of Goodby’s model for the
smectic A phase in which the carbohydrate moieties are located at the centre of the smectic bilayer and
assuming that hydrogen bonded aggregates persist into the isotropic phase. Within this framework, the
unusually low values of the entropy change associated with the smectic A-isotropic transition may also
be accounted for.
Received 27 October 2010
Received in revised form
24 November 2010
Accepted 25 November 2010
Available online 2 December 2010
Keywords:
Liquid crystals
Smectic A phase
Variable temperature FTIR
Hydrogen bonding
© 2010 Published by Elsevier Ireland Ltd.
1. Introduction
increases in chain length result in a decrease in the clearing tem-
perature. This apparently quite general relationship between the
There is now a surprising diversity of low molar mass molec-
ular architectures based around rod-like units known to support
liquid crystallinity (Goodby et al., 2009) and of these, carbohydrate-
based liquid crystals have attracted considerable interest. This has
arisen not only as a result of the wide range and low cost of the
carbohydrate building blocks used to construct these materials,
but also because mesogenic monosaccharides and polysaccharides
are important in many biological processes (Goodby, 1998, 2006;
Goodby et al., 2007; Van Doren et al., 2000). A carbohydrate-based
mesogen typically consists of a hydrophilic (polar) head group
attached to which are one or more lipophilic (nonpolar) chains.
Establishing the empirical relationships between molecular struc-
ture and thermotropic phase behaviour has been at the centre of
much of this research, although such materials also often exhibit
lyotropic mesophases. Of particular interest has been the rela-
tionship between liquid crystalline behaviour and the length and
structure of the alkyl chain or chains; for some recent examples see
(Singh et al., 2008, 2010; Milkereit et al., 2005a,b,c; Nguan et al.,
2010; Liao et al., 2006). In general, for short alkyl chains, liquid
crystallinity is either not observed or is monotropic in nature. On
increasing chain length, the clearing temperature increases rapidly
and a maximum value is reached for a chain length which is depen-
dent on the size of the sugar moiety (Bault et al., 1998a). Further
clearing temperature and chain length has been established using
a wide range of carbohydrate-based liquid crystals possessing a
single terminal chain, but has not so often been tested on com-
plete homologous series (Van Doren et al., 1989a; Van Doren and
Terpstra, 1995; Bault et al., 1998b). Thus, we have synthesised and
characterised the homologous series, the methyl-6-O-(n-acyl)-␣-
d-glucopyranosides, 1 in Scheme 1. A key issue in liquid crystal
science concerns the rôle played by hydrogen bonding in deter-
mining phase behaviour both in carbohydrate-based mesogenic
systems (Goodby et al., 1998) as well as in conventional low molar
mass liquid crystals (Janietz and Kohlmeier, 2009) and in order
to investigate this, we measured the variable temperature FT-IR
spectra for the dodecanoyl member of series 1.
2. Experimental
The synthetic route used for the preparation of the methyl-6-
O-(n-acyl)-␣-d-glucopyranosides is shown in Scheme 1. Identical
conditions were used in the preparation of all homologues and thus,
only a representative description for the synthesis of methyl-6-O-
dodecanoyl-␣-d-glucopyranoside is given.
2.1. Materials
Acyl chlorides were obtained by heating at reflux the appropri-
ate alkanoic acid (Aldrich) and thionyl chloride for 1 h. The excess
thionyl chloride was removed under vacuum and the acyl chlo-
∗
Corresponding author. Tel.: +44 1224 272567.
E-mail address: c.t.imrie@abdn.ac.uk (C.T. Imrie).
0009-3084/$ – see front matter © 2010 Published by Elsevier Ireland Ltd.
doi:10.1016/j.chemphyslip.2010.11.007

A.G. Cook et al. / Chemistry and Physics of Lipids 164 (2011) 118–124
119
with EtOAc/n-BuOH (4/1, v/v, 2 × 40 ml). The combined organic
layers were washed with water (3 × 20 ml), dried and the solvents
removed under vacuum. The residue was recrystallised at least
twice from acetone.
