www.elsevier.com/locate/cplett

Chemical Physics Letters 410 (2005) 417–422

Dipole-induced chiral smectic-C phase in a eutectic mixtureof cholesterol esters

P. Das *, S. Basu, R.K. Sinha, U. Das

Department of Chemistry, Bose Institute, 93/1 A.P.C. Road, Kolkata, West Bengal 700 009, India

Received 31 March 2005; in final form 20 May 2005

Available online 21 June 2005

Abstract

Both cholesteryl oleate (CO) and cholesteryl oleyl carbonate (COC) are thermotropic mesogens with a general cooling phase

sequence: isotropic ! cholesteric ! smectic ! crystal. They are almost identical with respect to molecular structure and the central

location of molecular dipole. However, the orientation of the latter is axial for CO and transverse for COC. A eutectic mixture of

these components shows a phase sequence like the above but involving an induced smectic-C* phase. We explain this discovery on

the basis of computer simulation results involving rigid core-dipole-flexible tail model of a mesogen and argue in favor of a ferro-

electric liquid crystal model of the induced phase.

� 2005 Elsevier B.V. All rights reserved.

Cholesterol esters are well-known typical examples of

thermotropic mesogens [1,2]. For example, cholesteryl

oleate (CO), one of the most abundant cholesterol esters

in living tissues [3], has a cooling phase sequence: iso-

tropic ! cholesteric ! smectic-A! crystal. The liquid

crystal (LC) phases, e.g., cholesteric and smectic-A, are

metastable and revert to crystal on keeping [4]. Notewor-

thy is the fact that cholesteryl oleyl carbonate (COC),which is structurally pretty similar to CO, has a similar

phase sequence with thermodynamically stable LC

phases [5] and just a different smectic phase identity [6].

Intermolecular interaction leading to the emergence of

LC phases has been explored theoretically [7–11]. To

summarize briefly, it has been hypothesized in a general-

ized van der Waals model [8] that the short-range steric

repulsive interaction is balanced by longer-range disper-sion interaction to confer stability on the cholesteric

liquid. Dipoles, though not an essential attribute of chiral

nematogens, may affect the thermal range of the chole-

steric. By contrast, the stability of the smectic phases is

0009-2614/$ - see front matter � 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2005.05.111

* Corresponding author. Fax: +91 33 2350 6790.

E-mail addresses: pdas@bosemain.boseinst.ac.in, pdas_123@

yahoo.com (P. Das).

a subtler issue to grasp. Considerable theoretical and sim-

ulation work has already been done to accomplish the

goal of appreciating the correlation between molecular

structure and mesophase behavior of mesogens with a

thermotropic phase sequence involving a smectic-A or

smectic-C phase or both. In the well-known McMillan

model [9] of the smectic-A phase anisotropic interaction

is supposed to occur between the hard rodlike sectionswhile the flexible tails have a role in determining packing

densities. The molecular dipoles have been supposed to

be weak or disordered in this treatment and thus effec-

tively ignored. In McMillan theory of the smectic-C

phase dipole–dipole interactions create the tilt [10]. The

presence of transverse, terminal dipoles was suggested

to be a prerequisite for smectogen-C. In a later model,

the quadrupole–quadrupole interaction was used as aperturbation of the smectic-A phase to give a smectic-A

to smectic-C transition [11]. It has, however, been exper-

imentally shown that molecules with only centrally

located, transverse dipoles may also generate the smec-

tic-C phase, but off-center, outboard dipoles yield greater

thermal range of stability [12].

Recently, there has been a resurgence of interest in

the phase transitions of systems of dipolar hard sphero-

Fig. 1. Molecular structure of (a) cholesteryl oleate; (b) cholesteryl

oleyl carbonate; in each of them, the fused alicyclic steroid ring is rigid

whereas the hydrocarbon and fatty ester (in CO) and fatty carbonate

(in COC) chains have some flexibility. The arrowhead indicates the

direction of molecular dipole. (c) Phase diagram of the CO/COC

binary mixture. The eutectic point, E has coordinates: weight % of

COC = 20 and temperature = 24 �C. P is a peritectic point and E 0 is

another eutectic point with coordinates: weight % of COC = 80 and

temperature = 5 �C. Sm stands for a higher-ordered smectic phase.

