Microscopic structure and dynamics of a partial bilayer smectic liquid crystal
Yves Lansac, Matthew A. Glaser, and Noel A. Clark
Condensed Matter Laboratory, Department of Physics, and Ferroelectric Liquid Crystal Materials Research Center,
University of Colorado, Boulder, Colorado 80309
Received 30 April 2001; published 16 October 2001
Cyanobiphenyls nCB’s represent a useful and intensively studied class of mesogens. Many of the peculiar
properties of nCB’s e.g., the occurence of the partial bilayer smectic-A
d
phase are thought to be a manifes-
tation of short-range antiparallel association of neighboring molecules, resulting from strong dipole-dipole
interactions between cyano groups. To test and extend existing models of microscopic ordering in nCB’s, we
carry out large-scale atomistic simulation studies of the microscopic structure and dynamics of the Sm- A
d
phase of 4-octyl-4 ' -cyanobiphenyl 8CB. We compute a variety of thermodynamic, structural, and dynamical
properties for this material, and make a detailed comparison of our results with experimental measurements in
order to validate our molecular model. Semiquantitative agreement with experiment is found: the smectic layer
spacing and mass density are well reproduced, translational diffusion constants are similar to experiment, but
the orientational ordering of alkyl chains is overestimated. This simulation provides a detailed picture of
molecular conformation, smectic layer structure, and intermolecular correlations in Sm- A
d
8CB, and demon-
strates that pronounced short-range antiparallel association of molecules arising from dipole-dipole interactions
plays a dominant role in determining the molecular-scale structure of 8CB.
DOI: 10.1103/PhysRevE.64.051703 PACS numbers: 61.30.-v,64.70.Md
I. INTRODUCTION
Liquid crystals LC’s, broadly defined as state of matter
intermediate between crystalline solid and isotropic liquid
are fascinating materials both from a fundamental and an
applied point of view 1. Due to their rich phase behavior,
resulting from the delicate interplay of fluidity and self orga-
nization, LC’s play an important role in biological systems
for example, lipid membranes and in technological appli-
cations, in particular electro-optic displays utilizing nematic
and, more recently, ferroelectric LC’s 2. Despite its key
importance for the synthesis of LC materials having optimal
properties for specific technological applications, the rela-
tionship of macroscopic properties to molecular structure and
microscopic organization is poorly understood. This is
mainly due to the extreme sensitivity of the LC properties to
small changes in the molecular architecture, itself a conse-
quence of the intricate interplay of a variety of energetic and
entropic effects responsible for the spontaneous formation of
mesophases.
Due to the rapid growth in available computer power, as
well as the development of efficient algorithms, Monte Carlo
MC and molecular dynamics MD simulations of chemi-
cally realistic molecular models hold considerable promise
for investigating the microphysics of LC’s and for illuminat-
ing the relationship between molecular architecture and mac-
roscopic behavior. Owing to the technical challenges associ-
ated with atomistic simulation of LC’s, however, there have
been only a relatively limited number of atomistic simulation
studies of thermotropic LC’s 3–26. The quality of these
studies varies widely, and whether or not specific studies
meet the criterion of ‘‘chemical realism’’ is debatable. An
early reported atomistic simulation of a thermotropic LC 3
was of total duration 60 ps, far too short a time to equilibrate
even a small sample 26. Subsequent work has focused on
the development of improved molecular models and on the
simulation of larger systems over greater spans of time.
The main fundamental obstacle to the quantitative model-
ing of LC phases is the need for highly accurate models of
molecular interactions, due to the sensitivity of LC properties
to details of molecular structure. The central aims of this
paper are to develop and validate the computational infra-
structure for achieving realistic modeling of LC materials,
and to test methodologies for deriving suitable interaction
potentials for LC’s. For this purpose, a well-studied experi-
mental system is needed to check the quantitative predictive
capabilities of our approach for specific LC compounds.
The partial bilayer phase (Sm- A
d
) of a of 4-octyl-
4 ' -cyanobiphenyl 8CB as been chosen for this study be-
cause it is the prototypal partial bilayer smectic. The layer
spacing of a partial bilayer smectic is intermediate between
that of a monolayer smectic ( d l , where l is the molecular
length and a bilayer smectic ( d 2 l ). The layer spacing is
d =31.432 Å at T =24°C 27, which is approximatively 1.4
times the fully extended molecular length of 22.1 Å, and d
increases with increasing temperature, in contrast to most
monolayer SmA materials. In fact, the Sm- A
d
phase of 8CB
is the ‘‘fruit fly’’ of smectic LC science, being undoubtedly
the most studied smectic phase.
The chemical structure and phase diagram of 8CB is
shown in Fig. 1. The most notable feature of the chemical
FIG. 1. Chemical structure and phase diagram of 8CB.
PHYSICAL REVIEW E, VOLUME 64, 051703
1063-651X/2001/645/05170312/$20.00 ©2001 The American Physical Society 64 051703-1