Yield 10%; mp 73 ◦C; elemental analysis: Anal. % Calcd. for
C
19H36O7: C 60.59, H 9.64. Found: C 60.31, H 9.52; 1H NMR (DMSO-
d6) ı (ppm): 0.86 (t, 3H, CH3–CH2, J = 6.8 Hz), 1.25 (m, 16H alkyl
chain), 1.5 (m, 2H, CH2–CH2–CO), 2.3 (t, 2H, CH2–CO, J = 7.3 Hz), 3.25
(s, 3H O–CH3), 3.05 (m, 1H, H4), 3.18 (m, 1H, H2), 3.41 (m, 1H, H3),
3.50 (m, 1H, H5), 4.02 (m, 1H, H6a), 4.31 (m, 1H, H6b), 4.52 (d, 1H, H1,
J = 3.7 Hz), 4.7–5.3 (3 broad peaks, 3H, OH2–OH4); 13C NMR (DMSO-
d6) ı (ppm): 15.44 (CH3–CH2 alkyl chain), 23.59 (CH3–CH2 alkyl
chain), 29.92–30.50 (alkyl chain), 25.97 (CH2–CH2–CH2–CO), 32.80
(CH2–CH2–CH2–CO), 35.01 (CH2–CH2–CH2–CO), 55.80 (OCH3),
65.06 (C6), 71.06 (C5), 71.91 (C4), 73.31 (C2), 74.68 (C3), 101.19
(C1), 174.28 (C O); IR ꢀ (cm−1) 1728 (C O), 3376 (OH), 2850–2900
(C–H).
2.4. Characterisation
The structures of series 1 and all intermediates were verified
by 1H NMR spectroscopy using Varian Unity Inova 400 MHz and
Bruker AC-F 250 MHz spectrometers, and by FTIR spectroscopy
using an ATI Mattson spectrometer. Elemental analyses were per-
formed by Butterworth Laboratories Ltd. The thermal properties of
the materials were determined by differential scanning calorime-
try (DSC) using a Mettler-Toledo DSC821e cooled by liquid nitrogen
and calibrated using indium and zinc as standards. All the samples
were measured in a nitrogen atmosphere. The time–temperature
programme used for the DSC analyses was (i) an initial heat from
−20 to 100 ◦C at 10 ◦C/min; (ii) held for 3 min at 100 ◦C; (iii) cooled
from 100 to −20 ◦C at 10 ◦C/min; (iv) held for 3 min at −20 ◦C and
(v) reheated from −20 to 100 ◦C at 10 ◦C/min. In order to obtain
the thermodynamic data associated with monotropic clearing tran-
sitions, the sample was heated into the isotropic phase, cooled
to a temperature within the mesophase but prior to the onset of
recrystallisation, and finally, reheated to the isotropic phase. The
thermodynamic data quoted are averaged values from measure-
ments of two aliquots of each sample. Phase identification was
performed by polarised light microscopy using an Olympus BH-
2 optical microscope equipped with a Linkam TMHS 600 heating
stage and a TMS 92 control unit. The FT-IR spectrum of the dode-
canoyl homologue of series 1 as a function of temperature was
obtained using a Nicolet Nexus FT-IR bench attached to a Contin-
uum FT-IR microscope equipped with a Linkam FTIR 600 heating
stage and a TMS 93 control unit. The sample was sandwiched
between a gold coated microscope slide and a 3 mm KBr disc. A
thin film of the sample was obtained by first heating it into the
isotropic phase. The spectra were recorded in a trans-reflectance
mode, i.e., the IR beam passed through the sample was reflected by
the gold surface back through the sample and then detected.
Scheme 1. Synthesis of the methyl-6-O-(n-acyl)-␣-d-glucopyranosides.
ride purified by distillation. All other materials were obtained from
Aldrich and used without further purification.
2.2. 3-Dodecanoyl-5-methyl-1,3,4-thiadiazole-(3H)-2-thione, 2
3-Dodecanoyl-5-methyl-1,3,4-thiadiazole-(3H)-2-thione was
prepared using the method described by Baczko and Plusquellec
(1991). Thus, to
a stirred solution of 2-mercapto-5-methyl-
1,3,4-thiadiazole (4.74 g, 36 mmol) and triethylamine (5.6 ml) in
CH2Cl2 (80 ml) at 0–5 ◦C was added a solution of freshly prepared
dodecanoyl chloride (8.75 g, 40 mmol) in CH2Cl2 (40 ml). Stirring
was continued overnight, and the reaction mixture subsequently
washed successively with 0.5 M HCl (2 × 20 ml), 5% NaHCO3
(3 × 40 ml) and water (2 × 20 ml). The organic phase was dried
and the solvent removed under vacuum. The residue was purified
by recrystallisation from n-hexane. The crystal structures of two
homologues of this series have been published (Low et al., 1999).