Note the immiscibility in solid state and partial immiscibility of LC

states in appropriately demarcated regions.

418 P. Das et al. / Chemical Physics Letters 410 (2005) 417–422

cylinder molecules [13]. The collective organization of

such molecules is the result of a balance between the

short-range repulsive hardcore and the longer-range

dipolar interactions. This governs another balance be-

tween the tendency of two dipoles to arrange in antipar-

allel geometry and the formation of domains with acommon dipole orientation. A primitive model for small

mesogenic molecules has been proposed, being com-

prised of a hard spherocylinder attached at one terminal

to a semiflexible tail and a dipole at the other terminal

opposite the flexible tail [14]. Monte Carlo simulation

taking account of dipolar interaction shows four phases:

isotropic, cholesteric, smectic and crystal. A terminal di-

pole without the tail stabilizes the cholesteric phase atthe expense of the smectic-A [15]. The latter is stabilized,

however, when the tail is present but the dipole is miss-

ing. Monte Carlo study [16,17] of spherocylinders with

central axial point dipoles shows that the cholesteric

phase is destabilized with respect to both the isotropic

and smectic-A phases. In the case of a system of mole-

cules, each with a large longitudinal, terminal dipole,

the cholesteric phase is stabilized relative to the smec-tic-A phase; the staggered dipolar pairs are difficult to

accommodate within the smectic layers [14]. Recently,

a number of off-lattice models have appeared of simple

molecules with rigid cores and one or two (semi)flexible

tails [18]. Of particular interest is a model comprised of a

rigid core with two flexible tails built up as a chain of

atoms. All atoms have repulsive interactions but be-

tween two rigid cores there are also specific attractions.Isotropic, smectic-A and crystal phases have been re-

ported but no cholesteric phase.

A Monte Carlo simulation of a spherocylinder model

of a mesogen with centrally located transverse point di-

pole showed the dipoles to stabilize the formation of the

smectic-A phase relative to the cholesteric phase; the lat-

ter disappears altogether at lower temperatures [13]. In

the smectic-A phase the cores of the molecules align par-allel to the layer normal with the dipoles in the plane. At

high temperatures the dipoles are disordered; as the tem-

perature is lowered ring-like domains appear and then

elongated antiferroelectric chain-like domains form.

Clearly the smectic phase in this model possesses greater

in-layer order than is exhibited by a fluid smectic.

Thus the contention of both molecular theory and

computer simulation is that spherocylinder moleculeswith central axial dipoles or such molecules with weak

dipoles but long tails prefer to form a monolayer smec-

tic-A phase with layer spacing almost equal to the

molecular length. A typical example may be the cho-

lesteryl oleate molecule, which is about 36 A long and

has an axial dipole of moment 2.02 D, due to the ester

linkage, located at the center of the molecule [19]. The

assignment of dipole direction seems to be consistentwith the molecular model (Fig. 1a). The polarizing light

micrographs consist of batonnets and focal conic fan

[20], which are typical features of the smectic-A phase

[1,2]. In view of the fact that the extended molecular

length of CO in the crystal state at 295 K is 41.37 A[21], the reported smectic layer spacing [3], e.g., 36.7 A

implies an average non-cooperative tilt of 27.5�. Inter-estingly, in agreement with the prediction of simula-

tions, the cholesteric phase has a much smaller thermal

range than both the isotropic and smectic states (Fig.

1c) [5]. The COC molecule, on the other hand, has a di-

pole moment of 1.14 D, directed transverse to the

molecular long axis [22]. It is only slightly longer thanthe CO molecule, having one extra oxygen atom to give

the carbonate moiety and one extra –CH2-group in the

fatty chain (Fig. 1b). Note that the static dielectric per-

mittivity e(T) registers an averaged polarizability deriv-

ing contributions from all the molecular structures,

which, for the isotropic phase, comprise the liquid-like

background and fluctuations with premesomorphic

ordering. For molecules with permanent dipole perpen-dicular to the molecular long axis as in COC, e(T) is a

Fig. 2. The polarizing light micrographs: (a) cholesteric texture; (b)

spherulitic and focal conic fan textures of smectic-A phase in pure CO;

(c) banded focal conics of smectic-C* phase in the eutectic mixture; (d)

schlieren texture of the smectic-C* phase; (e) banded focal conic fan

texture; (f) spherulitic and threadlike texture in 80% COC sample.

