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 manifestation 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 $\mathrm{Sm}{-A}_d$ phase of $4-\mathrm{octyl}{-4\mathrm{}}^{\prime }$-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 $\mathrm{Sm}{-A}_d$ 8CB, and demonstrates 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.

• Received 30 April 2001

DOI:https://doi.org/10.1103/PhysRevE.64.051703