Yield 68%; mp 50 ◦C; elemental analysis: Anal. % Calcd. for
C
15H26N2S2O: C 57.26, H 8.34, N 8.91, S 20.40. Found: C 57.62,
H 8.10, N 8.98, S 20.44; 1H NMR (CDCl3) ı (ppm): 0.86 (t,
3H, CH3–CH2-R–CO, J = 6.8 Hz), 1.24 (m, 16H alkyl chain), 1.7
(qn, 2H, R-CH2CH2CO, J = 7.5 Hz), 2.42 (s, 3H CH3–C), 2.95 (t,
2H, R-CH2–CH2–CO, J = 7.5 Hz); 13C NMR (CDCl3) ı (ppm): 15.15
(CH3–CH2 alkyl chain), 17.35 (CH3–C thione ring), 23.72–32.94
(alkyl chain), 25.16 (CH2–CH2–CO), 38.87 (CH2–CO), 156.02 (C N),
172.63 (C O), 189.51 (C S); IR ꢀ (cm−1) 1753 (C O), 2850–2900
(C–H).
3. Results and discussion
2.3. Methyl-6-O-dodecanoyl-˛-d-glucopyranoside, 1
The transition temperatures and associated enthalpy and
entropy changes of series 1 are listed in Table 1. The undecanoyl
and octadecanoyl homologues do not exhibit liquid crystalline
Methyl-6-O-dodecanoyl-␣-d-glucopyranoside
pared using the method described by Baczko and Plusquellec
was
pre-
(1991). Thus, methyl-␣-d-glucopyranoside (3.8 g, 20 mmol), 3-
behaviour while all other homologues show a monotropic
dodecanoyl-5-methyl-1,3,4-thiadiazole-(3H)-2-thione
(3.14 g,
mesophase. The assignment of the liquid crystal phase was made
on the basis of the characteristic optical textures observed when
viewed through the polarised light microscope. Specifically, on
cooling from the isotropic phase, bâtonnets formed which coa-
lesced to give a focal conic fan texture implying a layered
arrangement of the molecules. This formed in coexistence with
regions of homeotropic texture, see Fig. 1. On further cooling, the
10 mmol) and triethylamine (4.2 ml) were dissolved in dry DMF
(50 ml), and the mixture stirred at room temperature overnight.
After addition of acetic acid (1 ml), the solvent was removed under
vacuum at 40 ◦C. The residue was extracted from a phosphate
buffer (pH 7; 60 ml) and a mixture containing EtOAc/n-BuOH
(4/1, v/v; 60 ml). The aqueous phase was extracted twice more

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A.G. Cook et al. / Chemistry and Physics of Lipids 164 (2011) 118–124
Table 1
the whole series. This supports the view that the melting point of
this class of materials is largely determined by the strength of the
static hydrogen bonding network in the crystal (van Doren et al.,
1990). At the melting point this network is weakened by increased
thermal motion and becomes more dynamic in nature. The essen-
tially linear increase in melting point on increasing the chain length,
however, does suggest that the van der Waals interactions between
the alkyl chains also play a role in determining the melting point.
The crystal structures of two members of series 1, n = 10 and 12,
have been reported and are essentially identical (Abe et al., 1996).
The molecules are arranged in bilayers in which the alkyl chains
are interdigitated while the sugar groups are disordered. This dis-
ordering made it impossible to determine the extent of hydrogen
bonding within the crystal structure.
Transition temperatures, and associated enthalpy and entropy changes for the
methyl-6-O-(n-acyl)-␣-d-glucopyranosides, 1. ( ) indicates monotropic transition
temperatures. The thermodynamic data quoted for the SmA-I transition for n = 15
and 16 were obtained from cooling cycles, all other data were extracted from heating
cycles.