Magnification: (a) 57·; (b) 57·; (c) 228·; (d) 228·; (e) 228·; (f) 114·.Crossed polarizers are along the edges of each photograph.

P. Das et al. / Chemical Physics Letters 410 (2005) 417–422 419

linear function of temperature T up to the clearing

point, without any manifestation of pretransitional fluc-

tuations, owing to a lack of contrast between the back-

ground and the fluctuations [6]. Like CO, COC forms a

cholesteric and a smectic phase, but the latter is not ex-

actly of the smectic-A variety as in CO. Apparently, asthe spherocylinder molecules line up to define the smec-

tic lamellar distance, the transverse dipolar order may

vary from short-range (i.e., smectic-A) through med-

ium-range (ring-like domains) to long-range (antiferro-

electric chain-like) with progressive lowering of

temperature [13]. It is noteworthy that CO and COC

have almost identical hard rod and flexible chain sec-

tions so that the steric interaction between them in a bin-ary mixture is expected to be just about the same as in

an assembly of pure CO molecules. However, the ques-

tion of the effect of dipolar interaction between two

nearly identical molecular structures with extreme differ-

ence in molecular dipolar orientation on the binary

phase behavior is rather intriguing. Also interesting is

the question of how similar the smectic phase of COC

is to the smectic-A phase of CO, in view of their molec-ular structural similarity but variation in dipolar

structure.

In order to investigate these questions further we

undertook to construct a binary phase diagram (Fig.

1c) of CO and COC, using standard techniques such

as polarizing light microscopy, differential scanning cal-

orimetry and X-ray diffraction. Cholesteryl oleate and

cholesteryl oleyl carbonate, both stated to be of greaterthan 99% purity, were obtained from Acros Organics,

Geel, Belgium. Their purity was checked with thin layer

chromatography (TLC). The COC sample was found to

be reasonably pure as claimed. However, the CO sample

needed further treatment involving silica gel column

(Merck, mesh size 60–120) chromatography in a chloro-

form medium, followed by elution with a 1:10 metha-

nol–petroleum ether (boiling range: 60–80 �C) mixture.Requisite amounts of CO and COC were weighed out

and mixed in petri dishes of diameter 2 in. each to give

binary mixtures containing 10%, 15%, 20%, 25%, 30%

and so on, by weight of COC. Each binary mixture

was warmed to 50 �C to ensure a thorough mixing of

the components in the isotropic phase for about half

an hour; then cooled to room temperature and pre-

served with the lid on. A Prior Scientific polarizing lightmicroscope model no. MP 3500, equipped with a

Mettler FP-80 hot stage, was used to view the samples

with objectives of various magnifications. Differential

scanning calorimetry was carried out on a Mettler (To-

ledo) 827 instrument equipped with a liquid nitrogen

cooler. A temperature range from 0 to 50 �C was

scanned at the rate of 2� per min, both on warming

and cooling. X-ray diffraction was carried out on a Phil-lips make powder diffractometer (model no. CW1710)

operating on the software number CW 1877, equipped

with a temperature controller permitting low tempera-

ture as well as high temperature operation as needed.

