  We report experimental studies on optical (birefringence, $\Delta n$),
dielectric $(\Delta \varepsilon)$ and bend-splay elastic anisotropies ($\Delta
K=K_{33}-K_{11})$ of a few mixtures of two nematic liquid crystals, namely
CCH-7 and CCN-47, made of highly polar molecules with identical cores and
antagonistic orientation of permanent dipoles. In particular, the polar group
(-CN) attached to the bicyclohexane core of CCH-7 is oriented along the
longitudinal direction whereas, in CCN-47, it is oriented along the transverse
direction. We show that apart from the significant contribution to the optical
and dielectric anisotropies, the antagonistic orientation of strongly polar
groups plays a crucial role in determining the bend-splay elastic anisotropy.
The elastic properties are explained based on a model proposed by Priest,
considering the effect of intermolecular association and the resulting
length-to-width ratio of the molecules.