n
Cr–I (◦C) ꢁH (kJ mol−1
)
ꢁS/R SmA–I (◦C) ꢁH (kJ mol−1
)
ꢁS/R
11
12
13
14
15
16
18
69
73
78
82
84
84
90
39.8
43.3
50.7
53.7
54.8
42.5
54.3
13.7
15.0
17.4
18.2
18.5
14.3
18.0
(65)
(72)
(75)
(70)
(59)
1.06
1.10
1.05
1.00
0.52
0.38
0.38
0.36
0.35
0.19
On increasing the chain length, the smectic A-isotropic, SmA-I,
transition temperature initially increases, passes through a max-
imum for the tetradecanoyl homologue and then decreases, see
Fig. 4. There is no apparent alternation in the SmA-I transition tem-
peratures as the parity of the alkyl chain is varied. A similar overall
dependence of the clearing temperature on chain length has been
observed for other carbohydrate-based series (Jeffrey and Wingert,
1992) although an odd–even effect is often overlaid upon it (see
for example, Bault et al., 1998b). By analogy with the behaviour of
conventional low molar mass mesogens, the origin of this odd–even
effect may be the change in the average shape of the molecule as
the parity of the chain is varied which becomes less pronounced
as the number of conformations available to the chain increases
(Imrie and Taylor, 1989). The liquid crystalline members of series 1
all contain long alkyl chains and this may account for the absence of
the odd–even effect. In addition, for conventional low molar mass
mesogens, the magnitude of the alternation in the clearing temper-
atures of a homologous series is known to depend on the interaction
strength parameter of the mesogenic groups (Imrie and Taylor,
1989). Further studies on homologous series of carbohydrate-based
liquid crystals now need to be undertaken to establish whether
the interaction strength parameter between the sugar moieties is
related to the magnitude of the odd–even effect in the clearing tem-
perature. The initial increase in the SmA-I transition temperature
on increasing chain length not only reflects the increase in van
der Waals interactions between the alkyl chains but also reveals
the tendency of these systems to microphase separate. Indeed, a
very general observation for low molar mass liquid crystals is that
increasing chain length promotes smectic behaviour (Date et al.,
1992). The subsequent decrease in the clearing temperature for
longer chain lengths is attributed to the back-folding of the chains
which disrupts the molecular packing (van Doren et al., 1989b). It is
interesting to note that the thermal properties of four members of
the methyl-6-O-(n-alkyl)-␣-d-glucopyranosides, the decyl, dode-
cyl, tetradecyl and hexadecyl members, have been reported and
even though their melting points are ca. 15 ◦C lower than those
of the corresponding members of series 1, no liquid crystalline
behaviour is observed (Miethchen et al., 1992). This suggests that
the carbonyl group participates in hydrogen bonding which assists
in stabilising the smectic arrangement.
carbohydrate molecules showed a strong tendency to adhere to the
glass surface resulting in much of the sample becoming homeotrop-
ically aligned and hence optically extinct. This is typical behaviour
for carbohydrate-based liquid crystals (Bault et al., 1998a,b) and
is an indication that the phase is optically uniaxial. Thus, the
molecules are orthogonal with respect to the layer planes, and
the phase is assigned as a smectic A (SmA) phase. The monotropic
nature of the phase precluded the possibility of structural stud-
ies using X-ray diffraction. By analogy with structurally related
materials, however, it is reasonable to assume that the molecules
are in an interdigitated arrangement (see for example, Bault et al.,
1998a,b). We will return to a discussion of the local molecular
arrangement within the SmA phase later. The melting points listed
in Table 1 are in good agreement with literature values (Baczko and
Plusquellec, 1991; Abe et al., 1996) although the liquid crystalline
behaviour of this series was overlooked in these previous studies.
Fig. 2 shows the second heating and cooling DSC traces for series
1 while Fig. 3 shows the traces for the clearing transitions obtained
by heating the metastable smectic A phase from a temperature just
prior to the onset of crystallisation. On cooling to −20 ◦C each mem-
ber of the series crystallised with the exception of the undecanoyl
homologue whichinsteadexhibits coldcrystallisationonreheating.
This suggests that this homologue has a tendency to vitrify which
is often observed for liquid crystal carbohydrates (see for example,
Bault et al., 1998b; Letellier et al., 1997; Ewing et al., 1998). We
should note that the weak peak seen in the cooling DSC trace for
the undecanoyl homologue indicates a small amount of crystalli-
sation does occur (Fig. 3(b)) but microscopic studies show that this
peak is not associated with the formation of a liquid crystal phase.