Under polarized light cholesteryl oleate successively

shows, on cooling from the liquid state, a cholesteric

(Fig. 2a) and a smectic phase [20]. The smectic phase

shows a spherulitic and a focal conic fan texture whenthe sample is held between a glass plate and a cover slip

(Fig. 2b). These are characteristic features of the smec-

tic-A phase [1,2]. The Debye–Scherrer pattern (Fig. 3a)

of pure CO shows a Bragg peak at 2h = 2.407� corre-

sponding to d = 36.7 A and a diffuse scattering at

2h � 20�. Since this is a monolayer smectic-A phase

comprising molecules with axial dipoles, the phase struc-

ture is as shown in Fig. 4a, in which the in-layer order isliquid-like short-range giving rise to the diffuse scatter-

ing in the X-ray profile. Unlike COC, CO should exhibit

a pretransitional anomaly in the static dielectric con-

stant e(T) in the isotropic phase on an approach to the

clearing point [6]. The cylindrical symmetry of the smec-

tic-A spherulites (Fig. 2b) suggests a toroidal focal conic

Fig. 3. Debye–Scherrer pattern of (a) CO at 24 �C (smectic state); (b)

20% COC (eutectic) mixture at 28 �C; (c) 80% COC at 24 �C (smectic

state). Fig. 4. The smectic organization is determined by competing steric

(excluded volume) and dipolar interactions: (a) cholesteryl oleate with

the dipoles along the molecular long axes; (b) 20% COC (eutectic)

mixture with all molecular axes tilted with respect to the smectic layer

normal (u). One central COC molecule together with its four nearest

neighbor CO molecules constitutes an effective molecular unit or

block. The preferred direction (n) of alignment of block long axes (v) is

inclined to u at the tilt angle (h). The cross indicates that the transversepolarization is along the direction defined by u · n. (c) Cholesteryl

oleyl carbonate with the dipoles transverse to the molecular long axes.

Although very short-range order shows a common dipole direction,

the long-range in-layer organization is antiferroelectric [13].

420 P. Das et al. / Chemical Physics Letters 410 (2005) 417–422

structure where the inner torus radius is too small to be

resolved. Since free termination of the smectic layers is

associated with large surface energy, the appearance of

a focal conic texture where all layers terminate on the

free surface is expected. The addition of 20% by weight

of cholesteryl oleyl carbonate, followed by a warm to50 �C (i.e., isotropic phase, to ensure a thorough mixing)

and then cool to just a few degrees above room temper-

ature (e.g., 28 �C) results in the formation of the eutectic

mixture in which the spherulitic texture gives way to

banded focal conics (Fig. 2c) and schlieren texture

(Fig. 2d), signifying the appearance of the smectic-C*

phase [2]. In the schlieren texture, only four brushes

can meet at a point, as is typical for smectic-C and smec-tic-C* samples [23]. When the sample is held between a

glass plate and a cover slip, focal conic fan texture,

adorned with equispaced lines (Fig. 2e), are seen. The

latter show strong polarization colors and from which

it is possible to measure the helical pitch (e.g.,

18.94 lm). The Debye–Scherrer pattern (Fig. 3b) of

the eutectic mixture shows a layer thickness of 31.70 A

(i.e., tilt angle �40�) with Cu Ka radiation of wave-

length k = 1.5418 A, corresponding to 2h = 2.78� and a

fairly sharp diffuse scattering at 2h � 17.5�. The latter

corresponds to an in-layer short-range positional order

(Fig. 4b), to be described in next paragraph. When thesmectic-C* phase is warmed the banded focal conics

and schlieren texture disappear. In their place strongly

birefringent cholesteric textures (Fig. 2a) appear, which

P. Das et al. / Chemical Physics Letters 410 (2005) 417–422 421

vanish on further heating, yielding an isotropic phase.

Thus, a phase sequence: isotropic ! cholesteric !smectic-C* realizes on variation of temperature of the

eutectic mixture. The banded focal conics owe their ori-

gin to Dupin cyclides [1,2], with the equispaced lines

arising from a polarized diffraction of the helical macro-structure of the smectic-C* phase. The threadlike struc-

tures (Fig. 2f), which abound in the smectic �Sm� phaseof the 80% COC sample, suggest attractive interaction

between adjoining spherulites. The phase is thus pre-

sumably smectic-A-like with greater in-layer order.

The confirmation comes from the Debye–Scherrer pat-

tern (Fig. 3c) for the �Sm� phase (Fig. 1c), which shows

a Bragg peak at 2h = 2.4� corresponding to the smecticlamellar distance d0 0 1 = 36.81 A. Another Bragg peak

is observed at 2h = 9.5� corresponding to d = 9 A. There

is scarcely any diffuse scattering. This implies that the

smectic layer is not fluid but has a long-range order

(Fig. 4c).

The remarkable result emerging from this study of

the binary phase behavior of the CO/COC system is

the appearance of the induced chiral smectic-C phase,which does not occur in the thermotropic phase se-

quence of either mesogenic component. Since the molec-

ular weights of CO and COC are nearly equal, the

eutectic mixture has a CO to COC molecule number ra-

tio equal to 4:1. We have constructed a model, based on

competing steric and dipolar interactions between (chi-

ral) blocks, each being comprised of a central COC mol-

ecule surrounded by four nearest neighbor COmolecules, to create the tilt plane comprising the layer

normal and the preferred direction (n) of alignment of

block long axes (Fig. 4b). The whole smectic layer con-

sists of such blocks with head-to-tail randomization.

Alignment of steric dipoles occurs perpendicular to the

tilt plane. The component of the electric dipoles of CO

and COC molecules along the direction (C2 symmetry

axis) of steric dipole alignment constitutes the macro-scopic transverse polarization. The head-to-tail random-

ization ensures that the polarization in the tilt plane

cancels out over a smectic layer. All CO molecules in a

single smectic layer are similarly tilted but in a direction

opposite to that of the COC molecules which themselves

are similarly tilted. The induced smectic-C* phase has a

net cooperative tilt determined by the balance of the

opposite tilts of CO and COC molecules, since thereare four times as many CO molecules as COC molecules.

Clearly the model considered here is that of a ferro-

electric smectic-C*.

In conclusion, we point out that a layer-to-layer alter-

nation of tilt angle in the antiferroelectric fashion, i.e.,

smectic-CA* phase, is observed along with or even pre-

ferred, in some cases, to a layer-to-layer precession of tilt

angle as in ferroelectric smectic-C*, in some 1000 com-pounds known to date [24]. These molecules are fairly

similar and each possesses a dipolar group like carbonyl

in the vicinity of a chiral center located somewhat off the

molecular center. The alkyl chain attached to the dipole

is rather short and has been found to be bent even in the

smectic-A phase of some of the mesogens studied [24],

leading presumably to a strong dipolar interaction be-

tween adjacent layers. The smectic-CA* phase has aspontaneous polarization, related to chirality, normal

to the tilt plane, just as in the smectic-C* phase. How-

ever, unlike the latter, it also has a polarization parallel

to the tilt plane but at the smectic layer boundary. This

polarization is unrelated to chirality and is attributable

to dipolar interaction between adjacent layers [24]. For

the CO/COC case we note that both the components

have central dipoles, e.g., carbonyl groups, in the vicin-ity of chiral centers, e.g., steroid ring carbon with aster-

isk (Fig. 1a,b). Unlike smectogen-CA*, which generally

exhibits a direct isotropic to smectic transition, the

CO/COC eutectic mixture has a cholesteric phase of

four-degree thermal range. The central location of the

dipole, flanked by a long alkyl tail on one side and, on

the other, a bulky rigid steroid ring linked to a short al-

kyl chain, would most likely generate packing entropydue to the excluded volume effect, dispersion or

Maier–Saupe-type intermolecular interaction and in-

tra-layer dipolar interaction, which would cause tilting

in the same direction and sense except for a slight

layer-to-layer precession due to chirality [24]. In CO/

COC smectic-C* phase, end-chains are expected to re-

main partly untilted, and the hard core section plus

remaining parts of end-chains, though tilted [25], wouldprevent a close approach of the central dipole to the

layer boundary, thereby precluding an interlayer dipolar

interaction. We thus believe that the eutectic mixture has

a ferroelectric smectic-C* phase. The demonstration of

ferroelectricity and the nature of interaction between

the mesogenic components could be the themes of future

exploration.

Acknowledgement

P. Das wishes to gratefully acknowledge the financial

support received from ICMR (India) through scheme

no. 52/6/2001 – BMS.

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