Fig. 4 shows the dependence of the transition temperatures on
the length of the alkyl chain, n, for series 1. The melting points
increase essentially in a linear fashion on increasing the alkyl chain
length although the change in melting point is relatively small over
The values of the entropy change associated with the smec-
tic A-isotropic transition, expressed as the dimensionless quantity
ꢁSSmAI/R, are considerably lower than those normally associated
with the SmA-I transition of a conventional low molar mass meso-
gen (Imrie et al., 1993). Indeed the values listed in Table 1 are similar
in magnitude to those normally observed for the nematic-isotropic
transition (Imrie and Taylor, 1989). This appears to be a rather gen-
eral observation (see for example, Van Doren et al., 2000; Letellier
et al., 1997; Ewing et al., 1998) but surprisingly appears not to
have attracted significant attention. Indeed, for some systems the
entropy change associated with the SmA-I transition has been too
small to detect.
Fig. 1. Focal conic fan and homeotropic textures of the smectic A phase exhibited
by the tetradecanoyl homologue (68 ◦C).

A.G. Cook et al. / Chemistry and Physics of Lipids 164 (2011) 118–124
121
Fig. 2. DSC traces for the methyl-6-O-(n-acyl)-␣-d-glucopyranosides: (a) second-heating and (b) cooling. The label on each trace indicates the total number of carbon atoms
in the acyl chain.
Fig. 3. The peak associated with the SmA-I transition in the heating DSC traces for the methyl-6-O-(n-acyl)-␣-d-glucopyranosides. For n = 15 and 16 crystallisation occurred
on prior cooling. The label on each trace refers to the total number of carbon atoms in the acyl chain.

122
A.G. Cook et al. / Chemistry and Physics of Lipids 164 (2011) 118–124
free OH groups, i.e., the 3650–3590 cm−1 band is absent. On heat-
ing the changes in the spectrum are essentially identical although
the transition is now crystal-isotropic phase.
100
90
80
70
60
50
T / °C
I
The nature of the hydrogen bonding within a system containing
a carbonyl group as the hydrogen bond acceptor can also be mon-
itored by inspection of the band associated with the C O stretch,
as the frequency of this is reduced according to the strength of
the hydrogen bond. Thus, the dependence of the 1790–1650 cm−1
region of the infrared spectrum of the dodecanoyl homologue as
a function of temperature is shown in Fig. 6. Again the spectra
have been measured on cooling the isotropic phase. There is no
apparent change in either the frequency or shape of the band asso-
ciated with the C O stretch on cooling from the isotropic phase into
the smectic A phase. Again this indicates that the extent of hydro-
gen bonding is essentially unchanged at the SmA-I transition. On
crystallisation, however, the band shifts to lower frequency and
narrows, implying an increase in hydrogen bonding. On heating,
the changes in the spectra are essentially identical. These changes
support the view suggested earlier that the carbonyl group, through
its involvement in hydrogen bonding, is important in determining
the thermal behaviour of these compounds.
Cr
(SmA)
11
12
13
14
15
16
17
18
n
Fig. 4. Dependence of the transition temperatures on the length of the alkyl chain,
n, for the methyl-6-O-(n-acyl)-␣-d-glucopyranosides. ꢀ indicates melting points,
ꢀ smectic A-isotropic transitions. I denotes isotropic phase, Cr crystal phase and
(SmA) monotropic smectic A phase.
The variable temperature infrared spectra shown in Figs. 5 and 6
are consistent with the view, therefore, that the melting point is
driven to a large extent by a disruption in the hydrogen bonding
network while at the smectic A-isotropic transition, the hydro-
gen bonding is largely unchanged but instead, presumably, the van
der Waals interactions are reduced. There are two models for the
smectic A phase exhibited by carbohydrate liquid crystals. Goodby
and co-workers suggested that the sugar moieties are located in
the interior of the bilayers which are held together by hydrogen
bonding and the alkyl chains are on the outside (Goodby, 1984;
Pfannemüller et al., 1986). Alternatively, the interdigitated alkyl
chains are considered to form the core of the bilayer with the sugar
groups located on the exterior (Jeffrey and Bhattacharjee, 1983;
Jeffrey, 1986; Van Doren and Wingert, 1991).
Fig. 5 shows the dependence of the 3700–1900 cm−1 region of
the infrared spectrum of the dodecanoyl homologue of series 1 on
temperature, measured on cooling from the isotropic phase. The
band associated with O–H stretching is often used as a test for, and
a measure of, the strength of hydrogen bonding within a system.
Thus, a free O–H group gives rise to a strong and sharp band in the
range 3650–3590 cm−1 while involvement in hydrogen bonding
shifts this band to lower frequency and it generally becomes much
broader. The OH stretch is centred at ca. 3400 cm−1 in the isotropic
phase and shows no apparent change in either frequency or band
shape on cooling into the smectic A phase, see Fig. 5. On crystallis-
ing, the OH stretch shifts to a lower frequency, ca. 3300 cm−1, and
broadens. These changes are indicative of an increase in the extent
of hydrogen bonding. Thus, the nature and extent of the hydrogen
bonding is unchanged on passing through the clearing transition
but becomes considerably more extended in the crystal phase. It is
important to note also that there is no evidence for the existence of
Itis clear from the infraredspectrathatatthe meltingpointthere
is a reduction in the hydrogen bonding network. This gives rise to
hydrogen bonded aggregates in the smectic A phase, the structures
of which differ according to which model is invoked. If we assume
that the sugar moieties are located on the outside of the bilayer then
Fig. 5. The dependence of the 3700–1900 cm−1 region of the infrared spectrum of methyl-6-O-dodecanoyl-␣-d-glucopyranoside on temperature. The spectra have been
recorded on cooling from the isotropic phase.

A.G. Cook et al. / Chemistry and Physics of Lipids 164 (2011) 118–124
123
Fig. 6. The dependence of the 1790–1650 cm−1 region of the infrared spectrum of methyl-6-O-dodecanoyl-␣-d-glucopyranoside on temperature. The spectra have been
recorded on cooling from the isotropic phase and note that the temperature scale is reversed with respect to that used in Fig. 5.
hydrogen bonding presumably occurs both between and within the
layers. At the smectic A-isotropic transition this structure must nec-
essarily become disrupted with an accompanying change in the
infrared spectrum. This is not observed. Instead, if the sugar moi-
eties are located in the centre of the bilayer then the hydrogen
bonded network may be finite and essentially within a layer. At
the smectic A-isotropic transition, the change in the van der Waals
interactions between the alkyl chains destroys the smectic arrange-
ment while the hydrogen bonded aggregates remain intact. Thus,
there would be no apparent change in the O–H stretching region of
the infrared spectrum and indeed none is observed.
If we now return to the unusually low values seen for the
entropy changes associated with the smectic A-isotropic transi-
tion which, as we have already noted, are in fact comparable to the
typical magnitude of the entropy change observed for the nematic-
isotropic transition (Imrie and Taylor, 1989; Imrie et al., 1993). For
low molar mass mesogens, reductions in the entropy change asso-
ciated with the liquid crystal-isotropic transition are most often
interpreted in terms of an increase in the molecular biaxiality aris-
ing from, for example, chain branching (Yeap et al., 2009; Imrie,
1989) or changes to the mesogenic core structure (Attard et al.,
1992; Attard and Imrie, 1992; Donaldson et al., 2010; Blatch et al.,
1997). Such an interpretation, however, cannot account for the
entropy changes seen here. Instead, within the model of the smectic
A-isotropic transition proposed earlier in which the sugar moi-
eties are located in the centres of the bilayers, the persistence of
hydrogen bonded aggregates into the isotropic phase reduces the
changes in orientational and translational ordering that would nor-
mally be expected at the smectic A-isotropic transition. Hence,
a smaller than expected entropy change would be observed. A
similar explanation has been used to account for the similarly
low values of the entropy change associated with the smectic A-
isotropic transition observed for the hexafluorophosphate salt of
1-hexadecyl-3-methylimidazolium for which the aggregates are
ionic in nature (De Roche et al., 2003).
monotropic smectic A phase. The smectic A-isotropic transi-
tion temperatures initially increase on increasing chain length,
accounted for by an increase in the van der Waals interactions
between the alkyl chains, but subsequently decrease on further
increases in chain length. This decrease is attributed to back folding
of the alkyl chain disrupting the packing of the molecules. Variable
temperature infrared spectroscopy has revealed that the extent
of hydrogen bonding changes markedly at the melting point but
reveals no apparent change at the smectic A-isotropic transition.
These observations have been accounted for in terms of Goodby’s
model of the smectic A phase in which the carbohydrate groups are
located in the centre of the bilayer (Goodby, 1984) and suggesting
that hydrogen bonded aggregates persist into the isotropic phase.
Within this framework the unusually low values of the entropy
change associated with the smectic A-isotropic transition seen for
these mesogenic sugars have also been explained. The generality
of these observations and in particular, the changes in the infrared
spectrum on varying temperature must now be established for a
wider range of carbohydrate-based liquid crystals.
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